|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?
|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?
|slow6i||4/1/2018 8:18 PM|
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|
|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|
- 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|
|slow6i||4/14/2018 7:02 PM|
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|
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.
|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|
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|
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|
Q7 Pin 1 is the "top" of the D shape looking down at it.
|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|
- 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|
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|
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|
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|
|nickb||6/3/2018 5:13 AM|
|I think they are J113 but better check.|
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|
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|
|nickb||6/7/2018 9:02 AM|
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|
CR20 Stripe: 5.67 dcV
CR20 No Stripe:0 dcV
CR21 Stripe: 7.9 dcV
CR21 No Stripe: 0 dcV
CR20 Stripe:-2.50 dcV
CR20 No Stripe:-2.20 dcV
CR21 Stripe:7.92 dcV
CR21 No Stripe:-2.01 dcV
D(1): -.39 dcV
S(2): -.39 dcV
G(3): 0 dcV
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|
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
R2 Brn: -17.2V
R2 Gld: -2.9V
R3 Brn: -2.9 V
R3 Gld: -2.27 V
Clean: 15.94 V
Gain 1 (Y):-16.3 V
Gain 2 (R):-16.3 V
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|
|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|
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.
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|
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|
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|
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|
|g1||6/11/2018 1:10 PM|
|slow6i||6/11/2018 9:05 PM|
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
|nickb||6/12/2018 1:00 AM|
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|
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|
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|
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|
|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|
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|
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|
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|
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!