Marshall reverb 12 model 5205

[Bushbaby]

Hi All
Sorry I've not been around for a while but I have just undergone some minor back surgery which has put me out of action for a few weeks.

I've decided to return to work on this amp that I bought many years ago for the sum of £20 knowing that it had issues. These were that the volume and gain pots were stiff to the point that you could only really turn them with a pair of pliers and that they were horrendously crackly!! I replaced these with the equivalent Alpha pots only to find that at the time there was absolutely no output...I then left this and forgot about the amp until I found it at the back of my workshop again. 
Does anyone have any thoughts where the problem might be and where to start?
I've attached some pictures of a similar amp and a schematic.

Best Regards

               
  marshall_reverb12_12w_5205_sch.pdf (Size: 181.32 KB / Downloads: 18)

PS - I'm not working on my 1930 clone amp as my back is still very bad and I cant lift very much at present....this will have to wait.

[ppppenguin]

Standard faultfinding procedures apply.

First check the power rails. Many of us have been caught that way, even though we should know better. This amp has +ve and -ve power rails. Difficult to tell what they should be, but I'd guess around +/-15V from what I can see.

Then you can either connect and input an trace the signal forwards with a scope. Or inject a signal near the output and work backwards. At least you aren't dealing with a fully DC coupled design where a single fault anywhere can muck up the entire circuit.

[Murphyv310]

Hi.
In addition to Jeffrey's comments, I'd go over your work, check the pots for correct values and there is no broken print or dry joints.
90% of issues that occur after replacing parts are manmade.

[Bushbaby]

Thanks - and I will follow this up. The prices for these are becoming astronomical considering they were only designed to be a small practice amplifier. On average these are hitting £175......what a good purchase at the time.

[Mark Hennessy]

Hi Rob,

Hopefully you're on the mend, but presumably to get your back fixed, you had to describe your symptoms to the doctors?

"No output" could mean almost literally anything. What evidence have you gathered so far? It's OK if you can't analyse the evidence, but for us to be able to help, we need something to go on. What have you done so far. As Jeffrey tactfully says, standard fault-finding procedures apply.

With your ear to the speaker, is there anything at all - slight mains hum or maybe a hissing noise? Any pops or clicks when you power up/power down? Is the speaker OK? Have you tried headphones? (Only try headphones if the DC offset is OK, or at least, use cheap/disposable ones). Does anything get hot when powered up? Any smells? Does the mains transformer hum mechanically, indicating it's getting power (some do, some don't). Does the mains switch light up? All basic things that could be checked with no use of tools...

Beyond that, the schematic gives plenty to go on. There should be about 30V across each of the main PSU caps, give or take the usual variables. The supply to the op-amps is +/-15V, and these are regulated by zeners, so should be within half a volt, say.

The power amp is DC coupled, but a fairly basic one. Check the voltage at the output node - the junction of R37/R39. That should be 0V, though there will no-doubt be a small offset. Anything less than 100mV or so should be OK.

What might you have disturbed when you had it apart? Check for wires incorrectly replaced, or wires accidentally broken by the folding up of the PCB. Of course, inspect everything for dry joints, and remember Trevor's last sentence especially.

Check the headphone socket, as that switches the speaker.

I note R40 (if I've correctly identified it) is cooked. Hopefully that doesn't point to something more sinister. It's part of the headphone attenuator, and will potentially warm up if running at full output for long periods, but it will get hotter still if the power amp output is jammed up against a rail. Let's hope that hasn't happened, as it will usually take out the speaker in the process.

Assuming all these checks are OK, inject a signal and follow it with your 'scope, as Jeffrey suggests. It's all any of us can do - we're engineers, not magicians.

Such open-ended question like this might just about be be OK for "stock faults", but not here. More input required!

Look after yourself while you recover,

Mark

[Murphyv310]

Hi.
The fact that the amp was working prior to fitting the controls (assuming you had an output bar the noisy pots) makes me think there could be a simple problem.
Well spotted Mark on the cooked resistor and I didn't see it on the phone. It looks as if its a feed from the PSU, long now since I've repaired this model so forgotten. Like I say check your work before getting bogged down going too deep. While I respect others contribution, use ears, eyes and nose, recheck any work done in the first instance even before applying power, check for O/C speaker leads or plugs, it's surprising how many faults are found in these basic checks.
Hope you're feeling better. Don't over do things.

[ppppenguin]

There should be about 30V across each of the main PSU caps, give or take the usual variables.

The main PSU caps are rated at 25V according to the circuit. That doesn't really square with the 21V AC quoted at the transformer.

[Mark Hennessy]

There should be about 30V across each of the main PSU caps, give or take the usual variables.

The main PSU caps are rated at 25V according to the circuit. That doesn't really square with the 21V AC quoted at the transformer.

I agree, but I wouldn't be surprised. I've lost count of the number of times I've seen over-volted caps in audio gear. Perhaps the most extreme example I've come across is the Musical Fidelity B200. This uses 63V caps, and the PSU rails idle at 75V! But in percentage terms, this could be about the same.

Perhaps surprisingly, the caps I've found in the B200 are always in pretty good health - certainly no worse than you'd expect for the age.

Of course, in this case, we don't know the idle current, and we don't know the conditions that 21V is measured at. It's likely the transformer is more like 18V at full load, perhaps. It's quite a small one (20VA, maybe?).

Either way, these things are very much built down to a price.

Well spotted Mark on the cooked resistor and I didn't see it on the phone. It looks as if its a feed from the PSU, long now since I've repaired this model so forgotten.

Looking again, I'm still pretty sure the cooked resistor is R40 (in the headphone circuit). A better photo would help, but the two resistors at that point - next to the headphone socket - are the only two that are 1/2W.

The 220R pre-amp supply resistors are elsewhere - between IC3 and TR5.

I do agree that these 220R resistors will be leading an unhappy life. Assuming 25V rails, they're dissipating practically 0.5W, rising to 1W if the rails do indeed reach 30V. They are standard 1/4W carbon film. The zeners are running warm, if C21 is any indication. In round numbers, the op-amps are only taking around 12mA, leaving around 55mA for the zeners (0.75W) at 30V rails. That falls to 35mA/0.5W if the rails are 25V. A bit marginal, but in keeping with the rest of the design...

The FR4 board doesn't look too discoloured around these hot-spots, suggesting it's not had a lot of use. But I think I'd be upgrading some of the components and spacing them away from the PCB with some ceramic spacers or similar.

[Bushbaby]

Humble apologies - I did say that the photos were from a similar amp but NOT mine. I've attached some of mine now. 

When I indicated that there was no output I was referring to the fact that I added the replacement pots for those that had seized, certainly the volume and gain ones. After replacing, I put an input signal from a sound generator and nothing - no sound output at all!! I did stop at this point and left it until now. 

This may not be related, but I have noticed scorch marks around the diodes ZD2 & 3 but have yet to test them?

               


Best Regards

Rob

[Mark Hennessy]

I can't speak for others, but until you've answered the questions and done the tests in post #5, I can't advise any more. Aside from clarifying that the pictures in post #1 were not yours, you haven't given us any more information to chew over.

I discussed why ZD2 and 3 run hot in post #8. But the tests in post #5 will should tell us if they are OK.