Help with FET voltages on Lab Series L3

[glebert]

Working on this amp, power amp works, tracing signal shows it doesn't get past the first FET Q1. There is no DC on the Source or Drain, so I am guessing the 356 op amp is dead (I have found other threads with this problem). What should the voltages be when it is working? In the service manual is says the voltage at the collector(?) should match the -7.5V reference, but I don't know if they are talking about the op amp or it is a typo and talking about the FET. I am not used to dealing with negative rail only op amps, but at a quiescent point it seems like A1B pin 6 should also be -7.5V, so pin 7 should be something between -7.5V and the -19V rail to be able to pull current through the FET?

Next question: I am measuring -10V at the gate of Q1. I thought it should be almost equal to the -15V being fed through R6. I pulled Q1 and the voltage at that point is still reading -10V. I realize that 4.7M is a big resistor, so small current will drop that voltage a lot, including the load of my DVM. What should the voltage there be?

Thanks in advance,
Greg

forgot to add service manual:
http://music-electronics-forum.com/a...ice-manual.pdf

[J M Fahey]

Itīs confusing because itīs "upside down".
Itīs fed from -19V and its "+V" is Gnd/0V but otherwise, voltages must keep standard relationship between each other.
Meaning Source a couple V more positive than Gate (so gate is Negative biased) and Drain more positive than them all, so current flows.

What you "should" find:



PS: assuming 10M Multimeter impedance, and almost 5M series resistance; "-10V" at Q1 gate means itīs getting proper -15V when unloaded by multimeter.
Now 0V at source or drain means either a fully open Fet, not usual but not impossible either, or LM358 is not feeding it.

Or you might have broken/open tracks/solder between Op Amp output and Fet Source.
First measure DCV at LM358 pin 7 and follow through R14/13.

Or C4 might be shorted.

[glebert]

Thanks Juan, that is really helpful. Good tip to check for shorted C4, I hadn't thought of that.

[J M Fahey]

Me neither, it clicked when I was closing notebook cover so I had to add it on EDIT.
Oh well.

[glebert]

It ended up being the A1 op amp. Replaced it with a similar modern one and it works great. Pretty good sounding amp. Reminds me of a Yamaha G50 that I have (which is a really underrated SS amp). Once I knew I was getting sound through the amp I totally forgot to go and measure the bias voltages

[Tassieviking]

The link to the service manual no longer works, any chance of uploading it again ?
Thanks

[loudthud]

I found this version of the schematic.

[dmeek]

Here is the service manual

Lab L3 Service Manual.pdf