mikeydee

Ok so it is distorted when you go direct in to the FX return, then that means that it is only the power amp and the speaker. My money is on the speaker. All wound components hate damp - that includes speakers, transformers, reverb tanks and chokes. Other parts are more resilient but rule of thumb is never leave any gear where you wouldn't be happy to sleep yourself. Good luck with the fix.

Can you elaborate on the distortion. Is it a kind of signal strangulation or gross distortion like a robot voice of the signal that is effected by how hard you play. Also is the problem still present if you plug in your bass to the return Jack.

Exactly.

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Yep it's about 4 amps on that heater connection, so needs to be A1. All transformers hum to a degree but you can reduce it by tightening up the four screws. That will help - up to a point. So is it working now?

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Hi Bradwell, I repair amps all the time. So pre-amp is fine. Do any of the buttons on the front make any difference - esp mute if fitted. Next, do the valves light up (sorry) just asking. You've changed the valves already - Did you check the HT fuse ok? Check on a meter. Next: you can confirm power amp stage problem by seeing whether you get output by plugging guitar into return jack. If you get a strong signal, then the issue is probably send/return jacks - It's very common for low output to be caused by the send and return jacks oxidising. If output still low check the phase inverter (PI) valve. First is there any sign that the silver top has gone white? If so, then the glass has leaked. Better tho to replace the PI for another. If you don't have one, then take the one from V1 (nearest input jack) and then use your guitar via the return jack again. If it works then the PI was bad. If not, then you have not proven anything. At this point, you have to open the amp up to go any furthere and you need to know what you are doing in working with high voltage. There is a common problem of bias drift on TSL/DSL. This only effects certain serial numbers but can cause output TX to fail. Check the valve bases for signs of discolouration. They should be reddy brown but sometimes head black when this illness strikes. BTW I know everyone likes to rubbish cheap chinese parts but seriously Marshall will probably sell you the exact right TX for £30-50. They happily last 10-15 years. That's not too bad. Mike

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Yes I was expecting something in the range of 150-250 volts. The 175v is what is supplied to the HV connector on the input board. There are a couple of 10k dropper resistors one for V2 and a further one for V1 so the plate voltage on V1 is reduced a fair bit once biased up. I'll post measurements if folks are interested. See you are in Bristol too - so a local Hi to Subthumper.

Just in case anyone was anxioustto know, it turns out the high voltage supply to the pre amp is 175v.

Done

Dont know that exact model but my first check would be any signal interrupting jacks like send and return loop, headphones jack or similar. When the fault is there is it effected by tapping the aforementioned jack sockets. If so then they need replacement or may respond to a bit of cleaning or repeated in and out therapy. Also I have worked on a couple of ampegs this year with bad solder joints from the factory. This can give the same symptoms as the amps warm up the joints go bad.

I bought an old OBT pre amp board from obbm to create a little pre amp module. I have it working fine with the input ht set to 240vdc giving a plate voltage of about 140 on v1a. I would be keen to hear from any one that knows what the ht should actually be. And, if anyone has a schematic that would also be very welcome. Mike

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