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
The question of how to repair the inertia tuning gearbox on these nice old sets (from 1969) has come up a couple of times over the last decade as the original gear sets will have disintegrated. Suggested repair methods have involved rubber washers and bits salvaged from old VCRs.
There is a simple solution for anyone wishing to restore the heavy feel to the dial cord mechanism.
You will need two 0,5 mod gears, one of 20 teeth, and the other of 45 teeth. RS Components (in the UK) do them and the part numbers are 5217023 and 5217124. They are made of Delrin.
The bore of the larger gear will have to be drilled out to 6mm to accommodate the axle. For accuracy, I would recommend that this is done on a lathe, not a drill press. The smaller gear is a press fit on the shaft.
More on this resto/repair as it progresses..
Take care.
Even if it doesn't seem to recreate the original friction function, that saves the dial string if youtune up to the end of the dial at full speed, it's still a nice alternative.
Martin
Interesting hack!
Krolroger: You will need two 0,5 mod gears, one of 20 teeth, and the other of 45 teeth. RS Components (in the UK) do them and the part numbers are 5217023 and 5217124. They are made of Delrin. The bore of the larger gear will have to be drilled out to 6mm to accommodate the axle. For accuracy, I would recommend that this is done on a lathe, not a drill press. The smaller gear is a press fit on the shaft.
A lathe? Who's got a mini-lathe standing around as a hobbyist?
A good drill press should be fine as long as the workpiece remains movable so that the drill can center itself, don't you think?Is the big gear a press fit too? If so, I'd enlarge the hole with a 5.8mm drill and see how that works. Heating the gear to some 80 or 100C might help with mounting, since Delrin has a thermal expansion coefficient good ten times bigger than that of steel or brass.
Let us know your actual process, incl. successes and fails. Thanks!
I tried this on a drill press with not such a great result. Too much run out on my Chinese drill spindle, I guess. It might be fine if you have a Meddings or Fobco or similar quality kit.
I don't have a lathe either so I put a call out on a model engineering forum and was swamped with offers of help from good people who wouldn't even accept a bottle of wine for their trouble.
You need to remove the flywheel and assemble from the back. Both gears press on.
Getting the dial cord on was a bit of a faff. The manual calls for a string length of 160cm to be wound round the capstan and the various pulleys and magically knotted by fingers more nimble than mine with the knot in precisely the right place. Or you could pre-tie the knot but then it feels too tight to slip over the rim of the capstan. The vintage radio forums are full of arcane information on dial cord knots, if that is your thing.
The solution I adopted was to use a spring to tension the cord. It's not banjo-tight as I suspect it was intended to be, but it shows no signs of slippage. The knots are superglued.
I fitted a new mains flex. The tuner front end can be unscrewed and moved out of the way which makes just enough room to get a soldering iron onto the switch. There is no shrouding on the switch (and indeed elsewhere) to prevent inadvertent contact with line voltage, but I guess that was normal for 1969.
Krolroger: I tried this on a drill press with not such a great result. Too much run out on my Chinese drill spindle, I guess. It might be fine if you have a Meddings or Fobco or similar quality kit.
Granted, a damaged drill press may cause some problems, but any standard product - even a no-name made-in-China low-cost type - should be fine. The average accuracy you get on a lathe and on a drill press should be around +/-0.05mm. Add another 0.05mm for the drill and you'll see that you need to go for a 5.9 or 5.8mm drill to get a 6mm hole. For a real engineering fit you normally need a reamer anyways, i.e. you drill a 5.8mm hole and then you use the appropriate reamer to get a transition fit or press fit as needed.
Moving on to the pre-amp, I am getting a lot of noise on the right channel. Noise on the tape input is not a problem so I am assuming a fault somewhere on the pre-amp. The noise is a crackle and whine and hiss, evident at high volume levels. The left channel has some slight hiss only.
I replaced the both BC114 transistors on the right channel which I believe can be noisy and that has lessened the hiss component. On the circuit diagram, four of the resistors on each channel are marked Beyschlag, which I take to be a specification for low noise resistors. Though they measured fine, I have replaced them with 0,6W metal film just in case they were contributing to the problem.
I haven't touched the caps yet. Could the tants or styroflex caps be contributing to the whine/crackle?
Hoping to solve this without digging out the oscilloscope.
Simon
Yes, tantals can become noisy, but it could also be something else, - a contact failure perhaps.I wouldn't suspect the styroflex caps. They are usually incredibly reliable.
Well, I have gone through the phono pre-amp board and I still can't find the source of the whine on the right channel, a bit like a boiling kettle whistle, though other white noise has been lessened with the replacement of the low noise resistors with similar. I've changed the two right channel BC114 and the 2,2uF tant.
I'm assuming this is some sort of oscillation. When I press the Mono switch it lowers the pitch. When I release the Mono switch, sometimes the oscillation has disappeared - until I depress Mono again. The whistle is more apparent at volume levels below maximum. It is attenuated/increased by the treble control.
The fault is not apparent on tape input which means it must be prior to the first amplification stage.
Taking Martin's advice of possible contact failure, I have reflowed any solder points I can get to round the Phono and Mono switches and the preamp board contacts, and cleaned (as far as possible) both switches with Kontakt 60 - without improvement.
Next step advice gratefully received.
Are the power supply filter cap(s) good?And you didn't you fit low-ESR caps anywhere in the output stage or power supply, right?
Hi Martin,
I have used four Nichicon PS and PW series caps in each output stage to replace old grey Roe's. I haven't touched the Philips axials as the ones I checked were mostly ok (+20%/-50%) (or maybe not?)
The power supply caps I believe to be ok and are Nichicon.
Might this instability be related to the impedance of the input? But why is there is no whistling in the left channel or on any input except Phono?
Many thanks,
I think I have resolved the problem, at least partially.
The idle current was far too low in both channels. Adjusting it to the correct setting has removed the r. channel oscillation I was getting with the Phono button depressed.
However, if I engage Mono (with Phono) the oscillation starts up for as long as the button is depressed. Pressing Mono on other inputs is fine with no unwanted noises.
In practical terms, this is unlikely to be a problem, but I would like to understand what's going on.
How does engaging Mono alter the loading of the circuit/cause oscillation on this input only?
Cheers,
It seems as though one of the phono pre-amps is unstable,ie oscillating.This is the most sensitive input in the Beomaster as it needs to amplify the very low signal from a magnetic pu.Pressing the mono button simply joins the two channels together at the output of the pre-amp,so sends the noises to both channels.Each pre-amp has it's own decoupled 24v supply.My drawing isn't very clear,but it look's like item 633/696 (160uf)I'd replace those two anyway.
In the pre-amp itself,i think that the original resistors would have been carbon rather than metal film.It can make a difference.I'd be particularly suspicious of those 180k babies in the feedback network there too!Freezer spray may provoke/resolve the problem,giving you a clue as to where the problem lies!
There seem to have been two versions of this pre-amp,where the feedback network resistors are different values.If you have the version with 390k,I'd be even more suspicious of those!!
Keep at it sir.
Nick
Hi Nick,
Nice to hear from you again.
I'll look at the supply to the pre-amp board. Both the axial decoupling caps are easily accessible which is more than can be said for many of the others, so I'll change them and report back.
I've already changed the 180K feedback resistors on the board in an early attempt to fix the oscillation. Your point about carbon vs. metal film is noted; I was looking specifically for low noise components as they are marked Beyschlag on the schematic.
Thanks,
I changed the bypass caps on the supply to the preamp but the symptoms remain the same.
I hooked up the o'scope to the preamp output to see what happened to the waveform when I pressed the Mono button. The waveform seemed to be pretty much identical in both channels.
This picture is of the waveform without the Mono button depressed and seems to be the normal sort of noise you hear with no input and volume at max.
And this is what happens when the Mono button is depressed and the whistling starts up. Same in both channels.
Freeze spray didn't help me locate the fault, just created a lot of pops and crackles til it evaporated.
My suspicion is that the fault is in the preamp, unless instability further down the road is somehow feeding back into the preamp which is what Martin suggests.
Curious indeed!
Is the grounding of the pre-amp secure.The connectors that the pcb fits into will be oxidised with age I guess,but you should have no resistance between the ground pins and chassis,or the main pcb signal ground.
What amplitude is that waveform shown on your 'scope?
Any ripple on the two 24v supplies?
Just out of interest,what happens if you actually connect a turntable inc pu to the record player input.
Martin may be onto something regarding feedback from the main amp,and it might be worth checking the main power supply cap's.Is the 3000uf one of those grey ones?
When you say that the treble control affects the whistling,does it change the pitch or simply make it louder?
There are a couple of intermediate supply decouplers too,it looks like 566 and 717 (100uf)
I can't tell from the drawing,but normally good quality design's like this would have careful routing of signal wires and particularly ground's.Does it all look "original"?
Thanks for taking the trouble.
Is the grounding of the pre-amp secure?: Yes it is secure and there is no oxidation. However, there is a ground noise trace on the scope when I press the Mono button, measuring between the preamp ground and chassis ground elsewhere. (Picture below)
What amplitude is that waveform shown on your 'scope? 5mV pp and about 5.5kHz whistle
Any ripple on the two 24v supplies: Not that I can see. Main filter cap has been changed and the two output caps as well
Treble control makes it louder/more trebly
I already changed the two intermediate bypass caps in the supply to the board.
(I will try connecting a turntable later, but it involves some furniture moving).
In the meantime, any thoughts on that ground noise? Not convinced that the routing of signal wires/grounds looks particularly careful, more like a dog's dinner.
Could my using a very few low impedance caps in early audio stages be contributing to this? Should I look to replace them with general purpose ones?
That's certainly a "singing" Beomaster Simon!
Unless the cabling looks as though the "phantom bodger" has been in there,I think that you can assume that B&O would have carefully designed the earthing to avoid loop's and wotnot.It's just that any common grounding points might have become oxidised?
I wonder about the phono pre-amp itself again?General rule is to replace components with those as close in spec to the originals.If eg 'tant's were used,use again,etc.
It's significant that B&O changed the design of the pre-amp to overcome radio breakthrough,which suggests that the pre-amp was found to be a bit too sensitive to frequencies outside it's normal range.
If you are using the manual from this site,you will see the layout of the later pre-amp at the end of the manual in the "mod's" section.
I can't see the layout very clearly,but there is a capacitor (390pf ,probably ceramic?)between base and emitter of the first transistor.This is an rf filter,and it would be well worth trying one on your "singing" pre-amp.If it does the trick,fit one to the other channel also.
Rf breakthrough was a problem in the area where I worked during the 'seventies.Our Dealership was close to the main Sutton Coldfield(Birmingham)transmitter.We often got complaints from customers that they could hear tv programmes while listening to their record's!!!
B&O almost alway's had solutions to problems in those day's and were keen to hear from Dealer's and pool resources.Official "mod" sheets were then sent out to all other Dealer's.
Happy day's!
Well, I think I'm getting closer.
The oscillation problem I described earlier I thought might have originated in the pre-amp. Martin suggested that I might have caused instability further up the chain by using low ESR caps.
The problem, I now think, relates to the switching of the stereo light. When the stereo light is on, there is no oscillation. At the precise moment the stereo light goes off - or the Mono button is pressed - the oscillation occurs.
It is also just audible with the volume set to zero when an FM station is detuned, which is the point at which the stereo light goes out.
On the Multiplex board, the stereo indicator is switched by a BC113 and a BC140. There are a couple of 4,7uF bypass caps and an associated 47K resistor.
My question is: is it possible that either of these transistors is generating noise when the stereo lamp is in the off condition, and this noise is not being grounded by the associated caps?
The plot thickens Simon?
My circuit is not very clear,but it seems as though the mono switch simply disconnects the stereo lamp,and joins the left and right channels together.The output's from the stereo decoder and the phono preamp are switched by the phono selector though,so they are close together there.
The stereo decoder is still powered in phono mode,although it won't be getting a mpx signal from the tuner,so it should be inactive.
As you suggest,it could be unstable,as it shouldn't be producing anything in the phono mode.
Is it's 12v supply ok(check across the ZF12 Zener diode) on the decoder pcb ok,or the decoupling cap across it (10uf)
The 5uf decoupling cap(39)might be worth investigating too,otherwise,the frequency of the signal you are seeing on your scope might provide further clues?
This is shaping up to be another one for the gremlin book?
Krolroger: My question is: is it possible that either of these transistors is generating noise when the stereo lamp is in the off condition, and this noise is not being grounded by the associated caps?
No, that seems unlikely. Those transistors only switch the stereo bulb and that shouldn't directly impact the decoded FM signal.
I read this thread twice and I'm still confused. You seem to be jumping around with your conclusions quite a bit without actually trying to correlate those conclusions and without trying to isolate the source of the problem.As far as I can see from the circuit diagram (and that's not that easy with the way B&O has coded those countless buttons in the diagram), the pre-amp is ONLY active in phono mode, and the FM signal - which is the only signal affected by stereo decoding, I think - is connected directly to the tone control amp.So, I was asking myself: where's the difference between Mono ON/OFF. All the mono switch does is shorting the left and right input of the tone control amp. The effect of that is that you're doubling the input signal to the tone control amp. If everything is fine that shouldn't be a problem, but if there is a flaw in the amplifier section then this could be the cause of your distortions.Since you have an oscilloscope, you can easily compare the left and right signal in the amplifier signal path after each transistor. By switching mono ON and OFF and comparing the results between left and right channel, you should be able to identify the amplifier stage that is the source of the problem.
rgds,
manfy
[cross-posted with Nick]
Hi Nick and Manfy, and thank you for your helpful comments.
It's certainly true that I'm jumping around a bit here, but I should just clarify why I think the MPX board might play a part in this.
If I tune into an FM station so that stereo light goes on, and I then reduce the volume to zero and detune the station so the lamp extinguishes, the oscillation becomes faintly audible over both speakers. This happens at the precise moment the lamp goes out. This is with the Mono button disengaged.
The single condition that is common to the oscillation on FM, Phono and (barely audibly) on Tape inputs, is for the Stereo lamp to be extinguished. If the lamp is on, there is no oscillation. If the Mono button is switched in on Phono and Tape inputs, the lamp goes out and the oscillation starts. (Mono does not extinguish the lamp on FM input)
So, for this reason I suspect the problem might lie in the MPX board.
I'll pull it out and report back.
Cheers.
Have you ever checked and adjusted the 30V supply?
I'm asking this because there's another effect happening when the stereo light is on. It adds a load to the 30V supply. Selecting phono or tape switches the light on; pressing mono switches it back off. If the 30V regulating transistors don't work properly, this could have an impact on the supply voltage. I noticed that the BC114, which is used in the amplifier is specified at max 25V Vce and that's close to the actual voltage at some transistors. If the supply voltage is exceeding the nominal 30V it might affect those transistors. Or maybe it happened in the past and that's why one of them is not in perfect working condition. That's just a guess, but it's certainly possible.I'd monitor the 30V supply with the scope, hoping to find a clue.
Hi,
Yes, 30v supply is bang on as is the idle current. I have replaced the main filter cap.
I pulled the MPX board.
There are a number of 4,7uF tants in there. This one, C40, which should ground spurious noise at the base of BC113 (??), doesn't look too happy to me. Maybe it's still in spec, but I have a hunch.
I'll replace all the tants and see what happens.
Krolroger: This one, C40, which should ground spurious noise at the base of BC113 (??), doesn't look too happy to me. Maybe it's still in spec, but I have a hunch.
This one, C40, which should ground spurious noise at the base of BC113 (??), doesn't look too happy to me. Maybe it's still in spec, but I have a hunch.
Your hunch in God's ears... BC113 and BC140 only act as switches, and C41 at the base of BC113 is a smoothing capacitor similar in function to the main filter caps after the rectifier in any AC/DC power supply. C41 removes the ripple from the rectified 19kHz stereo pilot tone. It's more likely that the change of the AC coupling tantalums which are directly in the signal path on the stereo decoder has more effect on the tuner output signal.My money is still on a cap or transistor in the amplifier stage as the source of your noise.
Hi Manfy,
Well, you're not wrong. Changing out all the tants on the MPX board (some were a bit resistive) has made no difference to my oscillation on Mono, but was worth doing anyway.
As faults go, this is pretty insignificant, but I'll dig out the 'scope and try to figure out what it's telling me.
I'll have a look at transistor outputs along the amplifier chain.
Krolroger: As faults go, this is pretty insignificant, but I'll dig out the 'scope and try to figure out what it's telling me.
The internet says that your Hitachi scope is a 20MHz dual channel unit and that should work nicely for watching the same testpoints on each cannel simultaneously and by switching back and forth between mono and stereo, you should be able to narrow it down which amplifier stage needs closer attention.
Well, I got the scope out and the audible oscillation is visible on both channels (photo) when Mono is depressed, even as far back as before the tone network (C627 and C690)
Martin queried the power supply so I put the scope on the 30v rail. When Mono is depressed, this waveform is visible on the rail.
When Mono is not engaged, the power supply looks relatively clean to my untrained eye.
I've replaced the main filter cap and the film caps across the rectifier legs, but I'm just wondering if the rectifier might be behaving strangely when loaded differently?
When Mono is not engaged, the 30v rail looks relatively clean to my untrained eye.
Guidance much appreciated, as always.
Hi Simon,
whenever you post such sope pictures you should give us also the scope settings so we can make sense of it.
From the first picture I see you have 0.5ms/div and 4 waves per division, i.e. that's an 8kHz ripple that shouldn't be there. I can't make out the vertical resolution. Anyway, this is not the primary fault you should look at.
With Mono OFF you have a 2.5mV peak to peak noise on the 30V rail. That looks reasonably normal - it could be noise from the probe or setup. With Mono ON, however, you have a 60mV peak signal and that shouldn't be there. This is not "normal" noise; it shows a pattern. You should increase horizontal resolution to visualize the actual signal. This signal on the 30V rail is the primary problem you should look at. Once this is solved it might resolve the problem on the signal line too!
Where did you connect the GND lead of the scope? To the chassis? Or a clearly defined GND point at the board? Is it power GND or signal GND? This is an important thing that needs to be clarified first. Try connecting scope GND to different GND points in your system and see if and how the scope signal changes!
rgds, manfy.
Well, you live and you learn and that's what great about this forum, so my thanks to Manfy, Nick and Martin. Martin was on the money about this two months ago.
Looking at the waveform on the power rail, it was obvious even to me that all was not well, though I suspected the rectifier at first. The 8kHz ripple that Manfy identified was what I was hearing as audible oscillation. (In answer to Manfy's question, I was using a clearly defined GND point on the board at the Power Supply)
I squirted some freeze spray around the power supply and when I cooled the 400uF cap (909) which I had replaced with a Nichicon 390uF PW(M) series component, the oscillation stopped.
I thought I'd parallel another cap with my replacement to increase the capacitance, but that didn't work, so the oscillation must have been a function of low ESR. (What Martin said).
I still had the old cap which measured ok so I reinstalled it (Big blue one in the photo). Problem solved
If I'd had a general purpose cap when I replaced the original, I'd have used it, but it's quite hard to get hold of components that aren't described as low impedance.
Thanks, guys.
Congratz on this successful fix! Enjoy your BM1400.
Yes indeed,well sorted sir!
The information is recorded in my gremlin book for future reference,although I'm still puzzled by the effect that this faulty item was causing.
Keep on choogling!
It's when components get "to eager", that things can run astray.These older B&O things were built with the contemporary components in mind. - Not components produced 50+ years into the future.Basically, when talking about AC-signals (programme sound etc.), a positive power supply rail is just as good a ground as the negative.This is mainly because of the low output impedance of the power supply, which (when we talk about conventional power supplies) typicallyhas a huge filter capacitor sitting right across the supply, from positive to negative. It filters out all 50Hz, 100HZ etc. ripple but any other AC-signal,even very low frequency ones - and particularly higher ones - will pass through this capacitor as if it was nothing but a mere short.The original (conventional if you prefer) filter capacitor had a bit of series resistance, which is also "seen" by AC-signals, preventing themfrom passing through unhindered.A low-ESR capacitor has far (multitudes!) less resistance.When the supply line is suddenly a "just as good ground" as the real ground, it can cause circuits to interfere with eachother through the power rail, down to ground, - back and forth and round and round.Add a little resistance and it will calm down.
Not everything modern is per definition better than older or conventional.It can get to perfect.
Good job!
Martin, many thanks for that comprehensive reply; much more informative than Google.
I installed a new grain of wheat bulb (from Martin) in the tuning slider. The plastic runners on this had disintegrated, so I replaced them with some 2 mm id nylon tubing tube epoxied on. The tuning slider is glued to the dial cord.
I redid the stereo lamp wiring in 30 awg silicone flex for ease of movement.
And a completely shameless but entirely reversible hack..
To get the tension on the dial cord right calls for a loop of length 160 cm.
This needs nimbler fingers than mine so I came up with this idler wheel arrangement.
It was high time to wrap this project up, so I re-checked the rail voltage and the idle current.
The service sheet calls for the tuning voltage to be set at 3,4v with the varicap at max capacitance, but there is a modification set out towards the end of it that it is easy to miss.
There are two different tuners used on this set which can be distinguished by the placement on the scale of 104 MHz relative to 16m. In my case, tuner 8050034 calls for a tuning voltage at minimum frequency of 4,4v, so I set it thus.
There is still some error between the dial pointer position and indicated frequency which indicates the need for some alignment. (Way beyond my pay grade).
The mono/stereo switching is adjusted by means of the stereo poti on the multiplex board so that the base voltage of the BC140 is 0,4v when tuned to a mono station. I had to back it off a hair to prevent the light from remaining dimly on when mono was depressed on other inputs.
So, all back together..
And within 30 minutes the (new) dial pointer lamp blew.
This is a grain of rice lamp glued into the pointer that runs off a 6VAC tap on the transformer.
I thought I would try an LED replacement as I had a few warm white 3mm LEDS. Installing it was not particularly easy as the legs had to be cut back almost to the LED body and flexible wires soldered on to avoid shorting on the aluminium chassis as the pointer moves across the scale.
I made up a rectifier board with four signal diodes which is glued in place.