Are those TO-3 Packages.
You can desolder the BE and test for shorts

 

See the circles....I believe they are the power transistors and may be a driver.

If you desolder those pins then you can check for shorted power transistors

Still, I am wondering where the heatsink is !!

 

Are those TO-3 Packages.
You can desolder the BE and test for shorts

Yes. Good idea.

 

Still, I am wondering where the heatsink is !!

Between the PCB and the panel. It's quite an elaborate construction: they don't make things like they used to.

This is why I asked for advice on disassembly. I'm afraid I might not be able to put it back together.

 

See the circles....I believe they are the power transistors and may be a driver.
View attachment 344336
If you desolder those pins then you can check for shorted power transistors

Still, I am wondering where the heatsink is !!

Three TO3 devices in parallel each with its own emitter resistor. The emitter resistors are where there is something written in feltpen on the board.
The heatsink is on the other side. The black screws fix it.

 

The heatsink is on the other side. The black screws fix it.

Nope. The black screws tie the PCB to the back panel via two sheet aluminum U-channels.

I think (I can't see the whole thing yet) the heat sink is one monolithic chunk of extruded aluminum held on to the PCB only by the TO-3 (silver) screws. Somehow, the screws must be insulated from the heat sink (shoulder washers?).

What I don't know is if I can remove the PCB just via the black screws. It looks like some hardware is mounted to both the PCB and the back panel, and there are a ton of wiring harnesses underneath the board.

I'm going to delsolder the transistors first and ensure one or more of them are shorted. Then I'll deal with the next step.

I've got a real job, so I can only spend a few minutes a day on this. It'll be a while.

 

I need someone to agree with me that the EBC pinout of all the TO-3s are the same, regardless of NPN or PNP.

That's what the trace layout tells me.

 

Here's a video on the 680 plus. I don't know what the difference is from the 680S Plus*.

*It is still 300W per channel into 4 ohms, like the S version. I assume the power amp board is the same in both units.

 

I need someone to agree with me that the EBC pinout of all the TO-3s are the same, regardless of NPN or PNP.

That's what the trace layout tells me.

You can still diode check the TO-3's without desoldering. If shorted you will know.
Do discharge the big caps first

 

I need someone to agree with me that the EBC pinout of all the TO-3s are the same, regardless of NPN or PNP.

That's what the trace layout tells me.

NPN and PNP will be there as far as I can tell. If it is class AB.

If shorted you would need to pull them out, you would also need to check drivers too and other components in the driver stage

 

Remove black screws and lift the pcb.
Take pictures of cable orientations and mark which goes where.
It will be rather straight forward

 

Here are my in-circuit measurements, but they don't look all that useful. Perhaps transistors 3, 4, 8, and 11 are damaged based on Ohms CE?

 

I need someone to agree with me that the EBC pinout of all the TO-3s are the same, regardless of NPN or PNP.

That's what the trace layout tells me.

They are all the same.

 

Here are my in-circuit measurements, but they don't look all that useful. Perhaps transistors 3, 4, 8, and 11 are damaged based on Ohms CE?

View attachment 344394



View attachment 344393

All dead. You should get the "over range" indicator on C-E in both directions, and Vbe on the diode test function should be around 0.55 to 0.7

 

Remove black screws and lift the pcb.
Take pictures of cable orientations and mark which goes where.
It will be rather straight forward

Yeah....I'm taking it slow. I like this amp.

 

You should get the "over range" indicator on C-E in both directions, and Vbe on the diode test function should be around 0.55 to 0.7

Even in-situ? There won't be parasitic effects from the other attached components?

 

Even in-situ? There won't be parasitic effects from the other attached components?

I should pay more attention. I'm reading this while cooking my tea, and seeing a picture of a dismantled amplifier.
In situ, the lowest ones will be dead. Measuring across a good one will give the resistance of the bad ones plus one emitter resistor, plus several emitter resistors in parallel depending on how many dead ones there are.

 

I desoldered B and E of transistor 1.

Now I get:

Vbe = -0.525V
Veb = OL
Vbc = -0.525V
Vcb = OL
Ohms CE = 4.2MOhm
Ohms EC = OL

I'd say transistor 1 looks like a good PNP.

 

Short circuit CE on transistor 3. Vbe looks good though, so part of the transistor survived!

 

Do you have the transistor’s part number? A photo of the device will be nice.
I don’t want to discourage you, but TO-3 packaged transistors will give you a sticker shock.