Beogram 6500 performing erratic

[alf]

Hi All,

just when I thought everything is back to normal…..it isn’t !

press Turn and the pickup arm moves to the drop down point,

press Play and the PUarm lowers and immediately rises again, goes back t9 its resting position, rises the PUarm, lowers it again and after some attempts of pressing Stop the arm finally goes to its resting position but stays in powered position.

while this is happening I can hear the spindle attached to the servo motor tries to klick into gear after a couple of tries and finally manages to do so…..

it is quite difficult to explain what is happening here and I wonder if it could be a fault microcontroller (the 24-pon chip on PCB 1) ?

none of the gear wheels is damaged, the magnetic coil on PCB1 is ok

to the point: I have no idea what is going on and what is causing that
erratic performance.

can anyone shed some light on this problem ?

ALF

 

 

 

[filip_kbh]

It probably resembles the issue I had with my BG7000, and Martin (Dillen) answered that I should clean the switch near the top left of the deck – it’s marked S1. This helped.

Location: Copenhagen

[alf]

Hi,

thank you for your reply, certainly always good to have clean contacts !

i am afraid but I decided to reset the cam-wheels as per service manual – I wished that manual is a tick more understandable using phrases like ‘….opposite h9le J’  when it actually means overlapping holes !! It can easily be misunderstood…never mind, I got it now.

in theory resetting those cam-wheels should fix the problem – I will report back.

ALF

[alf]

Hi All,

finished the process of re-fitting all cam-wheels as per SM. Sadly the table still plays up.

looks like the next move would indeed be the removal of board 1 and give those switches a good clean ?!?

i do not feel overly confident this will be the problem fix as the table was working fine just before all this

started……but I shall try and will report back.

In the meantime please do not hold back with pointers …..

ALF

[alf]

Sorry, I forgot to mention the table has had a complete recap – so, that should not be a problem source.

ALF

[alf]

back from the workbench  after cleaning the 3 switches but no luck. The PU arm moves in, lowers but rises straight after again and so forth …l

i begin to wonder if the magnet inside the coil is moving as it should to move those two control arms for two

of the cam-wheels ?

one other question: does the hook of the metal rod which lifts and lowers the PU arm need to sit behind or in front of the white plastic rod the runs behind the PU arm assembly ?

your expert advice is badly needed 😁

thank you, ALF

[alf]

back to testing:

RL1 and RL2 seem to be controlled by TR4 and TR5 if I am not mistaken.

the question I would ask do those two transistors have to be closely matched ?

in my case they are about 15% apart in their hfe values  ?!

i simply can not find anything wrong with the cam-wheels as i checked their fitting as per SM.

the cam responsible  for the lift & lowering just is not stopping as per correct sequence.

hard to believe no one has any ideas or pointers to help with that issue………too hard perhaps I wonder ?

ALF

[alf]

Hi,

i must admit the obvious lack of interest in what I believe is a rather interesting issue to solve is surprising

to say the least unless this post has found the wrong audience ?

even the “big guns” are silent which is unusual and disappointing…..

well it looks like searching on and perhaps find what is causing the issue – this turntable is far too nice

for becoming a “for parts only” seller !

ALF

 

[Guy]

Hi alf!

I’m just ‘bumping’ your thread up again because I too am surprised that no one is able to suggest a fix!  It must be a very rare fault indeed.

I have a Beogram 6500 (a Beoworld Prize Draw win!) but it’s working perfectly (or at lease was when I last used it some time ago!).  I tend to follow most threads for equipment that I own, because I know that it is only a matter of time before I have to attempt repairs myself.

Anyway, hopefully someone can chip in with a repair suggestion.

Location: Warwickshire, UK
My B&O Icons:

[Mark-sf]

I have not observed your problem, but would approach troubleshooting to understand which circuit is at fault as it could also be the record detection section. TR4 and 5 are simply switches so they do not need to be matched. Check the action of the mute switch and check whether it is unmuting or staying muted as it transitions from lowering to raising. I would also check how it behaves with a 45 and without a record at all. Finally, I would monitor the base of TR% to see whether it is causing the arm to raise immediately. Report back with these results and I may be able to recommend either a fix or further testing.

• This reply was modified 3 weeks, 5 days ago by Mark-sf.

[alf]

Hi Mark,

thank you kindly for your reply.

S3 gets activated by the lower camwheel and IC1 sends the up or down signal via TR5 to RL2 which in return

activates the lower control arm that is connected to the lower camwheel responsible for blocking that camwheel once the PU arm is lowered. For that to happen the solenoid must generate enough energy to move that control arm via its piston.

as I understand if that control arm is not moved far enough the lower camwheel keeps turning and activating S3

over and over again.

I am happy to be corrected if my theory is wrong.

I accept that TR4 and TR5 don’t have to be matched, which was just an assumption. But what is actually energising the solenoid I like to understand and is TR6 alone responsible for the motor control or does it play a role towards the solenoid as well ??

yes, I can measure the muting switch which does open and close when operating arm N (SM drawing).

i will report back

ALF

[alf]

I measured the resistance of the muting switch as per SM:

0.1 Ohm with the PU arm raised – switch in pressed position

10.5 Ohm with the PU arm lowered – switch in depressed position

does that make sense ? I could not find any reference values of the switch’ resistance readings.

BTW, which one is TR% ??

ALF

[alf]

Apologies, a typo:

the muting switch resistance, measured in playing mode was 0.1 Ohm and 14.5 Ohm, not 10.5 Ohm !!

hope that still makes sense.

My guess is the mentioned TR% should read TR6 ?!

ALF

[Mark-sf]

I don’t believe you are measuring the correct contacts with those results. The muting switch is different depending on whether you have the version with the built-in RIAA preamp. In either case, when the tonearm is up, you should get 0 ohms or .1. This is a two pole switch meaning there are two sets of contacts. With it down, you should get 800-1K ohms w/o the module and around 5K-7.3K ohms with it depending on the input impedance of your amp..

• This reply was modified 3 weeks, 3 days ago by Mark-sf.

[alf]

All Beogram 6500 models are fitted with the RIAA module.

I measured as per SM 5-4 Muting, assuming the drawing shows the solder side of the muting switch PCB.

if your readings are the correct ones then my muting switch does not work which coincides with the fact

that there is no audio output at the moment no matter which cartridge is fitted !

in that case I shall try to bypass the switch…..

ALF

[alf]

What I meant was

i have never seen a BG6500 w/o the RIAA board, perhaps there are early versions w/o that board ?

i revisited the SM 2-2 , studied the drawing of the new version RIAA board and measured between

the RM and LM contacts of the switch but still, I can not verify your data.

this particular drawing there also  mentioned cuts (RM, LM and LG) – I  presume this is for bypassing the muting switch ?

after I removed those 3 wires I fitted a different cartridge to check the audio output….nothing again ??

strangely enough the BG plays again (at the moment).

Both cartridges appear to be in perfect health and were working before ??? So, I really don’t know where

all this is heading ? PU arm wiring ? RIAA board fault ?

If indeed I bypassed the muting switch by removing those 3 wires I may have a faulty RIAA board ??

any further thoughts ?

ALF

[Mark-sf]

I would first check the cartridge coils which can be done by measuring between the R & GR and the L & LG pins. Depending on which cartridge you have they should measure between 800-1K ohms for each channel. I would then check that the RIAA board is getting 12v  especially the collector of TR2 which is a quad bilateral switch IC that also acts as a mute. If the 12v is not on available from TR2 to pins 5, 6, 12, & 13, then you will not get sound even with a good cartridge. You should also have 12 on pin 8 of IC2 and IC3.

[alf]

First of all a big Thank You for your support – very much appreciated 👍

second, I did not check any cartridge coils as I was dead sure the MMC2 and MMC4 were working

perfectly before…..and still are.

Now everything is back to normal (for now but one can never be sure 😁) – shat happened ?

the RIAA board did not get any voltage because P2 at the power supply board wasn’t properly plugged in.

this must have happened when I had to replace the fuse which is right next to it. While levering out the fuse

it partly unplugged P2. the RIAA board is back in business and so is the audio signal 🙏🏼

i was certain the 12V supply was working but did not expect this small hick-up……and the muting switch

works fine as well !

why the previous issues have suddenly disappeared is not quite clear to me – one fact was that the wrong

fuse had been installed – a slow blow 500 mA fuse instead of 750mA.

anyway, i was certain all camwheels were installed correctly and the piston in the solenoid is not deformed

and moves freely…..

I rest my case for now, once again thank you for being so patient and helpful

ALF

[Author]

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