The soft-startup sequence is this:
- You choose a source on the front panel.
- A command to power up the amplifiers are sent from the processor.
- Relay RL2 is energized (from the low voltage power supply).
This initiates current flow to the large transformer, but through the two power
resistors so as not to burn the fuse in your house mains because of the very
low DC resistance of the tranformer primary when not induced.
It's still enough to provide some voltage on the secondary and to power and
trigger the delay circuit for RL1.
- So shortly after RL2 is energized, so is RL1, shorting out the two
power resistors, bringing full mains to the (now induced) transformer
which again will bring full voltage to the amplifiers.

If RL2 is energized but you have no secondary voltage check the condition of :
- RL2 contacts (double check this, these relays live a hard life)
- both resistors
- all connections between relays, resistors and fuses.
- large transformer primary winding

You can isolate the transformer secondary from the rest of the circuit
to make sure that no secondary things (the amplifiers) are causing problems
at this point.

Martin

Excellent explanation! That's exactly the information I was looking for.

Thank you Martin, you're a great help!

Well, I figured out a couple important things:

1. The circuit does not match the schematic. The circuit diagram shows the two 5.6 ohm resistors in series, for 11.2 ohms total. The circuit board itself has the resistors in parallel, for total resistance of 2.8 ohms. This appears to be from the factory, but I don't know for sure.

2. The brand-new 5.6 ohm resistors are toast, totally open. This explains why the second relay was failing to close, and why there was no power to the transformer. But the question is why the resistors failed--would they have been killed by being in parallel, and not series?

UPDATE: I just saw the alternate schematic for type 1903. The circuit does indeed match, as the resistors are paralleled. But what happened? The original power resistors both failed open, as did the replacements. But the fuse did not blow---what is happening?

I have ordered more output transistors and power resistors. Next step will be installing those sometime later this week, or this coming weekend. Then I hold my breath and switch it on again...

OK, that indeed explains the missing startup.
I suggest you fix the startup board, disconnect the transformer secondary and power the transformer up normally
(through the resistors) while measureing AC on the secondary to see if there is a fault in the transformer.
It's a huge transformer, rated at 700W and it's a sturdy build but that doesn't mean it won't ever fail.
I've replaced two or three in my time and I seem to remember that the faults were all at the primary side and
a shorted primary will put a huge load on the power resistors, relays and fuse(s).

Martin

Can I do a resistance test on the transformer secondary?

Do I remove the leads going to the rectifier? What about the big lead going to the main filter caps?

Thanks! 😐

It's hard to get a resistance test right since the DC resistance will be very low even if working, but I suppose you
can compare the two secondary outputs.
An AC voltage test would be better.
Just lift off the leads to the rectifiers. You can check the rectifiers too then.

Martin

Well, I got the parts today, and attacked the Startup Board first, putting the new resistors in. Now, I had previously removed the output transistors on the amp board that had blown up, and had disconnected power to that board. I powered the unit on, and it worked. 2 relay clicks, and 55V present.

Then, I put in the new transistors, and reconnected the amp board power wires. It powered on...

...and blew up.

The postmortem revealed that the emitter caps and TR209 physically blew apart. The other components visually look OK.

I disconnected the output board again, and the unit still powers up OK. I'm actually getting 52V across the main power rail, so I'm glad about that.

The output board is really frustrating me, though. The bias trim pot is a brand new, 10-turn 0-100 ohm Vishay. I'll have to replace those emitter resisors AGAIN, and go from there. Like I said, the output transistors are brand new, TIP142/147. They share a datasheet and are identically spec'd, except for Vce and Vcb, 100V vs. 80V for the TIP141/146.

Please stop me from throwing this machine through an up-opened window.

Yes, it's like new years evening with the fireworks and sounds...
Reminds you what a very powerful beast this is.

Did you replace all semiconductors on the board ?

Did you get a chance to monitor the idle current ?
I never liked the multiturn trimmers, often you cannot tell if the setting is near the center
or towards one end. Did you check yours before powering up ?

Martin