CRT power supply repair - Page 1
Forum section: electronics / repair

author: JKKDev
author_rank: Regular Contributor
timestamp: August 31, 2019, 09:13:31 am
content: Hey guys, I'm working on a Gould 4074 with a blown high voltage PSU (CRT supply) - my fault- it sparked half an inch gap between C29 and C39 (down by the Q15 in the schematic). I've got a few questions regarding some of the components (I'll attach a schematic and a picture of the circuit board for reference).1) D52 Zener diode is a BZX55C100V. I would expect that I'd measure a normal diode drop one way and an open the other way as the multimeter doesn't have the 100V to conduct in the breakdown region. What I've got, however, is a 0.21V drop one way and 0.19V drop the other way which is why I'm assuming the diode is toast. Does it have to be this exact zener or is it ok to get another 100V 0.5W zener?2) R66 2W resistor with a color code of red, violet, blue, yellow. This appears to be non-existent except if you use gold instead of yellow it's supposedly a 27Mhigh-voltage resistor which does make sense. Are these special resistors or is a normal E24 27M2W resistor ok?3) C31 This was a first for me. It's the same shape as a 1W resistor (pink colored) with a color code of brown, black, orange, black, gray (I don't know which way around is the right way). What type of capacitor is this?4) R63 is either a 110or 1kand I can't decide. Opinions? (Check attached image)5) The lower part of the PCB is populated with diodes (grey components). These I can't measure at all (open both directions). What kind of diodes are these? Should I be able to measure them in any way?Thank you for your time and answers (I'm not used to repairing old stuff so I got confused by a few of these) and have a nice day.

author: PKTKS
author_rank: Super Contributor
timestamp: August 31, 2019, 04:41:06 pm
content: The diode is gone.The resistors with 5/6 bands are from the 90s pre smd generationand several parts including CAPS and INDUCTORS also camein those format.. take care and don't assume "resistor" .inductors are green mostly. (mostly) and caps yellow (mostly)The bands you will have to figure out.. quite common and timeconsuming problem since the 90/00 and smd just made it far wosrtIn 4 band code i would take yellow as gold as wellPaul

author: JKKDev
author_rank: Regular Contributor
timestamp: August 31, 2019, 07:21:11 pm
content: The C31 appears to be a 3 pF +-0.25 pF across the CRT heater supply. All I can find that's readily available are SMD 0603 caps with this spec rated for 50V which should be enough for the heater afaik.Is it reasonable to use SMD caps as a replacement for axial ones?I'm also having trouble finding the 27Mohm 2W resistor. I should be able to just combine 4 0.5W resistors which add up to the value (measured ofc) correct?Thanks for the info PKTKSPS: If anyone else is looking for a color code guide for vintage electronics this is something I've found (attachment).

author: pbarton
author_rank: Frequent Contributor
timestamp: August 31, 2019, 07:54:25 pm
content: Quote from: JKKDev on August 31, 2019, 09:13:31 amThose are high-voltage silicon rectifier diodes and they are laughing at the puny 1.5V battery in your multimeter.You have to assault them with something significantly higher than 1.5V.I use an insulation tester, set to the lowest voltage output range (in my case 250V) then they act like diodes.