HenryL

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C1-C6 do look like hf stability to my rusty eye, maybe even a slight tonal rolloff? C17 seems to be just dropping the voltage a bit (and a bit of extra filtering) - it's the same AC load .

The service sheets I'm seeing don't seem to specify the cap types - just capacitance values. Will have a look later to see where in the circuit it seems to be. Ahh the old tants vs elects controversy not so long ago while doing some other work on a Neve preamp copy I replaced the electrolytics with tantalums as per the original Neve design - whilst crossing myself and facing mecca and consulting a psychiatrist. (ps no disrespect intended to anyone - just an inclusive faith or psychology metaphor applied to some apparently discernable acoustic differences)

Yes it's a poky and effective power stage that seems to have stood some tests of time... I'm comfortable working on it but I prefer a map or guide first given half a chance - I'm a read-the-manual-first kind of person I guess - I've designed and built a couple of valve guitar amps from the ground up running at high voltages and coulombs of stored energy - but I've never really got to grips with analogue semiconductor circuits in the same way over my embarrassingly large number of decades of tinkering and fixing things . And it takes a while to remember how even my own amp designs work these days!

DGBass - that's really useful guidance. Yes we have the 2-mosfet version of that board, and I previously noted the rail voltages as being +/- 70V (with 100V rated main reservoir caps) in line with your comments. I had been peering suspiciously at the horrible looking preset as a possible problem area. No tantalum - it's a little electrolytic on ours. Luckily not an overly busy PCB, though I have limited experience of mosfets. This is ours anyway:

That's great Bremen. This is the best set of stuff I've got so far (1986 schematics regardless of the 1999 on the front page). The preamp is rather different to what we have but not much changing as you say in the power amp end it seems! The main power circuits do look essentially the same across these variants though I need to cross-check more of the component values and compare with our own but it looks like we have something very much like channel A of an A350X (single pair of mosfets). I'm gradually assembling some of the history - these are by Stuart Watson who pre-dates Clive Button as I understood it. Thanks again

I just noticed that I said it had started misbehaving "after 20 years" - that should be: "after 40 years" !

Thanks Downunder - a good suggestion but sadly no I don't think it's corroded socket contacts. It doesn't have an FX loop, just preamp line-outs which don't interrupt the signal path. I've been over the internal plugged connectors from the preamp to the power amp to clean them, Deoxit etc. - though I should go over it all again to be on the safe side. Both the front panel input sockets behave the same (and after plenty of wiggling etc.), multiple good input leads tried (and the leads are fine in any case when DI'd into an audio interface). There is an open frame preset potentiometer on the power amp board - some sort of bias adjuster presumably - which looks a bit exposed to the elements so although it has been bedded in that position for years the contact point may have deteriorated I suppose. A bit more digging to be done today..

Hello Basspeople, I found this forum and thread whilst scouting about for information about the TE GP7 combo, and noticed with interest that @Stuart Watson had been here only a year ago - so I'm living in hope Our trusty GP7 combo has just started misbehaving after more than 20years. The amp part has had a varied career, out of the original combo box into a flight case and used with other cabs, then back in the 4x10 cab as a combo again... Handed over to me by the original purchaser (who I was in a band with) it's been in the family so to speak ever since its birth in the 80's. It has had a quieter life in the last ten or so, just some studio use, but now with a rare live gig looming naturally it has decided to malfunction so I am scouting about for technical info before diving in with a soldering iron. The symptom seems now to be mainly that it is breaking up / distorting at low signal levels. Higher levels are better but it seems a bit unstable - and untrustworthy obviously. It can still produce plenty of power - albeit not scientifically scrutinised at this point. The problem seems at first glance to be in the power amp since permutations of the pre gain and master volume on the pre-amp do not affect the behaviour. I mean for example low master gain and high pre-gain setting is not more prone to causing it than the other way around - it seems to be just the general level coming out of the preamp that is the factor. Turning down the master gain whilst someone is playing at an even level will cause the distortion to come in more noticeably as the knob reaches the low end. It's a 1983 production GP7, 4x10 combo, model 1004. (150W into 8ohms / 300W into 4ohms I believe.) The power amp main board is marked "MPB2 issue 2" I've just started casting about today, but so far I've not found much detail about this early model. A source of correct schematic for this model would be helpful, and any technical insights, pointers, etc. most welcome - I'm not so familiar with this type of amp (more familiar with valve amp circuitry..). Cheers

Hello.. More of a bass tech and engineer in effect than a player - my wife's been the bassist in our musical malarkings. I just get the gear, fettle it, fix it, re-fix-it etc.. /H