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,
Hard to say - I use a Fluke 8050A Meter with some cheap leads and my zero ie when I connect my leads together I measure is 0.37 ohms, so that is my zero. What's your zero number? If it is significantly lower than 0.7 ohms then maybe the way you re-soldered 1C13 back in after measuring it is not so great.
Just to check that we are on the correct circuit diagram the serial # for my BG 4002 is 2869XX, is yours above 257556? If it is similar we can also compare some measurements.
Did you change the lamp 1IL1? The resistance of the lamp is very important in setting the gain of the stage. When it is cold it has a lower resistance which causes a high gain and kicks the system into oscillation. As the lamp warms up the resistance increases and the gain reduces to a more stabilized value. The gain of the circuit is (R26 + Rlamp) / Rlamp. If you changed the lamp to an incorrect alternative value you can throw the system out to point that it may not oscillate in the first place.
How did you go with the photos?
Regards
Lonnie
Hi Lonnie,
My 'zero' is 0.6 Ohm and the ser# is 296172.
No, I did not change that lamp - only checked ts resistance cold which should deliver an amplification of about 7.5 according to your formula.
Tried a few shots of the solder-side of the main board - not great but will do some more.
This whole issue is a total mystery to me and all happened suddenly after replacing 1TR5, 6 & 7 with the correct type ?!?!?
Cheers
ALF
The resistance of the lamp should go up to around 70 ohms when it is on and has voltage across it which will set the operating gain to around 3.
Check if that is happening.
Hello Lonnie,
What is the expected time until the lamp reaches about 70 Ohm ??
What dc / ac voltage do you expect for that condition ?
My lamp only shows about 23.x V after turning on the table and practically doesn't change !
Wouldn't you expect the resistance of the lamp to change after a minute or so ?
Is this indicative of the lamp not getting sufficient voltage or is the lamp 'half dead' ???
If I would substitute the lamp with a 70 Ohm resistor for testing purposes it should work unless the problem is somewhere else, shouldn't it ?!
!!!! 23.x Ohm !!!! Not V !!!
It lamp resistance should change quite quickly.
If we look at the 45 rpm circuit again it is made up of the gain stage which consists of 1TR6 feeding 1TR7 with feedback from the collector of 1TR7 back to the emitter of 1TR6 via 1R26 and the lamp which is AC grounded through 1C13. AC is generated by the bridge and is amplified by the active stage and is what will heat the lamp. If there is no change in the resistance of the lamp then it is just confirming what we already know - the circuit is not working.
I would remove 1TR6, 1TR7 1C13, 1C6 & 7 and remeasure them to confirm that they are ok. Re-install the old 1TR6 & 1TR7 as they measured ok, make absolutely sure the polarity of the capacitors are correct. Then check the DC conditions around 1TR6 & 7 to make sure that can operate correctly.
Measure resistance across 1R17, it should be around 5k9 and measure resistance across 1R23. It should be similar.
1R17 = 5.5K and 1R23 = 6.1K - both on-board !
1C13 = 44uF, 1C6&7 = 0.2uF, all off-board and put back with correct polarity !!
1TR6 & 1TR7 replaced with original components and measured off-board !
DC voltages
1TR6: C = 0.68V, B = 8.9V, E = 8.8V
1TR7: C = 8.9V, B = 0.71V, E = 13.3mV
Resistance of lamp still does not change and remains at about 23 Ohm.
Does that make sense ?
The measurements on 1R17 & 1R23 should be a little closer to each other but they are fine for what we are trying to achieve at the moment.
Glad the caps are good.
The measurements on 1TR6 should be causing alarm bells.
Can you re-measure, use the ground point on capacitor OC1 or 2. The base should not be higher than the emitter! You have to now ask how did that happen, where is it coming from? Check if 1R11.12 &13 are ok but it looks like the issue comes from upstream. Do you still get 0.1V at the collector of 1TR4 when you select 45 rpm?
'45' has come to life after I remeasured B and E of 1TR6:
B = 7.05V, E = 7.88V at '45'
C of 1TR4 = 0.188 at '45'
I also checked the sine-wave with the oscilloscop - looks smooth at 57.xxHz - I am getting confused about the suggested Pk-Pk voltage ?!
Anyhow, '33' still shows no sign of life.
Correction,
With the motor running at '45' I measure 0.68V at C of 1TR4 !! And it stays the same when I switch back to '33' which should not happen ?!
But that seems only one of the logical outcomes caused by the original problem......?!?!?
You are most probably right with your up-stream suggestion, but where is up-stream ?
Sorry about the confusion but that is how it is !
Great, we are making progress. Output Amp works, 45 rpm Wien Bridge circuit works. Did you measure the lamp resistance to watch it go fro 23 to ~70 ohms?
Can you confirm that the collector of 1TR3 goes from 22v to 0.1V when you switch to 33 rpm?
C of 1TR3 goes down to 0.158V
1IL1 shows about 62 Ohm
Alf, there really was no reason to touch the TR6 transistor based on the original problem. If you look at the BG4002 service manual schematic for the Type 5501 - 5503, there are really two Wien-Bridge oscillators there...one for 33 RPM and one for 45 RPM. The 45 RPM Wien-Bridge circuit uses 1TR6 and its associated feedback network (includes 1R24 trimmer) while 33 RPM uses 1TR5 and its associated components (includes the 1R21 trimmer). So now, what is proven out, is that the 45 RPM oscillator works and the 33 RPM oscillator doesn't. That should isolate the problem to just the area involving 1TR5 and it's direct components. The 1IL1 lamp and 1C13 capacitor are attached to both the 33 RPM and 45 RPM feedback so it is unlikely that it is part of the problem now. Something specifically in the 33 RPM only path must be the problem.The components, wiring path and solder joints to check are then -1TR5 (do you think it is original?)1R81R91R101R151R161R191R201R212R11C61C81C10Compare again the input to the 45 RPM Wien Bridge (input to 1R11). It should be the step input that starts at 22 V and goes to 0.1 V......to the 33 RPM Wien Bridge (input at 1R8). What is going on at the base of 1TR5 in comparison to the base at 1TR6 as you switch between 33 RPM and 45 RPM?What is going on at the base of 1TR7 now as you switch between 33 RPM and 45 RPM?John
I attempted a circuit simulation of the Wien Bridge Oscillator in the BG4002, 5503 type turntable. Using the simulator I simulate power on with the S1 switch, 33 RPM selection with the S33 switch and 45 RPM with the S45 switch. Note that in this simulator I have to manually make sure just 33 RPM or 45 RPM are selected.In running this simulator, it appears that the most likely cause of a flat line output for 33 RPM would be either the input signal at 1R8 or somewhere in the feedback components - 1C10, 1R19, 1R20 or 1R21 (trimmer). Removing traces for the other 33 RPM oscillator circuit did not cause a dead oscillator (for just the 33 RPM selection).Note: The attached image is a case where I had to use a 1200 pixel wide image and even that is a bit small to read everything.
Great work John,
It's nice to be able to see some of the inner workings of the circuit.
I think you are correct about the input to 1R8 being the issue now.
ALF,
Can you re-measure TR3 & 4 as there was some conflicting measurements in your last couple of posts. Can you measure relative to 0C1 ground the following:
When 33 rpm selected - measure TR3 B, E & C then TR4 B, E & C, then TR5 B, then TR6 B
When 45 rpm selected - measure TR3 B, E & C then TR4 B, E & C. then TR5 B, then TR6 B
When you removed the "new" TR5,6 &7 and replaced them with the original ones, did you re-measure the "new" ones to confirm they still measure good?
Out of curiosity what do you use to measure your transistors?
Hi john,
Many thanks for taking the time to help......once again:-)
I amm aware some of my measurements differ but I have to stress that again.....
It is that crazy inconsistent perrformance the table shows - I hardly dare to post some findings as it simply may differ from day to the next
To no faut of my own !!!!
1TR5 to me look original with those typical colour stripes.
The input to 1R11 changes from about 21.3V to 172mV
The iput to 1R8 changes from 21.1 to 8mV
C of 1TR5 changes from 7.02V -33 to 8.1V -45
C of 1TR6 changes from 8.15V -33 to 7.04V -45
B of 1TR7 changes from 0.68 -33 to. 0.67 -45
I have not yet checked the rest on your list.
let me stress again how much I appreciate the help I am getting - inconsistent measurements are down to absolutely strange
behaviour of that turntable - I can only measure what is there.....and am almost hesitant to post any as it looks bad on my end.
there is nothing I can do about it, I am also trying to do my very best to get this beast running again.
in answering your questions:
measurments from this morning are as follows
at '33'
1TR3 B=0.93V, E=26mV, C=170mV
1TR4 B=170mV, E=27mV, C=22.4V
1TR5 B=7.1V, E=8.1V, C=0.7V
1TR6 B=8.24V, E=8.08V, C=0.7V
at '45'
1TR3 B=36mV, E=30mV, C=21.9V
1TR4 B=0.95V, E=30mV, C=192mV
1TR5 B=8.2V, E=8.1V, C=0.7V
1TR6 B=7.1V, E=8.1V, C=0.7V
I re-checked the new taken-out components again - all good !
transistors get taken out and measured with my DMM
cheers, ALF
Hi John,
Checked all R and C as suggested - all fine !!
It's learning for me as well.
When I look at those measurements I see one issue but It looks like you should now have the motor spinning at both 33rpm and 45 rpm. With the 33 switch engaged 1TR5 should be conducting and 1TR6 is cut-off while for 45 rpm it is the other way round. Is that now the case, do you have the motor working at both 33 & 45 rpm?
The one issue I see is with the 45 rpm reading of 1TR3. The base should be higher than the 36mV, can you re-check that measurement?
I found that with the BG4000 turntables so many of the intermittent issues are caused by all the switching that takes place. I wonder if it is similar for the BG4002. It may be worth pulling the keyboard PC board and cleaning all the switch contacts. It would also be worth cleaning the contact switches that the arm activates as it moves across the platter.
Lastly, is this a TT that you had for a while, worked reliably and now has only just gone faulty or is it a TT of unknown history?
Re-checked of B at 1TR 3 = about 33mV. (As we speak)
The table in 'play mode' start very briefly at '33' but switches suddenly very quickly to '45'
At '45' I am also unable to operate the arm lowering - only works in '33'
You see, I am absolutely confused about this table's performance and just don't know where to start!
I'd love to cut free the main board and send it to one of you experts :-):-)
This carriage does not activate any switches while crossing unlike the BG4000.
The table has been purchased from the US second hd and was working after some minor repair work.
The problem started with the 'up grade bug', replacing those grey cap reservoirs and the bulb-assembly for the servo !!
To be frank it is getting close to throwing in the towel considering the number of hours I spent with no progress !
Finally, the motor spins at '45' and '33' in principle.
ALF: Finally, the motor spins at '45' and '33' in principle.
Sadly this is only re-instating the old status - yes, the motor spins again at '33 & 45'.
I wish I could tell you whatthe ''fix'' was - all I did was re-inserting the original 1TR5&6 that I had replaced with
exactly the same type new BC212B transistors - if you want to call this a fix ?!!!?
But as described in some of my previous posts - again the table starts very briefly with '33' and a second later jumps to '45'.
Once in '45' the solanoid can not be activated to lower the arm - it feels like being attacked from all sides at the same time......!
In this context the 'again spinning' motor means very little progress, doesn't ?!
Wouldn't it not be a good idea to take off all components on the main board and start building it from scratch ?? Because frankly
I no longer know what and what to check :-):-)
Change one of the last 'whats' to 'where'
That seems the only time the brain is ahead :-)
You have to be careful in tackling too many issues at one time. Now that the oscillator is fixed did you check and calibrate the sine wave of the motor amplifier per the service manual? You should get all of the motor speed section checked out and working (and calibrated) before moving to the next issue. The arm raising and lowering problem is a whole different problem and can be checked out by checking the control panel switches with the control circuits that drive the solenoid control. John
So just to be clear:
1. When you switch the TT on it briefly starts at 33 rpm and quickly shifts to 45 rpm and can stays at that speed.
2. When you manually shift it back to 33 rpm it will stay operating at 33 rpm.
3. When you manually shift it from 33 rpm to 45 rpm it will stay operating at 45 rpm.
4. The arm won't operate when in 45 rpm mode but will in 33 rpm mode.
Sticking with the issue of the motor. The base of 1TR3 should not be sitting at 0.03V. Can you double check that you are measuring the base by measuring 1R7 as this will give the same measurement as the base. The base should be sitting at ~0.9V at 33 rpm and ~ 0.1V at 45 rpm. If you are sure that 0.036V is correct then can you check to make sure the 1C3 is within spec and that 1TR2 is measuring ok.
Fully understand - somehow all these issue seem connected ???
As far as the 'oscillator fixed' goes I do not yet trust this as it is still unstable, meaning sometimes the motor-trimmer requires
a little re-adjustment - and that to me is not a reliable stable fix - to make sure it operates at both speeds !!
I had both speeds adjusted as per SM - all fine but.....as said it is not reliable but should work everytime !!
Yes, most of the time for Pt1 - what that means is I can turn on the table, it starts at '33' the arm moves in position and lowers......
That would work a couple of times until it goes bad as described. But yes, it then stays at '45'.
To Pt 2 - yes, I can manually go back to '33', however I need to keep that '33' key depressed for a very short while to make sure it stays at '33'
Once it switched to '33' it stays.
To PT3 - yes, shifting back to '45', which is accepted immediately unlike the other way round it stays there.
To Pt 4 - yes, the solanoid can not be activated at '45' and only at certain positions with '33'.
It simply is so hard to explain all that in an email but I am trying to make it sound plausible.
To the other measurements:
B of 1TR3 = 0.92V at '33' and 32.8mV at '45'
B of 1TR7 = 0.695V at '33' and 0693V at '45'
1TR2 (B,C,E) = 490mV, 27mV, 0.93V at '33' and 558mV, 29mV, 33mV at '45'
1C3 = 2.02 uF
Incase you are interested
Cheers ALF
2nd picture of motor adjustment
The sine wave outputs look pretty good. Frequencies are a little off but before messing anymore with those I would set the measurement information on your scope from Peak to Peak to RMS and adjust 1R27 to get the signals set to around 6 VRMS.That being the case I don't believe your turntable problems are related unless there is a grounding or voltage source problem involved. Regarding the arm lowering problem and speed changing, look at Rudy's description of a similar problem on a type 5524 Beogram 4002. The circuitry is not identical of course but the problem sounds similar and both turntables utilize a similar plexiglass bar with markings to determine tonearm position.
Done, however I can only get as high as about 4 V rms to maintain a smooth sine-wave signal without distortion or clipping!
I could go as high as 5V rms only for the '33' mode but that would cause a distorted signal for '45'.
Thanks for pointing out that Beolover link - I had lengthy discussions with Rudy about my issue but sadly that did not delivered a desired
solution.
It is truly wheird the way that table performs and incredibly difficult to explain in a simple text! I have not yet given up but the light is getting dim....
Just to clarify the measurement I was after was the resistor 1R7 but your measurement of 1TR2 emitter re-confirmed your 1TR3 base measurement anyway.
Did you get a stable 0.558V DC measurement of the base of 1TR2 at the 45 rpm setting? Or did it drift lower?
Interesting to note the the circuit goes from the base of 1TR2 into the automatic lowering part of the circuit!
Here is a simple?? way to think about it. 1TR2 is a PNP the arrow on the emitter points inwards which is the direction of the current flow. Current flow and electron flow were supposed to be the same thing but they guessed wrong about how electrons worked. We now know that electrons are negatively charged and they will always travel towards a positive potential, so electrons will travel from the collector to the emitter. The collector of 1TR2 is grounded so it must be at zero potential and for the electrons to travel to the emitter it must have a positive voltage on it. You can think of the base as an electronic valve that sticks a little bit, if there is no potential on it relative to the collector it won't open and it needs a certain amount to overcome the stiction. The more potential on it the more it opens and the faster the electrons can flow through. If there is a higher potential on the base than the emitter then the electrons are just going to travel to the base and stay there, so the base always needs to be less than the emitter. Lastly, some electrons which are sitting at the base also get attracted to the emitter. The total amount of electrons which make it to the emitter are the sum of the electrons from the collector and the base. When you convert the electron flow back into current flow then current flow into the collector Ic = Ie - Ib which is the standard equation for a PNP transistor.
So you should be able to answer your question.
Also this is why I have not been happy with your measurements of the base of 1TR3 which is the same as the emitter of 1TR2.
Just a correction on my last post, the bias on the base is relative to the emitter not the collector as I said. In the PNP the base is negative compared to the emitter and for a NPN the base would be positive to the emitter.
Just another thought:
Did you by any chance measure the resistance increase of the opto-resistor when a black marker on the plexiglas marker board passes / blocks light to that resistor - the light bulb looked awfully dim in my table ?
I went through Rudy's blog about that sudden speed change at the start and obviously had to identify the apropriate involved components in the
Type5503 circuit diagram - it would be interesting to know whether you can confirm his meaurements for our 4002-type. 5503 model or are these different ??
Hi Alf,I have to say I am still a little confused as to where you are in your restoration but regarding the Beogram tonearm position logic, no I never had to dive into the opto-resistors like Rudy did on one of his recent Beograms. To find and fix tonearm position related problems, just be systematic in going through each function. I know that sounds like over simplifying the task but that is how I approach it.
John
hello John,
well, the plattermotor-function is restored for both speed settings in principle, only the 6Vrms setting has not been achieved without creating a distorted sine-wave - as said earlier 4.xxx V rms seems the working maximum.
the other issue is the sudden speed change at the start from 33 to 45 with tonearm not lowering - hence the link you provided for the Beolover website.
I identified the involved components as Rudy's Blog was for a different 4002 type - not the type 5503 !!
this is where I am at right now.
The resistance of that opto-resistor apparently changes dramatically whenever a black marker on the plexiglas position marker blocks the light to that resistor.
firstly I wanted to make sure my resistor operates in the ballpark with its resistance change and obviously I need to restore the arm-lowering at the 30cm mark with stable 33 speed playing records again.
cheers