Ray,

Do you get distortion regardless of modulation level? You may want to pull Q2, or at least lift the drain lead so that it cannot influence the signal. If the distortion is still there, then it is not due to the limiter/AGC/compressor, etc.

I have seen the CA/MC 1458 dual op amps in a lot of equipment and do not think they are necessarily troublesome. It is a standard dual op-amp. Nothing weird.

Shotgunning those four caps may not be a bad idea. I wonder if C41 is degraded. That would mess with the time constant of that AGC loop and could be causing the issue. Don't rule out C38, C23 and C24. 

I recall looking for the jumper that you mention. The schematic does not show one? I think I had the same problem when I was chasing a possible low transmit audio issue.

FWIW, I like my MSR8000A. Most of the time. 

n3kcb




------ Original Message ------
Received: Mon, 10 Aug 2026 01:11:17 PM EDT
From: Ray Fantini via MRCA <[email protected]>
To: "[email protected]" <[email protected]>, Rhett Isley via MRCA <[email protected]>
Subject: [MRCA] Mackay MSR-8000 Blues


I have been playing around with a Mackay MSR-8000 with audio issues. I am no big fan of the MSR being it's a good radio when working but when it develops issues it can be a Bear. That radio appears to be built around lower quality components stuffed onto cards that are difficult at best to work on. All that aside the issue with this radio is and have seen this on a couple others that the transmit audio is distorted. 
The microphone inputs, there are two one low level dynamic microphone input amplified by one half of U1 on the 1A1A3 card and a second high level input for carbon elements and line level that are routed directly to the second half of U1 that functions as a compressor - Limiter that in turn feeds the balanced modulator U2.
The second half of U1(B) works with Q6 to control Q2 that appears to be a FET that controls the input level on U1(B) by shunting the audio to ground.
At least that's what I think is happening; Q1 appears to be switched on to turn Q2 full on to mute transmit audio in Tune mode. 
Have had cases where this stage produces real distorted audio, no audio, or lots of random noise.  In short, this stage sucks on more than one radio.  The output of U1(B) passes thru R39 the modulation control to TP1 allowing you to see the audio that's delivered to the balanced modulator. 
In the CW mode, an internal 1 kHz oscillator is fed in at the same point to generate output in the CW mode. In both cases the transmitters produce clean audio, and clean outputs form the modulator and the radio itself so the balanced modulator, ALC and all that other stuff is good just the USB/LSB audio that's affected.

My questions are:
First, am I correct in my assumption on how their compressor limiter works? CR5 and 6 are developing a voltage that's amplified by Q6 to control the gate on Q2 and Q2 is acting as a shunt on the input to U1(B) to control the gain?
Second, is there any history of the CA 1458 (U1) devices going whacky or developing issues over time? Don't think I have ever seen one in anything else other than this radio. What about the 2N4338 Q2 ?
Think I may try a shotgun replacement of C39, 40, 41 and 42. I know that's not real troubleshooting but if it solves the issue can live with being a lessor technician.
Third , anyone else ran into this same situation on the Mackay? Maybe the radio is better than I give it credit for being, they do have a quite receivers and if I were just a bit smarter would be able to resolve this. Who knows.
 Anyway, attached is a schematic of the affected 1A1A3 card. 

Final trivia on the 8000, where the Hell is the audio jumper JP-1 (Carbon /Dynamic) on the schematic? The manual shows the jumper but cannot find it on the full schematic for the 1A1A3 card???
 

Ray F/KA3EKH


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