5154 PSU Repair - Page 1
Forum section: electronics / beginners

author: matpro76
author_rank: Contributor
timestamp: March 14, 2018, 06:33:25 am
content: I'm recapping the PSU from an old IBM EGA CRT from back in the 80's. I've come across the below which I can't quite figure out:2.2nF +/- 20% ceramics of some description but the voltage has me confused i.e. are these 400V~ or 4KV~? I've no idea what Up 4KV~ implies.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 14, 2018, 07:43:54 am
content: These are called X or Y caps today, X cap connected across the power input lines, Y cap connected from line potential to safety ground. Used for EMC.The rating is 400V AC nominal, but up to 4kV impulse rating (e.g. during line burst or surge events). The caps can stand a limited number of these events.They look like ceramic ones, check if the coating is OK (no cracks on the body, some smallish cracks at the leads would be OK) and don't replace them if they look OK, they don't degrade like electrolytics, and they aren't the well known exploding RIFAs.Otherwise you'd have to figure out if you need X rated or X rated caps by reversing the schematic (my guess would be Y), because they are critical for safety.

author: matpro76
author_rank: Contributor
timestamp: March 14, 2018, 09:23:36 am
content: Thanks for that, the schematic is below, these are C34 & C35 respectively. C34 is shown but not C35 for some reason, the schematic is from the US model (120V) and I'm working on the local 240V so there are probably minor differences. Both caps look fine however this all started when C6 went up in smoke (literally) which is a .1uF 250V~ X2 Film Cap, perhaps I'm being overly cautious in replacing them all but it is 30+ years old now. So anyway they look like they must be Y then as C34 is going to ground?

author: capt bullshot
author_rank: Super Contributor
timestamp: March 14, 2018, 09:38:18 am
content: Yes, these are Y caps.

author: matpro76
author_rank: Contributor
timestamp: March 14, 2018, 09:37:46 pm
content: Thanks, so I've removed all the electrolytics, the board as it stands is below. Is it worth replacing the all the film and ceramic caps as well or do these last forever? I'd rather spend a few bucks and do the job now rather than revisit this again if more caps go.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 14, 2018, 09:57:33 pm
content: Quote from: matpro76 on March 14, 2018, 09:37:46 pmNothing will last foreverThe board looks quite toast, IMO there may be failures due to bad solder joints, cooked semiconductors, ...Most ceramic and film caps will last quite a long time, longer than other components. Sometimes there are exceptions, e.g. if the particular cap is running at high current levels, usually film and ceramic caps in an older power supply board aren't running at high current. There might be some in the horizontal deflection circuitry.

author: matpro76
author_rank: Contributor
timestamp: March 14, 2018, 10:25:32 pm
content: Yeah there looks to have been some serious heat coming off those green resistors which has scorched the board over time, there are a few diodes around that area will check those out. The solder joints appear OK under magnification, the backside of the board is below.

author: matpro76
author_rank: Contributor
timestamp: March 14, 2018, 10:59:02 pm
content: Looks like D12 is toast, getting a voltage drop when testing in diode mode in both directions. Not surprising I guess sitting right in the middle of the heat damage.Its a BA159 according to the parts list which is readily available.http://au.element14.com/multicomp/ba159/diode-fast-1a-1000v/dp/9565280?st=ba159Maybe I should replace all diodes around the scorched areas as a precaution?

author: matpro76
author_rank: Contributor
timestamp: March 15, 2018, 03:29:16 am
content: Overlooked the smaller diodes, D10, D21 & D24 are also shot, all 1n4148. Do diodes usually have such a high failure rate?

author: capt bullshot
author_rank: Super Contributor
timestamp: March 15, 2018, 06:07:19 am
content: Quote from: matpro76 on March 15, 2018, 03:29:16 amNo I wouldn't expect a 1N4148 to fail here. Did unsolder at least one pin of the diodes before testing them?

author: matpro76
author_rank: Contributor
timestamp: March 15, 2018, 06:36:17 am
content: No I didn't, have done that now, all ok except D12 which is still dead. Does the diode test pick up voltage drops elsewhere when still in circuit?

author: capt bullshot
author_rank: Super Contributor
timestamp: March 15, 2018, 07:05:29 am
content: Yes, it does.It's a general rule for in circuit component testing to lift one leg of a two leg device before testing it. Exceptions apply to every rule.

author: matpro76
author_rank: Contributor
timestamp: March 15, 2018, 07:24:43 am
content: Thanks I really should take this to a pro to get looked at but I won't learn anything that way!Anyway will replace D12 and all the electrolytics and see how we go.