Trace Elliot, tell me about it

[SimonK]

11 minutes ago, TheGreek said:

I think that I have a port ring or two somewhere. I'll have a look tomorrow.

That would be amazing if you have one!

[sandy_r]

2 hours ago, SimonK said:

Does anyone know where to get the green and red caps for the gain and master volume knobs? I'm almost at the point of buying another TE just to strip for parts!

 

 

Greetings from a few clicks along the coast to the West!

 

Most quality component suppliers (eg. RS Online, Rapid Electronics, etc. shown below) handle this kind of top-secured  (collet-tightening) control knob with colour-coded inserts. Google is your friend (...minimum order P&P not so much!)

 

(...these days you can often find a slightly better deal, item(s) + p&p on am*z*n & eb*y - the former's search facility, however, is rubbish, so use google & include am.z.n uk in search term)

 

 

 

 

Edited February 21, 2024 by sandy_r

[SimonK]

47 minutes ago, sandy_r said:

 

Greetings from a few clicks along the coast to the West!

 

Most quality component suppliers (eg. RS Online, Rapid Electronics, etc. shown below) handle this kind of top-secured  (collet-tightening) control knob with colour-coded inserts. Google is your friend (...minimum order P&P not so much!)

 

(...these days you can often find a slightly better deal, item(s) + p&p on am*z*n & eb*y - the former's search facility, however, is rubbish, so use google & include am.z.n uk in search term)

 

 

 

 

Awesome - thank you - I will see if I can find something that fits.

[BassmanPaul]

@sandy_r Thank you for taking the time to help out.

 

[HenryL]

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

 

[Downunderwonder]

5 hours ago, HenryL said:

The symptom seems now to be mainly that it is breaking up / distorting at low signal levels.

I am not familiar with older stuff.

 

Presumably there is an fx loop. On mine it is a parallel loop but I think your is in series.?

 

Quite possible you can insert a patch cable onto it and all will be well.

 

The old insert contacts can get corrosion between them breaking the signal at the loop.

 

If it's a parallel loop you want to turn it all the way off and insert the cable for good measure.

[HenryL]

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..

[HenryL]

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

 

[bremen]

If it's the Clive Button mosfet board (based on a Hitachi applications note) it looks like it hardly changed as the product line evolved, and now Ashdown are using  it.

 

Any of these any good? There's more somewhere that show the resistor feeding the long tail pair split with a decoupler to ground, and some with gate capacitors added to the output devices

 

 

 

 

trace-elliot_GP11-AH350-AH500-MK-V-BASS-1.pdf trace-elliot-GP7-300w-7-band-1.pdf

[DGBass]

The MPB2 Issue 2 boards on the older TE models were usually 150 watt units. They just sound like 300 watt units. There is sometimes a circular sticker on the front that will say which configuration it is, either PM2-S ( two hitachi mosfets ) or PM4-S ( four hitachi mosfets). The issue 2 boards were usually fused on the board, wheras the more common Issue four boards were not, but had a seperate speaker fuse on the back panel. Here is a photo of an MPB2 Issue 2 board from a 150 watt combo unit PM4-S. These ran off a 40V-0V-40V tap on the transformer. The PM2-S version had a larger supply voltage to get the 150 watt rating. IMHE, servicing points I always look out for on these boards are the open frame preset pot, these often go open circuit, and the tantalum bead capacitor can cause signal gremlins eg distorition / intermittent noise. The mosfets rarely give any trouble. If I was servicing one of these boards, I'd change all five small transistors as well even if they test ok, renew the open frame preset with a sealed one, and renew the tantalum bead cap while the board was out of the chassis. Clean the multi plug contacts as well.  The resistors, diodes and passive caps also rarely give any trouble but its worth testing them anyway if the board is out on the bench.

 

[HenryL]

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

[HenryL]

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:

 

 

[bremen]

They're amazingly robust, simple designs. I shorted the output to the negative rail by careless placement of a Croc clip, which arced bright white, gave off a lot of smoke and welded shut. Power off, chiseled it off and it was fine.

 

[DGBass]

Changing the tantalum cap for a standard electrolytic is unfortunatley something i've come across in the past.  It will cause instability and random noise when done, but won't stop the board working. It's an easy fix done mostly because standard electrolytics are more commonly available in the parts bin. If you have a test meter you could try measuring the pot to see if you get a reading both sides of it. Be careful though as once you open up things, there will be hazardous voltages, especially from those very large smoothing caps. It might be time to consult a tech if you aren't confident in testing or swapping out a few components.

[bremen]

Is that the input coupler? I wonder why replacing a horrid tant with a nice shiny electro would cause instability. Genuine question.

 

I use film caps for input coupling and mlcc and electros elsewhere on my clones and they're plenty stable.

 

[HenryL]

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!  

 

[Chienmortbb]

35 minutes ago, DGBass said:

Changing the tantalum cap for a standard electrolytic is unfortunatley something i've come across in the past.  It will cause instability and random noise when done, but won't stop the board working. It's an easy fix done mostly because standard electrolytics are more commonly available in the parts bin. If you have a test meter you could try measuring the pot to see if you get a reading both sides of it. Be careful though as once you open up things, there will be hazardous voltages, especially from those very large smoothing caps. It might be time to consult a tech if you aren't confident in testing or swapping out a few components.

Tantalum capacitors are usually not recommended for audio circuits so why do you think the electrolytic is worse, or am I reading your reply wrong?

[DGBass]

9 minutes ago, bremen said:

Is that the input coupler? I wonder why replacing a horrid tant with a nice shiny electro would cause instability. Genuine question.

That is a good question. Wish I could answer it with 100% certainty. I can only say from experience working on these type of boards. Tant's are as far as I am aware more expensive, have a longer life span, and from experience seem to have tighter tolerance than comparable electrolytics so are more stable. Yes, on these boards, and also on most Ashdown amps in recent years, tants are often employed as input couplers. Most schematics for these type amps specify components are replaced with the same original part specification which for me at least is good enough reason to replace like with like.🙂

[bremen]

Traditionally, tants were used for coupling as they could take small reverse polarity volts better than the electros of the day.

 

Nowadays they're used where electros cant do small enough size/low enough esr/high enough capacitance all at once. Theyre a last resort at the last place i worked. They do flame quite amusingly when they lose their tempers though.

 

 

[HenryL]

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)

 

Edited April 7 by HenryL
clarification

[bremen]

 

How the choice of a tant indirectly brought down a company. Probably.

Edited April 7 by bremen

[bremen]

Fwiw, here's the Ashdown variant. The output devices are plastic package equivalents  of the originals. All now obsolete but equivalents available from Profusion PLC.

 

Additions: C1, 2, 3, 4, 5, 6 (for hf stability?) and C17 (lf stability? Power supply noise rejection?)

Ashdown ABM Schematics.pdf

[HenryL]

 

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 .

Edited April 7 by HenryL
minor tweak

[bremen]

Yup, C1-6 begin to roll off the tone at about 16MHz 😉

 

C17 will isolate the input stage from the +v rail and any nasties thereon, eg 100Hz ripple or any DC variation as big currents into speaker drag it down.

[HenryL]

👍