UREI LA3A trouble
« on: January 04, 2012, 11:35:48 PM »
Hi: Wondering if there are different schematics for the UREI LA3A mine does not have a bias pot. Bias voltage is 24 volts (should be 13). New T4B but no compression. Need some help! When I turn up comp pot, it pegs meter to the right and I get hum. I put the old t4B in and took off cover and turned out lights and it seems to react to light. I can hear a level change. When looking for ac voltages on T4B I don't find ac voltage like schematic says. thanks!


mjrippe

Re: UREI LA3A trouble
« Reply #1 on: January 07, 2012, 11:00:39 PM »
Having a very similar problem with two LA-3As my buddy is working on!  Bias at TP-1 is very high (19v on one and 22v on the other)  Ours are the versions without the trim pot for R7.  Instead R7 and R44(?) are 27k.  Power supply voltages are reasonable and the units have been recapped.  One will pass signal and reduce gain but has terrible headroom and distorts.  The other will not pass signal unless we reduce the mains voltage with a variac to 50-60v!  This brings the bias voltage to 8-10v but above that it cuts out again.  So strange!  We have also replaced all of the small signal transistors on that unit to no avail.  Checked resistors, diodes, etc.  Really baffling!

mjrippe

Re: UREI LA3A trouble
« Reply #2 on: January 08, 2012, 03:10:00 PM »
AHA!  C3 is mis-labeled on the PCB.  + sign is on the wrong side.  Reversed cap and it works!

Re: UREI LA3A trouble
« Reply #3 on: January 09, 2012, 09:07:26 PM »
Wow! I wouldn't have found that in a year without some sort of schematic! Thanks! Mine is working fine now. The new Drip T4B does not seem to work in this unit but the original T4B is working since the recap and reversing C3. Do you have a print for this model? Thanks so much for the help!!! ;D


 

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