Philips BM7502 Monitor 80 repair

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1980s_john
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Philips BM7502 Monitor 80 repair

Postby 1980s_john » Mon May 08, 2017 10:40 pm

Hi,

I was given one of these as dead from someone who had collected a load of beeb stuff, and would like to revive it.

When I got it the 2A fuse on the DC supply had blown. Attached is the schematic of the similar BM7522 (which is amber rather than green CRT).

I've checked all the electrolytic caps and only found C521 to be faulty so replaced it (it sits near a heatsink).

I removed the HOT TS521 and this appears to test OK (using a diode test on my multimeter it appears as two diodes back to back). With it removed I applied power briefly (using a 15V DC supply across C104) and R434 (5.6 ohm between the two push-pull transistors driving the vertical deflection coil) gets hot, has 6V DC across it, circuit was drawing about 1A from the power supply.

I haven't applied any input signal, I don't think the 15kHz horizontal and the 50Hz vertical drive circuits are oscillating.

I put TS521 back in and applied mains, the green LED came on (showing the DC is there) but no raster, and after about 5s the 2A fuse blew again.

Has anyone any experience in fixing these please? They are very common with beebs, it would be nice to fix it rather than dump it.

Thanks,
John
[EDIT] - Found a photo of service manual, similar monitors are BM 7500, BM 7502, BM 7520, BM 7522, BM 7542, BM 7550, BM 7552.
Attachments
philips bm-7522 monitor.pdf
(835.71 KiB) Downloaded 31 times

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sbadger
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Location: Farnham, Surrey

Re: Philips BM7502 Monitor 80 repair

Postby sbadger » Tue May 09, 2017 7:32 am

Hi John,

I've asked over on the Jamma+ forum on your behalf. Those guys have helped me out with CRTs in the past. It's their bread-and-butter if you will.
I'll let you know.

stew
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1980s_john
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Re: Philips BM7502 Monitor 80 repair

Postby 1980s_john » Sun May 14, 2017 6:00 pm

Thanks!

I found this post:

https://www.freelists.org/post/techassi ... deomonitor

which seems to be same failure as my monitor has, but mine didn't display anything (no HV) and brew the fuse.

I have removed the flyback (LOPT) and removed transistor TS431 / BC635, then applied following supply voltages:

15V to switch SK1
25V to R525
60V to R438

Note that 25V has to be applied after 15V, else transistor TS101 is switched off and then don't get 12V supply (found this out by experiment).

In this state the supply currents are all very small, eg less than 50mA each so all looks good and no obvious shorts.

With a video input (Freeview box) and a scope it becomes easy to trace the signals as per the service manual. I get a nice 15.7kHz input to the HOT TS521, even with the input signal removed. The 50Hz vertical signal gets as far as the base of transistor TS421, then due to the DC conditions caused by removing TS431 the TS421 does not conduct so emitter and collector are 0V.

I have ordered replacements, gone for BC639 rather than BC635 and BC640 rather than BC636 as these are more readily available and are identical apart from having higher voltage rating. I tried to test the removed BC635 and although it behaved as two diodes back to back my basic meter transistor tester gave hfe out of range (ie more than 4000 I think). I then removed the BC636 and this was toast (didn't act as a pair of diodes).

Fingers crossed the two new transistors fix it.

Regards,
John

1980s_john
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Re: Philips BM7502 Monitor 80 repair

Postby 1980s_john » Wed May 17, 2017 10:00 pm

Hi,

New transistors arrived today, so I fitted TS431 with BC639 and TS432 with BC640 and re-applied low voltage supplies, all looked good with low current drain. A check on test points 18 and 19 with a scope looked ok.

I then quickly (a mistake I think) refitted the LOPT, put a new fuse in and connected everything back up. I connected up a Freeview box (no aerial but with menus etc) and powered on. All looked OK, LOPT brought up the DC supply for 60V (measured 55V) and 25V (spot on) and screen had a faint raster so must have enough HV. I refitted the case and tried again. With power saw the raster again, and after about 5 minutes I unplugged the video input and almost immediately smelt burning so unplugged. I think (from the smell) it was R434 so probably bust TS431 and TS432 again.

I don't think the circuit has any protection if the 25V supply dies, this seems to cause the TS431/432 pair so go into DC mode rather than 50Hz ramps.

With hindsight I should have left the LOPT out and soak tested the circuits with the LV supplies for a day or so, but I was impatient (no need! :oops: )

Anyway, too late today to investigate further. I think my next project will be to build a variable HV DC low current supply to test some of the high voltage capacitors (a job I have had on my list - see examples such as https://www.vintage-radio.net/forum/sho ... hp?t=73265) and test C524 and C527 at their rated voltages too.

Luckily the transistors came in packs of 5 so plenty of spares!

Regards,
John

Richard_barsby
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Re: Philips BM7502 Monitor 80 repair

Postby Richard_barsby » Sun Oct 01, 2017 6:10 pm

Hi I know this is an old post but I have this service manual if it is of any use to you.

Rich
Attachments
BM7522.pdf
(716.26 KiB) Downloaded 13 times


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