Collins 390 bf problem

[iw5ci]

My 390 developed this issue this morning.

From the diode output the BF after amplification is crisp and clear. But on the speaker since this morning is low and somehow distorted.
If i tune quicky a strong BC , for a moment the volume from the speaker is high but immediately go down (i suppose when agc compensate
the strong signal input).
If i use mgc, and turn rfgain to 5 or 6 position, the audio volume is stronger even if not normal.
I have the same audio problems from the locat output and the balanced line 600 ohm output "line".

I have tried to change v601 af amplifier , V602 line and local af amplifier, V510b positive peak limiter, without resolving the problem....

i suppose that if the diode load output works good, the problem is after V507A detector.

Any suggestion?

[WA3VJB]

You're on the right track testing those tubes.

I never spent much time listening on the Line/Local audio out because the Diode Load sounds SO much better.

But a couple things come to mind --

Is there any difference turning on and off the Noise Limiter ?

How about when you change the setting on the AUDIO selectivity (not the Bandwidth selector).  I remember having a dirty switch on the NAR/MED/WIDE audio switch, and it created a dead spot and distortion like you describe.

Also check whether there's any dirt or loose screws in Terminals 11 & 12, the jumper for the Line Audio.

The fact you still get good audio to the diode terminals rules a lot out.  I would also check whatever you are using between the receiver and the speaker. Is there an external audio amplifier or bridging amp of any kind ?

[iw5ci]

Hi, yes! I track the problem. If I turn limiter completely off i got distortion and volume reduction, if i turn limiter on and i advance it al 12 for example on strong BC signals i got compression and some distortion (but noise reduction).
If i rotate the limiter ccw slowly there is a position when i ear a "click" and limiter is switched off and the audio is perfect, but if i continue to rotate ccw i return in the muffled low volume situation.
Maybe the limiter switch has a false contact in the full ccw position. Maybe i have moved the limiter for a reason and not repositioned it correctly. I never use the limiter.
Is it normal than if i increase limiter level there is some distortion on strong signals?

[WA3VJB]

OK, sounds like a combination problem , mechanical and electrical, in that switch for the Noise Limiter.

Maybe you can find the 'sweet spot' and leave it alone !
At least until you get the time to fully troubleshoot and repair.

The limiter is garbage on voice communications anyway.  It is somewhat helpful on CW depending on how bad the static crashes are.  Yes, the increased distortion at a higher clockwise setting is how it does its job, regardless of signal strength. It clamps amplitude peaks.

I've got a Collins and a Motorola R390 and they're among my favorites for AM reception.

http://www.youtube.com/watch?v=9RPInNAStjo

[flintstone mop]

Sounds like an AGC problem to me. AGC is not operating and the I.F. is swamped and overloaded.
I hope it's not the AGC.....hard to trouble shoot, but I found my long-hard-to-find problem.

Fred