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BG8000 fuse issues

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ALF
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ALF posted on Mon, May 1 2017 7:02 AM

greetings all,

a new arrival, a BG8000, which had a partial re-cap job done by the previous owner.

I noticed C24 was put in the wrong way as its positive should connect to the F1 ?!

anyhow, I corrected that, completed the re-cap job incl C27 - see pictures - and now F1 is constantly blowing

even with my DBT ?

I checked D16 to D19, all measured ok.

did I overlook something ? any ideas or suggestions are most appreciated :-))

cheers

ALF


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ALF
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ALF replied on Thu, Jul 13 2017 6:56 AM

Hi Lonnie,

The   -3.2V are DC !

ALF

Lonnie
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Lonnie replied on Fri, Jul 14 2017 3:15 AM

Hi ALF,

Ok, last time you indicated that it was positive 3.2V, if it is negative then it's less than the 0.7V maximum that the diode is supposed to control. 

Since you are holding a stable 33.33 rpm you can assume the AC integrating opamp is doing it's job.

Before you tackle the 45 rpm issue I'd look at the carrier sensor. Are you getting a square wave output at 1IC1 pin 8 and 1IC2 pin 14 per the section 1-5?

Regards

Lonnie

ALF
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ALF replied on Fri, Jul 14 2017 5:00 AM

Apologies Lonnie,

a typo, as it was always -3.2VDC !!

as for the 45rpm speed selection it seems to work now - it is hard to tell the difference when only the black sub-platter is spinning.

nevertheless I will check that square wave signal but am quietly confident it is a square wave.

as for the 'not lowering' arm,  I checked the output pulse of 2IC1 pin 18 and there is a short voltage jump to about 1.2V going out

the SM talks about a 'high' at pin18 - is this reading considered to be a 'high' ?

.....back to work !

ALF

ALF
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ALF replied on Fri, Jul 14 2017 6:10 AM

Correction:  2IC1 Pin 24,  not pin18

ALF

ALF
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ALF replied on Fri, Jul 14 2017 6:14 AM

Reason I mentioned pin18 was

P6 - pin18 !!!

cheers ALF

sonavor
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Since your speed control appears to be fine and other functionality dependent on the processor work I don't think you have a bad processor IC. When I see strange behavior with the Beogram 800x turntable regarding tonearm raising and lowering or sudden stopping of the carriage assembly I immediately suspect module 3 (control panel) R9, R10 and IL1. Those are the forward and reverse scanning controls. If that circuit is not correct you can experience all sorts of problems with the tonearm. I recently had an intermittent problem with one of the LDRs (R9 & R10) that was causing the Beogram 8000 tonearm to just stop and raise up during record play. Once I reworked the LDR and lamp circuit everything was fine.

To check this you can monitor the Lift signal at 2-IC1-24 to see if it is sending the command to lower. If it is and the ramp voltage out of 1-IC1-7 (for the tonearm lowering) doesn't fire then there is a problem in the Lift circuit. But I would guess you are either not getting a Lift command out of the processor or it is being interrupted before the tonearm can lower. The Lift command signal at 2-IC1-24 should be around 60mV when the tonearm is supposed to be up. When the processor sends the command to lower the tonearm that voltage at 2-IC1-24 should go up to around 680mV (Whatever the voltage needs to be to turn on 1-TR8). That Lift signal should stay at 680mV (around there) until it determines to raise the tonearm. An unwanted signal from the LDRs in the control panel could be the stimulus to cancel the Lift signal from lowering or keeping the tonearm lowered. The "at rest" voltage across each LDR should be from 600mV to 700mV. When in that range the circuit should be stable and not interrupt the tonearm lowering. 

That is my 2 cents for what to check first. 

- John

Lonnie
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Lonnie replied on Fri, Jul 14 2017 7:26 AM

Hi ALF,

If you look at the function table for the Microcomputer:

When you activate pin 2 by pressing play the arm should move across to the record with pin 24 in the low state so that it won't lower.

In the Playing state pin 24 has gone high which turns on TR8 as described in the Technical Product information. 1TR8 is a standard NPN transistor with a grounded emitter and the high state needs to be high enough with some margin to turn it on and keep it on while playing.

You should have the understanding of transistors by now to work out if 1.2V is high enough? 

The next thing to check is if there is current draw through 1TR8? You should be able to measure the voltage change at the collector caused by a change in the current between the off and on state. If there is no change then this would require further investigation as to why.

Regards

Lonnie

 

ALF
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ALF replied on Sat, Jul 15 2017 7:20 AM

Hi John,

thank you for engaging 😁

Here is what happened:

first I adjusted both LDRs on the control panel to about 650mV - both were way over 740mV at the beginning!

the next test:

with the platter off - press play, carriage moves right accross, arm does not lower, carriage stops at center and does not return. Brought back by pressing stop.

during that motion the Pin24 signal is about 71mV and does not change.

same again, press play but stopping the carriage by pressing >>  followed by pressing play again, the spindel turns slowly, the Pin 24 signal rises to 678mV and remains steady, signal at 1IC1 Pin7 rises from 3.5mV to about 12V

arm does not lower till carriage stops at center, carriage remains for a couple of seconds then returns, signal goes down again to about 71mV.

I traced the lowering signal to B of TR8, which was exactly 678mV. C of TR8 sees about 2.2V.

according to the data sheet of BC547B (TR8) I would need more than 0.7V VBE to turn on TR8 ??

this is as far as I got for now 👿

ALF

sonavor
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In your test with the platter off you should have seen the arm lower or try to lower. You need to see which input signal is preventing that detection. Does it always try to stop and lower at the center of the record?

If you see the ramp signal from 0V to 12 or 13 V at 1-IC1-7 then that should have caused the tonearm to lower. I typically measure a lowering voltage control signal at 2-IC1-24 of 680 to 690 mV on my Beogram 8000. My voltage at 1-IC1-7 is typically 12.6 V.  If 1-IC1-7 reaches 12 to 13 volts and the arm doesn't lower then maybe there is a connection problem with the solenoid. You have two sets of failure modes on the arm lowering though. One case where you are not seeing an expected lower tonearm command signal and another case where you see the lowering voltages but the tonearm doesn't lower. 

Very strange Alf. You don't have a second Beogram 8000 you can trade parts with? If you can't find it I can offer you an option to mail the main board to me and I can test it in one of my Beogram 8000 units. Even just the board might be too expensive to ship from Australia though. 

John

Dillen
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Dillen replied on Sat, Jul 15 2017 9:05 AM

Bad opto on the threaded spindle (Bad sender IR).
Or the related electronics.

No pulses received = it never reaches the count where to lower and just runs against the center end stop.

Martin

sonavor
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That makes sense Martin. If the opto is determined bad can it be replaced with a Optek OP240B device?

John

Dillen
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Dillen replied on Sat, Jul 15 2017 4:58 PM

I believe that's what I have in the drawer.

Martin

ALF
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agreed, that makes a lot of sense !!

I assume we are talking about this small device ?!

if so, couldn't I see a square wave signal on the osci ? or is there another way of testing if good or bad ?

Martin, would you be able to supply a suitable replacement  ?

thank you

ALF


ALF
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for good measure:

this is what I got taking the signal from P2-Pin4

press play, carriage moves right across, arm not lowering


ALF
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very similar signal from P2-Pin4

press play, stop carriage with >>, press play again, spindle turns slowly as carriage moves across, arm not lowering


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