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beogram4000 issues

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ALF
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ALF posted on Tue, Nov 13 2018 10:41 AM

hi all πŸ€—

my BG4000 has issues with a faulty engaging servo !

following the faul-tfinding flowchart of the SM all is pointing to the pair of ORP61.

they differ quite substantial in their resistance reading:

under direct light (bottom side) they show about 24kOhm and 16kOhm

under direct light throught the top slots they show  about 160kOhm and 105kOhm.

no matter how I adjust the servo (light intensity, diagphram height) it is just not working.

both ORP61 are getting about 23VDC, the light source is working fine and 1D7 and 1D13 are fine!

perhaps there is something else I may have overlooked but with those differences in resistance value

I can not imagine it would ever work ?!

comments are warmly received - I also posted a wantd ad.

ALF

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ALF
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ALF replied on Thu, Nov 15 2018 5:40 AM

Hi all,

getting a response from Martin  - he does not believe that ORP61 parts are to blame but pointed me

to a signal check at the H- bridge.

according to the circuit diagram I should be getting 0V both at E of TR28 and TR31  when ON and ^ are activated ?!

sorry, I do not have the 0V on either - one reason being is the carriage slowly moving outwards 🀬

in kept adjusting the diagphram height, shutter position and light intensity....just won’t do it.

I will revisit the diodes and test the four transistors involved with the H-bridge.

so far I only managed once to play an entire record, however after reaching the run-out groove the arm did not lift and return.

ALF

ALF
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ALF replied on Thu, Nov 15 2018 6:07 AM

Ok,

I finally got the carriage to stand still πŸ™„ so I can actually verify the voltages:

E of TR28/31 shows around 70mV, not 0V as per CD

B of TR27 is varying between 45 to 90mV, not 0V as per CD

B of TR33 is around 150mV, not 0V as per CD

B of TR25/34 show 0.69 to 0.72V which I think is ok.

to sum up: 0V at ON and ^ activated are not present ☹️

I guess it is off to a transistor check/test off board !

ALF

solderon29
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Hello Klaus.

Is it a silly question,but have you checked the tracking servo switch on the solenoid assembly?

The '4000 has a switch to enable the tracking servo once the arm has lowered.

It connects the two tracking sensor ldr's into circuit.The contacts can and do become oxidised,so reducing the current through the ldr's,and limiting the tracking sensitivity.

Nick

ALF
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ALF replied on Sat, Nov 17 2018 10:16 AM

Hi Nick,

yes, thanks for reminding me - I did indeed carefully sand those brass contacts - sadly no positive outcome to report.

just ordered a new servo belt from Martin and will see what that changes?? If anything ??

the carriage control does not perform as usual at all - does not move up to the drop-down point, arm drop right after activating ON.....

I tested TR26 to TR33 off-board.....all good - also checked the diodes D7 to D14,  but not off-board.

the main board has not yet been recapped...should that impact in the way the carriage performs ??

ALF

solderon29
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You should have 24 volt at the collector's of TR27,28,31 and 33.Are you confident that the 24 volt supply including it's reservoir cap is ok?It may not be supplying enough current.The reed switches are alway's suspect too.

Martin's "v" belt's are a good investment,they provide better traction than either a square or round belt.

Is the carriage motor seized?

The carriage motor drive circuit is pretty robust,with the motor forming a bridge between the two amplifiers.When a direction button is pressed,the circuit becomes active and applies the supply voltage across the motor in one or the other polarity to drive the carriage in or out.

Nick

ALF
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ALF replied on Sun, Nov 18 2018 10:21 AM

Well,

the 24V supply is showing about 23.4VDC - and that is the case at all suggested Collectors of TR27, 28, 31 and 33.

the two reservoir caps 0C1 and 0C  (2000uF/15V) measured actually about 3140uF and have been replaced - thanks for pointing this out !!

sadly, that did not yet show any improvement in the carriage performance.

the carriage motor is running freely - I can hear it clearly when its naked pully (no belt) spins (both directions).

as said, a new servo belt is coming but this alone won’t fix the issue. I am also not yet convinced a complete recap of the main board will do it ?!

question is though, where to from here ???

ALF

solderon29
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Just to clarify,are you saying that the carriage doesn't move,but the motor spins ok?

If so,does your '4000 still have it's original plastic carriage spindle pulley.If so it could be simply spinning without gripping the spindle?

Can you undo the securing screws for the spindle pulley bracket,and see if the carriage can be moved freely by hand?

Otherwise,you could try bypassing the 24v supply reed switch to see if it's reducing the supply under load?

I don't think that your problem is on the main board,but it will almost certainly need attention after you sort out the above issue.

It's not necessary to completely re-cap the board,but replaceing the electrolytics,particularly the 'tant's will improve the operation overall.

The speed change relay will probably be intermittent too,and of course the presets will be oxidised!

Nick

ALF
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ALF replied on Sun, Nov 18 2018 12:47 PM

What happens after ON is activated is a mm long move of the carriage inwards, just enough to clear the on/off nipple switch.

if I do that exercise without having the servo belt fitted the platter motor starts spinning and so does the little servo /carriage motor. 

Yes, the spindle pully is the old plastic one - I still have a metal replacement I could fit but the carriage was moving freely towards the drop-down point

before all that jazz started but will double check that pully/spindle issue.

when I mentioned recapping the main board I was only going to replace about 10 or so elcos and the few tants.

I did deoxit and fine-sanded all brass spring contacts in the control panel and underneath the nipple-switch assembly that has the carriage run over it.

To rule out any contact triuble,  meaning I do not expect any issues from that corner !!

it is just absolutely puzzling that all of a sudden it would no longer work without any obvious reason ?

what is going on here ??

solderon29
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This seems to be a start up problem then,rather than just a transport problem?

The slide switch that you refer to as "start/stop" is in fact just the stop switch,and it seems that if the turntable motor is running once you move the carriage inwards from that switch,the problem is to do with the logic control of the power supply at start up.

I think you will need to double check the operation of the "nipple" switches as you call them under the carriage mech.The stop switch should remain closed once activated.The power supply is controlled by a flip flop circuit which in turn is controlled by the logic system.

I have the training course notes for this machine,I may be able to copy the relevant pages and e.mail them to you,but in the meantime,check those switches and the reed relay's again?

Yes indeed,re-cap as you plan to,but I would overcome this other problem first?

Nick

Craig
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Craig replied on Sun, Nov 18 2018 8:43 PM

Nick

You may send me those training course notes too, if its not too much trouble ;¬)

Craig

ALF
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ALF replied on Mon, Nov 19 2018 2:51 AM

Yes Nick,

of course, it is the stop switch (what I actually meant) πŸ™„

I only mentioned the recap as it usually comes up as one of the “must do things”, but it was not on my agenda straight away.

well, the relevant course notes would be fantastic - thanks for your thought Nick...happy to receiveπŸ™πŸ€—

see how things progress....😳

ALF

ALF
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ALF replied on Mon, Nov 19 2018 12:08 PM

This is also an interesting example of an issue where the fault-finding flowchart in the service manual may!! lead you to “nowhere” 

or masking underlying issues:

quote: .....is the carriage moving inwards ?  N  is OM2 running ?   Y  is carriage blocked ? N. ===>. Repair spindle, fit belt !! 😬😬

it is not that easy and straight forward 😒

so, I am looking forward to study Nick’s training course notes he offered.

may there be some light at the end of the tunnel.

ALF

sonavor
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The Beogram 4000 can be a bear to get set back up properly. I remember on one of mine I was having some sort of logic error. The contacts were all good and the parts all tested good. It turned out to be a contact problem when everything was reassembled. A contact was not making good contact. It took a bunch of iterations to tweak the switches before mine was fully working again. 

ALF
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ALF replied on Fri, Nov 23 2018 12:07 PM

Too right John !

to put it blunt: right now I have absolutely no idea what is going on with this table ?

there are only a finite number of mechanical contacts to check.

I double and tripple checked they all provide good contact - all have been cleaned as much as it was possible.

the old servo belt had it and I am awaiting Martins shippment for a new one to put on.

why the carriage only moves a very short distance and almost immediately after drops the arm is puzzling.

don’t tell me there is a logical error ...... pleeease.....as the original problem was a non engaging servo.

at that stage the carriage was traveling nicely up to the drop-down point.

a mystery, or is it not ?

ALF

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