Marshall reverb 12 model 5205

[Bushbaby]

Ok, will do.
Yes, it did switch on last time and I don't recall any burning smells and it did have the speaker attached with no sounds, hum or crackling at the time I did this. I set it up again and investigate further.

Best Regards

Rob

[Refugee]

The first two things I would check for is the tracks around the pot connections as they have been out and back in again with thin tracks on the board close by.
Also wires and jack sockets as the board has been in and out several times since it last produced sound.

[Diabolical Artificer]

This is a classic case of fault finding 101 Rob and should take about five minutes to resolve. After making sure your power supply rails are there and in spec, stick your signal out of your sig gen into the IP socket, then follow the signal path with a scope probe. so, into the IP socket,have you got a signal out of it? Is there a signal at the volume gain pot? Then onto Q1 or IC1 or whatever. Where the signals disappears or is low, check voltages, pull and test components.

After mucking about with amps for 10 years you should have this down pat by now mate, it's very basic stuff. Don't wish to be harsh but if this sort of thing isn't familiar to you maybe some reading up or even better putting in some bench time might be beneficial. Do you keep notes of your work? Definitely worth doing so you can look back and get some pointers if and when you find yourself in a similar situation..

Good luck, Andy.

[Bushbaby]

Good and bad news, and yes perhaps I have been a wee bit lazy but its understandable given my heath issues at present.
I thought I would restart from scratch. I used my signal generator with a nbc connector to 1/4 jack into the amp input to see if any sound at all could be heard through the system? I connected the speaker with some leads with crocodile clips. I put a 500Hz signal through the amp and low and behold a great signal from the speaker could be heard. I tested all of the controls (treble, middle and bass) and in each case they are all working (you could hear the difference in tone in each case)........except, the reverb does not appear to be doing anything at all?? Furthermore, when I switched the amp off (volume and gain controls at zero) yet another loud pop through the speaker (I seem to have this initially with all the amps I've worked on).
I disconnected the signal generator and two things I did, rightly or wrongly - switched on a off a few time to see if it popped again and not this time, very tiny sound but not very loud at all. The second thing was I turned the volume and gain full up with nothing connected (just to listen for any crackles from the circuit) and a motor boating sound could be heard immediately which lasted for a few second before 'pop' the internal fuse blew. So, yes its working to some extent BUT does appear to have a fault somewhere - is this related to the reverb circuit/springs etc???

Best Regards

[Bushbaby]

Just another thought....could this be a ripple issue from one of the electrolytics at all?? The amp has not been used in years.....

[Murphyv310]

So what fuse blew and is it blackened?
Never a good idea to switch an amp on and off on quick succession especially with the gain right up. A sure way of creating subsonic oscillation.

[Bushbaby]

Hi Trevor
It's the internal fuse. I switched on an off with the volume & gain controls off. I switched back on a bit later and it was then I turned the volume and gain full up, it motorboated, I turned them down and then up again, motorboated again and shortly after, the internal fuse lit up brightly and blew.

Best regards.

[Diabolical Artificer]

Switching an amp on and off isn't a good idea, at least not without discharging caps. First thing I'd do is have the board out and check with a magnifying glass all solder joints, while your at it whip the big caps off and test. After that power it up with a bench PSU, current limited and check voltages on all op amps to see if they have power and are getting hot. If not bench PSU solder a resistor across the fuse, all initial tests at low volume/gain until it's running ok.

Andy.

[Murphyv310]

Hi.
Again I ask what does the fuse protect? Speaker, AC mains input, power supply rails?
I don't have a diagram.

[radioman]

The only fuse (T160mA) I can see in the diagram (available here :-https://elektrotanya.com/marshall_reverb...nload.html
is in the AC 'L' feed to the mains transformer primary.

Andy
EDIT - I've just noticed that Rob posted the circuit diagram in post #1 !