Retrocomputing: comx35 - Page 1
Forum section: electronics / repair

author: saronno
author_rank: Regular Contributor
timestamp: August 19, 2016, 01:12:51 pm
content: I have one of this little puppyhttps://en.wikipedia.org/wiki/Comx-35... my problem ismodulator don't work.As you can see in the photo below there is clearly a missing component,something connected to the "big pin" and the "little pin".There is a symbol in the PCB ... it's not clear but is like a capacitor and is very near to a circle.Any guess?

author: Ian.M
author_rank: Super Contributor
timestamp: August 19, 2016, 02:10:25 pm
content: Odds are there's nothing missing and its a crude variable capacitor, with a value of a couple of pF, adjusted by bending the small pin towards or away from the big pin. If there *was* something missing, you'd expect to find debris rattling around inside the screening can + traces of the component body on the pins.* Have you checked the modulator video input and 5V supply?* Have you got a TV win an analog tuner that covers UHF channel 36?* Have you checked its output on a UHF spectrum analyser?If there is no UHF output, odds are its a dry joint on the undeneath of the modulator PCB. The modulator would need to be desoldered from the main board to gain access. Worst case, if the video in and supply are OK, and the modulator is FUBARed (which I doubt) you can probably substitute an Aztec UM1233, which are much easier to find as N.O.S.

author: saronno
author_rank: Regular Contributor
timestamp: August 20, 2016, 12:38:26 pm
content: Quote from: Ian.M on August 19, 2016, 02:10:25 pmNever seen something like that. Is it very common is this kin of modulator?Quote* No, but I will.* Yes.* Unfortunately I have not a spectrum analyzer.As soon as I have any news I will update this thread.Thanks anyway.If there is no UHF output, odds are its a dry joint on the undeneath of the modulator PCB. The modulator would need to be desoldered from the main board to gain access. Worst case, if the video in and supply are OK, and the modulator is FUBARed (which I doubt) you can probably substitute an Aztec UM1233, which are much easier to find as N.O.S.

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2016, 01:47:18 pm
content: Quote from: saronno on August 20, 2016, 12:38:26 pmBefore the introduction of SMD technology, in the days of point-to-point wiring and then through-hole PCBs, physical layout of the circuit was absolutely critical and VHF/UHF tuners and modulators were usually aligned by squeezing or spreading air core coils to tweak their inductance and by bending wires or sheet metal vanes to tweak the capacitance. Only larger values of variable capacitor or inductor would have an actual screw adjustment.

author: saronno
author_rank: Regular Contributor
timestamp: August 20, 2016, 02:00:59 pm
content: Quote from: Ian.M on August 20, 2016, 01:47:18 pmVery interesting.ANyway, I have a very good news: it was just a cold joint !Now everthing if fine: this little puppy, as usually David says ...it's a winner.And from what I see on ebay, they are pricey too.I see auction here in Italy with a closing price of 300 euros.http://www.ebay.it/itm/RETROCOMPUTER-COMX-35-1983-VINTAGE-HOME-PC-THE-CLEVER-COMPUTER-COSMAC-RCA-1802-/112070517136?hash=item1a17ec6190:g:ef0AAOSwSv1XljmH