Traynor YCV40 Standby

[bobloblaws]

This amp was brought to me blowing fuses. It turned out to be a bad power tube. But another problem has surfaced, again with the standby/voltage regulation circuit (I have a YCV40 thread on here from a couple of years ago for a low B+ issue https://music-electronics-forum.com/...or-ycv40-low-b). This time the amp is operational even with the standby switch engaged. I still don't understand how that voltage regulation circuit works, but obviously U3 somehow shuts down the high voltage. The 24V supply from Q1 is working and R55 measures 4.6K (tested in circuit with the standby switch engaged). Pin 1 of U3 is grounded with the switch engaged (down) and has 1.1V on it with the switch released (up). According to the datasheets for the 4N35 the "Typ" forward voltage is 1.2 or 1.3V (depending on whether I'm looking at the 3.5KV or the 5KV version). The voltages on pins 4, 5, and 6 are the same with either switch position. I could use some guidance on how to proceed. Should I change the value of R55 so that the voltage on U3 pin 1 is at least 1.2/1.3V? Or should I replace U3? Another angle?

sm_ycv40_M581-7.10.pdf

[Jon Snell]

U3 is an opto-isolator that controls Q8 and its conduction. Allowing HT supplies to function or not.

[bobloblaws]

U3 is an opto-isolator that controls Q8 and its conduction. Allowing HT supplies to function or not.

Uh huh, but Q8 is conducting, so the problem must be U3, no? In normal operation pin 1 of U3 is grounded, then when the standby switch is engaged some voltage is applied to pin 1 which is supposed to turn off the HT supplies. In my case that is not happening. So to reiterate: "Should I change the value of R55 so that the voltage on U3 pin 1 is at least 1.2/1.3V? Or should I replace U3? Another angle?"

[The Dude]

I think 1.1V should be enough to light the LED in U3. What happens to the voltage on both sides of R102 with standby engaged and disengaged?

[bobloblaws]

I think 1.1V should be enough to light the LED in U3. What happens to the voltage on both sides of R102 with standby engaged and disengaged?

404V on both sides of R102 with standby engaged or disengaged.

[The Dude]

Is the FET (Q8) shorted?

[bobloblaws]

Is the FET (Q8) shorted?

In circuit I measure about 70 ohms between the D pin and either of the G and S pins. For the G and S pins alone I measure 4.5 ohm. I guess that qualifies as shorted? In any case I have an IRF80 in stock so I can go ahead and replace it unless you tell me different.

[The Dude]

Sure seems shorted. I'd replace it, but check the pads where it solders in before putting in the new one. You could also have shorted D12, D13, U3, Q12, etc. I'd check that whole turn on circuit for shorts.

[g1]

Like The Dude said, if the Q8 Fet is shorted, the amp will stay in 'play' mode. Shorted Fet is probably the most common fault in these standby circuits (they ditched the Fet in later versions).

[bobloblaws]

Sure seems shorted. I'd replace it, but check the pads where it solders in before putting in the new one. You could also have shorted D12, D13, U3, etc.

Check the pads for....?

[The Dude]

I edited my post. There are only a few SS components in that turn on circuit. I'd just check them all in circuit for shorts. It'll only take about a minute. In direct answer to your question, check both Q12 and D12.

[bobloblaws]

I edited my post. There are only a few SS components in that turn on circuit. I'd just check them all in circuit for shorts. It'll only take about a minute. In direct answer to your question, check both Q12 and D12.

Sorry, I didn't notice D12 at first. Anyway, I'm not sure what you meant to check for re: pads.

[The Dude]

If you check the solder pads where the FET soldered in for shorts and there are still shorts there, then other components are shorted.

[bobloblaws]

If you check the solder pads where the FET soldered in for shorts and there are still shorts there, then other components are shorted.

Ahhh, you mean check the pads on the PCB, while the part is out. I thought you were talking about the insulator pad between the back of the chip and and the mounting surface.

[Jon Snell]

The opto-isolator Q3 is unlikely to be the culprit, unless it has been damaged but it is cheap to buy and replace. But that means serious de-soldering etc.

What I would do;
Check for 24v on the 4k7 R55 top end.
Q8 is not a regulator, it is a switch. Measure the 410v with the gate drive resistor disconnected? Q8 is being run very close to its voltage limit. Should be no HT when off.
This information would allow me to make an educated if not accurate guess as to where the issue lies.