The Workbench

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Mon, 11 Jan 2010 08:42:22 GMT

It's been a couple of days of extended play, with no problems - the cause was definitely the Motor PCB's capacitor, which, I would hazard a guess, was stressed by the motor shaft occasionally seizing. Oiling the shaft with sewing machine oil took care of the stress problem, and new caps should ensure good current, without backpedalling to overload the 10ohm fusible resistor.

Just one other very nice point to close up this repair - When I did the restoration, I replaced all caps with my standard fare of Elna, Nichicon and Panasonic units. But I wasn't happy with the sound - it was too muted. It was good quality, with lots of presence, but something was lacking.

I went back and had another look at the circuit diagram. I was looking at the signal path, and around the LF353N dual opamp in particular, and its input and output paths. At the inputs I had originally installed 10uF 16V Elna Silmic II audiophile electrolytics (C208, C408). I immediately thought about 10uF Wima MKS2's which I don't have, and their $5 price tag (in singles). Then I thought - hey, I've got plenty of stock of Wima MKS2 4.7uF, with far superior characteristics than a 10uF audiophile electrolytic. Why not try it!

Wow! it's like being at a live performance. dB's are way up, and the presence is overwhelming. This is a cassette deck, not a SACD!!!!

Just goes to prove - numbers aren't everything, and quality is everything. The 4.7uF Wima polyester blows away the 10uF Elna audiophile electrolytic.

Caveat - I'm talking about the signal path here, not about coupling/decoupling power rails. Electrolytics are better on the power side, especially on modern SMPS's. But for the signal path, those Wima MKS2's are champs.

Menahem

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

lausvi — Sun, 10 Jan 2010 06:10:14 GMT

This sounds very similar to my experience with my Beocord 2000. I got it from eBay as non-working. After I replaced the belts (thanks Martin) it worked for a awhile, then the motor went dead. After Martin's suggestion I replaced a motor driver resistor on the mainboard and it worked again, but the speed was much higher than normal. I could adjust it from the motor but it was very heat sensitive, first 10 min it was fine but then it started to go faster. I replaced the caps on the motor control board inside the casing but accidently broke the board causing the motor to run without speed control at all. Martin sent me a replacement motor unit and since then the BC2000 has worked perfectly.

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Fri, 08 Jan 2010 07:41:10 GMT

Those transistors don't just fail willy-nilly, they fail because of poor current - guess from where. Right! Those damn electrolytic caps.

Check them (and probably replace them) before you waste your hard-earned gold on new transistors.

Some cassette motors have internal PCB's and some don't. On my BC2200, the control PCB is external, but even so, the Service Manual does not make any reference to it, nor circuit diagrams. They are generally factory adjusted as a single unit from the motor manufacturer, and B&O apparently takes the approach that anyone who is not familiar with them, should not be fiddling with them, and so they provide no info on them.

Actually, they're very simple circuits, and replacing the caps generally solves the problems.

You'll need a solder-sucker to remove the solder blobs securing the PCB to the motor tabs.

Menahem

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Stoney3K — Fri, 08 Jan 2010 07:35:11 GMT

Hey, I never knew those motors to have a control PCB inside them - I figured it was just a DC motor, as the Beocord 2000 service manual doesn't show any relevant stuff around it.

I have a spare BC2000 which (presumably) had a shot motor, the power supply comes up nicely and the CPU works without a hitch, but the motor doesn't spin, rendering the deck useless. The motor has the power supply voltage across it, but no current, so I ultimately dismantled the deck and put all of the stuff in my spares bin.

If the problem 'happens' to be the speed regulator PCB (I wouldn't be surprised if the transistor was kaputt), I can fix this deck up nicely.

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Fri, 08 Jan 2010 07:12:33 GMT

Now, I wanted to see the current draw.

Before I reassembled it, I sprayed machine oil into the shaft; just turning the shaft by hand, I could feel much easier rotation.

Now this is measured connected to the battery, but I was more interested in seeing stability of current draw, rather than an actual quantity of AC mA.

So, it came up immediately to 16.4mA, and didn't budge - that stability is what I was looking for.

Then after reassembly, I had to readjust the tape-speed. It's now been running for over an hour with no problems.....

BTW, the motor model is a XMI-6B2ML, I think Mitsubishi - with the 3 pointed emblem, but I've had similar Sankyo motors like this as well.

Could the Tray motor be identical to the internal Cassette motor, but without the control PCB and external casing?

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Fri, 08 Jan 2010 07:01:00 GMT

It's not in series with the motor, but between 2 power transistors - the in voltage is 23.8V, the out voltage is 23.0V.

As I am not in the habit of giving up, I cleared my lab table, and got to work disassembling the motor. It was not an easy task to remove the metal cover. I've done a good number of these, and I've broken the PCB tab with the power leads, so this time I was sooooo careful, especially when I know that I may not be able to get a spare.

On the motor's control PCB, there were 2 electrolytic caps - 4.7uF and 0.22uF. The 4.7uF was OK, the 0.22 was well-finished with a stupidly high ESR of 25. I decided to replace both anyway for good measure, and not relishing the thought of opening this up again.

I put in a Mylar 0.22uF, for which I had to move an 82ohm resistor to the obverse side of the PCB, to make space for the cap.

Here is the refurb'ed PCB.

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Stoney3K — Fri, 08 Jan 2010 06:35:22 GMT

Dillen:

It's not a common thing to see the fusing resistor blow but it's a common thing
to see the fusing resistor blow when something is not right.
That's what it was designed for.
The motor rarely shorts, but I suppose it could happen. Have you tried desoldering
one lead from the motor and put in an amperemeter in series and measure a bit ?

If the 10Ohms resistor is in series with the motor, you can also put a voltmeter across it, it will show 10 mV of reading for each mA of current.

(Consequently, if it's a 1/3W resistor, it will blow when the current exceeds ~6mA, with the resistor dissipating 360mW of power)

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Dillen — Thu, 07 Jan 2010 12:06:26 GMT

Yes, the cure must be to mount a higher rating of fuse...
NOT !
That would risk something else to blow instead or possibly set the whole thing ablaze.

It's not a common thing to see the fusing resistor blow but it's a common thing
to see the fusing resistor blow when something is not right.
That's what it was designed for.
The motor rarely shorts, but I suppose it could happen. Have you tried desoldering
one lead from the motor and put in an amperemeter in series and measure a bit ?

I don't have the exact motor specs but I do have a couple of factory new
replacements of the last brand and type of motor that B&O supplied.
But if the motor runs fine from a 12V supply, I don't think that's where the
problem lies.

Martin

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Thu, 07 Jan 2010 11:56:26 GMT

Any idea of the specs for that motor - new Mitsubishi cassette motors are pretty widely available as spares here.

Or if there are any spares floating around at your place?

And, what about using a fusible resistor of a higher wattage - Digikey has 1W and 2W versions available? Maybe 1/4W - 1/3W is just too low, if this is such a common failure.

Menahem

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Dillen — Thu, 07 Jan 2010 11:33:39 GMT

No, they are two very different motors.
The tray motor does not have any speed regulation and was made for rotating both ways.

Martin

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Thu, 07 Jan 2010 09:54:20 GMT

Well, it didn't last long - I'm beginning to think that there's a problem in the motor - brushes or something.

I tried prying off the motor cover, as I've done many times before with smaller cassette decks, but this one doesn't budge.

Do you know if it's possible to swap the tray motor with the cassette motor?

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Thu, 07 Jan 2010 08:40:40 GMT

I checked and rechecked the circuit, and I couldn't see anything electronically.

I think you're probably right that there was a physical load.

As far as I could see, the only thing was that the flat ribbon cables going to the LED-level indicators were passing on top of the flywheel, and touching it. It could have caused friction. I rerouted the cables, and it seems to be working OK.

Anyway, I'll do a little check from now on, before inserting a tape.

I'll press the Play button, and if I hear the motor/flywheel start rotation, then I'll know it's OK to insert a tape (yes, the rotation is very quiet, but audible in a very quiet room). If I don't hear the motor, then I know there's a problem.

Thanks for your help!

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Dillen — Thu, 07 Jan 2010 07:35:15 GMT

Hard to point a finger directly at something specific but definitely an overload of some kind.
I suppose you will have to measure around a bit.

Martin

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

yachadm — Thu, 07 Jan 2010 07:07:51 GMT

Well, we know that the belts are new, and I ran the test of the motor with the battery power.

I checked the strain-release pillars - all have their plastic shields, and there is no damage to the cables.

I put in the new 10ohm fusible, connected power, pressed Play to get the front display on, which it did. Then I put in a tape, to make the Tape-sense contact switch close, and pressed play again. Only a click of the relay, and then the smell of smoke - the new 10ohm resistor had blown again.

So, there's possibly something when the Contact switch is closed with the Play button pressed, blowing the resistor.

Menahem

Re: BeoCord 5000 (1980's) not responding correctly to control buttons

Dillen — Thu, 07 Jan 2010 06:57:27 GMT

The 10 Ohm resistor usually blows because the old belts in the tapedeck seizes up the motor or something else blocks it.
Could also be a fault in the motor itself but the belts are by far the most common reason.

Another reason can be the flexible ribbon cables that run from the power supply to the tapedeck drawer. If one of
the black insulation tubes in the strain relief arrangement is missing, the cable will wear through to the copper and short circuit
to the metal.

Martin