Apogee DA-Serise Repair

[daviddas]

I'm a little new to amp repair. But I'm trying to tackle an Apogee DA series. it seems similar to an older Crest - say a CA9. The channels come in 4 modules: a preamp module, and then 3 modules that make up the output stage. 1x -ve stage, 1x +ve stage & one that is connected in between both (but still involved on the output stage). I believe the pre-amp board works. I can use either one on my better channel & get the same result.

One channel is out of protect - but has distortion & low gain. (.6 amp on the kill-a-watt meter with this channel installed)
One channel is in protect with a +10V DC offset. (1.0 amps on the kill-a-watt meter)

I cannot find any shorts in the output. And this is where my experience ends. I'm not sure how to debug.

The output feeds back into the input - through what appears to be a differential circuit. So, I'm not familiar with how I debug where the initial offset is coming from - and what is simply a response from the differential circuit.

I attached the schematic. How do I start?

[52 Bill]

One channel is in protect with a +10V DC offset. (1.0 amps on the kill-a-watt meter)

I cannot find any shorts in the output. And this is where my experience ends. I'm not sure how to debug.

I' just downloaded the schematic and am looking into it, but by offset do you mean there is 10 volts dc on the speaker output?

[daviddas]

Yes - there is 10v DC on the speaker output of the amp module. The banana jacks on the outside of the case are on the outside of the protect relay - and show no output (since we're in protect).

[52 Bill]

Yes - there is 10v DC on the speaker output of the amp module. The banana jacks on the outside of the case are on the outside of the protect relay - and show no output (since we're in protect).

Shorted transistors are not the only thing that can cause voltage on the output, missing power supply voltages, open resistors, etc.

That schematic is as confusing as some of the newer Marshall ones with interconnects everywhere.

Because both channels show symptoms, I'd suggest that you start by checking all of the power supply voltages. Plus and Minus 120vDC, +&-60vDC, etc.

[daviddas]

Page 2 of the schematic is probably the best one to look at. It's a summary of the output stage - collapsing the 3 different circuit boards onto one diagram.

I believe my PS is good. I get all 4 readings correctly off of the big PS. I'll check the smaller supplies - +24, etc for aux voltages later tonight.

Probably should have included this earlier - but I believe that the better channel did work correctly when I first started with the amp. I swapped the pre-amp section onto it from the bad channel and it worked. Then, I put the output common module (which I did not know at the time was related to the output) from the bad channel on the good channel. This is where I think the good channel proceeded to be a half-good channel.

[52 Bill]

Probably should have included this earlier - but I believe that the better channel did work correctly when I first started with the amp. I swapped the pre-amp section onto it from the bad channel and it worked. Then, I put the output common module (which I did not know at the time was related to the output) from the bad channel on the good channel. This is where I think the good channel proceeded to be a half-good channel.

Be sure that the repositioned board is connected correctly and that the connectors are all good and tight.

Having a working channel will help you for comparison readings, etc.

[daviddas]

Be sure that the repositioned board is connected correctly and that the connectors are all good and tight.

Having a working channel will help you for comparison readings, etc.

Put new regulators in the 15v pre-amp power supply. This brought those voltages from being uneven to 15 volts. I'd bet that the uneven pre-amp voltages ultimately is what caused my amp damage. We're out of protect - but it doesn't sound right yet. When I put a sin wave in, and turn up the volume, (to maybe +-10V on the output) I get a clip light - & a triangular pulse at the point marked Clip+ on the output module schematic. The Clip- shows nothing abnormal. The speaker output does not appear distorted during the clip - but is reduced drastically. Looking at the schematic, the clip infrastructure loops back into the volume control VCA - so, I assume it's backing off the volume in relation to the clip.

I do not yet understand what the Clip+ signal is measuring. It doesn't seem to be on the output - so are we measuring saturation of the pre-output transistors - the 2sc2336 & 2sa1006?