BeoGram

Re: Beogram 6000 Quadraphonic problem

yachadm — Tue, 04 Mar 2008 05:55:11 GMT

Mika,

Here is a picture of my CD4 PCB, complete with all the ugrades.

http://forum.beoworld.org/forums/permalink/66317/66396/ShowThread.aspx#66396

Best of luck, and thanks for your help!

Menahem

Re: Beogram 6000 Quadraphonic problem

tournedos — Tue, 04 Mar 2008 05:32:18 GMT

I'll have to try the cap upgrade too. I replaced all electrolytics elsewhere to get my turntable to work correctly, but didn't touch the CD4/RIAA board. Sounds good as it is, but I'll certainly take a cheap improvement like this!

Those red drop tantalums B&O used in the 70s seem to be a problem always.

-mika

Edit: I gave a little thought to how these components in the muting circuit might have failed. I think it might be a good idea to check that the diode over the arm solenoid coil is OK. It's there to prevent the other components from reverse spikes generated by the coil. The coil driver transistor would be in greatest danger, but I suppose it's much heavier than the small-signal transistors in the muting circuit.

Re: Beogram 6000 Quadraphonic problem

yachadm — Tue, 04 Mar 2008 05:19:26 GMT

Hi Mika,

You are spot-on. It is the muting circuit on the CD4 board.

I put in the 1N4148, and all is OK.

For your problem, look on the picture above - the 2 transistors close to the big brown Elna capacitor are the other 2 transistors I had to replace.

As you see from the above photo, I have replaced all capacitors (including Tantalums) with Wima MKS2 (for 0.22uF to 4.7uF) and Elna Silmic II for 10uF and above.

The sound improvement is simply stunning.

This Beogram, with the upgraded CD4 Decoder as the RIAA amplifier, beats everything else on the market, for pure clarity.

Love it!

Re: Beogram 6000 Quadraphonic problem

tournedos — Tue, 04 Mar 2008 04:32:38 GMT

Oh yes, and 1N60 is definitely not a zener. It's a plain old germanium diode (I overlooked you previous post, sorry. I'm pretty sure this is the muting circuit).

-mika

Re: Beogram 6000 Quadraphonic problem

tournedos — Tue, 04 Mar 2008 04:24:07 GMT

Is this on the CD4 decoder board? The only section that seems to match this in my schematic is the muting circuit there. It's not critical, try it with 1N4148 and see what happens...

The idea is that the transistor should start to conduct after a delay when the arm is lowered. If it doesn't, you won't have sound, as the next transistor stage shorts the signal paths to ground. There's an RC circuit implementing the delay. When the arm is raised (the line from the connector goes low), this diode is supposed to allow the cap to discharge immediately, so the muting happens without delay. If a 1N4148 doesn't work in this application, you'll have a "pop" when the arm is raised. Just like in most other manufacturer's turntables

There's a chance I'm talking nonsense, so take this with a grain of salt... in any case, it's not a place for a zener.

Good thing you brought this up, the muting circuit in my Beogram doesn't work either. At least I know now where it is

-mika

Re: Beogram 6000 Quadraphonic problem

yachadm — Mon, 03 Mar 2008 15:46:04 GMT

Mika,

I realized that if I wanted some good help, I'd have to put in some good information, so here's a sketch of the circuit in the AA143's immediate area.

Does this help you to determine whether we need a Zener, or is a 1N4148 just fine?

Double Click on it to enlarge for clarity.

Re: Beogram 6000 Quadraphonic problem

yachadm — Mon, 03 Mar 2008 11:47:57 GMT

Hi Mika,

Unfortunately, I don't have the Circuit Diagram, so I can't see its function.

On this CD4 PCB (C8005015), it is D106.

The Anode is connected to one of the Thumbwheel variable potentiometers (R122), and the Cathode is connected to the Blue wire (pin 4) on the 9-pin round INPUT to this CD4 PCB.

See picture.

This then goes up to the smaller upper PCB (light blue wire), which has the switches for the clear plastic optical sensor.

The 1N60xx series are Zeners, which I have no clue what voltage, and from a basic look at the circuit, I'm not convinced that this should be a Zener.

The AA114 is cross-referenced at Mouser as 1N914 / 916 / 4148 !!!

And I've got the 200mA 4148's in stock here!

What do you think?

Menahem

Re: Beogram 6000 Quadraphonic problem

tournedos — Mon, 03 Mar 2008 09:18:03 GMT

yachadm:
The 3rd part, Germanium diode AA143, is a challenge. This is NLA at any reasonable price. Oh, of course, there are a few vendors willing to sell at outrageous MOQ's, or have a minimum order value of $250.

What part is it, or what function does it serve? I can't find it mentioned anywhere in the service manual I downloaded from this site. It's probably a germanium type for the low drop-off voltage, in which case you could probably replace it with a modern schottky diode, but there may be other reasons too.

Direct replacement types according to my cross-reference book would be AA114, AA116, AA138 and 1N60, but the engineers at B&O might have done something tricky with it. In any case, 1N60 should still be quite readily available.

Hope this helps,

-mika

Re: Beogram 6000 Quadraphonic problem

yachadm — Mon, 03 Mar 2008 09:02:40 GMT

Hi All,

Problem almost solved!

The following 3 semiconductors were defective:

Transistor NPN BC182B - replaced

Transistor NPN MPSA13 - replaced

The 3rd part, Germanium diode AA143, is a challenge. This is NLA at any reasonable price. Oh, of course, there are a few vendors willing to sell at outrageous MOQ's, or have a minimum order value of $250.

Can anyone tell me what a suitable widely available substitute would be.

Because the MicroSemi datasheet describes it as "General Purpose, Low Noise", surely there must be any number of modern diodes which can substitute for it?

Any good ideas out there?

Derek, I took your advice - the epoxy is a great solution for cracks!

Regards

Menahem

Re: Beogram 6000 Cracked Pully

beoaus — Sat, 01 Mar 2008 00:39:30 GMT

Sorry to alter the thread though I have a BG6000 problem and the picture two posts above details it well..

The picture above shows a white spool connected generally via a belt to the black pully. Does anyone have a spare white spool, mine has melted and broken...

in hope, beoaus.

Re: Beogram 6000 Cracked Pully

auric — Fri, 29 Feb 2008 16:24:33 GMT

Menahem:

Fill the crack with super glue to stabilize it. Then fill the cavity of the crack with epoxy. Before the epoxy cures use a razor blaze to cut off the excess amount. When cured, use a black marker to colour the epoxy.

Should be good as new.

Derek

Re: Beogram 6000 Quadraphonic problem

yachadm — Fri, 29 Feb 2008 07:14:03 GMT

Hi Mika,

Thanks!

This was a compound fault!

Yes, first the arm didn't recognize the LP, and then later it recognized the LP, but didn't lower.

It appears that it was a bad solder joint.

On the Control PCB (the large Rectangular one on the left), I resoldered all the joints with wires on them.

Everything MECHANICAL seems to be working OK now.

BUT....

There is no sound output from the CD-4 decoder.

The red 4ch lamp illuminates when a CD4 LP is played, which shows that the CD4 PCB is getting power.

Maybe a short blew a diode or transistor.

I'll have to roll-up my sleeves and look for the problem.

Actually, I have the original B&O service manual for the BG4002-6000 Type 5501 and 5502.

But mine is the 5505, with a different CD4 PCB.

Peter kindly sent me the parts list, but the Circuit Diagram for this PCB (8005015), would really be appreciated

Also, I discovered that the black plastic wheel, which is driven by the servo motor, is cracked, and therefore slightly crooked (see picture).

It still rotates now, but at some point the crack will break it completely.

If anyone has a spare in good condition, I'd really appreciate it.

Regards

Menahem

Re: Beogram 6000 Quadraphonic problem

tournedos — Fri, 29 Feb 2008 06:32:18 GMT

yachadm:
No response. I can't remember ever pressing < to start playing an LP, so I don't know if that is ever supposed to work from At Rest?

No, only "<<" starts the machine. But once it is running, you can press < or > (or V) to cancel the automatic operation and control it manually from there on with the other keys.

In your first post, you mentioned that it ran the arm to the end and returned, which would suggest a fault in the sensor circuit, being stuck to the "platter seen" status. But now you say it stopped at the right position but didn't lower. Is this consistent? This is a bit odd, since it won't stop if it sees the platter, so the fault would be opposite to the first case.

You may have two separate problems, an intermittent sensor circuit (the first case) and a non-working arm solenoid circuit (the problem you have now). Do you hear anything when you try to lower the arm with "V"? If not, I'd expect the solenoid driver circuit to be at fault. I don't think the problem is mechanical, if it doesn't make any sound when trying to lower.

BTW, the schematic can be found elsewhere on the net, but the complete service manual is invaluable. I upgraded just for that

-mika

Re: Beogram 6000 Quadraphonic problem

yachadm — Fri, 29 Feb 2008 02:13:55 GMT

Tried that!

Didn't work.

From At Rest position, I pressed Slow Move <.

No response. I can't remember ever pressing < to start playing an LP, so I don't know if that is ever supposed to work from At Rest?

Then I pressed START <<.

The arm moved to the cue position, and stopped (as it should), but the arm didn't lower.

I Pressed v ^ - no response.

When I replaced all the caps, I did clean and lubricate that little arm/piston/spring arrangement at the rear of the deack.

Any ideas?

Menahem.

Re: Beogram 6000 Quadraphonic problem

Peter — Fri, 29 Feb 2008 01:45:40 GMT

No! Press the slow move button and then when you are in the right place the up and down button.