ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello All,
Finally got round to fixing up (or attempting to) a cosmetically nice BM901 I've had sitting around for a couple of years.
I replaced (with Panasonic FC) all the red Roederstein electrolytics. These were mostly in spec, but a few had failed open. I will in due course replace the big red ones, but for now they measure ok.
The few voltages I measured seemed ok, so I connected a pair of speakers.
I am missing a left channel (on either speaker output) but the right channel seems fine. Tone controls work. I have a green stereo light but no tuning indicator (probably blown bulbs). The tuner pulls in FM fine with no aerial.
I have poked around for dry joints/cracks, but I'm beginning to suspect a semiconductor issue.
Can anyone give me any pointers?
Many thanks,
Do you have any sound at all from the left channel?A thump-sound at power up/down or when switching source?
Any DC on the output rail of the amplifier (before the series capacitor)?
Martin
Hi Martin,
Getting thump sound on power up/down. But not on left channel, nor even the faintest hint of a sound. Phono and tuner input play normally through right channel.
Voltage on output rail shows 21.9 v between anode of 3000 uF series cap (good channel) and ground, and only 7.6 v on the left channel. (Think it should be 23 v on both).
Thanks,
Simon
21.9V is acceptable.7V is not.You will have to diagnose a bit. Fortunately, you have a working channel right there to compare to.
Well, my diagnostic skills are non-existent, but what I have found is that working backwards through the transistors in the non-functional left channel output stage, I am getting out of spec voltages (around 8.5v instead of 25v) until I get to TR19. The comparable transistor on the other (working) channel is TR27.
At the base of TR19, I am seeing 15.8 volts, but only 8.2 volts out of the emitter and nothing on the collector. This compares with the functioning TR27 which has a base voltage of 17.8, and 18.5v and 0.6v on the emitter and collector respectively, which is acceptable.
While 15.8 volts at the base of TR19 is a bit on the low side (it should be 19v), am I right in thinking that the problem may be centred on TR19?
Thanks again,
Has TR20 shorted B-E?
Hello Martin,
Measured in circuit, TR20 B-E measures 1.54 KOhm, test leads either way round.
I am getting 47,8v at the collector of 2TR22 which is correct, and 31,8v between the R130 and R131 which seems about right.
But if I follow the 48v supply back to the base of TR22, I get 32,5v after R136, but then only 8,7v after R138.
Suspecting R138, I dismounted it, but it checks out OK.
So, something appears to be pulling the voltage down after R138. (There exists 600 ohms between C-E of TR21).
All pointers, hints, gratefully received. I've attached a pdf which might throw some light.
Thanks.
Martin pointed me in the direction of 2TR20 which appears to be shorted or defective.
Am I better off getting new-old-stock BC171B to replace exactly what's there already, or should I look for a more modern equivalent?
I presumably should replace the identical 2TR28 at the same time.
I'm new to this semi-conductor stuff, so grateful for advice.
Regards,
I replaced the diode string D19 - D22, but still have one channel operation.
TR21 appears to be non-functional, so I have a replacement on the way.
I thought I would check TR22 (TIP120 Darlington on the heat sink) so I demounted it but it checks out ok.
Is it good practice to replace it with a new one, or can I safely reinstall it?
If I replace it, should I replace TR23 at the same time?
Thanks for guidance,
You can safely put the same darlington transistor back again.If one of the pair has shorted, the other one will have seen an overload (since it would then in effect have been sittingright across the supply) and they should both be replaced.
Thanks, Martin.
Does the same advice apply to re-using demounted transistors in general, or just those with heat sinks?
Components that are good can be put back in.However, components that are fault-prone from experience, such as trimmers and electrolytics, I like to replace once out.
I still have one channel operation, which is frustrating as I've replaced TR21 and TR20 and the diodes D19 to D22 and thought I might be onto something.
I only have 11.5 volts rather than 19v at the base of TR19 which suggests to my very untrained eye that there is some sort of short to ground somewhere pulling the voltage down. Apart from TR19 (which seemed ok when demounted and tested on a Peak analyser) is there any other component or area I should be looking at?
Also baffled as to why I have only 2.5v at base of TR22.(which checked out ok when demounted).
48v rail is good.
Any help appreciated, as always.
I eventually stumbled across the cause of the loss of my left channel: a B-C short on TR23. I don't know whether failure of TR21 and Diodes 19 and 20 caused TR23 to fail, or vice versa, but the machine is now functioning again, following replacement, which is satisfying.
In line with Martin's recommendation, I will change out TR22 as well.
Apart from that it just remains to replace the idle circuit trimmers and a couple of blown bulbs as I've already recapped it and changed a few out of spec resistors.
I thought this one was pretty much done and dusted, but today it gets interesting.
I changed out TR22 (per standard recommendation to replace Darlingtons in pairs) with a TIP120 and set the idle current with no load to 10mV across emitter resistors R142 and 191. So far so good.
This was, prior to changing TR22, a working set, so I had no hesitation in plugging in my loudspeakers.
I switch on, and immediately get a loud hum in the left speaker followed by a puff of smoke from R143 (the other left channel emitter resistor) which is now toast.
Does anyone care to hazard a guess as to what the cause of failure might be?
Both of the new output transistors have shorted, maybe as a result of running the set for an hour or two without simultaneously replacing them as a pair?
Or maybe they're chinese counterfeits...
Will replace all transistors in the output stage and check other component values and see what happens...
Regards
Well, it all came right in the end.
I replaced the two output transistors and all the other transistors in the output stage (even where they tested ok) and the fusible 0,39 ohm resistor in the idle circuit. The diode string was intact.
I reset the idle current and connected it up and it sounds great into S45-2s - another underrated amplifier from B&O. I don't find lack of power to be an issue with this set up.
However, pressing any other button appeared to cause the loss of the left channel, so I opened it up again and prodded around until I located the fault.
The push button mechanism (which had taken a fair bit of contact cleaner to get to latch properly) bears straight onto the circuit board. The spring loaded action transmits a certain amount of stress to the board when pressed and I think this had caused some defective connections where the signal wires join the board. A bit of reflowing and new solder seems to have cured the fault.
That'll be a great sounding setup indeed.
Job well done.