Bang & Olufsen M20 - Standby but not powering on

[daveyheats]

Hi all,
Long time lurker on the forum but first post looking for some help!

I'm trying to restore a Bang & Olufsen M20 CRT set that I recently acquired. It was sold as a donor / parts chassis for an MX200 I received at the same time, but upon inspection it all seems complete and in good condition, so I'd love to get this set working too.

Using the MX2000 set as a reference for how to connect everything up, when I plugged the set in and switched it on the dreaded RIFA cap on one of the front small boards exploded and stank the place out. In it's dramatic demise it seems to have ripped off the solder pads too. But I've seen a few posts online saying it should work ok without this being replaced?

Since then, I have the standby light, but when you switch it on there is no picture, and the set goes completely dead after around 2 or 3 seconds.

I've been through and reflowed any dry looking joints, recapped as many capacitors as I had available with Panasonic replacements, replaced the battery and started going through and checking any suspicious looking resistors & diodes. But the problem remains.

I also replaced the transistor at TP01, but upon checking this seems to have now been shorted.

The community here seems incredibly knowledgable so any advice on where I can check next would be a massive help. I'm a beginner when it comes to electronics repair, but really enjoying learning about these old beauties and would love to get this old girl going again.

Thanks so much for any pointers.

[bobbyball]

Just looking at your pictures, and on the second one with the red coil and the big "Roe" branded capacitor (these are garbage now), there is what appears to possibly be a small crack on the circuit board adjacent to the right hand (visible) pin of the red coloured coil assembly. This could be an optical illusion but it is worth checking that there isn't a copper track on the other side of the circuit board that might be "open circuit" due to the crack, if it is indeed a crack. Just a thought...

Also "RP03" appears to be missing - presumably a resistor. Check with the other set or the circuit to see if this should be fitted as sometimes certain components are not fitted in some sets.

The "Rifa" capacitor should ideally be replaced in time as it helps to reduce interference from the set going back into the supply, and also helps deal with spikes coming in from the mains to the set. These are "specials" designed to be connected across the mains, and needs to be a "Class X" (across the mains) or "Class Y" (between the mains poles and ground) - depending on the circuitry concerned.

[daveyheats]

Thanks so much Robert, that's all super useful. I'll take a closer look at that area in case there is a crack and investigate further.

Oh, and I forgot to mention RP03 is currently off the board as I was testing it, but it seems fine so will be going back on tonight.

Sounds like I probably should get that Class X replaced too. I did buy some replacements before I realised the pads were gone, and also that tiny board is a right pig to get to. But will man up and get that sorted too.

Really appreciate the pointers chap!

[bobbyball]

Hello again!

It's not vital to replace that Roe capacitor right now - again, in time assuming the set is repairable. There is nothing special about it apart that some of them had three mounting lugs (for secure mounting) making up the negative side, with the positive one in the middle. You can just use a normal capacitor and use "one" of the three negative ones. However, sometimes the three connected pins are used to "bridge" the ground side of the circuit board over other tracks so the three holes may need to be connected together. You can check this by looking at the tracks and pads on the copper side of the board - if the pads don't go to anything else you can discount that pin.

In time, assuming the set does prove to viable to get going, replace all those "Roe" capacitors as they will be "bad" by now and often the replacements will be smaller and easier to mount.

It would be worth trying to get a copy of the service manual unless you already have done. The manual can be found online, although the copying is not the best, or sometimes the manuals can be found for sale.

[Red to black]

This set uses a Thomson designed and made chassis (Loosely based on the ICC3), it utilizes a somewhat unusually designed psu, in that it has a step down isolation transformer for several of the LT rails, then one of these (110V rail) is converted via a step up chopper circuit to give a 145V HT rail.

Even the frame stage is unusual in that it is very similar to the ICC4, ICC5 and IKC2 chassis in using a single Thyristor as the output device, even most engineers whom have not been involved with these chassis would struggle with this part of the set.

As usual with typical Thomson "quirkiness" complete with comprehensive protection circuits coupled with B&O complexities in the mix then this is definitely not a set for a beginner!

No disrespect intended here, however I think you will need hands on help from someone whom understands and is familiar with this set/chassis.

[Maarten]

It's a full blown ICC3 of the second generation (ICC3500?), not just loosely based on it, with some added B&O parts. I definitely agree that having experience with this chassis certainly helps. There might be a write up in television magazine?

[Red to black]

Hi Maarten,
The core chassis is a full blown ICC3 chassis, however the sound stages and some of the microprocessor customer facing bits are vastly different, the sound stage in particular!

There is a write up in the October 1992 TV mag by Nick Beer, whom is I believe a member on here, alas he seldom posts

[daveyheats]

Thanks guys, it definitely does seem massively complex (for me at least!). I'm working on a Trinitron as well and that is infinitely easier to pull out, work on and reconnect.

I'm going to replace a few more capacitors and some fairly burnt looking resistors and give it one more try. If that doesn'tt work I'll definitely be looking for someone to work on it for me (any recommendations of someone in the Cheltenham / Gloucester area would be amazing!).

[Red to black]

Actually Maarten whilst you are here, the first ICC3 (and ICC4) sets I came across were in fact Nordmende badged models named as spectra series, a lot more simpler than the B&O models, this was because Thorn here in the UK had some sort deal going on with Nordmende/Thomson quite a while before they finally sold out to the latter in about 1988.

OT a bit I know, sorry mods

[Maarten]

Hi,

I forgot about the sound stage... Added parts may sometimes be recognisable by them following the B&O position numbering scheme for the components.

Interesting to know that Thorn already did business with Thomson before selling out. I did know they switched over to Thomson designs with the ICC5 chassis, around the same time as Telefunken.

[bobbyball]

Another bit of general advice on the B&O - look out for dry joints, this chassis has so many fine tracks and interconnects. The tube base was another area for them. We had this model from new many years ago, never let us down apart from the aforementioned dry joints on the tube base but that was while it was still under guarantee. I still have it in my storage unit. Definitely not one for the beginner though, not that I am implying anyone is a beginner here!

[John123]

Originally Posted by Maarten

Interesting to know that Thorn already did business with Thomson before selling out. I did know they switched over to Thomson designs with the ICC5 chassis, around the same time as Telefunken.

I think I may have spotted a few of these elusive Thorn EMI and Telefunken sets in a recent re-run of (not-so) classic BBC TV soap Eldorado (don't ask!). The show originally aired in mid-1992.

Again, apologies mods I hope we're not veering too far o/t.

[Sideband]

Heres's the TV magazine that contains the article by Nick Beer. https://www.worldradiohistory.com/UK/Practical-Television/90s/Television-1992-10.pdf

You mention that the set is in standby. Do you have the remote control? It may well be that the set is just in standby and you need to bring it out of standby either by using the remote control or by pressing the 'Tune +' button under the control flap. Many of these sets used to 'remember' the last command and if it was unplugged in standby, then it will remember standby when next powered.

Worth trying incase there is no fault.....!

[dave walsh]

Tim "Studio 263" may be able to help you davey. He's a B+O specialist and has his own website!

Dave W

[1955APREN]

Just read through this thread. Got say that Icc3/4/5 where not my favourite
sets by a long way. I don't remember ever seeing this version, possible Currys
where not selling many Ferguson's during this time period.
In the 1st post the op states that the led goes out when switch on . So set must be coming out of ST/By , but the shuts down after a few seconds indicating
some type of short .Transistor Tp01 is mentioned as being replaced , but reading
faulty. Is this when still in the set or when removed.The base /emitter will read
low 180 Ohms when in circuit . If its ok when removed from the pcb then the
fault is else where .Is the heat sink at ground if so is the collector of tp01
isolated ( metal Tab), also are any resistor/diodes across the collector/ emitter
for TP01. Also is TL01 ok this also needs to be tested.
Hope this might help.
Regards Derrick

[Maarten]

TP01 is probably the BU184 or BU806 darlington? This would result in strange values measured. As long as it isn't shorted, it's likely good.

[Max Ripple]

These sets were indeed essentially Thomson designs,and B&O did'nt need to enhance them too much,to fit into their "ethos"!
The sets can produce remarkably good pictures and stereo sound too(from external sources)and are popular with gamers as a result.
I have a spare remote control somewhere if you need one?
It has to be said again,that they are not for the faint hearted as a repair prospect!!
The symptoms you have are in fact a fault mode.The set is designed to try three times to power up,and if unsucessful due a to fault,it will revert to standby.In common with many tv designs,this one "powers itself" once running,in as much as the line output stage provides the various supply voltages,so if it can't run,nothing does,and the set sulk's!!!
The exploding Rifa may be coincidental,as they tend to "spontaneously combust" when presented with mains voltage anyway!!!
"Usual suspects",are shorted Line output transistor or transformer,but there are myriad other reasons.
A good circuit diagram is essential.I may have a manual,where B&O helpfully sometimes included a circuit description.
Good luck with it,and take care in there,as any telly,particulalrly crt ones,can "bite"due to high voltages,even when off!!
Nick

[daveyheats]

Thanks so much guys for all the help so far. I appreciate it so much, and it's good to know if I can't get it working by myself I'm not utterly useless, as these sound like a tricky set to repair even for someone who knows what they're doing!

I've got a few more things to try out first, but failing that I'll be handing it over to a specialist to hopefully bring her back to life for me. I can't turn back now!

I knew this forum would be useful but I'm blown away by the depth of knowledge on here, so once again a big thankyou from me!

[daveyheats]

Oh, and Max I'd love to take that remote off your hands if I can, so let me know what you'd want for it. And if you have a decent circuit diagram / manual you could share / sell too that'd be fantastic. The only version I can find online is a really bad scan, where half the components and values are unreadable!