Another Saturn issue

[Hawkeye351]

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.

 

[brandon7861]

Everything after TR8 is probably good then. I'd put my money on FL2.

 

[brandon7861]

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.

 

[Hawkeye351]

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......

 

[brandon7861]

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.

 

[brandon7861]

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.

 

[Hawkeye351]

Haven't checked FL2 yet, but I did notice if I push it back a little and then pull it back forward a little I can hear a change in receive on AM, makes no change in SSB though. Is that an indication of a bad FL2?

 

[Hawkeye351]

Aha, I think I found the problem. A lead on L7 had a solder blob touching to its IFT ground tab. Will give an update later when it's more active around local. Seems like SSB came up also.

 

[brandon7861]

dang solder blobs, how they get where they get lol. Hopefully that fixed it

 

[Hawkeye351]

Got a question though...

I just noticed in these Saturn type radios that C151 and C220 are 4.7uf 50 volt caps. I've been using 4.7uf 25/35 volt caps instead. Would this drop in voltage rating affect performance of any type?

 

[brandon7861]

The schematic I am looking at shows C151 as a 25v part. I can't find C220 on this schematic. Maybe we have different schematics.

 

[brandon7861]

OK i found it. I had to look in the G.Saturne manual to find it. In that, it shows 50v, in the schematic I have, that cap is listed as 25v like the other one by the audio IC.
Your C220 appears to be on the PLL lock detect line, and if that ever saw 50v, I think it would be toast.

The cap on pin 2 of the audio IC is for ripple reject. And if the ripple is 2x the supply voltage, you got a problem.

I think you are just fine using 25v caps, and I have a schematic that shows them used. Maybe someone sees something I missed though.

The only downside that I know of using a cap rated at a higher voltage is that it will eventually become the lower voltage cap if it only ever sees lower voltage. If a 25v cap spends 25 years at 8v, you should apply anything higher by slowly bringing it up to that to reform the dielectric at that voltage.

 

[Hawkeye351]

Ok, so it's all good.

I'm still gonna do some voltage checks on the AGC circuit. Reason being, I'm having to crank VR1 above 3/4 way up for signal reading to correspond to sound (what I think the signal should be) but VR2 seems right (actually a little too high).

Reason for concern is, on these Saturn type rigs VR1 hardly ever goes past the halfway point, maybe slightly over but never this far past halfway point. VR2 is about where they always land at though.

Still curious as to why I can tap/scratch/peck (not very hard at all) at FL2 and it makes a static scratch noise in AM receive.

SSB receive is so strong that it hits AM hard when sitting on AM while someone is on SSB, but AM is still slightly quiet, could be conditions also. I'll check it out again tomorrow but it is better than it was.

Anyway, it's close enough to figure out the small stuff.

Thank you Brandon, preciate ya brau...

 

[Hawkeye351]

Just compared AM signals with my Saturn (had it hooked up and turned on for past hour) on top of this Saturn by quickly switching the coax.

All signals on this Saturn are around 3db lower than on mine. Mine rests around 3.5 s-units on hash noise (with my AC whine in it) while this one rests on 0.

My locals show around 3db more on mine than this one.

I also tapped/scratched/pecked (not hard) on the FL2 in my Saturn and it doesn't affect receive at all, whereas this Saturn does affect the receive when tapping/scratching/pecking on the FL2. I think you may be right on a previous post above about FL2 being bad. Gonna swap it out in the morning with another one and let you know.

 

[Hawkeye351]

Yep, you were 100% correct Brandon. I yanked out FL2 and replaced with another one.

BAM, AM receive immediately jumped up to exactly what my Saturn shows on AM, actually a little too high, lol... Realigned receiver section and now AM receive matches my Saturn.

I did tap/scratch on this other one after putting it in, and it made no noise in receive or affected receive like the one I yanked out did. So I guess that's a good way at checking those crystal filters in circuit, just tap or scratch lightly on it and listen for a change in receive audio, if no change then it's good, if there is a change in receive sound/level then it's bad.

Thank you Brandon, you guys are the best brau...