Hiwatt Custom 50 Biacrown era Loud Scratchy Master Volume..Help

[vmazz]

Hello to the forum of experts and the like. It has been a while since I sought out help from your cumulative wealth of knowledge and experience. My own has increased over the years but im still pretty green.
Ive got a friends Hiwatt 50 combo from 84 , it was making some buzzing and noises he said, I checked it out and the first thing I noticed was an extremely loud(like imagine a wide open Custom 50) scratchy when you barely even touch the Master volume(like its on 0 but when you touch it ..Wham!)
So I opened it up and the first thing I noticed visually was the filter caps where bulging(all but 1 had bulges and white coming out) So I new that had to be done and that could possibly cause some noise issues. So I replaced all electrolytic filter caps and the bias cap as I read that should be done when the others go. So now that is done but we still have the loud scratching on the master. It only does it when touching the master. I have pulled the board partially(posts are pcb mounted) and tested the master pot out of circuit . It reads 282k its supposed to be 220k but no intermittance that appears to me with multimeter.
Just hoping for some input before I continue to replace parts.
Ive heard of DC current on the pots causing noise problems but not sure what cause is. Maybe bad coupling caps?
Ill post closest schematic I could find and some pics before I did the filter caps.
Any help appreciated.

[Dave H]

The wiper is directly coupled to the next tube grid (V4). Have you tried replacing V4?

[vmazz]

No I have not tried that Dave. Worth a try , simple enough. I just figured it such a loud scratch, Ive never heard a bad tube cause something like that. Its like the master is shorting out , its a full blast of volume!
Do you think a bad tube could cause the circuit to act that way?

[vmazz]

Changed v4 but that did not cure it. Eventually,reduced it to a bad pot.
Cleaned out and its a bit better.I will replace the pot.

But I am now dealing with what I think is blocking distortion. Ive read about that problem with this particular amp. But I am confused,because things where working clean before. I just changed the Filter caps and Bias Cap?
Now I get a real nasty splatty distortion and lower volume. Very little clean headroom, which is the opposite of what Hiwatts are supposed to sound like.

Anyone have experience with changing/adding grid stoppers to v2 to address blocking distortion?

[drewl]

Hey!
I'm in the middle of restoring one of these!

Thought the PT was bad, but the first node filter caps are bad.
It was sounding awesome for a while there, caps should be here today so I can finish it up.

[g1]

If things were working fine before, then you have some kind of fault. I think blocking distortion is more of a design issue, not something that just starts up where it did not exist before.
The description of "splatty" distortion and weaker volume makes me think of some kind of parasitic oscillation. Recheck your work, especially grounding.
I don't think adding grid stoppers would not fix an issue that was not there before.

[vmazz]

THanks for the input G-one. That was what I was thinking too. I had to pull up the rear pcb to change out the bias cap, I will go over all my solder joints there as well as the filter caps.

[g1]

If you don't find anything, a good place to start would be to check all the DC voltages shown in red on the schematic. Report any differences you find in excess of 10% or so.

[vmazz]

So I went over the voltages and nothing screams out way over 10%. Voltages were a little off but the mains was a little low. Plates were only about 490 so others were off accordingly. When you say parasitic oscillation, could that be lead dress, or strictly loose ground wire? All wires seem to be in place. im trying to avoid having to pull the whole board up. One other thing I forgot to mention..when I redid the filter caps I didnt double check polarity and I popped the fuse instantly, I had the 2 main 220uf caps reversed, I corrected and everything powered up well and voltages all seemed right. Nothing seemed to be effected by it, although now that im running sound tests this splatty low volume is an issue. Do you think anything could have been damaged in that instance? It seems like it must be in the preamp. Its way overdriven and this amp should be very clean til the last bit of the preamp volume.

[g1]

Yes, lead dress could cause oscillation. But oscillation is just one of the possible causes, a scope would show you whether it is oscillating.
A bad tube or even poor socket connection (dirty socket) could probably cause your problem, as could a bad solder connection.
Signal tracing with a scope through the circuit would probably be the easiest, but you could also do it with an AC voltmeter or audio type signal tracer.

[vmazz]

I have 2 scopes in storage, both need repair. How do you test with AC voltmeter? Check the grids for appropriate AC voltages?

[g1]

Yes, inject a sine wave at the input, follow signal through checking AC volts on grids and plates (except power tube plates for safety reasons).

[vmazz]

Wel I dont have a signal generator. I could probably rig up a boombox? However, while running my guitar into it and strumming then checking voltages I am able to make some observations. PI plates go up to about 90v ac when I strum than drop down to about 3.2v ac ( same as heaters?)one grid on V1 goes up barely to .9v the other doesnt seem to change much. Any clues here?

[The Dude]

Just an FYI, there are MP3 and WAV files all over the web that have everything from various test tones to full sweeps. I have a tone generator, but still use my PC more often for this. It's there, it's on, and I have it wired into my bench setup. It's just easier most of the time. Plus, I can quickly go from say a test tone to music to guitar tracks, or whatever else I want. You could even put some test tone tracks on your phone or MP3 player if you wanted to and have a portable tone generator.

[vmazz]

Thanks dude! I had not realized it could be so simple. I will try it.