BeoGram

Re: Beogram 6000 Quad 33/45 Speed problem

tournedos — Sun, 09 Mar 2008 09:46:13 GMT

yachadm:
Also, I have never seen this large IL1 bulb on this Control PCB illuminate - what is its function, and when should it illuminate? It tests OK, so it is not burnt out.

It's an old trick to stabilize a Wien bridge oscillator and reduce its distortion. The bulb is used as a temperature / current dependent resistor at a low current and therefore doesn't illuminate (as a side serving of trivia, this circuit with a bulb was first used by Mssrs. Hewlett and Packard in their first product, a precision sine wave generator).

In any case, if your turntable rotates at a correct and stable speed once you manage to select it, I think you can forget this part of the circuit (right of R8), it's working. It's just an oscillator with two preset frequencies and a power amp to run the motor, with no feedback to the rest of the circuitry.

I believe the real problem will be found in / around the flip-flop which is drawn under the selector switches in the schematic. It should stay at the state it begins with (other test point high, other test point low), unless "disturbed" by the switches or the automatic selection (TR2).

-mika

Edit: I see you already solved this (I was reading new posts in reverse order). Congratulations, and thanks for making a summary of all this!

Re: Beogram 6000 Quad 33/45 Speed problem

yachadm — Sat, 08 Mar 2008 15:24:19 GMT

Hi Mika,

You are correct that I replaced the bulbs with LED's.

However, this did not cause the problem, but actually identified a pre-existing problem.

Here's how:

When I bought the BG6000 last year, the 33 indicator light never illuminated - I thought it was burnt out (when I put in the LED's, I looked closely, and saw that it was actually not burnt out - just one lead not soldered!!!).

The 45 light did work, and it illuminated at all the correct times, so, the speed switching worked fine.

Because of the heat damage on the white plastic, I never got around to replacing the 33 bulb, intending to put in LED's at some stage.

Now, when I put in the LED's, and therefore completing the 33 circuit, the problem showed itself.

Now I measured the voltage on both sides of the bulbs - 22.1V on the power supply side, and 17V on the Flip-Flop Circuit side.

My first thought was - yes the LED's have caused the problem - the LED is taking too much voltage - 5V , so I must go back to regular bulbs.

So I took out the LED's and resistor, and soldered in both bulbs again.

I pressed the START button, and the problem still existed as above - 33 for a split second, and then remains at 45.

The only difference is that now there is 21.8V on the flip-flop circuit instead of 17V.

So I figured, OK, let's disconnect the 33 bulb (like when I bought the BG6000), and see what happens.

With the 33 bulb disconnected, everything works as it should!!!!

So there is a problem in the 33 circuit, whether or not there are LED's!

It now appears that this problem existed before I bought the BG6000, and a previous technician found this novel solution to solve it - just disconnect the 33 bulb.

D5, C3 (new Wima MKS2), TR2 all OK.

So, any ideas here?

Also, I have never seen this large IL1 bulb on this Control PCB illuminate - what is its function, and when should it illuminate? It tests OK, so it is not burnt out.

I will go back to the LED's again, probably on a modified circuit (to not take so much voltage), but not until I have this speed problem solved!

Regards

Menahem

Re: Beogram 6000 Quad 33/45 Speed problem

tournedos — Sat, 08 Mar 2008 06:27:08 GMT

I vaguely recall that you replaced the speed indicator light bulbs with LEDs sometime earlier? Rethink / recheck that area. The flip flop circuit which controls the speed selectors looks like it will be sensitive to correct load by the bulbs.

Other than that, I can't see other solution than start measuring around, perhaps around D5 / C3 / TR2... It's yet another RC integrator / delay which might not be operating properly.

-mika

Beogram 6000 Quad 33/45 Speed problem

yachadm — Fri, 07 Mar 2008 06:44:32 GMT

Hi all,

This is a never ending adventure!

Just when I think I've taken care of everything, something new comes up.

When I press START <<, The 33 indicator light (on the left of the red 4ch light), illuminates for a split second, and then immediately, the 45 indicator light illuminates, and remains illuminated (with platter speed 45, as indicated by the light).

This is while the tone arm is still moving to the left slowly over the flat aluminium plate, BEFORE the tone arm even reaches the edge of the platter, let alone the LP.

I can press the 33 button to reduce the speed to 33, instead of 45.

Any ideas?