I'm working on a BG6000 with AC motor.

The machine does play records perfectly in every way, except that it runs very hot.

Because it appears to do all it's supposed to, the SM troubleshooting guide appears to be useless in this case.

The voltages off the bridge (SM in brackets) are OFF 40.5 (41), ON 33.9 (35), V 23.7 (31).

Yes, when the arm is down (ie playing), voltage is leaking somwhere - 7V has run off somwhere. Within 5 minutes, the whole machine is hot to touch!

I've done a lot of troubleshooting, with no solution yet.

The main 22.8V rail is OK at all points, but R1 - I replaced it with a 2.2K 3W unit - gets HOT!

All diodes (including Zeners) appear to check OK.

All transistors also appear to check OK.

I have replaced all electrolytic and tantalum caps, and the 6 trimpots, and few out-of-spec resistors  on the main control PCB.

Unless I've missed something, as far as I can see, the components which are on board are all OK.

I have disconnected the CD4 PCB. No mods (such as LED's etc) have been done yet.

I've gone over the main PCB witha magnifying glass, looking for solder whiskers to see if there are any inadvertent connections (normally during soldering, I am extremely careful with this issue anyway), but I could not find anything.

I also double-checked trimpot R35 - the detector adjustment - it warns of a load danger, but I cannot see any voltage change when adjusting this.

The solenoid is measuring 9.2 ohms, 3.2mH, and indicates 2 yellow lights on the Blue-ring tester, so unless someomeone has another solenoid to corroborate results, it appears OK.

I concentrated troubleshooting in 2 main areas:

1. The Solenoid circuit - TR14, TR15, TR16, and 0TR4 - all the step-up voltgaes on the Circuit Diagram check out OK.

2. The detector arm circuit - TR10 11, 12, 13 is where I'm looking now.

I replaced TR11 BC337 - it was very hot, with BC639, and D10 15V Zener, but I still have only 8.4V (12V) at TR11 collector. D9 (Z6.2) checks OK - I do have 6.2V at TR11 base.

TR9 Base is 0.62V (0.7), and Collector is 2.85 (3.0). I do not get any of the scope forms for these 2 positions (or for TR10 Base (OK at 22.8V), as in the circuit diagram, rather I get a nice straight stable voltage line.

Un the picture, 2 things are important -

1. see the incandescent lamp illuminated - that's the voltage leaking only when the arm is down (When the arm is up, the lamp is not illuminated) - The BG6000 is connected via that series lamp to save fuses!

2. The scope form is of the detector arm 0PH1  - it is 0.1V, not changing voltage, only form. When the arm's down, that's the form. I'm not sure if that's what it's supposed to look like.

Any ideas where to go for this leakage?

The seller advertised this in Perfect Working condition, and according to a quick check, it was, until it's been on a few minutes and heats up!

Menahem

Hi Menahem

Since the unit only gets hot when the arm is down and the solenoid has been activated, I can only assume that it is the solenoid that is overheating.  Perhaps the switch by the side of the solenoid is remaining closed when it should be open and that would simply be an adjustment issue.  That switch must open to reduce the current through the solenoid.  Possible tests are to examine the temperature of the solenoid after it has been energised and to check the emitter voltage on 0TR4.  If the latter is close to zero, the switch is remaining closed.

I will be interested to hear how you solve this problem

Regards

Geoff

Hi Geoff,

Thanks for the tip - I wish it would be that simple. I had checked that already, and it's fine. When the arm drops, TR4 emitter goes from 0 to 4.2V.

I disconnected the TR4 collector, to remove it from the faulty rail. I pushed << and the arm moved to the set down position, and even though the solenoid could not actuate, my lamp illuminated, showing that the load is still there.

I've had some difficult ones in my time, but this ranks as one of the killers!

Think about 1R1 2.2K getting hot - something must be pulling there.

Any others with ideas, throw them at me - the worst is that I already tried it, the best is that you come up with a solution.

Menahem

Hi Menahem

I too didn’t think that the solution would be that simple but one has to start somewhere.

In my notes, I have the solenoid resistance at ±10ohm so that agrees with your measurement and the solenoid appears to be OK.

You give me the impression that everything is working as designed and the machine plays a record in the way that it should, it just overheats after a short time.  Can I assume that if the record starts playing and you press the raise button to pause the playing, the turntable will continue to operate and there is no overheating evident??

If that is the case I would conclude the following:

The detector circuit operates all of the time after the start button is pressed, whether the arm is raised or lowered.  I wouldn’t expect to find a problem there otherwise the arm would not be able to be lowered in the first place.

When the arm is lowered, the solenoid operates continually so I would check that part of the circuit thoroughly.  You seem to have done that.  I would also monitor the temperature of the solenoid to see if it was getting excessively hot.

When the arm is lowered, the slide transport detector is activated and operates intermittently thereafter.  This provides a variable 1 to 5volt signal to the forward side of the slide motor.  I would have a good look at transistors 1TR24,25,26 and 27 to ensure that there is no evidence of high current leakage around there.

When the arm is lowered, the electronic switch has some of the transistors grounded.  Check to see if any of the transistors 1TR17 to 21 are running hot?

Can’t think of anything else at the moment but it remains a fascinating problem.

Good luck

Geoff

Geoff,

"You give me the impression that everything is working as designed and the machine plays a record in the way that it should, it just overheats after a short time.  Can I assume that if the record starts playing and you press the raise button to pause the playing, the turntable will continue to operate and there is no overheating evident??"

Yes, Geoff, that is exactly the case. It's real easy to see with the series lamp connected. The lamp is off until I press the V button - the lamp illuminates - Main Rail Voltage drops to 24V (SM 31V). Press the /\ button (or any other button), the solenoid retracts, the arm rises, and the lamp goes out, the rail voltage rises to 33.9V (35V), and all is OK.

ONLY when the arm is down, does this leak occur - at no other time.

OK, I'll go back and check TR17-21, and TR24-27 again!

Menahem

OK, I've been patiently through each transistor (including the ones Geoff mentioned), and although everyone checked OK, some were tired.

Changing the following transistors produced the following results of increasing the bridge rail voltage.

TR3 and TR4 - up by 0.1V

TR5 and TR6 - up by 0.2V

TR14 and TR15 - up by 2.7V - wow

TR26 and TR31 - up by 0.1V

So, the bridge rail voltage is now up to 26.8V (SM 31V) - 4 volts is still missing, and so the AC series lamp still illuminates, but much less! It's quite dim now.

I'm going to go through each of the 1N4148 diodes now - again, they all checked fine on the multimeter, but there could be a couple of tired ones.

1R1 is quite cool now - it does not get hot at all.

The transistors which remain hot (and I've already changed them with new BC639) are:

TR7 and TR16 - very hot; TR8 and TR11 - hot. Funny though that the Circuit diagram voltages are spot-on.

Menahem

I'm done!

I checked all the diodes individually - none needed replacing.

I went back over all the voltages, and all are spot-on, except one:

TR11 Collector 9.5V (SM 12V). Well I racked my head, and couldn't find a reason for the discrepancy. In that part of the circuit, all resistors were fine, and the 2 Zeners were fine - double-checked.

Then I thought, take a look at the previous circuit diagram  - one for early serials, and one for later production. Sure enough, R43 has been changed from a 220ohm to a 270ohm. Well, if you increase resistance, voltage has to drop. B&O's graphics dept forgot to revise the 12V label, and proportionately, 9.5-9.7V would be just fine here with the bigger resistor.

It's not the first time I've found mislabeling in the circuit diagrams - I'm sure it won't be the last!

The solution to the desired 31V bridge voltage came as a natural progression of work.

I always work with the AC series lamp, always. As much as I try to be super-conscientious working around electronics with live power, once in a while, I do something dumb. That AC series lamp takes the load of any possible short, rescuing me from what could be an additional unnecessary time-consuming repair.

I figured that I had done everything possible to solve this problem, and I wanted now to see the effect of connecting the BG6000 straight to live power without the Series lamp.

So, after connecting it, I measured voltage. Yes!

ON - 41.6V; < - 35.3V; V - 31.9V

So, logically, it was the Series lamp which was eating those last 4 volts. Yes, even though at V position, the machine does eat more power, the AC motor version eats much more than the DC version.  On DC 4002 and 4004 versions I've repaired, the AC Series lamp does not illuminate at all, at V position.

In summary - replacing the above mentioned transistors played a large and measurable part - about 3V worth - (not to mention also replacing the capacitors).

Now, the BG6000 warms up after extended play, which I would class as normal, but definitely does not get hot like previously. Attention now to the rest of the machine - CD4 rebuild and alignment, and my LED mods!

Menahem

Well done Menahem.

I'm full of admiration of your patience and electronic knowledge.

Stonk:

Well done Menahem.

I'm full of admiration of your patience and electronic knowledge.

Seconded.

Since there's no emoticon available that says "I'm not worthy...": 

Thanks all of you!

My major motivation is that I can't allow myself to fail - if I can't do it, no-one else is interested in them here, and so these will end up in the bin.

Very sad, but that's the way the world goes today.

Menahem