Cobra 138XLR

[poindexter1]

Having issues with Cobra 138XLR on power up. It has a squeal in receive mode only for a few seconds and then goes away. Other than this issue the radio is fine (both on the bench and live radio checks). Did all the typical cleaning of relay, pots, switches. Replaced caps around IC4 - C89, C92, C94, C95, C107. Still has the squeal on cold power up. Anybody run across this issue before?

Poindexter

[MDYoungblood]

On CB Tricks there is a list of bulletins on that radio.

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3's

Greg

[poindexter1]

Thanks Greg. The tech bulletins did not cover a squeal in receive issue. Also noted that the volume control does not increase/decrease the squeal. So it is after the volume control. Any ideas?

[poindexter1]

PS - got the schematic diagram from your webpage reference. The schematic diagram helps but squeal/feedback issues in receive mode only can be very difficult to troubleshoot. Even wires that are close together can cause this problem.
Poindexter/Art

[MDYoungblood]

Does it squeal with the mic disconnected? Check the capacitor going to ground near the antenna jack, also I think the problem might be the #1209 bulletin, worth checking.

3's

Greg

[poindexter1]

Still does it with or without mic plugged in. Bulletin 1209 a and b deals with the AM Transmit mode only. This is a receive mode only problem. I think we can safely discount that issue. The capacitors connecting to the ground lug of the antenna jack are for RF decoupling of the PA/EXT speakers. Will test those capacitors next. Thanks.

poindexter

[poindexter1]

Looks like those caps going to the ground lug of the antenna jack (C405, C406, C406) are good with an ohm meter. Sure acts like a leaking electrolytic cap in the audio amplifier section (TR21, TR22, IC4) - some where after the volume control. Checked diagram again and found a electrolytic cap going from the TR relay to the Speaker/PA Speaker/Ext Speaker jack (C103) which I did not notice before. Replaced and no change. Your ideas have opened my eyes to look at other possibilities. Thanks Greg.

[poindexter1]

Well I finally found the bad electrolytic cap causing the receive squeal - C104 (330 uF/10V) which is in the 9.2V regulator circuit. Problem solved. Wow that had me chasing my tail. Thanks Young Blood for your two cents.
Poindexter