LA2A not compressing issue.

[Ampzone]

Our classic reissue (early 2000's low serial #) has stopped compressing. It passes signal though fine. So we just replaced with a new Kenetek T4 cell, still no compression function. The unit is mostly stock save a long while back we located an original set of trannies the rarer black UTC HX100 and A-24 and swapped out the OEM UA audio trannies. As a last ditch we just replaced all the tubes with spares but still no change. It passes signal no but compression reduction. Any ideas or clues would help. Thanks much!​

 

[ruffrecords]

If it passes signal from input to output then the basic audio chain is OK. You now need to trace the signal through the sidechain.

Cheers

Ian

 

[Ampzone]

The voltages on 6Aq5 are way off. I swapped the tube out and its still way off with either tube (both are bad tubes possibly?). The v4 reference voltages I found are in bold:
pin 2 = (7 - 7.5V) Im getting 1.4v
pin 3/4 6.3v
pin 5 = (135v) -0.5v
pin 6 = (100v) 23v

The rest of the tubes V1-V3 seems to be in range. Any ideas?

 

[ruffrecords]

R3 possibly open circuit. Or R34 open circuit. Clearly the 6AQ5 is not being biased properly. Check all the components around the tube and make sure HT is available to R34, R35.

Cheers

Ian

 

[Ampzone]

Ian great leads on that! So, r34 was dead no voltage was dropping across it to V4. The schematic r34 called for 2/3w 10k but reading the bands on the dead resistor it looks like brown brown orange on this dead r34, so it was 11k?? Anyways I replaced with a 3w 10k and and its compressing again. That must have been it.

 

[ruffrecords]

If the resistor was really old then it would be unlikely to be 11K. Probably was been brown black orange but years of heat made the first two look the same.

Cheers

Ian

 

[Ampzone]

I bet your right Ian. Great point!