View Full Version : Lisa Analog Card issues

October 16th, 2012, 10:50 AM
I'm futzing with my Lisa's analog card. Originally when I had it all you got on the screen was rasters. I could make out a proper image from the rear composite out and I can could confirm that there was a good TTL video signal going to the card.
After recapping the Datapower PSU (C1, C3-C7 had all either failed or were about to and were replaced with newer replacements) I then tackled the card again and first replaced the TDA1170N.
I put everything back together and now I don't even get a raster. :(
The neck of the tube is glowing but I don't know how to confirm if I'm getting high voltage from the flyback anymore. I poked at the card socket (card not installed) and reading the schematics I can confirm I do have +12v however when I checked for +33v I was reading +70v!
the +33V pin on the card feeds straight to U1 which is an MC7824CT positive fixed voltage regulator. I checked it while soldered to the board. The diode check mode says 1-2 is infinite (and 2-1 is reading) and 2-3 reads both ways.
Bit confused here and no, I did not check to see if I was still getting rasters before the TDA replacement. I was troubleshooting the card last year when the power supply blew those caps and I put it on a shelf until now.

Edited: My Datapower supply is Rev. G but the two pages to the Rev. J schematic are here (http://lisa.sunder.net/LisaDataPower-1-B.pdf) and here (http://lisa.sunder.net/LisaDataPower-2-B.pdf). According to it, the +33v doesn't really have all that much between the pin edge and the transformer.

October 16th, 2012, 12:50 PM
Are you getting 28V at the collector of the horizontal output transistor ?
If you have a scope, you could check the base and see if there's a sawtoothy waveform present.
Depending on how a voltage your DMM can handle, you might try measuring the voltages at the RGB grids directly.

October 16th, 2012, 01:01 PM
I have both a scope and a decent DMM but is it safe for me to continue plugging in the analog board if the +33 is so far out of spec?
Edited: Meh. I checked anyways. It's reading +3v so yeah, the buck stops at the 7824. Testing found that (in the power supply) R23, C22, CR21, CR22 and C25 are okay.

October 19th, 2012, 12:06 PM
I replaced the 50v 1000uf capacitor. The voltage dropped to 68v. Still madly puzzled on what is going on.