Hi,

I've looked into the electronic cueing system in my Beograms.

All I could find was C9 which is supposed to delay the full voltage to SL1 (lift relay) after the processor has sent its signal, hence braking the arm slightly down to the record. That's the theory at least.

On my two decks I think the arm is too fast lowering the stylus (exactcly the same really) so I was contemplating:

1)altering the value: doubling it to 2μF makes the arm lower very gently indeed but the muting circuit then stops working, less than that has little or no effect.

2)bad caps on both decks, although these aren't electrolytics but plastic, usually very reliable.

Any ideas?

Thanks,

Jacques

Anyone please? Martin?

Hi chartz

I guess that you too have hit on a topic where there is little help available.  Earlier this year, I posted a similar enquiry regarding the arm on my BG8002 dropping way too fast and all I got was a deadly hush.  I would have thought someone would have a good idea of the reason for this.

In my case, I had to lubricate the solenoid because it was sticking and once it was free, it dropped very rapidly.  I thought that the reason might be related to some form of damping lubricant which I had washed out in the process but I guess that I will never know. 

I found your thoughts on the possibility of a faulty capacitor /electrical delay very interesting and must admit that that option had never crossed my mind.  Since I have two decks, the BG6006 which drops normally and the BG8002 which drops too fast, I will check out the action of the transistor TR9 and try to see if there is any significant difference in the base voltage characteristics.  If that turns out to be the case, I will then remove the C9 capacitors on both units, replace them and see how the voltage characteristics and dropping characteristics change.  That will be an interesting exercise for me.

Unfortunately, I haven't got the time at the moment to work on this one so it will have to wait for a while.  In the meantime, I hope that someone will provide you with a useful answer to this problem and I hope that I can let you know the results of my tests one day. 

Regards

Geoff

   

Thank you Geoff.

I am therefore looking forward to hearing from your experimentation results.

I think there's no damping fluid involved here. The coil behaves much like a 'speaker coil, albeit at a very low frequency. Altering the value of the above mentioned cap (part of the op-amp feedback loop) tends to confim this. 

Hello!

So, has anybody got any ideas about the cueing problem on these decks?

I tried changing the feedback/delaying cap (C9) with a new Wima 1 μF/63V but it then refuses to lower, or at times it does lower veeerrrry slowly and the muting relay never triggers!

Re-fitting the original, possibly defective (?) cap returns operation. Lift signal comes from the processor at P6/18 (a simple low voltage DC signal).

From cold, the arm lowers nicely, but after one side it just drops!

 

I seem to remember that the pointy rest under the counterweight can become sticky.
Have you checked the mechanical parts ?
You could try placing a piece of paper under the counterweight to prevent direct contact between the parts.

Martin

Thanks Martin.

The pointy rest? When I try lifting the arm while playing (with a Q-tip) it is very free, it doesn't stick at all in the upper position.

Yes, if my memory serves me right, it's the pointy end of the adjustment screw for the
tonearm rest position. It sits vertically, head down, pointy end up at the rear of the carriage (not easy to adjust
but that's not the point here either).
I've seen it stick for a fairly long time after the solenoid releases and then suddenly letting go and the tonearm
falls to the record by pure gravity. In some cases first letting go when touched.

How fast is it lowering and is it dampened (controlled) or free fall ?

Martin

The screw doesn't stick, I've checked this.

The relay always clicks after the stylus has reached the disc.

When the deck is cold, it is normally dampened, no problem at all. But after a few minutes' use, it falls quickly, almost freely.

Hmm..
Cooling spray on the components could maybe shed some light on it.
Or on the board itself, I've seen lots of cases where a copper track has breaked away from
a solder pad, right where they meet, and the symptoms can then easily be temperature dependant.

Martin

So how do you analyze the role of C9 then?

chartz:

So how do you analyze the role of C9 then?

I have no idea of the specifics of this Beogram, but that is a pretty standard integrating amplifier constructed around an op-amp (a lowpass filter of sorts, if you wish). C9 and R58+R61 determine the time constant, while D12 makes it shorter for the other direction (arm going up I suppose).

This circuit may be affected by the leak current of the original cap. It's negligible in a new cap of course, but perhaps some other marginal component or other problem has altered the operation point. I would double check even the passive components in the circuit and all solderings of course. If IC1 is some cheap and common type, I suppose you could try replacing it as well.

I agree.
It's a LM324N if I remember correctly, so a very cheap and common IC.

Martin

Thank you both.

Okay then, but the same circuit triggers the muting relay too, doesn't it, and that works just fine. There's something I don't quite get here. I know I am being a pain, but I hate it when I can't figure things out... All solders triple-checked by the way! Also, do you think the dampening is purely electronic or not?

As far as I can see from the schematic cut-out, the the muting relay circuit doesn't care about what speed the integrator works at, it just trips at a certain voltage.

If it is indeed a LM324N, it consists of 4 identical circuits. That's 4 individual OpAmps that can be set to work in each their function.
One circuit (or part of one circuit) can easily go bad with the other three working normally.
Nothing strange about that.

Martin

Yes, it is an LM324N. And yes, the muting circuit depends entirely—and only—on the signal given by the lift/lowering circuit. So the delay circuit works fine for me. If the IC was defective, the muting relay would not work at all, or the delay would be wrong. Which then excludes the amp/delay circuit failure! Back to mechanics then? Well I don't know, because for me everything is fluid and free. The unknown part is that coil though...

It could be a little dust or other debris or it could rub on something.
Hard to diagnose per remote.

Martin

OK, I see what you mean... But the muting signal is taken directly from the output of the integrator op-amp, while there is still T9 before the signal reaches the coil. It might be useful to observe the voltage/current across the coil, but you will need a scope or an analogue meter for that to make any sense. And I'm still very suspicious about how replacing C9 would affect anything if the integrator circuit is working correctly.

Thank you.

I will look further into this and shall keep you informed of course. But I'm beginning to think this is a purely mechanical issue.

Yes, but don't rule out a broken PCB track. I've seen too many.

Martin

Hello,

Since this was never solved, I'd like to know whether someone has another hint, or the same problem...

As a reminder, when I double the value of C9 the arm's descent is slow but the muting relay never clicks. LM324 is fine. Passive components are fine, no cracked tracks or joints.

Are you sure the coil is not damped? 

Since your schematic cut-out above seems to be from the older(?) service manual scan, have you looked at the other one we have on site (larger file size)?

It mentions a modification to the raise/lower circuit (you seem to have it though if your turntable matches the schematic you quoted) and also shows an extra 10n cap across B/C of TR9. Dunno if that has any effect on your problem, but just a thought...