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Another Saturn issue

Hawkeye351

Sr. Member
Jun 27, 2021
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Got another Saturn on the desk. The issue is low receive (it's got receive but just a little low).

Evaluation:
All the electrolytic caps were either high esr, shorted or dried up. Everything else looks great.

Replaced all electrolytic caps with new ones.

After:
Receive came up some but not quite there. If I place my finger on the top of TR8 then the static level jumps up from 0 to about 5 (which is my normal noise floor level here) but TR8 tests as good with right voltages.
Also, when changing channels, the channel display and receive corresponds immediately but the frequency counter kinda searches a second before locking on.

That's all I could find wrong after recapping.
 

Sideband goes through the other filter. If you have good receive in SSB, that supports the bad AM filter theory. If it is low in SSB too, then its upstream of both filters.
 
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SSB seems ok.

Both USB and LSB show about a constant 2 to 3 db of signal on noise, as though the receive is too high. AM on the other hand shows 0 signal on noise floor and quiet until someone keys up.

VR1 is set a tad above halfway (about where all these Saturns rest when adjusted).

VR2 is just a tad below halfway (again, about where Saturns normally rest when adjusted).

There are no sharp spikes when adjusting receiver IFT's, they all adjust smoothly and none of them bottom out or go flush with the top.

RF gain control does control the amount of receive, smooth transition from all the way down to wide open.

One other thing I noticed along with the low AM receive is, people sound slightly off frequency on AM, but people on SSB sound fine.

VCO set to 3.20 volts on 27.205mhz

Buffers maxed out on 27.205mhz

PLL oscillators for all modes dead on

Carrier oscillators for all modes dead on

Receive adjusted best I can get it on all modes

Power supply set to 14.20v (as I normally adjust Saturns to)

All electrolytic capacitors replaced with new caps of same value, even the 50v caps.

Hmmm......
 
One other thing I noticed along with the low AM receive is, people sound slightly off frequency on AM, but people on SSB sound fine.
I think this is the nail in the coffin. If sideband makes it through the IFT's sounding good, it is hard to believe the IFT's could degrade AM sound quality much. The sidebands reside in the same place as the AM modulation products, so IFT misalignment should hinder them similarly (at least in one of the sideband modes).
My bet is still on FL2.
 
According to chatgtp, those little ceramic filters almost never fail. Toss a 100pF cap across it and see if AM noise level goes back to normal. Maybe swap it out for the cap if that dont work so internal filter shorts don't hide the problem from the bypass cap.

May be worth seeing what the AGC is doing in AM if the filter checks out.
 

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