LA3A Issue

[wave]

Hello everyone!
I've got an issue with an LA3A that's not passing any signal. I'm trying to trace the signal through the unit and I'm seeing my 1K tone at the input transformer primary but not at the secondary. I'm using an o-scope and my input signal is a 1k sine at 1.2V.

I should be able to see a sine at the secondary provided the transformer is working, correct?

Thanks,
Dave

 

[wave]

Update:
Transformer is good. I pulled the secondary and scoped it. Is there an easy way to check the audio path transistors in the LA3A?
I'm not having much luck with my searches as to what voltages I should see in places...

Thanks!
Dave

 

[Rob Flinn]

First I would check the PSU voltages, make sure that's working OK.

Places you could scope are gain pot wiper, C1, base Q3, C5.  If you look at the manual there are some test points with voltage readings, one in particular TP1 is worth checking.

 

[David Kulka]

You should see signal on the input transformer secondary without disconnecting anything. If connecting the secondary kills signal, that's a red flag. Presumably this would happen with power off, which points to a faulty passive component or a short somewhere.

If all is ok around the transformer check Q1-Q6 carefully as they are all common failures. And slight amounts of leakage in the few film caps in the audio amp circuit can cause all sorts of havoc with DC voltages, which can cause the circuit to fail, so check or replace those caps. Be sure polarity is correct on all the electrolytics. In some units, the silkscreened + symbol is backwards on one cap, so best to verify correct polarity with a volt meter across the cap.

Does the unit indicate compression when you input signal, meter GR, and turn up Peak Reduction, even though it doesn't pass audio?