LA3A weirdness suggestions?

Fixed a pair of LA3A's for a client a few weeks ago. They heard them and they sounded great. Since then They brought it back to me saying the unit is now noisy. This is true. I hear a hiss on both units that was not there before. If I play with the gain knob the hiss will increase as I bring the gain knob up. I am not 100% familiar with the la3a's and I know a few of you guys are. So might as well ask what are potential reasons this could be happening? This sounds exactly like when you have an amp cranked too loud and you get that SSSSSSHHHHHHHHHH. in the background.

what I really am asking is this, these are an original pair of la3a's not reissues. They were recapped 3 years ago. What would be an exceptable amount of noise/hiss when no audio is present.

Hi, Pucho

What was the original fix you did to the units?

It sounds like your problem is before the the gain pot (or maybe the pot itself).

Probably T4b related.

well the plot thicken's I just ran audio threw it. When I adjust the  peak reduction and have the meter set to GR the unit stays at 0VU then if I run the same audio with the meter in output level adjusting peak reduction does nothing. and the level stays the same.

Originally there were vu bulbs out and the unit was missing the 3 prong IEC plug. The ground had been cut off or pulled off so I replaced that plug. other then that it was thought the units were working well and they sounded great. Now the still sound great but no compression at all. I would highly doubt that the t4's in both units failed.

edit it is compression but the test in the manual indicates there is major no compression.

test is as follows
gain switch at 30db
limit switch set to limit
vu switch set to output
HF contour at full CW position which would be flat
stero link pot full ccw which would be mono setting



+2db on the input which I would guess the mean signwave
peak reduction set to full CCW

Set level on gain pot till VU hits 0

VU switch back to gain reduction

peak reduction pot full clockwise should give -6 to -10 db reduction.
but it full clockwise the level goes down by 1 DB on the VU meter

well, have you opened it to check if the ELP lights up?

Kit said:

well, have you opened it to check if the ELP lights up?

Click to expand...

yes. Swapped it with 3 different t4B/t4A's and all results were the same. the 1 DB drop...

time to hint around the PSU trasistor as well as the others.

PSU trasistor checks out. time to hunt elsewhere

Ok, Im a little confused.

Did you just swapp the T4´s? Or did you actually pop the lid on one, plugg it back in circuit and watch it light up?
Big difference.

Because if the ELP did light up in a normal fashion, like you say, then your problem lies in the shunt LDR or its connections.

Have you checked/cleaned the connections to the T4b?

Also, do you have acces to a scope? Makes this sort of thing alot easier.

faulty transistors seem to be the problem.  Base to emmiter yields low resistance which is good. Base to collector yields high resistance in the kohm range which is bad. Diode checker yields base to collecotr with a proper voltage drop base to emmiter yileds open short i.e. no voltage drop.