Any Trace Elliot repair-guys here ? 1215 question.

[lorbas]

Hello and greetings from Germany,

bought a Trace Elliot 1215 combo-amp yesterday off Ebay quite cheap, just 50€.

It's GP12, 200W, Series 6.

Everything works quite fine, BUT: there's a 50Hz cycle noise in the speaker all the time, even when master is turned down and no instrument connected.

Bought the manual online, took the amp apart. It's divided into a preamp- and a poweramp-board and even with the preamp-board disconnected the hum is still there, so i think it's the poweramp-board that causes the noise. 

There's just a very minor hum-voltage on the 4700uf main capacitors which i think is unlikely to cause the hum, but i ordered a new pair anyway. 

Any other ideas or experiences ?

Thank you,

Ralf

 

 

 

[BassBunny]

Contact @Ashdown Engineering. They are repairing TE gear now I believe. Makes sense considering the Trace/Ashdown history.

[Chienmortbb]

Hi Ralf,

 

Those 4700uF capacitors are fairly standard, they are on a number of the Trace power supplies. Use a reputable brand, 105 degree temperature rating and the lowest ESR you can get. Going to a higher voltage will increase the cost, but there will be benefits as it will last longer and probably give a better ESR than a 63V capacitor in the same range.

1 hour ago, BassBunny said:

Contact @Ashdown Engineering. They are repairing TE gear now I believe. Makes sense considering the Trace/Ashdown history.

I think they have stopped that since Guy Morel retired.

[Bill Fitzmaurice]

It could be the caps, but it's more likely a loose component joint on the power amp PC board. They can be almost impossible to spot, so hitting them all with a soldering iron and a tiny bit of solder is one way to be sure all that joints are good.

[itu]

A bamboo chopstick is a good tool to tap components, if there's a cold joint.

[paul_5]

4 minutes ago, itu said:

A bamboo chopstick is a good tool to tap components, if there's a cold joint.

Also, and perhaps more importantly, it’s non-conductive

Edited November 19, 2023 by paul_5

[lorbas]

OK, when i install the new main capacitors i'm gonna re-solder the complete board, Thank you.

[lorbas]

So i've done all that i could: re-soldered all the pcbs and connectors, changed all the relevant caps in the power-supply of pre- & poweramp, but to no avail: still an audible mains hum on the speaker. 

 

I measured the mains-hum: it's 'just' 20 milli-volts but is's annoying. My old SWR WorkingMan's 160 got an mains-hum of just 5-10 milli-volts, that's much better.

 

And there are minus 40 milli-volts of DC on the speaker-output that i can't get rid off...

 

Any tips ? Anyone in the UK who deals with that questions ?

 

Thank you.

[3below]

Could it be  noise picked up from fluorescent, led or other noisy devices e.g. pc/screen?   Do you have access to an oscilloscope?  If so investigate the rectified power supply quality.  Is the noise present on the dc power supply after rectification and smoothing before reaching the poweramp stage?  Even better if you can disconnect the power supply from the  pre and poweramp boards when you investigate.  If you don't have a 'scope, I would replace the rectification diodes on the basis that a) they are the other main parts in the rectification and b) they are cheap.  Looking at the schematic (If I have the correct one), C11 0.22uF capacitor might also be worthwhile changing (although an unlikely candidate).  Another possibility is a poor earth connection (causing a ground loop) somewhere between the various pcbs, transformer, case metalwork etc.  After that you are into checking power amp capacitors C3 -> C8.  Good luck!

Edited November 26, 2023 by 3below

[lorbas]

Hey 3below,

I have a scope, but somehow i'm not able to locate the source of the hum anymore. (Seems to be everywhere... but my days working with electrical stuff is like 30 years ago now, i used to repair TVs and HiFi when i was young...) It's seems already present at T1. 

Fun thing: when i short the signal-input of the poweramp-pcb to signal-ground the noise is gone. So i think it originates from the poweramp-pcb, though i'm not shure about that at all.

I can completely disconnect the power-amp from the preamp-section just be unplugging one connector.

Those massive diodes that form the bridge-rectifier seem to be OK. I unsoldered them on one side and tested them with the component-tester of my Hameg-scope and they looked OK. But as you said: they are cheap and i can replace them quite easily.

I changed all the electrolytic caps on the power-amp-board, but payed no attention to those other caps so far. Can they cause hum, too ?

Ground/earth-connections are done quite massive and well, thick cables going to one central point on the chassis. Have seen that done much worse.

 

 

 

 

[3below]

Just now, lorbas said:

... when i short the signal-input of the poweramp-pcb to signal-ground the noise is gone.

 

That suggests a ground loop problem to me even though it has good ground/earth connections (as expected in TE kit). Have you undone the G/E connections,cleaned, contact sprayed and then re-fixed?   This thread is relevant https://www.diyaudio.com/community/threads/trace-elliot-gp12smx-bass-preamp-hum-problem.56245/

 

This is going to be a tricky one but should eventually be fixable. FYI, my amp fixing is from being a Physicist/Engineer - not an amp tech, no serious electronics background, just learned as I went along.

 

 

[sandy_r]

5 hours ago, lorbas said:

...

And there are minus 40 milli-volts of DC on the speaker-output that i can't get rid off...

Any tips ...

 

The DC offset at the speaker output seems too high - but is it a cause or a symptom?

 

If you are confident (and competent! ☺), you could look at the schematic, slightly earlier in the output stage - there is likely to be a preset trimmer (maybe a few 100 ohm?) which gives adjustment of the DC offset.  Photo the existing rotation position of that trimmer before adjusting it (with power off!) in 1 direction slightly. Re-apply power and see if DC offset has improved or worsened. Repeat adjustment relative to result just found and see if DC offset can be reduced much nearer 0mV 

 

If DC offset can't be improved much from 40 mV, replace preset position as per your photo of it

 

Edited November 26, 2023 by sandy_r

[lorbas]

Hello Sandy,

 

there'a just one trimmer on the PCB (P1, 220 ohm) just before the Mosfets, but rotating in in any direction does not seem to change anything at all... is that trimmer supposed to adjust the DC-offset ?

 

I monitored the voltage on my scope and put it back roughly where it was before.

 

What happens if i adjust it 'wrong' ?GP12 200 watt series 6 poweramp.pdf

[sandy_r]

2 minutes ago, lorbas said:

...

there'a just one trimmer on the PCB (P1, 220 ohm) just before the Mosfets, but rotating in in any direction does not seem to change anything at all... is that trimmer supposed to adjust the DC-offset ?

I monitored the voltage on my scope and put it back roughly where it was before.

 

What happens if i adjust it 'wrong' ?GP12 200 watt series 6 poweramp.pdf

 

No it looks they only provide adjustment for the Fet biassing, not offset  😞

 

It can be tuned to minimise crossover distortion, but your symptoms are hum, not distortion? you have replaced it back in approx same position

 

Another thing you could check (if not done already) is the quality of connection on the FX Send/Receive loop sockets, and the parallel cable connectors going to the PCB (clean any oxidation) which can sometimes cause hum and noise

 

[Chienmortbb]

11 hours ago, lorbas said:

Hello Sandy,

 

there'a just one trimmer on the PCB (P1, 220 ohm) just before the Mosfets, but rotating in in any direction does not seem to change anything at all... is that trimmer supposed to adjust the DC-offset ?

 

I monitored the voltage on my scope and put it back roughly where it was before.

 

What happens if i adjust it 'wrong' ?GP12 200 watt series 6 poweramp.pdf

That is the quiescent current setting for the output MOSFETS. It will not affect the 40mV offset at all. Move it without either marking its position with have one of two consequences, The first is that too much quiescent current will be drawn, and the output stages will run hotter than normal. The second is that the MOSFETs will not be biased correctly, and you will get increased crossover distortion.

 

The 40mV DC offset is almost irrelevant IMHO. The Circuit Diagram (what our US cousins call the Schematic) for a typical Trace power amp is shown below, but I suspect your problem lies elsewhere. The fact that connecting the input to 0V or ground solves the problem means that the issue is in the preamp/EQ section. Start at the front by shorting the input to the preamp. However, the link to DIYAudio above suggests an earth loop problem, and I would not attempt that unless you are confident that you can do it safely.

 

 

The power amp Circuit Diagram linked above shows that same connections to the preamp/main PCB as the GP7. See Below: Check the ripple (AC voltage) across C29, C26,C27 and C28 as well as the DC voltages across them.

 

 

 

 

[sandy_r]

 

18 hours ago, sandy_r said:

...

No it looks they only provide adjustment for the Fet biassing, not offset  😞

 

It can be tuned to minimise crossover distortion, but your symptoms are hum, not distortion? you have replaced it back in approx same position

...

 

18 hours ago, sandy_r said:

Photo the existing rotation position of that trimmer before adjusting it (with power off!) in 1 direction slightly. Re-apply power and see if DC offset has improved or worsened. Repeat adjustment relative to result just found and see if DC offset can be reduced much nearer 0mV 

 

If DC offset can't be improved much from 40 mV, replace preset position as per your photo of it

 

7 minutes ago, Chienmortbb said:

That is the quiescent current setting for the output MOSFETS. It will not affect the 40mV offset at all. Move it without either marking its position with have one of two consequences, The first is that too much quiescent current will be drawn, and the output stages will run hotter than normal. The second is that the MOSFETs will not be biased correctly, and you will get increased crossover distortion.

 

[3below]

17 minutes ago, Chienmortbb said:

..... The fact that connecting the input to 0V or ground solves the problem means that the issue is in the preamp/EQ section. Start at the front by shorting the input to the preamp. However, the link to DIYAudio above suggests an earth loop problem, and I would not attempt that unless you are confident that you can do it safely.

 

The power amp Circuit Diagram linked above shows that same connections to the preamp/main PCB as the GP7. See Below: Check the ripple (AC voltage) across C29, C26,C27 and C28 as well as the DC voltages across them.

 

 

 

 

Agree about safety issues and the Earth wire is essential. I just took the link as a further pointer to check the earth/ground connections.  In an ideal world (no experience of inside TE equipment) you would be able to check the power supply on its own, then the power supply board shown above without any downstream connections.