Okay, 3Watt ones seem to be a little scarce would it hurt to go up as far as 5Watts? Or is that too much?

Thanks roly, I'll replace both. I'm a little unsure as to which diodes to buy though as I have two schematics for the 8080 power amp and one tells me the zeners are 15v at 3W and another tells me they're 15v at 1.3W. Do you know which one is correct?

Sorry I got R101 and R102 the wrong way round when I gave you my measurements, it's actually R101 that's giving me the -0.08V reading not R102. And yes R101 seems to have shorted, it's VERY hot. So I'm guessing it's ZD1 that needs replacing. And that's a 15V 1.3W zener? Guess I'll order some. How exactly does a zener fail? Is it something that can wear out gradually? I'm just wondering if it would be worth replacing both of them as if one has failed the other could have have problems too. yes?, no?   

Okay Enzo R101 and R102 aren't giving me what you say they should. R101 is giving me +47.3V on one end and +16.2V on the other. On R102 I have exactly -47V on one end and -0.08V on the other.

Regarding the op rails, which IC is it that I need to measure pins 4 and 8 on...Is it IC8B? 

Okay, I've gone ahead and replaced both TR10 and TR8. For what it cost I thought it was worth it just to be sure. However, the problem is still there with no change whatsoever. I've read elsewhere that bad filter caps can cause similar problems. Could this be the case? If it looks like there's component damage that'll be difficult to isolate or that I can't fix without taking it to a tech, I may give up on it. There's only so much money I can put into it before it becomes false economy, becuase I've decided that regardless of whether or not I can get it working I'm still going to sell it afterwards as I simply don't trust it enough to gig with given the problems it's had. I'd rather sell it, make my money back and get something newer and more reliable. Whether or not I can fix it will be the difference between selling it for parts or repair, or selling it as working, with the latter obviously being preferable as I'll make more money from a working amp (I obviously won't sell it without mentioning its history). But yeah...if you have any other suggestions let me know. Thanks.

okay, I've measured what you said and I'm getting +46.7v across C61 and -46.9v across C62. But on the output I'm only getting -8.3v.
Pretty sure I'm measuring it correctly but then I'm a bit of a novice so just to double check it's + to the speaker output wire and - to the chassis (ground)? Sorry if I'm being an idiot. Thanks again for the help though.

Hi, thank you for your response. I just checked your first suggestion and yes the speaker moves back hard when the amp is switched on. Bridging the fx loop send and return makes no difference at all. Plugging an MP3 into the fx loop return does nothing until the MP3 player is turned up past a certain volume, where the amp begins to crackle. The crackling intensifies slightly as the gain is increased and is worse on the boost channel, but disappears when the 20db cut is engaged. When plugging the fx send into another amp it just produces the same buzz as the valvestate. Does this mean it's a problem with the speaker? Should I disconnect it? Thanks for your help.

Hi just joined this forum as I'm having trouble with my amp and thought someone might be able to give me some insight into how to fix it. I recently bought a Marshall valvestate as not working (only £20 so worth a shot). The overdrive channel was very quiet but would sometimes cut back in at regular volume, splutter a bit and then cut out again. The pots were noisy too so I  sprayed some cleaner in them and replaced the ecc83. This fixed the problem and it started working fine, except for when the volume would occasionally drop as it did before the valve was replaced. Hitting it always worked to make it cut back in though. A couple of days ago during a rehearsal there was a popping sound from the speaker and the amp cut out entirely. It powers on okay but the only sound I can get from it is a low hum/buzz from the speaker which remains constant regardless of what any of the controls are set to (volume and gain controls neither reduce nor intensify the hum, it is just one constant volume). Plugging a guitar into it makes no sound at all and also does not affect the hum. Touching the tip of the jack lead whilst the other end is plugged into the amp also makes no sound and I've tried a couple of cables. I've checked the bottom of the pcb for any dry or cracked solder points and can't seem to see anything wrong. Any ideas as to what could be wrong with it?