slow6i 4/1/2018 3:54 PM
Fender Roc Pro 1000 - Output is SUPER low
Hey all! Ive been reading on the forums for a couple days now trying to figure out how to fix my amp, but alas, it has come time that I must post.

Amp is a fender roc pro 1000 combo. When I got it, I had a problem with the foot switch only switching between clean and the yellow distortion channel. So, me being me, I opened it up, and resoldered all of the contacts on the board. However, I accidentally bridged one -- in all of my infinite skill -- and obviously broke something because of that. When I powered on, there was no smoke, so thats good, but now Im trying to find the components that need to be replaced and Im hoping you all can help me.

Here is what I know:

- Amp has low output. Volumes all the way up, and I have to be sitting right next to it to hear it.
- Channel switching works with the face plate controls perfectly fine.
- Both clean and distortion work at this lower output.
- At TP22, I am getting ~30VAC, at TP23 I am getting ~80VAC (thats twice what it should be according to the schematic.)
- I was stupid and careless and torched my amp
- I want to fix it if possible.

So... Is there hope? Or should I scrap it and buy a different amp?

Thanks!
 
nickb 4/1/2018 4:30 PM
Welcome! You're not the first to make a mistake and I'm sure it can be fixed.

In order to measure tp23 you must put your meter on the DCV range, not ACV.

To fix this we will need to have a means of generating a test signal and then tracing it signal through the amp. So, what test equipment do you have?

Schematic
[ATTACH]47955[/ATTACH]
 
slow6i 4/1/2018 8:18 PM
Quote Originally Posted by nickb View Post
Welcome! You're not the first to make a mistake and I'm sure it can be fixed.

In order to measure tp23 you must put your meter on the DCV range, not ACV.

To fix this we will need to have a means of generating a test signal and then tracing it signal through the amp. So, what test equipment do you have?
Well, now that you say that, of course it needs to be measured in dcv... *derp*

Anyway, as far as test equipment goes, I really only have my volt meter. By signal, I assume that you are talking about an input signal from something like a guitar or keyboard or the like? Could I use my PC as an input with a tone generated by audacity or something?

My circuits experience is limited, especially diagnostics, so please bear with me. To me, its more worth it to diagnose and replace cheap parts myself, regardless of how long it takes then to take it to a shop and have them fix it. I don't learn anything that way.
 
glebert 4/1/2018 10:10 PM
I usually try to first figure out if it is a preamp or a power amp problem. Do you have another amp that has either an effects loop or a pre amp out / power amp in? A mixing board or mic preamp can also work if you have 1/4" outs. If you can use a known good preamp into the effects loop Return you can see if the power amp is working ok, and vice versa test the preamp by lining out of of the Send into an amp or mixer to see if you have a strong signal there. For testing the power amp you want to turn the effects mix all the way up.

Also, a smartphone with a function generator app can make a good test signal generator, but if you don't have a good meter or an oscilloscope it may not help that much. I am lucky to have an oscilloscope, never tried my DMMs to track small signals.
 
Enzo 4/1/2018 10:38 PM
To test the power amp, you can just plug the guitar into the FX return or Power AMp In jack. To test the preamp out or FX send yes you will need another amp.
 
slow6i 4/2/2018 12:07 AM
Quote Originally Posted by glebert View Post
I usually try to first figure out if it is a preamp or a power amp problem. Do you have another amp that has either an effects loop or a pre amp out / power amp in? A mixing board or mic preamp can also work if you have 1/4" outs. If you can use a known good preamp into the effects loop Return you can see if the power amp is working ok, and vice versa test the preamp by lining out of of the Send into an amp or mixer to see if you have a strong signal there. For testing the power amp you want to turn the effects mix all the way up.

Also, a smartphone with a function generator app can make a good test signal generator, but if you don't have a good meter or an oscilloscope it may not help that much. I am lucky to have an oscilloscope, never tried my DMMs to track small signals.
Portland, Or. Huh? I'm just across the river in Vancouver... anywho, I have a little Dean practice amp that has high and low inputs, and can go to an external speaker via 1/4 jack, but that's my only other guitar equipment. I will report back with results tomorrow evening.
 
glebert 4/2/2018 12:22 AM
Whatever you do don't put the Dean speaker output into the Roc Pro effects loop, that will blow it. You could probably get away with lining the Send from the Roc Pro to the Low input on the Dean but keep your volume on the Roc Pro way down at first as you don't want to blow the input on your working amp. Do what Enzo said and just plug a guitar into the Return jack on the Roc Pro and see if you get much sound out. Usually you don't get as much volume to the speaker when you do that, and since your problem is low volume it may be somewhat inconclusive, but worth a try.

You may want to probe the op amps and make sure they have correct voltages (+/-16V on pins 8 and 4 respectively), and one other test you can do is to touch the op amp input pins (2,3,5, or 6 depending on config) with your meter probe and see if you get a pop through the speaker. If the problem is in the preamp as you go through the signal chain eventually you should get louder pops once you get past the problem.
 
glebert 4/2/2018 1:01 AM
Also, with your original problem of it only switching between clean channel and yellow drive, are you sure you have the right footswitch for it? I have a Performer 1000 (same circuit as Roc Pro) and when I use a different (wrong) Fender footswitch I can only switch between clean and yellow. Correct footswitch should be a 3 button P/N 0064465000
 
slow6i 4/2/2018 10:45 PM
Quote Originally Posted by glebert View Post
Also, with your original problem of it only switching between clean channel and yellow drive, are you sure you have the right footswitch for it? I have a Performer 1000 (same circuit as Roc Pro) and when I use a different (wrong) Fender footswitch I can only switch between clean and yellow. Correct footswitch should be a 3 button P/N 0064465000
Tonight:

- Plugged my guitar into the FX return; nothing at all with the volume all the way up on both the instrument, and the amp.
- Plugged the FX send from the Roc into the low input on the Dean amp. Nothing.
- Plugged my guitar into the Dean, both inputs work.
- Decided against plugging the line out from the Roc to the input of the Dean.
- Tested the above mentioned pins, and results can be found at : https://docs.google.com/spreadsheets...it?usp=sharing
-- U7 is a different chip, so I got some questionable results initially, but after looking at the datasheet and retesting, it seems to be ok.

The foot pedal I have is PN 040409, from what I can find it is the correct one, but alas, I reserve the right to be wrong...

Whats next gentlemen?
 
glebert 4/2/2018 11:48 PM
Nice spreadsheet, BTW. I wish I was half that organized. This was done with clean channel? Was the volume up at least somewhat? Interesting that the first op amp where you get a response is pin 6 of U5, and a loud squeal at that point. If you crank the volume or the EQ pots all the way up do you hear the background noise increase? Wonder if there is something going on with Q7. Did you have a cable plugged in while you were doing this? Maybe Q7 (and Q6 on the drive channel) mutes the signal when there is nothing plugged in? Kind of weird how that inhibit line is fed with the -16V being connected through the switchjack in either position.
 
nickb 4/3/2018 1:00 AM
When doing these 'pop' tests you need to have something plugged into the input jack in order to defeat the muting of the reverb and the signal path prior to U5. You have a similar problem when probing U4 pin 3 as the return jack shorts (most of) the return signal to ground. So, redo these tests with something plugged in.

In any case, given you hear things when touching U5 you I'd expect to get something when plugging into the return jack. Redo that test but try different settings of the level switch and effect mix pot, just in case.
 
g1 4/3/2018 11:18 PM
This amp has a parallel FX loop, so when you plug your guitar directly into the FX return, you have to turn the FX mix pot up to 10.
 
slow6i 4/8/2018 4:50 PM
Quote Originally Posted by g1 View Post
This amp has a parallel FX loop, so when you plug your guitar directly into the FX return, you have to turn the FX mix pot up to 10.
Thank you for all the reply's, unfortunately, it will be over a week before I will be able to test these suggestions... Thanks for sticking with me!
 
slow6i 4/14/2018 7:02 PM
Quote Originally Posted by nickb View Post
When doing these 'pop' tests you need to have something plugged into the input jack in order to defeat the muting of the reverb and the signal path prior to U5. You have a similar problem when probing U4 pin 3 as the return jack shorts (most of) the return signal to ground. So, redo these tests with something plugged in.

In any case, given you hear things when touching U5 you I'd expect to get something when plugging into the return jack. Redo that test but try different settings of the level switch and effect mix pot, just in case.
Alright! Back from my work trip and I was able to sit down, albeit for only a minute, with my amp.

After plugging my guitar in through the FX return, and making sure all volumes / mixes were up, I get my guitar through the FX return (needed to turn up the effects mix as suggested. I had it down because my reverb wasnt working.)

Ill try to get to the pop test tomorrow during a homework break. So what is this telling me? Power amp seems good?
 
nickb 4/15/2018 1:14 AM
Yes, power amp is good.
 
slow6i 4/18/2018 3:02 PM
Quote Originally Posted by nickb View Post
Yes, power amp is good.
Alright, I have finally gotten time to redo the pop test on the op-amps.

Results can be found https://docs.google.com/spreadsheets...it?usp=sharing

Im still only getting +- 25 volts at U7 (pins 4 & 7), which is low compared to the schematic of +- 40V thought im not sure what this is telling me.

This was dont with volumes all the way up, mix all the way up. and a guitar plugged in with its volume up.

Next steps?
 
g1 4/18/2018 7:00 PM
+/- 25V at U7 is correct, look at TP17 & TP20 near U7.
Were your pop tests on the chart done with clean channel selected? If not, please redo with amp set to clean channel. Also set mix to 'dry' if you are not using the FX return jack.
 
slow6i 5/27/2018 11:06 PM
Quote Originally Posted by g1 View Post
+/- 25V at U7 is correct, look at TP17 & TP20 near U7.
Were your pop tests on the chart done with clean channel selected? If not, please redo with amp set to clean channel. Also set mix to 'dry' if you are not using the FX return jack.
Finally got some time to redo the test.

https://docs.google.com/spreadsheets...Azg/edit#gid=0 This spreadsheet is now up to date with the current information. I also included all my knob settings, and this was done with a cable plugged into the input jack.

Thanks for all the help!
 
g1 5/28/2018 6:50 PM
That says the mix is set to 10. If you are using the input jack you need the mix pot set to dry, which is 0.
 
slow6i 5/29/2018 11:28 PM
Quote Originally Posted by g1 View Post
That says the mix is set to 10. If you are using the input jack you need the mix pot set to dry, which is 0.
Ok, RE-re-did the tap tests, and updated the results on that sheet.

Im starting to think that I should just take it somewhere...
 
nickb 5/30/2018 1:09 PM
It looks like the signal is not getting thru Q7. Did you do the tests with something plugged in to the input? If you did, then measure the voltages at Q7 gate, CR20 cathode and CR21 cathode. If not then plug in and redo.
 
slow6i 5/30/2018 8:46 PM
Quote Originally Posted by nickb View Post
It looks like the signal is not getting thru Q7. Did you do the tests with something plugged in to the input? If you did, then measure the voltages at Q7 gate, CR20 cathode and CR21 cathode. If not then plug in and redo.
Yes, the test was done after plugging a cable into the input jack. For confirmation, I unplugged it as I was measuring your suggestions:

Q7 Pin 1 is the "top" of the D shape looking down at it.

Pin 1 Pin 2 CR20 No Stripe CR20 Stripe CR21 No Stripe CR20 Stripe
2 mV 17 mV 220 mV 5.69V 27 mV 7.95 V
0 V -15.24 V -15.84 V -16.22 V 15.83 V 7.95 V Cable Unplugged

Thanks!
 
nickb 5/31/2018 1:15 AM
With those voltages Q7 should be enabled. If Q7 were bad it would explain the clean channel volume but there would have to be a second fault to explain the overdrive signal that comes through Q6.

So, more data is needed. Select the o/d channel and measure the voltages on CR15 and CR17 both sides with the input plugged in. Also measure the voltages on U5 pins 5 and 7, they should both be close to zero.
 
slow6i 5/31/2018 11:47 PM
Quote Originally Posted by nickb View Post
With those voltages Q7 should be enabled. If Q7 were bad it would explain the clean channel volume but there would have to be a second fault to explain the overdrive signal that comes through Q6.

So, more data is needed. Select the o/d channel and measure the voltages on CR15 and CR17 both sides with the input plugged in. Also measure the voltages on U5 pins 5 and 7, they should both be close to zero.
Thanks for the response, here are the latest results.
Yellow Distortion Channel
CR15 Stripe: 6.48 V
CR15 No Stripe: 37 mV

CR17 Stripe: 5.70 V
CR17 No Stripe: 41.2 mV

U5-p5: 0 V
U5-p7: .2 mV
Red Distortion Channel
CR15 Stripe: 6.49 V
CR15 No Stripe: 38.3 mV

CR17 Stripe: 5.70 V
CR17 No Stripe: 42.6 mV

U5-p5: 0 V
U5-p7: .2 mV

Notes:
- When measuring the stripe side of CR17, the voltage started at ~10 V, then worked its way down to ~5.7 over the course of about 2 seconds, kind of like a Cap discharging or something.
- All settings are exactly the same as the google sheet save for the distortion channel selected.
 
nickb 6/1/2018 1:10 PM
Hmm. Well that checks out and I'm not buying that Q6 and Q7 are bad until proven. Please double check: select clean, clean vol up, plug into the input jack and do the noise touch test on U1 pin 5 again. Also measure the DCV on I1 pin 7 (should be about zero) .

Put you meter on ACV and measure the ACV on U1 pin 7 while you strum your guitar or connect a signal generator or music source - whatever works for you. Do you get and significant change in ACV reading when you play? If you do I'd say that U1B is OK and the problem is later. In that case try bypassing Q7 by linking CR19 stripe to U5 pin 6. More likely you'll get nothing in which case I'd replace IC1.
 
slow6i 6/1/2018 5:23 PM
Quote Originally Posted by nickb View Post
In that case try bypassing Q7 by linking CR19 stripe to U5 pin 6. More likely you'll get nothing in which case I'd replace IC1.
Do you mean bridging it with a piece of wire or something on the underside then measuring U1 again? Or checking to see if I get amp output with my instrument? Comfortable with both, I just want to make certain I understand what you mean.

Also:
U1 pin 5: 0 Vdc
U1 pin 7: -5.5 mVdc
Pin 7: 3.2 acV (strumming with tone knob at 10, using bridge pickup, clean vol. at 10 on amp.)
 
nickb 6/2/2018 1:11 AM
Well U1B is OK.

Yes, bridge with wire. You should be able to do this from the top without removing the board. I would use a piece of wire wire ez-hooks. But first let's do some more ACV testing. Turn all the tone knobs to max and measure the ACV on U1 pin 5 and R24 with the strum test.
 
slow6i 6/2/2018 2:54 PM
Quote Originally Posted by nickb View Post
Well U1B is OK.

Yes, bridge with wire. You should be able to do this from the top without removing the board. I would use a piece of wire wire ez-hooks. But first let's do some more ACV testing. Turn all the tone knobs to max and measure the ACV on U1 pin 5 and R24 with the strum test.
On U1 Pin 5 with all tone knobs (amp and guitar) maxed: 0 V on all channels (clean, dist. 1, dist. 2)

R24
Band 4 Side (Gold) Band 1 Side (Brown)
1.1 acV 1.0 acV Clean
1.1 acV 1.1 acV Dist
1.2 acV .9 acV Dist 2

Again, I really appreciate your help with this back and forth!
 
nickb 6/2/2018 3:28 PM
Well I'm feeling kinda dumb. I'd already got you to test the ACV at U1 pin 7 but I simply forgot. That meant we didn't need to do the last test. I was obsessed with finding a common cause for all modes failing.

So, in clean ONLY, bridge Q7 as we talked about and see what happens. Also check the DCV on U6 pin 1.
 
slow6i 6/2/2018 9:11 PM
Quote Originally Posted by nickb View Post
Well I'm feeling kinda dumb. I'd already got you to test the ACV at U1 pin 7 but I simply forgot. That meant we didn't need to do the last test. I was obsessed with finding a common cause for all modes failing.

So, in clean ONLY, bridge Q7 as we talked about and see what happens. Also check the DCV on U6 pin 1.
I get normal amp output in clean channel after bridging as we talked about (CR19 stripe to U5 pin 6)

I have 0 V at U6 pin 1 with input plugged
 
nickb 6/3/2018 1:10 AM
In that case Q7 is bad and I bet Q6 is also a gonner. Time to replace them. This would be a good time to update your profile with your location so we know who to direct you to for the parts.
 
slow6i 6/3/2018 1:35 AM
Quote Originally Posted by nickb View Post
In that case Q7 is bad and I bet Q6 is also a gonner. Time to replace them. This would be a good time to update your profile with your location so we know who to direct you to for the parts.
Location is updated. If you think that they are both bad, my recent experience in this thread would suggest that you know what you are talking about, so Ill just replace both.
 
nickb 6/3/2018 5:13 AM
I think they are J113 but better check.


https://www2.mouser.com/ProductDetai...Pvl0z7hwtK7ec=

I recall Mouser does have a cheaper USPS shipping option. The 'bay can be OK for small quantities but I'm always a little wary of the quality.
 
slow6i 6/3/2018 9:31 AM
Quote Originally Posted by nickb View Post
I think they are J113 but better check.


https://www2.mouser.com/ProductDetai...Pvl0z7hwtK7ec=

I recall Mouser does have a cheaper USPS shipping option. The 'bay can be OK for small quantities but I'm always a little wary of the quality.
I just ordered from Digikey. They were J111's, and I needed some stuff for a head tracking rig that im building anyway so it worked out well!

Ill report back when they are replaced. Thanks!
 
slow6i 6/6/2018 9:08 PM
Alright, so Q6 and Q7 have been replaced, and the symptoms are still the same. Next steps?
 
glebert 6/6/2018 11:18 PM
OK, if jumping across Q7 works and replacing Q7 doesn't help then it seems like it must be what is happening at Q7's gate. Could it just be a broken input jack not releasing the inhibit?

edit: guess that was answered back around #22
 
glebert 6/6/2018 11:46 PM
Quote Originally Posted by slow6i View Post
Yes, the test was done after plugging a cable into the input jack. For confirmation, I unplugged it as I was measuring your suggestions:

Q7 Pin 1 is the "top" of the D shape looking down at it.

Pin 1 Pin 2 CR20 No Stripe CR20 Stripe CR21 No Stripe CR20 Stripe
2 mV 17 mV 220 mV 5.69V 27 mV 7.95 V
0 V -15.24 V -15.84 V -16.22 V 15.83 V 7.95 V Cable Unplugged

Thanks!
The CR20 and CR21 no stripe are the same spot, right? How is one positive and one negative? Is that just a typo? Is the last column CR21 stripe? Seems like that should be more like 15V for clean channel (same as output of U8B).
 
nickb 6/7/2018 9:02 AM
Quote Originally Posted by glebert View Post
OK, if jumping across Q7 works and replacing Q7 doesn't help then it seems like it must be what is happening at Q7's gate. Could it just be a broken input jack not releasing the inhibit?

edit: guess that was answered back around #22
Exactly what I was going to say

So go and redo the test but this time put the probe on the gate directly and also check the DCV on the source (of Q7).
 
slow6i 6/8/2018 11:41 PM
Quote Originally Posted by nickb View Post
Exactly what I was going to say

So go and redo the test but this time put the probe on the gate directly and also check the DCV on the source (of Q7).
Clean channel, volume / tones up

Input Plugged
CR20 Stripe: 5.67 dcV
CR20 No Stripe:0 dcV
CR21 Stripe: 7.9 dcV
CR21 No Stripe: 0 dcV

Input Un-Plugged
CR20 Stripe:-2.50 dcV
CR20 No Stripe:-2.20 dcV
CR21 Stripe:7.92 dcV
CR21 No Stripe:-2.01 dcV

Input Plugged
Q7
D(1): -.39 dcV
S(2): -.39 dcV
G(3): 0 dcV

Input Un-Plugged
Q7
D(1): -.59 dcV
S(2): -.59 dcV
G(3): -1.15 dcVa

(Pin numbers per http://www.onsemi.com/pub/Collateral/J111-D.PDF )
 
nickb 6/9/2018 4:33 AM
There is something screwy going on there.

1) When unplugged there are different voltages on CR20 anode ( non stripe) and Q7 gate. Check the connection between them with an ohm meter.

2) When unplugged the inhibit signal (cathode CR20) should be close to -16V. Not that that explains Q7 not passing a signal, in fact it means that Q7 would always pass a signal

3) When plugged the inhibit signal (cathode CR20) should be close to +16V. Not that that explains Q7 not passing a signal either since Vgs>0 => pass a signal.

I suggest you check you do some DCV measurements around the input jack area first on both sides of R2 and then the inhibit side of R3.
 
glebert 6/9/2018 8:55 AM
If you get a chance check the output of U8 pin 7 with clean and dirty channels.
 
slow6i 6/9/2018 10:58 PM
Quote Originally Posted by nickb View Post

I suggest you check you do some DCV measurements around the input jack area first on both sides of R2 and then the inhibit side of R3.
Continuity at CR20 and CR21 anode to Gate on U7 is confirmed

Plugged
R2 Brn: 10.5->8.45V
R2 Gld: 13.5-8.45V

R3 Brn: 10.5-8.45 V
R3 Gld: 12.3->5.65

Unplugged
R2 Brn: -17.2V
R2 Gld: -2.9V

R3 Brn: -2.9 V
R3 Gld: -2.27 V

U8-7 Plugged
Clean: 15.94 V
Gain 1 (Y):-16.3 V
Gain 2 (R):-16.3 V

U8-7 Unplugged
Clean: 15.98 V
Gain 1:-16.3 V
Gain 2:-16.3 V

@glebert I am going to be in Portland tomorrow around noon for rehearsal, I would be more than happy to meet with you and have you take a look at this thing if you are available / willing. I can DM you my contact info, if you are available.
 
glebert 6/10/2018 10:38 AM
I'm totally booked up, and really am stumped on this one anyway, so I don't think me poking at it is going to help.
 
nickb 6/10/2018 11:43 AM
Your measurements are inconsistent since we have different voltage reported for the same node under the same conditions (more than once) . Without reliable data it's impossible to draw meaningful conclusions.

What make and model of meter are you using? Does it have a known good battery? Where are you clipping your negative lead?
 
slow6i 6/10/2018 12:06 PM
Quote Originally Posted by nickb View Post
Your measurements are inconsistent since we have different voltage reported for the same node under the same conditions (more than once) . Without reliable data it's impossible to draw meaningful conclusions.

What make and model of meter are you using? Does it have a known good battery? Where are you clipping your negative lead?
Its a Cen-tech 95670. Its certainly not a top of the line meter or anything. The battery is good afaik, though I will change it and check the voltages again. Negative lead is to the chassis just above the power switch. I am not using clips though, I'm using probes if that matters.
 
g1 6/10/2018 12:25 PM
Are all the boards etc. assembled? If not, your chassis ground may be intermittent or non-existent connection to circuit ground, which would explain the inconsistent measurements.
 
nickb 6/10/2018 1:57 PM
Quote Originally Posted by slow6i View Post
Its a Cen-tech 95670. Its certainly not a top of the line meter or anything. The battery is good afaik, though I will change it and check the voltages again. Negative lead is to the chassis just above the power switch. I am not using clips though, I'm using probes if that matters.
I can't find the all important impedance specification in the data sheet I found for that. I would not be surprised if it was lower than usual and that will skew some measurements. It does not explain the inconsistencies.

If the board is installed and the pot nuts are tight then there should only be 4.7 ohms from chassis to ground but I would not count on it. Better to connect directly to the real ground. The sleeve of the input jack (or footswitch) is a handy place.

So, with the new ground, plug in and measure the DCV on either side of CR1 ( same as R3 but the colors tell me nothing) , either side of CR20, the gate of Q7 and the source of Q7.

Note:
The anode CR1 should be the same as the cathode of CR20
The anode CR20 should be the same as the gate of Q7

If still not consistent then put your meter under the rear wheel of your truck and back over it
 
glebert 6/10/2018 2:50 PM
OK, someone correct my thinking on this if I am wrong. The voltage at U8 pin 7 is +15V for clean channel, but the cathode (stripe) of CR21 is about +8V, right? The only thing between those points is R152. This is DC so the capacitor C68 should be blocking it from ground (but we should not assume this is working correctly). There has to be current flowing through R152 for there to be this voltage difference. Current should not be able to flow through CR21 as it is reversed biased. So if the voltage measurements are correct, it seems like either C68 is leaking, CR21 is faulty, or there is some other current leak on that net.
 
nickb 6/10/2018 3:32 PM
Quote Originally Posted by glebert View Post
OK, someone correct my thinking on this if I am wrong. The voltage at U8 pin 7 is +15V for clean channel, but the cathode (stripe) of CR21 is about +8V, right? The only thing between those points is R152. This is DC so the capacitor C68 should be blocking it from ground (but we should not assume this is working correctly). There has to be current flowing through R152 for there to be this voltage difference. Current should not be able to flow through CR21 as it is reversed biased. So if the voltage measurements are correct, it seems like either C68 is leaking, CR21 is faulty, or there is some other current leak on that net.
I think the thing you're overlooking is the meter input impedance. That is why I tried (and failed) to find that info. Generally, if crucial info is missing I'm suspicious that it was left out on purpose.

I'm focusing on the inhibit path since we know for certain that bypassing the FET makes it work.
 
glebert 6/10/2018 4:23 PM
I have and have used similar Harbor Freight meters and never had them pull down that much. Low battery will give wrong values though.
 
g1 6/10/2018 6:06 PM
Quote Originally Posted by nickb View Post
I think the thing you're overlooking is the meter input impedance. That is why I tried (and failed) to find that info. Generally, if crucial info is missing I'm suspicious that it was left out on purpose.
A bit of an oddball meter having a tach function, but pg.7 of manual (attached) says input impedance 1Meg for all DC voltage ranges.
Having said that, if this is a gate reading, I've had my share of situations where you can't measure DC at the gate due to loading of high impedance circuit, even with pro meter.
 
nickb 6/11/2018 12:25 AM
Quote Originally Posted by g1 View Post
A bit of an oddball meter having a tach function, but pg.7 of manual (attached) says input impedance 1Meg for all DC voltage ranges.
Having said that, if this is a gate reading, I've had my share of situations where you can't measure DC at the gate due to loading of high impedance circuit, even with pro meter.
Thanks G1 - that was the manual I found but missed the impedance. I was guessing 1meg based on the numbers measured. I also have a cen-tech meter (different model) I got as a disposable one and it is 1meg also. A good meter will be at least 10 meg. I miss the "like" button,

Now that we know what we are dealing with we can correct for it.

PS: slow6i, can you add measuring the DCV from source to gate of Q7 when plugged in please?

PPS: g1, I guess the tach function is handy for shredders
 
glebert 6/11/2018 10:09 AM
Quote Originally Posted by nickb View Post

Now that we know what we are dealing with we can correct for it.
You know, without really thinking about it my initial response was "1 MEG input resistance should be good enough" but in this case it really will hugely impact the measurements. For the voltage at CR21 cathode it makes sense he is getting about 1/2 of the 15V from the U8 pin 7, because R152 is also 1 MEG. Guess that is my "new" thing I learned for today
 
g1 6/11/2018 1:10 PM
Quote Originally Posted by nickb View Post
PPS: g1, I guess the tach function is handy for shredders
lol. 'How many NPM are you up to?'
 
slow6i 6/11/2018 9:05 PM
Quote Originally Posted by nickb View Post

So, with the new ground, plug in and measure the DCV on either side of CR1 ( same as R3 but the colors tell me nothing) , either side of CR20, the gate of Q7 and the source of Q7.

Note:
The anode CR1 should be the same as the cathode of CR20
The anode CR20 should be the same as the gate of Q7

If still not consistent then put your meter under the rear wheel of your truck and back over it
I hope I have the anode / cathode terminology right so its not confusing. (Im taking Stripe side of CR1 as Cathode)

CR1 Cathode: 12.2->8.54 dcV
Anode side: 5.68 dcV

Anode CR1 - Cath CR20 are equal.
Anode CR20 - Cath Q7-Gate are equal.

Source to Gate Q7: -7.83 dcV

Thanks!
 
nickb 6/12/2018 1:00 AM
Quote Originally Posted by slow6i View Post
I hope I have the anode / cathode terminology right so its not confusing. (Im taking Stripe side of CR1 as Cathode)


Source to Gate Q7: -7.83 dcV

Thanks!
If the negative lead was on the source for this measurement.then this is wrong. I should have specified to put negative on the source (sorry!)

If you switch off and wait a few mins for the voltages to fall you should measure < 1k ohm between source and drain of Q7. Otherwise there is a problem with new FET - wrong type, wired in wrong, busted

The results for CR20 are missing (mainly sanity check as I'm questioning my own right now).
 
slow6i 6/12/2018 7:51 PM
Quote Originally Posted by nickb View Post
The results for CR20 are missing (mainly sanity check as I'm questioning my own right now).
Sorry! I missed that part of the comment. Newly measured values are below.

Im assuming you wanted the measurement on clean channel, so this is done on clean with everything at 10 except effects mix at 0:

Q7-2-3: -32.5 mV
between Q7 1(d)-2(s): 24.5 ohm
CR20 Cath: 5.69 V
CR20 An: 6.9 mV
 
nickb 6/13/2018 2:06 AM
The set up is correct. We'll keep it like that.

Now we have a new contradiction. The FET is low resistance as it should be and the voltages are correct. Bridging the (low resistance) FET made it work yet remove the bridge and it doesn't. That makes no sense.

Do you still get a noise when touching:

pin 6 of U9?

Source of Q7?

Drain of Q7?

R23? One side is the same as the drain while the other is rather low impedance so the effect will be small so it's not a very good test.

Pin 5 of U1?
 
slow6i 6/16/2018 10:47 PM
Quote Originally Posted by nickb View Post
The set up is correct. We'll keep it like that.

Now we have a new contradiction. The FET is low resistance as it should be and the voltages are correct. Bridging the (low resistance) FET made it work yet remove the bridge and it doesn't. That makes no sense.

Do you still get a noise when touching:

pin 6 of U9?

Source of Q7?

Drain of Q7?

R23? One side is the same as the drain while the other is rather low impedance so the effect will be small so it's not a very good test.

Pin 5 of U1?
U9-6: no
Q7S: no
Q7D: no
R23: no
U1-5: no

Im going to redo all of the measurements through this thread to confirm them.
I will update the google sheet previously listed.
 
slow6i 6/30/2018 7:28 PM
https://docs.google.com/spreadsheets...it?usp=sharing

Sorry that took so long.

Voltages have all been checked and verified with settings shown on sheet, and guitar at mid pickup and tone / volume all the way up.
 
nickb 7/4/2018 3:39 PM
I'll look these over...
 
nickb 7/4/2018 4:08 PM
On the spreadsheet the pin on Q7 that behaves as the gate i.e varies from 17mV to -15.24 when you unplug is given as pin 2. The Onsemi datasheet says this the gate is pin 3. Therefore there is some confusion over the pin numbering or the orientation of the FET. Using the schematic, figure out which hole of the FET is connected to what. From that you can identify which hole is gate, source and drain. Then use the datasheet to check you have the FET in right.
 
slow6i 7/5/2018 11:56 PM
Quote Originally Posted by nickb View Post
On the spreadsheet the pin on Q7 that behaves as the gate i.e varies from 17mV to -15.24 when you unplug is given as pin 2. The Onsemi datasheet says this the gate is pin 3. Therefore there is some confusion over the pin numbering or the orientation of the FET. Using the schematic, figure out which hole of the FET is connected to what. From that you can identify which hole is gate, source and drain. Then use the datasheet to check you have the FET in right.
These are the values that I just measured. The pin 1 / 2 naming was in place before i knew that there were actually 3 pins on the FET, that was my mistake.

Q7 test
Pin1 Pin2 Pin3
-.37 dcV -.39 dcv 0 dcV Plugged
-.56 dcV -.59 dcv -2.93 dcV unplugged
 
nickb 7/6/2018 1:09 AM
Again, the data is not consistent. We see the anode of CR20 (non-stripe) as 0.22V/-15.4V yet pin 3 of Q7 as 0V/-2.93V (plugged / unplugged). Either the measurement is wrong or the gate of Q7 (pin 3) is not actually connected to the anode of CR20 - I think we tested this for continuity once before
 
slow6i 7/28/2018 10:18 PM
Quote Originally Posted by nickb View Post
Again, the data is not consistent. We see the anode of CR20 (non-stripe) as 0.22V/-15.4V yet pin 3 of Q7 as 0V/-2.93V (plugged / unplugged). Either the measurement is wrong or the gate of Q7 (pin 3) is not actually connected to the anode of CR20 - I think we tested this for continuity once before
I've just gone through everything again, and with two different meters, I get two different readings. I dont have a meter that goes into the mVAC with any resolution other than 10's so I cant get a fully accurate reading with those anyhow (I started checking all the TP's.)

When we bridged over Q7 and Q6, what exactly was that doing? Why did that make this amp behave normally?

Im at the point now where ill probably just order all new op amps, jfets, replace them, see if that works, then scrap this thing and realize that I made a $100+parts dollar mistake by trying to fix something that I had no business fixing.
 
glebert 7/28/2018 10:22 PM
Bridging past Q6 and Q7 was getting past the muting circuit. IIRC the muting circuit should silence the unused channel during normal operation and both channels with nothing plugged in.
 
slow6i 7/29/2018 11:01 AM
Quote Originally Posted by glebert View Post
Bridging past Q6 and Q7 was getting past the muting circuit. IIRC the muting circuit should silence the unused channel during normal operation and both channels with nothing plugged in.
So, by bridging over that, I effectively had both channels active at the same time?

I did some more poking around, this time reading the actual test procedure for ACV on the schematic, results are listed on the "Test Points" tab at https://docs.google.com/spreadsheets...it?usp=sharing

I used a tone app on my phone as an input at 1kHz as thats the best I can do with the equipment I have.

Those MC1436 buggers are expensive! (Probably because they dont make them anymore.)
 
nickb 7/29/2018 11:09 AM
Quote Originally Posted by slow6i View Post
I've just gone through everything again, and with two different meters, I get two different readings. I dont have a meter that goes into the mVAC with any resolution other than 10's so I cant get a fully accurate reading with those anyhow (I started checking all the TP's.)

When we bridged over Q7 and Q6, what exactly was that doing? Why did that make this amp behave normally?

Im at the point now where ill probably just order all new op amps, jfets, replace them, see if that works, then scrap this thing and realize that I made a $100+parts dollar mistake by trying to fix something that I had no business fixing.
The idea was to bridge just one of them, not both. Replacing stuff at random is a horrible idea and will almost certainly be an expensive failure. I'll bet it's a simple fault to do with the muting.

It's time to hand the amp to someone with the right skills. It'll be fixed in in hour at minimal cost.
 
glebert 7/29/2018 1:26 PM
Quote Originally Posted by nickb View Post
It's time to hand the amp to someone with the right skills. It'll be fixed in in hour at minimal cost.
Considering how much this amp has been tinkered with I wouldn't assume that this can be fixed for minimal cost. Was the original problem (foot controller switching) ever fixed? Local guitar store is $70/hr. and this amp is maybe worth $150 around here on a good day. I'm not taking in outside work at this time BTW.

If bypassing the mute FETs worked you could permanently jump it. It would be fixed in one channel. I don't love the drive on this amp, and I have had three of them (still have one) so I would do it clean and add a overdrive pedal.
 
slow6i 7/31/2018 5:03 PM
NickB - I think you are right. Probably better not to just start replacing things. I am just going to bypass the muting circuit as glebert suggested and run a pedal setup.

If that fails, I will save the speaker, and scrap the rest of it, or break it down for components for other projects.

Thank you g1, glebert, and NickB for your responses and attempting to help me get this thing fixed, but unfortunately, I must admit defeat on this one.

I now turn my attention to a Baringher Powered PA speaker! Yay!