PERRY, Need help with Relay Drive circuit on Yamaha YPA amplifier.

I have a yamaha amplifier that has problems with the relays engaging. Tested the relays and they are working (engaging with a power supply). Also keep in mind the relay was bypass with a couple of jumpers but this is causing a turn on thump and it is annoying, especially knowing how well the relay delay method works (have another YPA-100 without issues and the relay works perfectly). I already removed the jumpers, want to make the relays engage work properly.

The amplifier is very similar to the YPA-720 so am using those schematics and they are identical. It uses a uPC1237HA IC as the driver for the relays.

These are the voltages I got from 1237HA:
1) 1.97 (should be 0)
2) 0
3) 0
4) 0 (should be 5-6v)
5) 0
6) 22.4 (should be 0.6)
7) 0.045 (should be 2.1)
8) 3.3

E) 22.4
C) 22.4 (should be 0 as it is tied to pin 1 of 1237HA in series with a 10k resistor)
B) 6.85 (should be 22.4 and is tied to each ch's TR604 transistor's collector pin).

Now while I was testing and measuring several transistors in the circuit of the relay driver, the relay engaged (clicked) and audio was present this was after a good 20 minutes of the amplifier being in idle. However powering the amplifier off then turning back on causes the same "no audio" issue due to the relays not engaging.

Am thinking the problem is in each ch's TR604 transistor as it's in circuit with the "Overload" pin of 1237HA.
YES it did work Perry, you are the man. There's about a 2 second delay which is just enough to avoid the turn on thump/pop.

Perry PLEASE tell me how you figure this out, I don't want answers handed to me rather an explanation why it fail or not working as it should. I don't know why I didn't bother to remove it when I spent over an hour just messing in that area around the transistor.

If that transistor failed, I found a replacement for it or is it safe for the amplifier to run without the TR605? No protection?
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Perry Babin

Paid Member
2003-11-20 11:01 am
I didn't figure anything out. Removing that transistor disconnected the protection circuit transistors from the protection IC.

I agree that having a Vbe of greater than 0.7v is a problem. TR605 may well be defective. It's not a critical component. Virtually any PNP transistor with the correct pin configuration could be used as a (temporary) replacement.

After you get a known-good transistor in that location, if the amp still malfunctions, remove one TR604 at a time to see if one channel is causing the amp to go into protect mode.

If you find one channel that's causing the problem, you'll troubleshoot from there.

It's not safe to operate the amp (for more than testing) without that transistor.
I went ahead and purchased the service manual for the 700, was getting a bit confused as this one has an extra component and are not label the same. So to avoid confusion I went ahead and got the correct schematic.

The component dedication numbers (TR###) don't exactly match my board but they are exactly the same lay out. For example on the schematic's TR819, it's actually 607 on the board. But they are the same component. Hope this doesn't cause confusion.

Perry let me know if you want a pdf copy, I'll email it to ya.


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