T1.4000DL

[Hexhclass]

T1.4000DL SOUNDSTREAM

This amp came in with half the PS FET shorted and one high side output FET shorted.I replace all outputs.Amp is producing audio but it it drawing about nine amps at idle and the outputs are heating up fast.Q126,127,123,122,(MPSA56),I believe are the drivers i replace them with A92s.

Any idea what the problem could be?
Is it safe to power these amp without the driver board?

 

[Perry Babin]

I've never worked on one of these but similar amps have no problem with the audio driver board removed.

Do you have a clean rail-rail waveform on the output transistors?

Could you have a shorted inductor?

 

[Hexhclass]

With driver board out amp idles ok.With it in circuit it bring down the 15-+ down to 8v.
.I have a limiter on the b+ so maybe that is why?

20v/dv 10/us
high side leg 1 and 3

 

[Hexhclass]

low side same setting leg 1 and 2

 

[Perry Babin]

If the inductors are in series, confirm that the rail-rail waveform is precisely 1/2 of the waveform on the FETs where the inductors connect to one another.

 

[Hexhclass]

Wave form look like half.

20v/dv 10/us

 

[Perry Babin]

Is there a reason that you used the A92 instead of the A56?

 

[Hexhclass]

I have no A56

 

[Perry Babin]

Increase the value of R175 to about 1k.

 

[Hexhclass]

With 1k the idle current goes down to about 3amps

 

[Hexhclass]

With another meter it shows 2.3amps

 

[Perry Babin]

It may need to go up a bit more (1.1 or 1.2k) although 2.3 amps is probably acceptable. This probably isn't the best solution. The best solution is installing the MPSA56.

 

[Hexhclass]

I will try to find some tomorrow.

 

[Hexhclass]

I pull four A56 from an amp and install them along with the default resistor.Amp idle is at 1.2A.
I apply input signal and all seem ok.After about 10 minutes I could hear a sequel coming from the amp and it is starting to draw more current.The 200v caps by the filter caps are getting very hot and wave forms on the output FETs change.
I try it with the A92s and got same results.

same scope settings were used 20v/dv 10/us
high side leg 1 and 3

 

[Hexhclass]

Low side leg 1 and 2

 

[Perry Babin]

That sounds like you have a shorted inductor. Are you seeing any residual of the carrier wave on the speaker output? If so, how much?

 

[Hexhclass]

Both inductors produce about the same heat.What can i do to get the bad one to show it self?

20v/dv 10/us

 

[Perry Babin]

Generally, twisting will cause an intermittent short to make/break.

If the output filer caps are not within tolerance, the residual on the output may be greater than expected.