President Lincoln TX Issue

[metrologist]

I blame @joeqsmith for this.

I dug out my old "Ham" transceiver and discovered it was having a TX issue. It started out by turning off the TX after a second or so. I verified I was not having a contact issue in the PTT button. Then it would stay in TX longer, but show high SWR on the indicator, even into a dummy load. The MOD/RF indicator is intermittent on sideband. A sloppy OTA spectrum view shows the carrier drops and presents multiple harmonics/spurs when modulated.

The notorious issue is with caps C75 and C60, 220uF electrolytics. These are connected to pin 7 of IC104, a dual package op-amp, which is just left of center.

http://w.datasheets.pl/telekomunikacja/cb_radio/cb/PRESIDENT-LINCOLN-5.PDF

I measured C75 (I think the caps are connected in parallel anyway) and see 390uF, ESR 0.2, and 165R. I think the low resistance means they're bad but cannot tell what else is connected to that circuit. The few resistors I suspect are in the kohm range, but they didn't bother to indicate nodes on the schematic. I'd also expect the capacitance to be farther off.

I could just shotgun them and see, but that requires some more disassembly and would like to have greater confidence before heating the old board. Also, I have not found this failure mode associated with those caps, but I suppose anything is possible because it seems these shorting caps would load the entire 8V TX supply circuit.

What do you folks think?

[TheDefpom]

These radios will need a full re-cap by now if they have not been done yet, they get so many subtle issues from the caps not performing as they should, such as VCO unlocking at certain ends of the band, audio issues, SSB detector issues etc.

[Wallace Gasiewicz]

I have not seen that sort of failure mode.   I have not seen many of the big caps go bad either. 
I would just check to see if the transmit voltage  is present when the PTt.  Otherwise I would start with a full tune up.     
A common problem with these radios is a bad board connection, a bad solder joint.

[floobydust]

I recommend a Bird dummy load lol.

It's best to take some measurements instead of assuming it's a shorted cap. Check the switched TX 8VDC rail is holding up, coming out of switch Q125.
Or the 8V reg IC is working IC107.
Sometimes a miswired/defective mic can damage the PTT circuit. But I can't quite follow the schematic few dots and fuzzy.

But at this age, I'll just shotgun and outright replace all electrolytics in a radidio.

[metrologist]

Thinking of that

https://www.ebay.com/itm/275790538180

[floobydust]

It looks convienient, as it does take time to make up a cap list unless you have stock. Just take pics and use a red felt pen.
For 1uF and less I'll use film caps instead of electrolytics.

This store sold out but offers a link to pics which might make it easier. I don't like the smaller electrolytics included, they don't last I find.
https://www.ebay.com/itm/144711101565

I've been doing recapping when on a useless phone call, you know where a friend is venting about something or telling a story....
Then they hear the vac pump "what's that noise?" lol

[pienari]

Hi.
Have you removed ugly glue every place on board?.
I have found that traces disapeared under glue.
No tx that case.
Gray caps have to go sooner or later.

[CaptDon]

Does anyone consider the President Lincoln a 'ham' transceiver? Is it kind of a CB rig with 10 meters included? Being named after President Lincoln perhaps it has 'Freed the electrons'

[floobydust]

I dumped a few old CB radios at the eco center, they were cheap designs. Single-conversion, discrete transistors etc. Fanon, Midland and the last one I forgot the model but it was featured in magazine ads, endorsed by a porn star. So much for my schematic search lol.
Back in the day of the boogie van and truckers, Patriots, FCC action and sexy advertising.
Cobra has dual-conversion IF and I have one still, in case of zombie apocalypse.
The Galaxy 10m units are popular.