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 again,
Couldn't resist this one off ebay as it was in good cosmetic condition but advertised with a tuning fade problem. The seller apparently didn't notice the humming transformer...
The flexible pcb was in very poor condition with broken traces and missing pads so I removed the front panel and the flexible pcb and remade all the switch solder points. (I suspect someone has got at it from the quality of some of the wire to board connections). I ran some jumpers and refitted the flexible pcb having changed out the two tuning voltage potis.
What I have now are the following symptoms: the unit picks up stations so there is no lack of sensitivity.
The stereo and discriminator lamps light up appropriately and the signal meter functions. Pressing the AFC button has some effect - briefly - so that circuit is doing something.
However, there is no stereo output (even if the lamp is lit), very bad distortion, and having selected a station on the presets, it fades immediately to about 60% of the initial volume.
I have replaced the 22v Zener (138) and the blue 470uF smoothing cap on the tuner board, but I don't think this is a voltage supply problem as the voltages in that area of the board seem to be in spec.
So, my uneducated guess would be that this is something to do with the stereo decoder.
As ever, I would be very grateful for pointers.
Simon
Perhaps a dead BF transistor. It wouldn't be a first.
Replace the rectifier on the tuner board.Replace electrolytic capacitors and tantals.Replace trimmers.
I know, I normally recommend diagnosing and repairing before upgrading/rstoring, but there are cases whereit can be difficult to diagnose eventual remaining faults with aged component still in circuit. The tuner in question is such a case.
Consider this:https://www.bonanza.com/listings/Beomaster-3000-3000-2-Beocenter-3500-Service-kit-capacitors-trimmers-lamps-/400100617?st_id=70245489
Martin
Hi Martin,
Thanks for that. (The four BFs check out ok).
Might be simpler to swap the board but where's the education in that?
Whatever I do, it's going to end up humming.
- Until you replace the transformer.
As the signal meter and lamps seem to function ok,it suggests that the actual tuner/if section is ok?
Usual suspects in the stereo decoder are capacitors 174,175,176,181,212,215.It's worth replacing these anyway,and it may be all you need to do.
The tuner power supply transistor can become dry jointed as it's heatsink is fixed to the chassis,and movement over the years put a strain of the soldering
Idealy,use Martins service kit,as it's comprehensive,but you will need to set everything up to spec afterwards.
Don't be tempted to "just twiddle" anything yet!
Nick
Hi Nick,
Wouldn't dream of just twiddling!
That's very helpful, so thanks. I'm keen to avoid as far as possible any re-alignment as I have neither the equipment nor the expertise to do it, though I do have an old analogue oscilloscope but no signal generator.
I'll report back.
I've replaced all the electrolytics (bar one) on the IF board and the problem persists, though the distortion/fade seems marginally better.
Martin helpfully suggested that it might be a bad detector diode and that I check for leakage.
There are four OA90 germanium diodes towards the front end which I guess function as detectors and I was surprised to measure (out of circuit) a forward voltage drop on these in the range 0.5v to 0.7v using a Peak DCA55
I was expecting 0.25v or thereabouts.
Does this mean they've gone bad?
Which one didn't you replace?
The diodes you mention are for AGC and AFC functions,both of which seem to be functioning judging from your earlier description.
Have you done any voltage checks around the decoder transistors TR19,20,21? this might provide more clue's.You can also check that the tuner,if and detector stages are functioning ok by bypassing the decoder.
Run a wire between the negative side of capacitor 175,to either the negative side of 230 or 237.If you hear nice clean audio through one speaker,you can apologise to the tuner etc for swearing at it,but it will lead you back to the decoder ?
The plot thickens?
The only tant not yet replaced is 2,2uF (109) as I'm waiting for the postman. The other tants were all in spec, but I replaced them anyway.
Voltages around the IF board seem pretty much ok, and measurement at the base of TR20 was spot on, and at the emitter of TR21 within 0.05v. I'll go back and do some more measuring around the decoder.
Presumably, I could inject a signal onto the neg of 230 or 237 but I only have an android based signal generator with a line output and I wouldn't want to mess anything up.
But I suppose that would tell me if the signal degredation was occurring in the switches or further along the chain.
Thanks for your help,
Simon,
You may want to replace the tuner box to board jumpers (black wires). I saw those broken inside the black sleeve where the current and signal were barely passing causing me a nightmare to troubleshoot.
Thanks, Jean, will check them out.
Quick update on the decoder transistors: Trs 19, 20 and 21 are all showing appropriate voltages, but TR22 has 0.75v (spec 0.8v) at the base and only 0.16v on the collector where it should show 0.4v.
Will check it and replace adjacent signal diode while I am at it.
Cheers, all.
I should have read the book of words more attentively.
TR22 only draws current on stereo signals. If I tune into a mono FM station, the stereo lamp goes out and I have 14.75v on the collector which is close to the supply voltage, as it should be.
Mono comes through (on headphones) clearly and without distortion or fade.
The problem remains on stereo though. I will try bypassing the decoder as Nick suggests.
Spot on, Nick.
Bypassing the decoder as you describe brings clear mono audio on all stations with no distortion or fade. The stereo lamp goes out.
Assuming the fault lies therein, and that I've recapped it the board and that the voltages are pretty much on the money, what should I check next?
Diodes?
Thanks,
Yes indeed,I would replace the switching diodes 201,202,205 and 206(in4148)
The fact that the stereo lamp illuminates normally,suggest's that everything before the switching stage is ok.
I've got a BM3000-2 on the bench just now,so I'm able to tinker if you need to compare notes?
You can also use the "bypass wire trick"again in lieu of an oscilloscope to trace the audio through the decoder.
Add a 0.1 cap to the end of the wire,and connect 'tother end of the wire to the yellow wire on the balance control.
Tune in a station,then depress the "tape" key,this will interrupt the normal signal path.
Touch the capped end of the wire to capacitor 174 positive side,and you should hear clean audio.
Move the capped end of the wire to any of the switching diode connections,and you should still hear clean audio,but at much reduced volume.Move the wire to the base connections of Tr23/24,and again you should hear clean audio.
When you move the wire to the collector's (reduce volume first!)you should hear much amplified clean audio.
This is "Heath Robinson" method of testing,but worthwhile if you don't have the use of an oscilloscope.
Go forth again sir.
Very helpful, many thanks.
Followed your instructions and the last audio signal I can get is at the right hand side of the switching diodes (viewed from the front), despite the fact that I have replaced the back two (202 and 206) as they are easier to get at.
I should presumably be getting something downstream of them. but there is no signal thereafter and obviously nothing at the base of TRs 23 and 24.
Thought I'd measure some voltages at TRs 23 and 24; voltages are maybe 10-15% under spec.
Do the switching diodes need a certain voltage present in order to function?
I would change all four switching diodes,otherwise it will be "mixed bag"of new/old spec devices-not ideal.
There should be low level audio at the output amplifiers base connections,so something is suspicious there?
I've measured about 8.5 volt (tuned off station)across capacitor 212.This drops to approx. 100mV when tuned to a station.212 is crucial from experience,and one of the favourite "usual suspects".Make sure that your replacement is in the correct polarity?
I imagine that the switching diodes need some biasing,but it may be sufficient from the switching signal itself?
I've measured about 130mV at the junction's of 244/216 and 243/217 which both seem to be biasing networks of some sort,as they are across the supply.
Something is not right with the output pre-amp stage,judging by your voltage readings there
Does the stereo light definately go out off station?
Stereo light does behave; I changed out TR22 just in case and the 18v Zener. Getting 0.75v at base of TR22 on station and nothing when off.
I'm going through the switching diode network checking resistor values - some of which seem to be a bit high (between 10 and 15% over) - and will replace the AA119s shortly.
C212 is correctly installed. Will check some voltages later.
What's the best way to get the IF board out as the wire trough is in the way - or can you work round it? I could jury rig from on top, but it's not very pretty.
Cheers,
There is a risk of "chasing ones tail",and going around in circles with these type of problems!!
Don't make unnecessary work for yourself Simon.The diodes 190/191 must be functioning ok as you have the stereo light coming on when the set is tuned?
The tuner/decoder pcb is awkward to get at underneath.Happily(famous last word's?)its not often necessary,but you will have to undo the securing screw's all around it,and the tuner fixings(but not the connections)to gain access to replace the decoder switching diodes.
You state earlier in this thread that you have "mono" sound?I'd overlooked that.Subsequently,the crude audio bypass trick I suggested seemed to prove that all was well before the switching stage too,which leads us back to that and the output pre-amps.
You mentioned earlier also that the unit appeared to have been "got at" previously.I wonder if all is well on the function select switch assembly pcb?
Can you fire up your analogue oscilloscope and trace the signal through?
Martins offer of another tuner/decoder pcb becomes more attractive?
This post on vintage-radio.net throws some useful light on how this switching diode circuit works:
https://www.vintage-radio.net/forum/showthread.php?t=75698
(Sorry, not sure how to create link)
I remade all the solder points on the switch (and flex) pcb's as my initial thought was that was where the problem lay. There was some pretty poor soldering which didn't look ex-factory.
I'll replace the remaining two switching diodes and check resistor values in the area, particularly the 5% ones.
Yes, it's about time I learned to use a scope, so I'll get to it..
Point taken about tail chasing, but I'm not quite there yet!
I think I've misunderstood your original thread Simon,and I may know where the problem is now!!!
The stereo switching matrix won't work without biasing.This is provided by the received (38khz) switching signal which is recreated by the doubler circuit of diodes 190/191,and amplified by TR21 when a stereo signal is received..For mono signals or when tuning between stations,bias is provided by the network of resistors 210/211 and decoupling capacitor 212.This network is disabled by the stereo lamp drive transistor in stereo mode,so it seems that the 38Khz switching signal is missing?
This must be due to TR21 or it's associated resistors,and should show up as dodgy voltages there.
Otherwise,it may be the tuning capacitor 197,meaning that the primary of the switching transformer 8022051 is not "peaking" and providing optimum loading?
I've been able to replicate the same symptoms you describe with my Beomaster 3000-2 ,by tinkering with these components .
If you have been able to "get at" the underside of the pcb to replace all the switching diodes,I would replace TR21 too.
Hope this helps.
The Vintage radio forum is very interesting too,thanks for the link.
I really MUST get out more though?
To recap, the story is that I can get clean mono reception (and there are a few stations broadcasting mono FM in London). The stereo lamp goes out, and I can get clean mono audio on both sides of all four switching diodes using your jumper to the balance slider.
As I am getting an audio signal (on mono broadcast only) at the base and collector of TRs 23 and 24, I can assume that the output stage works.
So, yes, the stereo switching matrix is not doing its thing correctly.
I set up my scope and connected it to TR19 base. I am getting a 19 kHz signal, but only 40 mV pp (spec is 70 mV). The distortion I am hearing is visible in the trace. (Picture below)
So the first question is, why is the waveform at TR19 below optimum? I will check resistors in the signal path, and C197. Just hope it's not an alignment problem further back.
TR21 could well be dodgy/noisy (as could any of them) though the voltages around them measured with a DVM seemed to be OK.
Thanks for your help.
And at TR 20 collector:
And at TR21 collector:
Stereo is far more sensitive to error than mono.Perhaps bad detector diode(s) or misaligned last IF stage.
Did you replace trimmers?Did you fiddle with the IF alignment? - Or can you tell if someone else did before you?
Martin,
I haven't fiddled with the IF alignment as I wouldn't know where to start. The cans don't look as if someone has been messing with them.
I haven't replaced any trimmers yet though some of them look a bit ratty.
You mentioned a bad detector diode before but I wasn't sure which (presumably germanium ones) you were referring to.
Afternoon, all,
IF board out. Wot a palaver.
Tr19 (BC208B) looks a bit suspect as hFE under spec.(170 as against min. 200) and Vbe of 0.94v.
Can I use BC548B (plentiful, cheap) instead in this application?
Regards,
I've checked my BM3000.2 using a 'scope,and I'm reminded that the voltages and waveform values stated in the manual,are not "carved in stone"!!!
The criucial stereo switching waveform at TR20 collector,in my BM is only 6vp.p instead of 10v,and yet the stereo sound is super,hey ho.
Voltages around TR20 in this working example are more or less correct.
I'm still convinced that your problem is in this stage!
The 19khz stereo pilot tone is transmitted with the signal of course,so can indeed be affected by shortcomings further back in the tuner/if stages.
But it seems to be intact judging by your "scopings".There must be the 17.5 volt at the frequency doubler 190/191 in stereo mode,as the lamp is switching on ok?
Which leads us back to TR20.I'd replace it anyway and see what happens.
The diode 198,is part of a damping network to reduce response in mono mode(disabled in stereo).It could be culpable if leaky,as it could dampen response in stereo too.?
The "phantom twiddler" may have been about at some stage in the units past,but you can see if the waveform at TR20 (c)can be "peaked" by careful and minimal movement of the tuning core of 8012051
Don't worry too much about the stated voltages and waveforms,they were taken after all,a long time ago under ideal conditions.
Nick,
Thanks again. While the board is out, I'm checking/replacing anything that would otherwise be inaccessible.
TR20 and 21 checked out ok, but I will replace them as you suggest. Can you suggest a replacement for TR19 (BC208B) per my previous post as it looks a bit down on gain?
Point taken about waveforms/voltages.
Can I replace the selenium rectifier (one of Martin's service kit items?) with a silicon bridge rectifier that will handle 30v at 300mA?
http://uk.farnell.com/multicomp/2w10mg/bridge-rectifier-1000v-2a-wobm/dp/1861433?ost=1861433&scope=partnumberlookahead&exaMfpn=true&searchref=searchlookahead&ddkey=http%3Aen-GB%2FElement14_United_Kingdom%2Fw%2Fsearch
Stereo lamp functions fine and yes to 17,5v at 190 and 191.
Major guess work with all but one of the replaced presets as it was impossible to get any sensible measurements off the old ones.
The two AA119 detector diodes in the module seem to be fine.
Apart from the presets and a few high resistors, I have yet to find a failed component.
Sorry,there was a typo in my last post.I meant to investigate the circuit around TR21 of course.
I think BC548B will do for TR19 and 21,with BC547B for TR20.
I've never had to replace the bridge diode,but I guess that while you're there it makes sense to replace it too?
I don't imagine that it needs to be too critical,the main thing is the "footprint" of the old one.The connections are "inline",with ac one side and pos/neg 'tother.So plumbing in a modern silicon version would mean having to twist the wires about,which might end up looking a bit "gash"?
Hello all,
Well, I can report some success with this though I'm not 100% out of the woods. Big thanks to Nick and Martin for all their suggestions and help.
I reinstalled the IF board, having replaced TRs 19, 20, 21 and 22 and the four switching diodes, a couple of Zeners and the rectifier and a few high resistors.
Switched on (Variac first) and still, disappointingly the same distorted sound with no stereo - a bit like listening underwater. Full functionality otherwise of stereo lamp, signal meter and discriminator lamps
But I then blasted the switches with contact cleaner (again) and after a bit of fiddling got clear stereo sound - reminds me why I like the sound of these tuners so much.
Sometimes the sound fades back to the distorted earlier version after a few seconds. If it does, I can often get clear stereo audio again by partially depressing Tape, Phono 1 or Phono 2 until the stereo lamp changes intensity. I've just listened for 30 minutes with no sign of fade or distortion.
I'm just wondering if either:
I do find myself wondering about the polystyrene film caps (C178, 183, 189, 197) as I understand they can be a bit temperamental. Expensive to replace, though and I don't want to desolder them unnecessarily. (Nick has already suggested 197).
Anyway, basically it works apart from this one thing. Which of course may have been The Thing all along!
Any thoughts, anyone?
Not too gash, I hope.
The plot continues to thicken?
The function switches are notorious of course for causing all sorts of problems.The mono/stereo section is particularly complex as it was designed to allow individual selection of l/r etc for tape monitoring.Thats also why the tape key has it's odd function,ie not releasing the other keys when pressed.Open reel tape recording was the "to do" method when these amps were current product of course.
I remain puzzled by the action of your Beomaster though.The switches can indeed cause intermittent sound problems,but I can't imagine how they could affect the operation of the stereo decoder?
After some 45 years of tinkering with(mostly B&O) electronics though,I'm still finding out things!
The only long term solution to the switch problem I've found,is to strip the switch down and clean the contacts by hand.They don't wear out in fact,they just become seriously oxidised.Switch cleaner will effect a temporary cure,but as it dries out,the problem returns.
You may get the chance to sit down and enjoy the sound of this Beomaster yet sir?
Neato solution to the rectifier replacement too!
Much too nice a day to even be thinking about this, but my latest finding seems to me to point to a solution.
If I connect a grounded DVM on DC volts setting to the collector of TR21, the test point adjacent to it, or the connected side of C197, full stereo sound is restored. This works most, but not all of the time. Often just a brief touch of the probe is enough. The measured voltage is fine.
This suggests that briefly (or not so briefly) loading the circuit in some way is restoring normal function which presumably was also happening when Tape or Phono were switched in.
As TR21 is new (genuine Motorola BC182B), I am inclined to suspect C197 styroflex cap.
Nick, you got there ages ago, but does this point to the culprit?
Can I use WIMA type replacement?
Curiously,the manual suggests a cap(197) with only 10% tolerance,but polystyrene types(5% or better) seem to be fitted to all the ones I've seen.The circuit has "PS" noted by these caps too,which adds weight to that?
The meter loading is significant,but replacing the cap anyway seems the way to go?
Waiting for styroflex caps to arrive.
This is what I'm getting at TR21 collector after briefly loading the circuit with DVM.
More like 1.2v pp rather than the prescribed 10v but sounds pretty good.
Normal service is now resumed as C197 was open circuit. It was pointed out to me elsewhere that the increased frequency I was seeing on the scope at the collector of TR21 is an indicator of reduced capacitance in a tank circuit.
Big thanks to Nick for walking me through this.
Now on to the pre-amp and output boards which are looking pretty manky...
Just a postscript to this saga.
Unit is almost finished with pretty much a full recap. It sounds excellent and I have full functionality.
However, I am getting some sibilance on FM when listening to speech.
This seems to be a known thing - not necessarily with Beomasters - with FM reception in general and apparently can be due to:
I didn't change any of the caps within the front end or the screened discriminator module 8002057 and I'm just wondering if C123 in the latter is a known culprit?
No evidence of previous IF twiddling and I'm in a very strong signal area, so inclined to suspect a slightly duff component.
Any ideas?
Before you dive in again,it's worth considering that you may have too much signal?
Try an attenuator in series with the aerial.