Philips PM6669 hot transformer and battery - Page 1
Forum section: electronics / repair

author: Shimonu
author_rank: Regular Contributor
timestamp: July 15, 2025, 03:21:56 pm
content: A couple of days ago I noticed the internal lead acid battery of the instrument got pretty warm, I had also heard a bubbling sound from it which made me conclude the battery was likely old and defective. I replaced it yesterday but today I again noticed the battery was very warm after leaving the instrument plugged in over night. The transformer is also very hot, so much I can't touch it without getting burned. Obviously the instrument will be disconnected from now on until this is solved, I'm changing the IEC filter anyway.Now my question is if I should be thinking that there's something wrong with the charger circuitry to the battery and maybe pulling excessive amounts of current making the transformer warm or if there could be something wrong with the transformer? I haven't seen any obvious signs of damage and the previous battery that I changed was working and took charge and the instrument could be run off the battery.I'll need to do some measurements of voltages. Could this be caused by excessive ripple?Writing this I realized I should at least check some caps. So I took out a cap on the battery charging board that sits very close to the transformer, it measured a little low at 2 mF instead of 2.2 but assuming it's a 10% tolerance it's within spec and the ESR was fine. The other two big rectifier caps measured spot on.Hmm, I noticed there was something that looked like a bodge and looking at the schematic I think I'm now realizing the thermal fuse in the transformer has likely popped and someone has jumpered the connection. Oh man that would be a bummer assuming that means the transformer is bad.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 15, 2025, 03:35:29 pm
content: i would say remove the battery, and before doing anything else check if an sv manual existususal pointers are bad capacitors on many supply linescheck if you find a manual for the x-former bodge and voltages on each regulated sections ... if you have a meter who do both ac and dc, you can see the ripple voltage(s) if any

author: Shimonu
author_rank: Regular Contributor
timestamp: July 15, 2025, 04:21:38 pm
content: There is a service manual available but it's mostly containing the schematics, which of course is great.https://www.jackenhack.com/slask/pm6665_pm6669.pdfIt brings up the thermal fuse and just says to replace the transformer if it's blown.I've made a quick test. I plugged it in again without the battery and monitored the temperature of the transformer. It went up a bit and settled around 36 C. I then plugged in the battery and it started rising again and got to about 41 C before I unplugged it, so I suspect it's related to the battery. I also checked the voltage on the battery and it was around 8.1 V which is too high for a 6 V battery. It should be no higher than 7.5 V when being cycled according to the battery. I'll go through the voltages and see what's going on.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2025, 04:36:46 pm
content: Trace through the circuit from the transformer from the battery. Something that is supposed to limit the charging current or max. charging voltage isn't working, so its killing batteries, and eventually the transformer. See the left hand side of the Battery Unit schematic, page 27 of the PDF you linked.

author: Shimonu
author_rank: Regular Contributor
timestamp: July 15, 2025, 06:24:41 pm
content: Yeah that's a great idea, I've started looking at the charging regulator and it shouldn't be too hard to find the issue. I just need to rig up something so I can test it outside the instrument.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 15, 2025, 08:47:45 pm
content: the charging regulator is not too complicated you seem to have current adjustments, some temperature sensing, maybe some stop thresholds / adjustmentswith an 0.22 ohms you may get around 2.7 amps out of this circuitnormally 0.6v divided by 0.22 ohms = 2.72 amps BUT you have an 1.6 amp fuse in serial of the battery + line ... adjusted lower than 2.7 amp for surebut after all theses years, the battery may have gone resistive more than anything, if you remove it and all seems good or better, the battery is overduebut now the problem is adjusting that regulator section / treshold, limitter etc ... BUT i think i would not play with thesesguessingpt1 would be the voltage outpt2 would be current limitpt4 would be the low battery set pointr19 should be near the battery, it depends if its negative or positive coefficient, meaning the more the battery heat, it should drop down the voltagethe 2x gr9 diodes should act as some switching for the low battery stop when you are on battery charge or battery supply mode

author: Shimonu
author_rank: Regular Contributor
timestamp: July 16, 2025, 03:43:07 pm
content: I've been analysing and measuring but I haven't found anything broken. Without the battery connected(and even with) there's a lot of heat coming out of R6 which is on the base of the pass transistor TS1. It gets up to 75 C but it measures correctly at 150 so I figured it's just because TS2 is fully open.So I tried adjusting PT1 and it's kind of acting like a switch if I monitor the positive battery terminal. If I lower it it seems to either close TS1 and I see basically a floating terminal or the battery voltage depending on if the battery is connected. If I try to carefully nudge it up again there isn't really any adjustment, it just jumps up to 10 V or about 8 V with the battery.Now I'm thinking that maybe this is a bit strange to do with a full battery, since I just got this new and kept it plugged in previously. It should not try to charge it when it's full of course but maybe the potentiometers was just set wrong and tries to charge to a too high voltage? Clearly someone has worked on this before and maybe tried playing with the charging.I think I'll just keep the instrument running on the battery for a while to drain it, while monitoring of course, and see how it behaves when actually charging.

author: Shimonu
author_rank: Regular Contributor
timestamp: July 17, 2025, 01:37:00 pm
content: It's not really too different when charging. Both the PT1 and PT2 are very twitchy when adjusting. I can reduce the charging current to about 50-60 mA and that makes the voltage drop to about 6.5 V but there's no smooth adjusting here. If I get a little too high it goes up to 500 mA and 8.5 V.On PT1 pin 2 I can sometimes measure close to 10 V, could someone explain if that's possible without pin 7 of IC2 driving that voltage? Otherwise I would imagine there's a break or poor connection between PT1 pin 1 and ground to make it a pull-up. But that just makes it turn off the charging so it wouldn't quite explain the behaviour, on the other hand if it breaks on pin 3 it would make it a pull-down and completely open TS1 all the time. Maybe I could try some contact cleaner

author: Shimonu
author_rank: Regular Contributor
timestamp: July 17, 2025, 08:26:14 pm
content: That might have been it. Used some contact cleaner and turned PT1 and PT2 a bit. When I turned on power I was still getting around 9V but then I turned the voltage pot and it didn't get the same floating output but a stable voltage around 5.5 V. So I kept turning and now I was able to adjust the output voltage up to about 6.8 V which the battery recommends for standby use. Now it's charging at a comfortable 130 mA or so I feel like I've read that a full charge can take up to 24 hours so I don't think it's an unreasonable current.Should the pots be replaced or could this be an okay fix?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 18, 2025, 02:54:34 am
content: depends on the footprint, sometimes you can put multi turns instead ... for me if they are scratchy and sensitives i would replace them