ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi ALF,
Based on your CMOS readings of 1SD = Low, 1CD = Low, 1D = High you are getting a correct response of 1Q = High . Can you remove the input voltage, maybe by removing 1R50 or P4(7) and confirming if the output goes low.
Does your BG8000 have the additional board PC7 which controls the uC chip? From jpeg P1010309 it looks like it does. If CMOS 4013 is working correctly this board might be the next thing to check.
Regards
Lonnie
Hi Lonnie,
my mistrust towards ICs has just had a "boost"
I had some suspicions about 1IC1, 2 and 3 and replaced those - the outcome was an improvement 🤓
the previous signals in respect towards the speed sensor are now a sine and upright one corresponding to 33.33rpm.
however, there are more issues to deal with such as:
45rpm speed change is unavailable
carriage moves right across and does not return, only when 'stop' is pressed
arm does not lower with record present
so, in other words there is more investigation neccessary - I will look into that CMOS on board PC7 !!
ALF
In your post it is not clear whether you have solved the high speed platter issue. You only indicate that the output of IC1 is a square wave. Can you confirm that the TT now holds 33.33 rpm? If it does have stable speed then did the replacement of IC1 or IC3 fix that issue ie. did you replace them one at a time and measure the outcome?
If you have stable speed then there is no need to check PC7 at the moment. The PC7 board was added to help solve a platter speed issue - see the bottom of 8-1 in the service manual.
Hello Lonnie,
yes, at the moment it holds 33.33rpm - I not sure though whether you would see a square wave signal at IC3 pin14 with a speed issue ?
I did replace the three ICs all at once but should have done it step by step.......
bear in mind I took over that table not working mainly for its transformer when I started investigating....
there are still issues as described such as:
- arm does not lower at record point
- carriage moves right across but does not automatically return, only when 'stop' is pressed
- speed change from 33 to 45 does not work
- gate of TR26 still sees about 3.2V which is way more than the suggested 0.7V ??
yes, I have all 3 service manal downloads 😎, read frquently through them trying to make sense out of it 😂
I like to rule out the main processor as the source of all those remaining issues, as this would mean the end
of all further attempts to rescue the table due to its unavailability !
Cheers
AL
For the 3.2V you measure at the gate of TR26 are you measuring AC or DC?
The -3.2V are DC !
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?
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 !
Correction: 2IC1 Pin 24, not pin18
Reason I mentioned pin18 was
P6 - pin18 !!!
cheers ALF
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
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.
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 👿
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
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
That makes sense Martin. If the opto is determined bad can it be replaced with a Optek OP240B device?
I believe that's what I have in the drawer.
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
for good measure:
this is what I got taking the signal from P2-Pin4
press play, carriage moves right across, arm not lowering
very similar signal from P2-Pin4
press play, stop carriage with >>, press play again, spindle turns slowly as carriage moves across, arm not lowering
The question is, is this optical device really faulty or is there still a different devil to uncover ?
That's the right way. Stop guessing and measure!Are pulses output from the two receivers when the threaded shaft rotates?
same procedure as before,
signal taken from P2-Pin6
first pic is a bit out of focus, sorry- just carriage moving right across
second pic is with the slow moving carriage, play pressed after carriage has been stopped with >>
second pic from P2-Pin6, showing a very similar signal as the one from P2-Pin4
Difficult to tell, but if that is 100mV amplitude, the sender IR is not emitting light.(Those small signals could be caused by ambient light reflecting onto the opto wings on the threaded shaft ). Check the power to the IR sender LED.If power is OK, check its light output with a digital camera.IR light typically shows as pale blue on a digital camera.(On some iPhones only the self-facing camera will work for this).
Thanks Martin,
I assume you are talking about the single optical device in the fixture of three, not the two next to eachother (wings you called them)?!
Its voltage is 4.98V, the other two next to eachother are getting about 14V
I could not see any pale blue traces on my digi camera, providing I checked the correct element ?!
yes, the signal amplitude was about 100mV.........! It looks like I will have to replace that single opto element - can you supply that ?
Yes, the bottom element is the IR sender LED.The wings I refered to are the four interruptors at the end of the threaded shaft. (It was a guess based on the signal and the fact that they rotate).
In the meantime:
I did replace the sender IR diode - and this is what happened:
with only the sub-plater on the carriage now moves to the 30cm drop point but arm does not lower....only a weak attempt to lower can be noticed.
at the lower/drop point I measue at P6-pin18 (signal from 2IC1 pin 24) voltage goes up from about 70mV to 0.67V and very briefly reaches 1.2 V!
same at base of TR18.
at 1IC1-pin7 I measure voltage goes up from about 2mV to 12V
at 1IC1-pin6 I measure voltage goes down from about 2.2V to 0.76V
looks like the counter is working as the new IR diode delivers a p-p signal of about 18V both for pin4 and 6 at P3.
still a long way ahead !
here is something I absolutely can not understand:
the moment I put the main platter on to the sub platter the carriage goes viral, briefly moves over, not getting anywhere near the 30cm drop point
and returns quickly to its resting position.
leave the main platter off and put a record on to the naked sub platter, the carriage moves in position to drop at the 30cm mark ???
however arm does not lower ?
i begin to wonder if anything is blocking the arm mechnically ? The main platter business I most certainly can not explain nor understand !
any theory on that matter ?
thank you for your kind offer to test the main board - I may indeed consider that option, just have to check postsge rates.
not sure whether you saw my last post but it seems the voltages I can measure are pretty similar to yours in respect of the
arm lowering issue !
now that the am moves into the correct position to lower and seem to be getting the correct signal would it be fair to suspect a mechanical issue
in the arm assembly itself ?
thanks btw for suggesting the OP240B device which obviously works - I had to settle for the OP240c
Hi Alf,
With the Beogram opened up you should be able to watch the solenoid mechanism in the tangential arm assembly slide forward and backward as it is commanded to control the tonearm. Watch it and see if it is moving correctly or sticks. I have one Beogram 8000 myself where the solenoid is sticking occasionally and causes a slight hitch in its lowering operation. So something mechanical with the solenoid will affect the arm lowering.
Hello John,
thank you for pointing this out.........this table is just so different from the BG4xxx series and lots to learn 😸
You were as usual 'spot on' - knowing where and what to look for certainly helps.
the solenoid was indeed stuck and would have caused some interference with procedure and other signals?!
a non-conductive lubricant-cleaner did the trick and the arm is moving freely up and down thanks to a moving solenoid 🥃....just one for now!
This BG8000 has a strange looking tacho disc, more like a Kodak film negative with a plastic-feel ? The whole thing looks like someone tried to cut it out
from a sheet trying to stay on the circle line ?? i wonder if there ever was a replacement in circulation for the original tacho disc that had the peeling-off issue after long-service?
Perhaps it is time to replace that for the metal version that Martin offers ?
I still have the occasional issue with the carriage trying to move towards the drop-down point but when a record is on the platter, but returns after a very short motion straight back to its resting position??
trying 'play' a couple of times Gets it going eventually - the interesting bit is in that case occasionally the first two digits '33....' can still appear on the display ?
The original Beogram 8000 units had a plastic tacho disk with the black film. Almost everyone I have seen have started to peel. Changing a Beogram 8000 to a metal one (supplied by Martin) is a must do task.
The Beogram 8000 will display 33 on the display if it returned to the park position due to a Pause command. When you press Pause the tonearm lifts and remains over the record for a short amount of time. If Play isn't pressed to resume playing the record the tonearm will return to its park position and wait. Pressing Play again should result in the tonearm returned to the spot where Pause was pressed.
yes John,
that is while its working correctly, displaying only the first two digits '33' but not '33.33'.
I am not sure whether the carriage not moving correctly, meaning behaving erratically, towards the drop point is related to that
tachodisc issue ? I have attached a picture of the current tachodisc and contacted Martin via the site"s email (hope it worked)
it seems to work if I give it some minutes rest and then start 'play' again ? looks almost like some old data have to be deleted first or the uP has to be rest?
once the carriage reaches the drop point and the arm is lowered onto the record everything is fine !!
The new tacho disc from Martin is more precisely made and will be a good upgrade for sure.Did you say you re-measured your arm position signal since you replaced that sensor? Are you getting good pulses from that sensor assembly now?
What is the condition of the arm start/stop switch that is mounted to the chassis? Is it operating correctly?
Well, I am still hopeful Martin can supply one of these tachodiscs !
yes, I did revisit the sensor signals P3-pin4&6 and the pulses look absolutely fine.
the start/stop switch I will recheck to make sure - so far it has not yet been amongst the suspects.
I am looking at a lenghty wait for that tacho disc - if currently available - and will reinstall the original film-disc
so I can check that on-off switch and table performance.
hi john,
I reinstalled that flimsy film tachodisc for now to get some signal pictures.
the chassis mounted on/off switch seems to work fine, sees about 5.04V which goes down to 10mV and about 550Ohm to 0.5Ohm.
I have attached a picture taken from the speed sensor under the platter and was wondering whether this is due to the ageing tachodisc ?
is the signal on your BG8000 the same ?
this second picture shows the signal on P2-pin4 or 6 coming from the IR sender diode op240 at the spindle
the double lines are from an unsteady hand of mine while taking the shot!
I had a Beogram 8000 on my bench for an adjustment last week but didn't see your request about the signal you wanted me to compare. I will try to remember when I start on another Beogram 8000 turntable. I'm not sure yet when that will be yet. I might be opening one of mine up soon to change the muting relay. When I do I will take some pictures of those signals on an oscilloscope.
Regards,John
not so much a "request", more a favour 🙄
In the meantime I borrowed a metal tachodisc - also bought one from Martin - but sadly that did not fix things.....
the carriage still moves over just a bit after 'Play' is activated but does not reach the drop down point and returns straight back to its resting position.
only after a couple of tries I may get lucky and everything works as it should ?????
i begin to wonder if there is a fault in the uP control circuit - board 7 ? Suspect being the cemos IC ?
I also see 4 dots on the display, not just the center stand-by dot ?
anyhow, the investigation seems ongoing .
regards
I have seen 4 dots on the display before. It was due to bad connections inside the metal box that houses the microcomputer. The 4.7nF capacitors for the display had problems (bad capacitors and bad connections). Those capacitors are tricky because there are solder connections on both sides of the board. I reference that problem here and here. I can't really help with the operation logic problem you are having without being able to test it myself. There are too many variables at play there. From your earlier reports of the Beogram functioning though I would be inclined more to suspect things that interface with the microcomputer rather than the microcomputer itself.