Eden WT-400 CE Solid State Amp help?

[Anticpunk]

Good day,

I've been working on an EDEN WT-800 that came to me with a blown power amp on the left side.

Upon inspection, I found it to have 2 shorted power transistors, both NPNs. I ordered exact replacements for all of the power transistors, driver transistors, bias and differential transistors. Replaced them all along with the .47R emitter ballast resistors. Basically everything that could have seen stress or big DC spikes when the power transistors went south. All caps are new on the amp board except the 2@10mfd/100v rail filters and the 2@22pf caps, waiting on the order containing those parts. Replaced both 1n4148 and the 270R 5w also. Zeners still test good.

After the rework and reassembly, I brought it up on the variac and it runs stable, biased right up to the factory spec of 7mv at the test points and holds there just fine. No significant DC on the speaker output either.

Problem is that with a speaker load and a guitar as input signal, I'm getting crackly distortion out of it, like a dying battery. This leads me to believe that I'm running out of current since power rails show steady bipolar 73vdc. Either that or the differential amp that is hung on the feedback circuit is clamping down on the wave for some reason when I load the output...

I first went through with my sound probe to try to hear where I'm losing clean signal.. with a speaker or a dummy load, I've got distortion all the way back at the input of the module, just after the muting FETS. I removed the fets to eliminate them as culprits. No change.

If I remove the load and sound probe it, I have clean and amplified signal all the way from end to end, and with a scope connected to the speaker out, I can see the same.

The scope on the output with a dummy load connected shows serious overshoot and some ringing on both sides of the wave. So something is breaking down under load. The psu rails are clean, no hot or leaking filter caps, resistors in the rails test in spec.

If I inject signal at various points (2.7k resistors on the input, 6.8k resistors at the drive transistors, etc) with a speaker load, I do get strong output, but with noticeable distortion...

Comparing voltages between the properly working right module and the left module shows only tiny differences of a few mv here and there.

Tests are conducted with bridge mode disengaged and a 1khz test tone right into the effects loop left or right return. I have attached the correct schematic also.

I should note that I'm not really a solid state repairman, my strengths are in tube land and with solid state preamp circuits, etc., but I do understand the basic concepts of the BJT push-pull amp that I'm looking at. Just missing something and need a "boost" from someone more skilled and experienced than myself with high power solid state equipment.

Any help is greatly appreciated!
Thank you,

J

[shooter]

All caps are new on the amp

that's couplers and ecaps?  it does sound like a cap ain't happy, is ain't.....

I'm a shotgun repair'r so...

do you get clean if you roll back input signal or is it distorted with any level?

what condition is the pcb in, baked on dust, multiple repairs, missing foil jumpers...

I'd hold out til all the new caps are in and evaluate, while you wait surf up a C-made monoblock that works with the PS and upgrade 

[PRR]

So this is a 1979, 200+ Watt, stage amp, two output pairs, with no protection?

I'm astonished it lasted this long.

I'd take it out back and give it a respectful funeral.

But let's look at basics. Are the voltage drops noted reasonable?

[Anticpunk]

Thanks for the replies!

Voltages taken with no load on output.
Rails are 76.9 and -76.1

All measured across the resistors, negative signs ignored:
Pnp side
6.8K is 70.6v
100R 1.18v
.47R measure 1.6mv and 2.7mv

Npn side
6.8K is 75.7
100R is 1.56v
.47R measure 3.9mv and 3.2mv

10R/2watt, as noted in schematic in PRRs post above, gives me 37mv.

The 4x 100K resistors measure
75.4v, 75.8v, 74.9v and 75.4v across.

2.7K resistor on the 8099 measures 2.14v

2.7K resistor on the 8599 measures 1.72v

Looking at it written out this way, the numbers I'm getting on the smaller transistors seem reasonable, so I'm leaning towards a problem with these nos Sanken PNPs. Those numbers don't look right to me compared to the NPN side. Thoughts?

Thanks!
-Jay