Eros left channel no output

[Jamus]

My Eros had been working well for the last several months but today I got no output from left channel. After localizing the issue to the Eros and popping the hood, all 4 LEDs on the left channel C4S board are out. All other LEDs are good. I repeated the voltage checks on both C4S boards. Right channel is all in spec. Left channel is...
IA : 0v
OA : 0v
Breg: 0v
Kreg: 0.3v
bA : 0v
OB : 0v

Any suggestions on where to start troubleshooting?

Thanks!
Jamus

[Doc B.]

Reflow.

[Jamus]

I did reflow everything on that C4S board even before checking voltages. Should I be looking outside that board?

[Paul Joppa]

IA should have some volts on it, to power the rest of the board.

[Jamus]

IA should have some volts on it, to power the rest of the board.

Thanks Paul. So would this confirm the problem is upstream from that C4S board? Since the other channel is working I'm hopeful this would help to narrow down the issue a bit.

Thanks,
Jamus

[Doc B.]

Yes, it sounds like the wire to IA does not have any voltage on it. Check the voltage at the connection of the other end of the IA wire.

[Jamus]

So I ended up pulling the shunt regulator board to re-flow all connections and then reattached it hoping that would hit all possible upstream issues. The A side board is still dead.

[Doc B.]

Did you check the voltage at the other end of the IA wire as I suggested?

[Jamus]

Did you check the voltage at the other end of the IA wire as I suggested?

Yes, sorry, forgot to mention 0vDC on D1 which is the other end of IA. OB on the shunt regulator board also 0vDC for what it's worth (292v on the IB). After seeing that I pulled the shunt regulator board and reflowed everything figuring the issue was there somewhere.

[Doc B.]

OK, the next thing I would check is to see if the transistors on the A side of the C4S board and also the shunt reg board that feeds that side are OK. I know I have posted that procedure a zillion times but I'm not finding it in a post here.  With power off and preamp unplugged set your ohmmeter to the diode test. Test across each of the three possible combinations of the three leads of the transistor, for example red to C, black to B; then red to C black to E; then red to B black to E. Then swap the leads and test again. In one orientation of the test leads for each pairing you should see infinite (or very high) resistance. When you swap the red and black leads you will probably see roughly around 500 ohms to 2K ohms or so on a couple of those measurements. The critical thing is if you see 0 ohms anywhere - or really anything less than 50 ohms - and it measures that same low value when you swap the test leads. That indicates that the transistor is blown.

Check the bigger black ones and the small ones too.

[Paul Birkeland]

I would suggest removing the red wire from the shunt regulated power supply to the front C4S board that shows no voltage.  I would just pop it out temporarily at the front C4S board (at IA).

Either something in the shunt regulated power supply is yanking the voltage down, or something in the actual preamp is yanking the voltage down.  If you disconnect the connection between them, either the voltage coming out of the power supply will pop back up to what it's supposed to be (and you know the issue is on the preamp side), or the voltage in the power supply will remain low (preamp is probably OK, power supply not so much).

[Jamus]

Ok, first I did the transistor resistance measurements that Doc prescribed, I didn't get any measurements of 0 ohms. The only transistors I couldn't access were the 2N2907s on the shunt regulator board. Happy to remove the board to access if it's critical.

Next I removed the red lead at IA of the A side C4S board. Volts at D1 and OB of shunt regulator remain at 0 VDC.

[Paul Birkeland]

Pull the 12BH7 and recheck (leaving the other wire disconnected).

-PB

[Jamus]

I got busy with a few projects around the house but was finally able to redirect some attention to this Eros again today. I was about to try PBs suggestion above regarding the 12BH7 when it occurred to me that I never tried swapping the EF 86 tubes. Sure enough the dead channel followed the tube swap. I'll be ordering a replacement and will follow up here.
Thanks all,
Jamus

[Jamus]

Success!!! I replaced both EF86 tubes and powered it up and all LEDs came on...  but then the left channel LEDs started to flicker and went out again
I was able to localize the issue to one of the 2N2907s on the shunt regulator board (chop stick trick). Reflowed and all is good. Interesting that I had 2 parallel issues going on here which made things tough to localize.
Thanks for the help all!
Jamus