Trace Elliot AH300-7 ( GP7) output fault

[Arbutt]

Hello all
This came to me and i was told that it didn't work,

Anyway on tested it powered up and i heard hiss from the speaker. found it working but scratchy pots
Cleaned pot and tested all good until i turned up above 1/4 way then it go just sound like the output stages were blown ( Loud hum dc going to spk)
I quickly turned off and then when you turned on again it was ok working at low volume until you pushed it again

There is signs of a repair. Should have 2 x BUZ901 and 2 x BUZ906
I have BUZ900/901/905/906.
R2 ( 4.7R) was burnt out and has been replaced now . I would normally just order and replace all output stages and drivers, But they are no longer available and if they are expensive, so i would fit equivalents But i'm not 100% sure in this case will fix it

any thoughts on this before i replace the mosfets
Many thanks
Mr A

Trace Elliot GP7 150-300watt SM.pdf

[glebert]

I would try to test it with a sine input with no load first (or a high resistance load) and look at the output on a scope. I'd probably put it on a light bulb limiter too. I'm wondering if the circuit is going into an oscillation when you get up to higher drive levels.

[Jon Snell]

If R2 has got hot, the amplifier has been overloaded and produced square waves from the distortion however, nothing to be concerned about ... yet.
BUX900 is a 160volt version of the BUZ901 likewise the other transistors.
If they fail, that's it ... dead short.
I would leave them alone as they are obsolete and unless you purchase from a reputable supplier, they may be fakes.
I would replace C3, C5 & C7.
Then test whilst monitoring the DC offset at the input R8. It should stay at or around zero.

[J M Fahey]

I would doubt anything but power Mosfets; transistors don´t self repair, they are either alive or dead and once dead that´s definitive.

Meaning scope it as suggested above, and then decide.

Spending a King´s Ransom on transistors will not make it work any better.

That, IF you get real ones.

[Arbutt]

Thanks for the help.
tested with scope and signal gen on a lamp limiter.. Without a dummy load .
The picture below is what I get .

I have changed all the small electrolyte caps 100mv -135mv on r8 at jnc of c7
all the to92 case transistor are original as far as I can see. Tested all . All test good
R13 has been changed prior to me ??

if I test with dummy load . The limiter just glows bright as you turn up the vol gain .

77v+ and 77v- UN loaded

Hope I've explained it
Mr a

[Helmholtz]

Glebert was right: Scope pic shows strong bursts of HF oscillation.
As C10 acts as a short to HF, R2 gets all the HF power.

Can you see the HF at the power amp input?

[Arbutt]

Glebert was right: Scope pic shows strong bursts of HF oscillation.
As C10 acts as a short to HF, R2 gets all the HF power.

Can you see the HF at the power amp input?

Many thanks for the reply.
I can't see any HF on the power amp input. I bypassed it this morning and put some audio straight into the power amp
still doing it, so the fault is 100% the power amp
I can see some of that HF noise on the grounds if that is any help
many thanks again

[glebert]

I'm assuming that there is no DC on the output until it starts to oscillate, on your scope pic is the oscillation going down to the -77V rail? I would check C21, C22 to make sure they have good connection to the FET gates and to ground, and also check D3 and Z2 as well. On the blown Trace Elliot amps that I have seen I noticed that sometimes they used really narrow PCB traces and when their was a failure they would blow right out. Since you said this had already been repaired once before I would check and make sure that all the connections in that area are being made properly.

[g1]

Anybody want to speculate whether some of those replacement outputs might be fakes?

[Arbutt]

Anybody want to speculate whether some of those replacement outputs might be fakes?

I know these are hard to come by and the ones that are ... very expensive. any thoughts on using equivalents
I have found a list of some for the Buz901 that some are available and not much, Just so i know.
would like to replace all of them,

https://alltransistors.com/mosfet/cr...1.4&caps=TO247

[Arbutt]

I'm assuming that there is no DC on the output until it starts to oscillate, on your scope pic is the oscillation going down to the -77V rail? I would check C21, C22 to make sure they have good connection to the FET gates and to ground, and also check D3 and Z2 as well. On the blown Trace Elliot amps that I have seen I noticed that sometimes they used really narrow PCB traces and when their was a failure they would blow right out. Since you said this had already been repaired once before I would check and make sure that all the connections in that area are being made properly.

Ok thanks again for the help
If i connect the lamp limiter and test with signal gen / dummy load and scope. I get no HF oscillation at output nor do i get DC at output.. , But clearly drawing way to much current
If i test without lamp limiter and monitor the output once i get to a set level the amp just nose dives and i can see the oscillation and on the DMM i get up to -15v ish on output, turn the volume back down don't stop anything , I have to switch off and turn back on again

I have checked what you stated and resoldered up , visual checking the tracks. Tested all semi's
Its taking its toll on my patience's

thanks again
Mr A

[Arbutt]

Just thought I would say now the job is done. I made the decision to replace all the semi conductors. Started with the transistors bf422 and bf423 . Removed them all tested again . All good . But fitted new ones . Tested all good . Thankfully it's done and ready too work
thanks again people