HM203-4a replacing capacitors how? - Page 1
Forum section: electronics / repair

author: Aldo22
author_rank: Super Contributor
timestamp: October 08, 2025, 10:16:10 am
content: Hi!I've been thinking about replacing a few electrolytic capacitors in the HM203-4a because one of them gets a little hot.However, the scope is working perfectly so far.I took a look at how that might be done. It looks like a nightmare.To get to it for soldering, you probably have to take the whole thing apart?The mechanical parts of the switches and controls, as well as the components attached to the heat sink, and above all: you have to unsolder some wires, which in turn could be difficult if you can't access them properly.Has anyone ever done this before?Is it as tedious as it seems, or is there a trick to it?The way it looks, I think the best thing to do is wait until it really stops working. Right?The capacitors in question would be:- Cosonic 47μF 250V 85°C- Cosonic 1000μF 40V 85°C- Cosonic 1000μF 25V 85°CAre they special in any way, or can you use anything with the same values?Thanks!

author: Gertjan
author_rank: Frequent Contributor
timestamp: October 08, 2025, 02:13:24 pm
content: Hi Aldo22,For a long time I used the very similar Hameg HM204-2.I replaced those capacitors without taking anything apart. Just (de)solder from the side. if you are very careful you can get to those capacitors with your soldering iron without touching anything else. But it is tricky!So, because it is a bit risky, I would only replace them if necessary.Measure (with a scope) The ripple before and after the regulators. If the output of a regulator is clean, and the regulator has enough dropout voltage, the capacitors are still OK.Another issue I had with my HM204-2, is that a few round bridge rectifiers got really hot. After many years, their soldering connections turned bad.Re-soldering solved that problem.regards & good luck, Gertjan.

author: Aldo22
author_rank: Super Contributor
timestamp: October 08, 2025, 02:37:45 pm
content: Quote from: Gertjan on October 08, 2025, 02:13:24 pmThanks for your reply!I don't think I can do that without removing the circuit board.To make things worse, the "hot" capacitor is surrounded by these wires from the transformer (attached photo).I can hardly reach it to measure it in circuit.Quote from: Gertjan on October 08, 2025, 02:13:24 pmThat's probably the best thing to do.If it breaks, I can try to repair it. Then it won't matter anyway.Thanks!

author: Doggone
author_rank: Contributor
timestamp: October 10, 2025, 10:43:54 pm
content: Hello Aldo22,I have just bought a 203-5 and have a couple of minor faults - first one has taken many hours to find - a jitter on the CH2 trace - after isolating the fault to the pre-amp board I finally nailed it to C115, a 22nf ceramic disc Cap - this had become unstable and was moving the CH2 trace up and down at random but this was literally the last component on the board to fall under suspicion. So if you have that problem you can go straight to C115!One of the psu smoothing caps in my scope needs replacing ( C1016 a bit corroded at one end - think it has been leaking) but fortunately it is close to the edge so should be able to get soldering iron in - I think it would be a nightmare to try and lift one these boards so would only do it if absolutely necessary!My next fault trace is something that has already been touched on in previous posts - an unwanted vertical line left-hand side of screen joined to the start of the trace - this is something awry in the unblanking cct so that should be good fun to trace! some have said the opto-coupler causes this and needs replacing, others have said the opto-couplers rarely fail so will have to take it one step at a time. I have been lucky enough to acquire a complete 203-5 for spares so have the luxury of being able to swap parts over which are plug-in such as the opto-coupler to 'prove' bad/good components before moving on to the next stage of fault finding.I also did a voltage test on the PSU outputs - the 1900v outputs are running at over 2000v and a few are on the low side (allowing for 5% tolerance) so I also may need to change some of the smoothing caps - go for a higher working voltage if you are going to change them.Regards Nick

author: Aldo22
author_rank: Super Contributor
timestamp: October 11, 2025, 12:49:33 pm
content: Quote from: Doggone on October 10, 2025, 10:43:54 pmThanks for your reply.Yes, I won't do that until it really stops working.Luckily, mine still works perfectly.In case you're interested, I posted about itherewhen I bought it.

author: Jan Steinman
author_rank: Contributor
timestamp: October 13, 2025, 09:09:32 pm
content: Quote from: Aldo22 on October 08, 2025, 10:16:10 am