Marshall 8080 - clean and boost channel sound distorted

[vegaluthier]

Hi, folks, last year I bought a 90's 8080 amp, and it worked fine, but last months it had a intermitent fail, suddenly went low volume and sounded distorted. I kept using it because the amp started sounding well again, and failure wasn't that often, but now it sounds distorted all the time, in both channels.

I've been reading posts about issues with this amps and I started measuring some things, but I'm getting a little lost with this.

Today I tried all +15 and -15 V lines and readings were really close to 15, but never exact. Positive line changes a little when I plug the guitar, increases some, but sometimes is below 15, I don't know if I'm measuring wrong, the tester is bad or if this is normal, really.

I've also checked that transistors are getting around +42 and -42 at the center leg, but I don't know if they are working well. All three of them must be isolated from the heatsink, right?

How you'd start repairing it?

[Enzo]

First, this is a guitar amp, not precision laboratory gear, so close is close enough. If your 15v rails are within half a volt, I'd be happy as a clam. In fact it matters less to me if they are 14v, 15v, 16v, than it does that whatever it is, the positive and negative are pretty similar in voltage.

The 42v or whatever, same deal, just ball park is good. So you have them.

First we look for loose connections, so ball up your fist and whack the top of the amp sharply. Does this affect the sound in ANY way? Does it make the volume return, or does it crackle when you whack it? Turn down any reverb first.

Turn the FX loop balance control back and forth a few times, any change? Plug a cord from FX send to FX return, any change?

Plug the guitar into the FX return to bypass the preamp, does the sound still have the problem or is it fairly clean and strong? Likewise, run a cord from FX send to some other amp, how does it sound over there?

[vegaluthier]

Hi, Enzo, thanks for your quick response... Well, at first I tried whacking the top of the amp a little but nothing happened. Later I tried what you proposed, tried to turn the fx pot but nothing changed too. Then I forgot to plug a cord from send to return and directly tried to plug the guitar into the return, and the sound kept being horrible... And finally, I tried to use the line out to another amp... And the amp DIED!!!

I thought I blown the fuse but he was there just fine... Made a continuity test and it was fine, so I thought I killed the amp definitely. Then, triying not to desesperate, I tried another power cord, and it went back to life...

So, with many doubts repeated all the process from the whacking part, and very reticent to try again that line out since back to life was partial, clean channel sounded fairly good and boost channel didn't sound almost nothing... And best part is... Line out sounds good too!

Selecting boost channel, I did some gentle wacking on the output transistors, on the resistors, and all the IC's (these are innocent) and I found that seems to be many broken solder joints, perhaps boost volume pot, T65 center leg, and R101 and R102, those are indeed the most dubious. Tomorrow I'll resolder all that seems little suspicious and tell you guys the result, but I think you solved my amp problem, it sounded fairly good for a while... From time to time needs some wacking in those components I said and all revived well.

I'll also use the fx loop to see if it works well...

I'll get a hammer, never thought this was the right answer for amplifiers!!!

[Enzo]

I use my fist, but to be official, we would use a rubber mallet. My fist just happens to be the mallet god provided me with first. Whacking the amp ends a vibration through it, and it often exposes loose connections, bad solder, broken wires, etc.

[vegaluthier]

Good Morning, Enzo and anyone reading.

Well, the funny thing about this is that I got a fairly good volume in both channels, but assembled the amp and it got back to sound bad.

So, took the circuit out again and I measure AC and DC changing channels and I found that D4 gets an oscilating between 2-3 VAC on one end, 0 on the other, D5 gets 2.2 and 1.9, on clean channel. On boost channel, D4 gets 9.4 and 0 VAC, same ends as before, while D5 gets 13.6 and 12.8. Measuring DC, D4 get 9.4 and 0, D5 gets 6.6 an 6.3...

Resistors 101 got crazy readings too, at VAC 101 gets 0 on both ends, 102 gets 93 and 33 VAC. In DC, both got normal readings of 42 and 15 V. ZD 1 and ZD2 beneath them got +15 and -15 on one end, 0 on the other, and D3 got 243 and 253 AC, and -253 and -10 DC... I think the transformer got crazy or D5 and D3 melt... Really don't get it.

I've also checked all bad solder joints and I think I got them all, I'll check again at noon if I see something else!

Should I measure a particular component? Autotransformer gets some current too, I believe only VAC, and R112, 113 and 114, but I didn't write results...

Got back to whacking or I should write down all measures, double check all an pass to you??

Thank you in advance guys!

[vegaluthier]

Hello Enzo, What do you think about these diodes readings? D3 and D5 shorted? (Readings were from positive probe to grounded probe to the chassis... Is this the way to do this?) How should I check them for sure? Continuity or resistance reversing tester probes, with the amp off an unplugged? Should I take them out of the circuit, right?

So frustrating, I thought it was solved.

Is there a reason to fail when you assembly the amp, after making it work dissasembled?

Good night fellows, hope better luck tomorrow with this.

[vegaluthier]

Thank you all, finally I solved it changing the t65 and t64 for tip142 and 147. Thanks!!