Hi Alex

I have never worked on the BG4000 but recently overhauled the BG6000 which does appear to be fairly similar in many respects.

I am not sure whether you have access to the service manual but I presume so because of your knowledge of the relevant transistors.  0TR4 provides the main switching capability for the solenoid.  When the BG6000 is in the off state, the collector reads around 25 to 30V and the emitter is grounded.  If you haven’t got this high voltage on the collector then your problem is back at the power supply but since the turntable is working normally, I think that this is not going to be the problem.

If you have 25 to 30V at 0TR4c, then this transistor is not being activated properly or it has failed.  Again, in the off state, the base of 0TR4 should be grounded. It rises to ±0.7V when it gets the signal to lower the arm.  Check out these basic functions and the results should start to point to your problem area.

I agree with your curiosity on that bulb, I wondered what it was for as well.  Perhaps one of the BG4000 experts will provide us with a good answer and shed more light on your solenoid problem as well.

By the way, welcome to Beoworld and have lots of fun.

Regards

Geoff

Hi Alex:

This is a crap shoot... but I had exact same problem on a BG4002 that I repaired.

Follow the solenoid to the transistor. Along the control signal path there is a zener diode the feeds the transistor. I don't have my notes handy (I'm thinking 24 volts but I don't remember!). This diode was bad in my unit.

Perform a diode test. If this is your problem, send me a note and I'll look up the correct value for you.

Derek

geearr wrote:

I agree with your curiosity on that bulb, I wondered what it was for as well.  Perhaps one of the BG4000 experts will provide us with a good answer and shed more light on your solenoid problem as well.

It belongs to the Wien bridge sine wave oscillator that generates the driving signal to the platter AC motor. It is not supposed to light up, it operates as a kind of current / temperature dependent resistor and stablizes the frequency. This is an old trick that was first commercially used by certain gentlemen Hewlett and Packard in their first product, a precision sine wave generator.

Oops!  I'm a little embarrassed here folks as it appears I do have some voltage readings at some of these points.  I rechecked them again this morning and I can only suspect that my meter probably didn't have a good ground connection.  Thanks for your help so far guys, it has given me an idea where to start looking.  Geoff, I have the circuit diagram that I acquired with the unit - I don't know if this is the full service manual or whether that contains any additional information.

This is what I have with the arm cued and ready to lower:

0TR4 emitter  0v

         base      6v approx

         collector 43v

solenoid has 43v at both terminals

1TR11   emitter   6v

             base      6v

             collector 6v

1TR10   emitter   0v

             base      0v

             collector 6v

The fact all voltages across 1TR11 are 6v brings me to resistor 1R28 that I am about to remove and check for open circuit.  The other side of this resistor is 0v - in fact this is the point where I am grounding my meter.  Please let me know if you think my diagnosis so far is correct.

Result!  Removed 1R28 only to find the value was fine around 150ohms.  Replaced it back into the board and now it works!  Must have been a dry solder joint though it looked ok before I removed it.

Just for the record the voltage at the base of 0TR4 when activated is a little over 3v.  The voltage at the solenoid is 31v and 34v referenced to ground, so it only needs 3v to work, but it is possible that it needs a higher peak voltage to draw it in.

Now I'm going off to google Wein bridge sine wave oscillators...

Thanks for your help guys!  I guess this was a pretty basic fault, but without your help and knowing roughly what voltage readings should be I'd have been p***ing in the wind.

Now onto the next project...I've got my eye on a faulty BG2200 on ebay...not in the same league as the 4000 but...SOMEONE PLEASE STOP ME NOW!!!

alexb1406 wrote:

Now I'm going off to google Wein bridge sine wave oscillators...

You will get better results with

Thanks tournedos

I continue to learn something every day

Regards

Geoff

Hi Alex

Thanks for your voltage measurements, they have cleared a few things up.

Firstly, you have 40v on the collector of 0TR4 and both terminals of the solenoid so you don’t appear to have a power supply issue.

Second, 0TR4 has 6v on the base and 0v on the emitter and is not conducting.  Hence there is a good chance that 0TR4 is cooked. 

You measure 6v on the three terminals of 1TR11.  When the signal is sent to activate the solenoid, 1TR10 should be off, providing the 6v to the base of 1TR11.  My guess is that 1TR11 is in the correct state and 1R28 could well be OK.  I am assuming that you are measuring to the nearest volt and are not seeing the 0.7v drop that exists across the transistor.

When the arm is being instructed to drop and the solenoid is being activated, the base of 1TR10 should be low and that seems to be the situation from your measurements.  My money would be on a faulty 0TR4 at this stage.

Regards

Geoff    

Hi Geoff,

I think you may have misunderstood me.  It now works after refitting 1R28 - hence why I suspect it was a dry solder joint.  I now have a range of voltages across 1TR11 and 3v at the base of 0TR4 when the solenoid is activated - 3.37v to be precise, but I don't fully trust the accuracy of my meter so I have rounded off all the figures I provided.  Across 1TR11 I have 3.37v, 4.05v, and 5.87v again when the solenoid is activated.  When the arm is in the up position the voltages are 0v, 0.73v, and 0.08v.

Of course this is purely academic now and I am only providing these voltage readings in case it may help someone else in the future.

I have however since fixing it discovered another problem for which I may start another thread - that is once lowered, the arm cues across the record very quickly and skates across the surface.  Is this likely to be an adjustment of the arm that moves across the photoelectric sensor?  I don't want to mess with it until someone can confirm this.