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IC16 is an 8-pin DIP, 458.458 dual op-amp. It is used for amplifying the mic audio, and for handling transmit/receive switching. I've circled its location in yellow in the uploaded image. Incidentally, this is the same chassis as the original RCI 2950/2970 radios and their base-station derivatives.

I am about 99.999999% sure that the IRF520 was never used in that model from the factory. This is most likely after-market, either a botched repair attempt or power upgrade, but if your friend had it from new and never had any mods done, it does raise an eyebrow a bit. As for your current issue...

I agree with previous posters in that you need to identify the type of noise. Is it just general static or is there some sort of "note" to it, as in a discernible tone under neath it? If it sounds a bit like a 50/60 or 100/120 cycle hum in the static, then some of your issue is likely from the...

Amen!

I appreciate the vote of confidence, but hopefully we can figure it out without you having to make two 3-hour drives!

I'd go a step further. Check for continuity between each MOSFET's center leg and ground after lifting that leg from the board. If you see continuity, it is shorted to ground. Check final and driver MOSFETs, but final is more likely from my experience.

No problem!

This is a very rough direct-entry frequency generator I threw together several years ago using an old telephone keypad, a cheapo 4-line LCD display, an Arduino Uno, and an SI5351 clock generator chip. There is no filtering as it was only intended to generate quick test signals to check for basic...

I have a Moonraker 4 beam antenna that was never installed for sale for $600. It includes all parts. The original box was deteriorated and had fallen apart when I obtained the antenna in an estate buy-out, but all parts were present. I've since put all of the parts except for the two longest...

The 636 is not an electret or condenser mic as the OP was asking about. Niether is the Stryker noise canceling mic. Between the two, I'd pick the 636, but I don't like either one. I've attached a quick audio file doing a direct audio comparison of the Ranger SRA-198, the Astatic 636, and the...

Stating that your "audio is off" is a bit ambiguous. It could mean that its low, and/or distorted. If it was low, then increasing the mic gain or using a stronger mic would be the obvious solution. If distorted, it may be an improperly-adjusted (on non-functional) AMC and/or ALC circuit, which...

Did you manage to fix this unit? There was one switching diode in the rx/tx switching circuit that seemed to fail at a fairly high rate in radios using that circuit. Unsure of its part # in your radio, but in the older Galaxy DX series (33HML, 44V, etc) it was D77, a KB362 part number. In a...

One possible explanation as to why some radios using 520's seem higher or lower in output to others of the same make/model is that not all IRF520's are created equal. You can find slightly different transconductance and on resistance specs between different 520 series parts.

Did you ever find the problem?

I have the same radio and that was the procedure.

That's what I was wondering myself, lol! If that was the case, shorted final/driver would jump to mind.

I finally got a chance to dig up my old SR490 and snapped a photo of the cap location. The fix was to add 100pf across those points.

Something else occurred to me. Years ago in the Stryker 440/490 series first gen models, there was an occasional issue with PLL unlock that displayed similar symptoms. the cure was to add a 100pfcap in parallel to another cap near the PLL chip. I'm not at the shop atm to get any better info, but...

I think you are on the right track with looking at the audio amp section. I've run into other makes/models with similar issues that traced to audio amp issues. As was mentioned, look for cap faults, especially bypass and feedback caps.

Nothing specific jumps to mind, but have you tested all PLL circuit voltages when the problem is happening? There would likely be other signal levels/voltages that are changing when this problem occurs.