How to properly tune L4 on Uniden 858

[nomadradio]

C42, C43 and C44 are more-likely culprits in a AGC circuit that's too fast.

And if it only does this in sideband modes, C43 looks more likely. It is switched into the AGC timing only in sideband.

That radio is old enough, that any quirky fault is probably caused by a failing electrolytic cap.

73

 

[Osscott]

Thank you Nomad. I already replaced all the caps in the radio with non-Chinese ones from reputable manufacturers and verified their values before installing. I will take another look at these specific ones. Perhaps try some higher values.

 

[Osscott]

Checked all three caps. within spec. so i played with increasing the capacitance of C44(AM+SSB) and C43(SSB Only but added to C44). All that did was appear to increase the recovery time. Makes sense. So i put those back to stock. AGC recovery time was fine with 4.7uF and 10uF, respectively.

C42 was increased from 10 to 22uF. No obvious change in AGC i could tell. That cap seems to feed the squelch and then AF amplifier. Not sure how that plays into the popping sound, though. Could you elaborate on what this cap actually does? Maybe that could give me some insight.

I then watched a video on how an AGC works from YouTube and learned about AGC threshold setting. So I added a 500KOhm VR between the cathode of D16 and ground.

This now adjusts the level where the AGC kicks in and how much it attenuates. I think by adding some resistance here (i think about 70K Ohms or so) I got the AGC to open up just a bit. My theory is that smoothed out the abrupt off/on switching that the shottky (or even the stock 1N60P) diodes present to the circuit. Maybe that's the cause of the popping sound?

It's better, but still not perfect. More signal gets through the 3rd IF amp now with heavy signal input (less AGC attenuation and more amplification of the low level signals with this adjustment).

So thanks for your help. You got me in the zip code. still gonna poke around. If you or anyone else have some other thoughts, I'd be grateful.

 

[Osscott]

Would be great if UNIT_399 or another SSB 858 guru could comment.

Another update...

I am now getting static and other noise coming from the speaker. same noise on internal and external speaker. Happens with volume up or completely off. volume knob does not influence this noise nor does the squelch. Also happens on PA function when external speaker is connected to PA output.

A couple of times the audio output cut out completely when the static/noises were real bad.

So now I suspect that uPC1156, the AF amplifier, may be on it's way out.

I found one on eBay. Hopefully it works not only to fix this latest issue, but maybe it also ties to the strong stations popping the speaker on initial reception of signal (before the AGC can reduce the signal). Maybe the uPC1156 amp is compromised to the point of generating the pop sound (distortion) when it receives a strong AF signal from the 2SC945 AF amp (TR21) that feeds it.

That would be great if this cures it all! So much work put into this radio to date.

 

[Osscott]

Would be great if UNIT_399 or another SSB 858 guru could comment.

Another update...

I am now getting static and other noise coming from the speaker. same noise on internal and external speaker. Happens with volume up or completely off. volume knob does not influence this noise nor does the squelch. Also happens on PA function when external speaker is connected to PA output.

A couple of times the audio output cut out completely when the static/noises were real bad.

So now I suspect that uPC1156, the AF amplifier, may be on it's way out.

I found one on eBay. Hopefully it works not only to fix this latest issue, but maybe it also ties to the strong stations popping the speaker on initial reception of signal (before the AGC can reduce the signal). Maybe the uPC1156 amp is compromised to the point of generating the pop sound (distortion) when it receives a strong AF signal from the 2SC945 AF amp (TR21) that feeds it.

That would be great if this cures it all! So much work put into this radio to date.

Well the uPD1156 was not the issue. that's the bad news. The good news is C88 a tantalum capacitor was the culprit. tested ok but clearly it had issues. the bacon sizzling noise (even when volume off) vanished when i replaced it with same value electrolytic! that problem solved.

i realigned (de-tuned from max signal) L8 and L3 ever so slightly clockwise. This appeared to reduce the AGC popping of strong signals without sacrificing receiver sensitivity on my favorite frequency. But this clearly is not "the fix."

i also realigned L1 and L2 (the can that has the NO ADMIT sticker on it) using a signal generator set to 23.5Mhz at the antenna input and used a spectrum analyzer scanning the same. was able to raise the signal strength going through the NB circuit from -60dBm to -30dbm at TP5! Seems to have eliminated the AGC popping!!! I do recall the service manual explained the noise blanker and AGC are closely coupled. Wonder if the NB circuit being out of alignment was impacting AGC performance?

So much for L2 NO ADMIT. Maybe it should read NO ADMIT UNLESS THE AGC IS POPPING/DISTORTING THE AF AMP WHEN STRONG SIGNALS ARE INITIALLY RECEIVED AND THE RADIO IS 50 YEARS OLD. lol.

I also replaced R43 with a 1M resistor (was 680k) per another thread for good measure.

It's been a long journey with this TRC-457. Fun project.

I hope this benefits someone in the future.

Scott