Beocord 5000 (Type 4715) Capstan Motor Speed

[Krolroger]

Hello All,

I posted here previously about the motor run cap in this player. I put it back into use with the original cap but was still having some capstan motor speed issues so I've pulled it apart again to have a look at the motor servo board. (In fact I have now put in a couple of back to back elkos which seem to be fine).

The problem is that in Play mode I can't adjust the capstan speed to bring it to pitch. In Stop mode, I can tweak poti R13 to achieve the specified 500Hz signal from the capstan flywheel inductor but in Play mode the signal falls to around 475Hz and no amount of adjustment will bring it back up. This means it plays about a semitone flat / 5% slow.

When I invoke Stop, the signal will rise again to 500Hz or more depending on where R13 was set.

Rail voltages are correct (-11 and +12) and in Stop mode specified voltages and waveforms seem to be pretty much on the money. All caps have been replaced, diodes and resistors checked. In addition, I have changed power transistor TR2 which acts as a resistor in series with the motor and the bridge rectifier D3.

Would I be right in thinking that the 8 pin op-amp (MC1458CP) is now the prime suspect as the feedback loop appears not to be working? Easy enough to replace, but I would greatly appreciate any input.

Thanks again!

[TonyDuell]

Try shorting 4TR2 collector-emitter in play mode. The motor should then try to run much too fast -- does it? If not, it doesn't matter what the speed control circuit does, the thing will run slow.

If it won't run fast enough, check for too much mechanical load, the motor run capacitor, the bridge rectifier 4D3,etc.

[Krolroger]

Thanks for that, Tony.

Shorting TR2 C and E in Play mode makes no difference to capstan speed. I am seeing only 0.08V drop across C and E in Play, as opposed to 9V in Stop. Does this suggest motor is already running flat out?

Run cap and bridge rectifier have been replaced so I'm not inclined to suspect those. Ditto belts from Denmark.

[TonyDuell]

Yes,sounds lke the speed control circuit is trying to do the right thing. It's sensing the motor is running too slowly, so turning TR2 on as hard as it can. Alas the motor can't run fast enough due to some other fault.

Where does the motor supply come from (the 2 connections going off to the right from the motor)? A secondary winding on the mains transfomer? I assume the voltage from that is not dropping significantly under load.

My guess at this point is ether excessive friction somewhere in the mechanism or a fault in the motor windings.

[Michael Maurice]

With the machine switched off try to rotate the motor by hand

1) with belt removed

2) with the belt fitted

3) with the belt fitted and the when tape heads pushed up so the pinch engages with the capstan.

Does the motor rotate freely?

Is there much difference in the force needed to rotate the motor in all three modes, if there is then you've found your fault?

[Krolroger]

Thank you chaps.

The motor has its own 27V winding on the transformer. (Schematic attached).

Have to say I am slightly puzzled by some of the motor side voltage readings I am getting, particularly the 15 and 20V~ across the run cap, which are way off. (I take it these voltages are not referenced to ground).

When I changed the belts, the motor was free running. I will be able to get a sense of whether there is a mechanical glitch by rotating the flywheel.

I will check for voltage drops etc and report back.

Thanks.

[Jan Zodiac]

Hello,

I had a few of these Beocord's over time, and they all suffered the same problem except one, that had the motor replaced. That motor did not look quite like the original, but it was the correct replacement part. I assume the original motors were not very reliable, so they switched to another supplier. They were used in several other types of their cassette recorders back then. A lot developed problems with speed. I know this does not fix the problem, but I recall that technicians talked about it as being common for these motors to fail.

Regards,

Jan

[Krolroger]

Thanks Jan. The motor runs quietly and the flywheel turns easily so at this stage I will assume the motor is ok.

I measured some of the AC voltages prior to the motor and am perplexed by them.

Instead of the specified 15V or 20V (Stop and Play modes), I am getting 27V~ and 38V~ across the motor run cap.

Across the dedicated 27V transformer winding, I am seeing 35V~ dropping to 33V~ in Play.

(The voltage carousel is set to 240VAC, btw!)

Is there some transformer failure mode that could give rise to these elevated voltages or could the servo be pushing DC back into the motor windings?

Can I disable the servo by temporarily removing TR2?

Thanks.

[Jan Zodiac]

I am still pretty sure the motor is the problem here. It has nothing to do with the motor/flywheel not turning freely. By shorting C-E on TR2, you take the servo out of function, and it should run significantly faster. Since it was decades ago I worked on those, I don't remember the voltages. As far as I remember the problem was either the motor windings or the rotor (short circuit copper rods developing bad contacts). It is a small asynchronous if I remember correctly.
The symptoms you descripe with a slow running/weak motor, was the exact same symptoms as on the ones I had with bad motors. Once they were replaced, all worked as supposed to. This still does not help you a lot, but if you short the transistor as mentioned, the motor should run faster. I assume the run capacitor has the correct value?

Regards,

Jan

[Jan Zodiac]

Oh, and what about the 2,2R resistor that connects to ground below TR2? Does it have the correct value?

[Michael Maurice]

No. If you remove TR2 the motor will not run at all.

Have you replaced the motor run capacitor? If not, I suggest you do so. Use one rated at 250V ac or higher.

[Krolroger]

Michael and Jan,

Thanks for your helpful comments.

Shorting TR2 C and E makes no difference to motor speed.

2R2 to ground is correct value.

I replaced the run cap some time ago with two back to back elkos (33uF/250V)

Any ideas about my elevated AC voltages?

My next move may be to strip the motor.

Simon

[Michael Maurice]

Electrolytics are totally unsuitable in that position.

Try this or this

[TonyDuell]

The symbol for the motor run capacitor (8C1) on the diagram posted is that of a non-polarised electrolytic. Normally such a capacitor can be replaced by 2 normal electrolytics of twice the value in inverse series.

[Michael Maurice]

We will have to agree to disagree. That capacitor was designed to take a large AC current which two standard electrolytics were not designed for.

I again suggest you replace it with a proper motor run capacitor.

[Jan Zodiac]

I believe the original run capacitor was a standard Wicon bipolar electrolytic, as used in crossover networks for speakers. It is a low voltage motor and there is no way a large MP run capacitor would fit inside the cabinet. I am sure the two inverse connected 33uF will do the job. At least as good as the original Wicon.
I am off work this week, and tomorrow I will visit the local radiomuseum, which I am a member of. There is usually a lot of old B&O stuff in the basement, mostly for parts. I will check if there is a similar model and if so, I will bring it home.

Regards,

Jan

[Krolroger]

Thank you all for your contributions.

I have ordered the film cap which will probably fit if I 3d print an adapter board.

(Back to back elko solution was adopted by Beolover in his 4715 restoration but it may not be optimal as Michael suggests).

https://beolover.blogspot.com/2015/0...ing-power.html

Jan, I've fixed up a couple of Beocenter 4000 (Type 1603) which have an almost identical tape deck, but the motor (looks identical) has a different part number. Though both run off a 30V~ supply and see 18V across them and use a 16uF run cap. I wonder if they are in fact interchangeable.

Interestingly, the motor doesn't seem to lack torque. If I slow down the capstan spindle, TR2 collector voltage goes towards positive which confirms that the servo is functioning.

Simon

[PJL]

TR2 is going positive because it is unable to sink the current demand from the motor - no servo action. One thing you have not tried is to run the motor directly on the AC supply although it would be safer to use a 2.2 ohm resistor in series.

[Krolroger]

Thanks for that.

TR2 collector should be approx -10V in Stop and -6V in Play mode.

I have the -10V but it's at -0.65V in Play.

As you point out, this suggests that TR2 is turned on fully to sink the motor current.

I could run the motor directly off the supply by removing the rectifier and bridging the two ac terminals with a 2R2 resistor but as it's maxxing out already, I'm not sure what it would prove except that it's drawing too much current.

[Maarten]

Originally Posted by TonyDuell

The symbol for the motor run capacitor (8C1) on the diagram posted is that of a non-polarised electrolytic. Normally such a capacitor can be replaced by 2 normal electrolytics of twice the value in inverse series.

Sometimes non-polarised electrolytics will be used as a start capacitor, but using them as run capacitor is quite unusual (which means B&O would certainly do so) because it would become a relatively fast wearing part. Substituting them with a 'close but not quite there' equivalent circuit could lead to premature wear of the capacitors if they're not selected very carefully since they're effectively used as electrolytic rectifiers half of the time.

A large high voltage run capacitor would not be needed, maybe a few 63v or 160V MKP or FKP capacitors in parallel? 3 times 4,7u and one 2,2u would make up 16,3u which would be within tolerance if the original was 16u.