|Begbie||8/30/2017 9:44 PM|
|Marshall VS65R powers on ..No Sound|
I've got a Marshall Valvestate VS65R that I love. Unfortunately its stopped working. It powers up just fine and will output sound at very low volume if the Volume pots are turned right up, but nothing like it should.
Ive tried various things based on searches in this forum and elsewhere but have had no luck. So here I am cap in hand asking for help.
I replaced the Valve a while back with a similar one 12AX7 and it was working fine. About two weeks ago I had the house to myself and was going to make some noise, but got nothing from the amp. I thumped it and still nothing!
I ran to google and found that there were reported problems with the large cement block resistors getting hot and causing bad solder joints and the dreaded switch fault with the jacks on the back. So I reflowed all the solder joints that looked even slightly dodgy (I tested continuity on these joints too) and tested all the jacks on the front and back. No change.
I had some spare electrolytic caps lying around and have replaced the ones in the amp no change there either.
I've tried the headphone jack and had no luck there.
So I took out the Muting gate (T16 J111) and tested without it and still no result.
Im not sure where to go next.
The best schematic I can find is this one which is hard to read (due to the original scan methinks) and not enetirely the same as my board (there are some discrepancies between my board and the schematic -probably just revisions)
I'm a shiftworker so my response may be delayed or erratic, but I am happy for any help getting my amp working again.
Also living where I do (Perth Western Australia), some parts can take months getting here, so it can take a long time for me to get results. So Id rather replace several components at once than one at a time.
|Pedro Vecino||8/31/2017 3:55 AM|
|Two basic tests:|
- Check the +15/-15 voltages on the zeners.
- Check what happens when you connect a signal on the return jack. To isolate the problem in the preamp or in the power stage.
|Begbie||8/31/2017 4:58 AM|
|Thanks Pedro, |
The return jack gives slightly louder signal than normal but is still very quiet and unaltered by adjusting volume pots.
I Measured the Zeners Number one (ZD1) was -15.7v
Number two (ZD2) however was 3.74v
Both were reading -7.5v on the shared rail which I believe should be the Earth. So something is wrong there.
Also after testing (only a few minutes) One of the large resistors was incredibly hot.
|Pedro Vecino||8/31/2017 5:27 AM|
|It could be that C25 capacitor and/or zener associated is defective, or that some active element behind it generates a greater consumption. Try the first one. Itīs relatively common in these amps. |
If the problem comes from there, itīs advisable to replace them on both sides, and if the zener is 1/2 watt, better use 1.3 watts models.
|Begbie||8/31/2017 5:35 AM|
|OK I will try and source a couple of 1.3 watt Zeners tomorrow (its getting late here now).|
I have replaced the C25 Capacitor already as I had a supply here.
I wil let you know how I get on. Thanks
|g1||8/31/2017 1:01 PM|
|Both of those big resistors should get really hot. |
Is one cool, or are they both real hot and one even worse?
|Begbie||8/31/2017 7:20 PM|
|It appears to be one much hotter than the other and it gets that way really quick! And yes g1 one is definitely a load hotter than its companion.|
I have replaced Z1 and Z2 and the adjacent caps but no luck still! I did manage to lift one of the tracks on the board in doing so and have repaired and tested the continuity there.
|g1||9/1/2017 12:28 PM|
|What I was getting at was that if one of them does not get very hot, then it is more likely the problem, not the hot one.|
You measured both resistors and resoldered them?
Which one is hotter, R17 or R18 ?
|Begbie||9/1/2017 3:58 PM|
|Its R17 that gets hot.|
I rechecked them after I discovered the hot one. They are both OK and the continuity of the solder joints is good too.
|The Dude||9/1/2017 4:10 PM|
|If you have +-15v rail issues, my bet is a shorted op amp loading down the supply. See if one of your op amps is getting hot. The best place to start is the first one in from the input jack. Lots of times these can short from someone plugging something into the amp that shouldn't be.|
|Begbie||9/1/2017 4:48 PM|
|OK I will check them out this morning.|
|Enzo||9/1/2017 8:38 PM|
|Would seem to me that the 15v supply that is measuring only 3v is likely to overheat the resistor. My first vote is a bad zener there.|
|Begbie||9/1/2017 9:19 PM|
|I've replaced both Zeners and still no luck. I tested the ones I removed too and they seem fine. I think The Dude might be onto something with the OpAmps and I think I might order some and systematically change them out. |
Its crude and not the most graceful solution but given the time it takes to get parts here sometimes I'm inclined to replace them all at once, but curiosity is going to be the better of me here I think and I will end up working through them one a t a time until I fin the culprit.
|Enzo||9/1/2017 9:31 PM|
|We call that an Easter egg hunt: just start replacing things until it works. Not very efficient. If the zeners are OK< then touch each op amp and see if any are getting hot. Same with the caps around the low zener. If push comes to shove, carefully remove the ICs one at a time to see if the supply recovers.|
|Begbie||9/1/2017 11:09 PM|
|I tried the touch test on the ICs this morning none were getting hot. I didn't try any of the passive components though. I will try that next thanks Enzo.|
|1ampman||9/2/2017 8:07 AM|
|Just had one in with the same problem. The fuse mounted on the circuit board was blown. It's for the heater of the 12ax7 All signals go thru it.... Check for filament voltage .......since it's hard to get to jumper the fuse or solder a quick shorting bridge.|
|Begbie||9/2/2017 6:32 PM|
|Thanks 1ampman, |
I did test for continuity on the mounted fuses but think I read somewhere that the tube gives a false indication on the fuse.
How do I test filament voltage?
And do you mean : Jumper the fuse as an easier option?
|g1||9/2/2017 6:39 PM|
|The heater circuit and it's fuse can not affect the +15V rail in this amplifier.|
|Enzo||9/2/2017 6:46 PM|
|Check for heater voltage by pulling the tube and sticking your meter probes in pins 4/9 and 5/9.|
|1ampman||9/2/2017 7:09 PM|
|By a quick jumper. I mean solder a piece of wire to take the place of the fuse. First see if the tube is lighting up? If not the power transformer has a separate winding for the filament (AC) . The fuse is a small round cylinder soldered to the circuit board, that can't checked by looking at it like a normal fuse. I believe it's 250ma. Not a fun easy 2 minute job. I had to pull the circuit board. Getting the board out requires unhooking a bunch stuff. If you mark the wire & board with a colored magic marker, You'll know where the wire goes when installing. So you will need many different color magic markers. The amp does sound good for rock&roll....Hope this helps|
|1ampman||9/2/2017 7:46 PM|
|Enzo, the filament is 12vac , so pins 4&5 would be used to check for heater voltage. Make sure your meter is on AC not DC|
|Enzo||9/2/2017 10:11 PM|
|Thanks, I didn't look at the schematic. The basic idea was pull the tube and measure whatever was coming its way on the hater pins of the socket.|
|Begbie||9/3/2017 1:59 AM|
|Thanks for the help everyone. If nothing else I'm learning a lot more about this amp than I knew before. I'm back to work so my ability to test/try/curse will be limited by over the next few days. |
I am so desperate to get this thing working again though so please don't mistake my tardiness in replying for apathy.
Thanks again for all your help, I would be lost without it!
|Begbie||9/10/2017 11:08 PM|
Ive managed to source a collection of OP amps and have swapped out IC3 the first one near the Input and IC7 because it was the same as IC3.
No joy at all.
The 15v +/- rails are at 5/-7 respectively.
The resistors are still behaving the same way, that is one is getting real hot and the other just warm.
|Enzo||9/10/2017 11:10 PM|
|Are you sure the IC is in the socket facing the right direction?|
|Begbie||9/10/2017 11:21 PM|
|Begbie||9/12/2017 1:32 AM|
|So I managed to change out ICs 1, 2 and 4 in an effort to eliminate them from my inquiries. |
I double checked the orientations of the ICs the continuity of my soldering and replugged it all in to test it.
I had a moment of sincere joy as the tube lit up and the LEDs on the front panel illuminated brightly (a symptom of my problem was the LEDs were lighting but at a less bright level than usual -probably due to the bad 15v rails.
I tried to input a signal and no joy
I then tried to switch channels from 1 (clean ) to 2 (OD) and nothing
I then tried the volume pot on the OD channel and all the lights (valve /leds ) dropped out.
Could this all be a Pot issue?
|g1||9/12/2017 1:02 PM|
|Do you have the pots assembled back to the chassis and tightened?|
What are your +/-15V rails reading now?
|Begbie||9/12/2017 7:50 PM|
15v rails are worse now -2 and 0
|Begbie||9/13/2017 12:00 AM|
This is getting me down now seems like it's going backwards. But I changed out ICs 1 and 3 and the volume pot on channel 2. Now I have +-/15v rails back yay. No sound still though.
Any suggestions on where to try next. I'm tempted to change out all the pots anyway as they are getting on in years.
|DrGonz78||9/13/2017 1:47 PM|
|You might want to consider the idea of building a home made signal tracing device since you don't have a scope. Very simple to just inject some sort of audio into the input of the amp and then trace the signal through the amp. If you have a cheap practice amp you can build something like this where you plug the tracer output plug into the little amp. In regards to the picture shown I would make sure the ground clip wire was longer too. |
Here is something similar and with an actual picture. This one is from the billmaudio website. Do a search for more info on it too.
|Begbie||9/14/2017 1:24 AM|
I will try that when I can find a suitable cap.
As this is my practice amp ill have to try another avenue to listen to the signal I'm inputting. Would love an excuse to buy a scope but I dont think i'd get enough use from it!
Where I'm at now is the 15v-+ rails are good again.
I get a really weak signal through the FX loop input but its not much louder than the one I get maxing the OD volume pot.
I bi-passed the reverb box and had no luck there so I'm figuring its somewhere between the 15v rails and the reverb which doesn't narrow it down much!
|m1989jmp||9/14/2017 10:43 AM|
|g1||9/14/2017 3:48 PM|
|Begbie||9/14/2017 5:10 PM|
|Begbie||9/14/2017 5:15 PM|
|The Dude||9/14/2017 5:18 PM|
|Plugging a guitar into the effects loop will be weak. It expects a line level input and you are giving it an instrument level. It's just a test to see if the amp works. If you feel the need to verify the level, try plugging something in with a line level or variable level (like a CD player, mixer output, your phone, etc.) into that jack so you can give it a bit more juice.|
|Begbie||9/14/2017 5:43 PM|
|Ive been using the drum machine function on an FX pedal for the test signal. I will try with something else today!|
Update: using an iPod I can vary the volume through the FX loop but even at max volume on iPod and amp its still really low.
|The Dude||9/14/2017 7:02 PM|
|Have you tried a known good speaker or tested the speaker in the amp with something else? I know, ...... you can call me Mr. Obvious if you like. Just want to make sure before we go chasing all over the amp. A shifted speaker magnet would/could cause very low output.|
|g1||9/14/2017 7:04 PM|
|Also try headphones.|
Do you still have T16 (muting fet) removed?
|Begbie||9/14/2017 7:29 PM|
|Dude, the speaker works fine, but was something I hadn't considered thanks!|
g1, Headphones produce the same weak signal, I put T16 back in after testing it originally but will remove it and try again. Thanks.
|Begbie||9/18/2017 6:52 PM|
|Tested with T16 Back in and out no change!|
|g1||9/19/2017 5:37 PM|
|Have you built that tracer/probe yet?|
|Begbie||9/19/2017 11:46 PM|
|Begbie||9/23/2017 12:25 AM|
|Ive built a tracer probe. It is working as it should (the probe the, not the amp). |
I'm afraid I'm going to miss the fault though as I'm not sure sure where I am supposed to get a signal and where I shouldn't.
|g1||9/23/2017 1:29 PM|
|Try comparing the wiper (middle lug) of the mix pot with pin7 of IC7. (constant tone injected at main input jack, nothing in FX loop)|
|Begbie||9/24/2017 7:56 PM|
|Begbie||9/24/2017 7:57 PM|
|Enzo||9/24/2017 8:24 PM|
|Think about the overall circuit. You have a signal at IC7-7. The signal at the wiper of VR10 is where that signal comes from. SO even though you are not detecting it, it MUST be there for it to come out the IC.|
|Begbie||9/24/2017 11:41 PM|
|Going to plead stupidity here. I went to recheck my previous test and was a whole IC out, I was testing IC1 not 7.|
There is no signal at IC7-7.
Thanks Enzo for prompting me to double check.
|Begbie||9/25/2017 1:47 AM|
|Have started at the beginning of the circuit and slowly working my way through IC 2 has output but nothing into IC5 so I'm narrowing my search down. |
Thanks again for all your help guys.
|Begbie||10/6/2017 10:43 PM|
I'm back and hopelessly lost. Im not sure how I should be testing the signal through the Amp. That is should i have the amp set for the clean channel and have all the pots turned down or is that going to give me false negatives?
Also is it worth checking the tubes power supply as suggested earlier? I really want to get to the bottom of this but as I said earlier feel a bit lost, not sure if I'm finding the signal where it should be or more importantly not findiing it where it should be.
IC6 and 7 have no output but not sure if that is due to how the amp (pots/channel) is set up.