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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello All,
I have a question related to how power can be going to a circuit board if the turntable is supposedly turned off. What leads me to ask is i am doing a restoration on a BM 4002 with the AC motor. It was non-working when I got it on Ebay. I got a capacitor kit from Martin some time ago and have been taking my sweet time restoring the turntable. Cleaned it all up, replaced the capacitors including all of the large ones and the old carbon resistors. When I replaced resistor R1 on the main board I noticed the board was really dark there, heat was my guess. Anyway tried to turn on the turntable after components were replaced to test. The AC platter motor was not turning but the tone arm carriage moved to the 12 LP position and arm dropped, but without a record in place. The other thing is the 33 light on the control panel does not come on but the 45 display is lit up. If you push the 33 button it lights but does not stay lit, not a bad bulb it appeared.
So with a meter in the open circuit setting I checked for shorts in the board and discovered that near the outside edge of the board where a screw hole is that is nearest the front of the turntable there was a crack. Nine breaks in the traces. I reinforced the board from the top and soldered all the cracked traces. Now the motor runs but the arm does not drop at all or recognize a record on the platter and the 33 display is still not lit, just 45. The sensor light behind the ruler is visible as well as the light under the tone arm so those are fine. I have reflow wd all the solder joint I could as well. And I have checked for any additional broken traces.
So I got ready to start testing all the transistors and diodes. I then unplugged the turntable and grabbed the board near the edge where resistor R1 is and it was really hot. The table had been off for sometime so I think power is leaking to the board and guessing that maybe that this is related to what is causing the problems mentioned before, power I wouldn't think should be getting to the circuit board of the table when it is supposed to be off. This explains why the board was burnt I guess with that resistor under power as long as the table is plugged into power. Any thoughts would be so welcome as to how this might happen.
John
I have two Beogram 4002 type 5503 units I am currently working on. One is on the bench and getting close to complete. Looking at the R1 resistor on both of mine, that area of the board is darker as well. It looks like it must dissipate a lot of heat. Hopefully I will be up and running soon and I can check mine.
-sonavor
Thanks for the reply Sonavor. But I don't understand why the resistor would be getting power when not in use which is my case. As long as it is plugged in the resistor is hot. Can't imagine that would be the normal state and how it was engineered but I have little knowledge of how these were designed.
In the schematic it looks like R1 will always have power connected through it when the Beogram is plugged in.
The current when the unit is off should only be about 18mA.
If it helps for anyone who is looking at this post, the resistor is specified as R2 in the service manual for the type 5501-5502. R1 for the type 5503. My meter is not working in the amperage setting so can't get a reading. The resistor is getting hot to the touch however in the off position but plugged in to the outlet. If anyone who had the earlier model 4002 with a AC motor could see if their R2 is really warm even when the unit is not playing that might be helpful for me to know.
Measure the voltage drop across R1 - for both the on and off positions of the Beogram. You can calculate the current from that.
Looking at the BG4002 (AC motor version) power supply schematic. It appears that the "Off" voltage should be 41 VDC. I'm not sure if the switch designated as "SO" is on at that point. If the "SO" switch is engaged, then the current through R1 should be 18mA. If "SO" is disengaged, then the voltage across R1 is something like 41 minus 24 or 22 VDC. So the current through R1 would be in the 7mA to 8mA range. The most current then is when the "SO" switch is engaged which results in around 0.7 plus Watts on R1. R1 is rated for 1 Watt. Once I get my BG4002 ready to plug back in (maybe this weekend), I'll measure my actual voltages.
As far as I understand it the SO switch is closed in the 'home' position of the carriage to pull down the base of 1TR1, which turns off the 21V rail. In that case R1 sees most of its dissipation.
Rudy
http://beolover.blogspot.com
http://beolover.com
Beolover: As far as I understand it the SO switch is closed in the 'home' position of the carriage to pull down the base of 1TR1, which turns off the 21V rail. In that case R1 sees most of its dissipation. Rudy
SO is short for "switch off" and you're right. The ON switch << overrides that and so needs to be pushed long enough for the carriage to clear the SO switch, after that the supply stays on by its own.
--mika
So the R1 (2200 ohm) resistor will typically dissipate around 0.7 Watts when the turntable is off. I was a little surprise on both of my BG4002, 5503 units that the PCB around the R1 resistor is so dark (from the heat). Maybe adding a heatsink to the resistor would help?
Pertinax board doesn't need that much heat to darken over the years, so it's nothing to worry about unless it's actually charred. Anyway it is a good idea to install potentially hot resistors so that they don't touch the board, that will also allow air to circulate around them.
but this is a good case for checking the voltage selector; if it is still at 110 / 220V from the old days, it is a good idea to switch it to 120 / 240 to help the components that were designed to work near their limits.
tournedos: Pertinax board doesn't need that much heat to darken over the years, so it's nothing to worry about unless it's actually charred. Anyway it is a good idea to install potentially hot resistors so that they don't touch the board, that will also allow air to circulate around them. but this is a good case for checking the voltage selector; if it is still at 110 / 220V from the old days, it is a good idea to switch it to 120 / 240 to help the components that were designed to work near their limits.
Thanks Mika, that is good to know. Both of my boards look okay, no charring. Just darkening. I will also make sure the voltage selector is set to 120. My line voltage measured around 125 VAC the last time I checked it.
I really did wonder for some time what "SO" stood for...;-). Thanks for the illumination! ES = 'end switch'?
Beolover:ES = 'end switch'?
I believe so
I just measured the voltage across resistor R1 and got 43 VDC with it plugged in but not running.
I finished my electrical work on my BG4002 tonight and ran a quick check on the R1.Here is my measurement with the Beogram plugged into AC and in the off position.
This is the R1 voltage when I pressed start.
The voltage across R1 varies between a little over +10 VDC to over +13 VDC for various modes (play, forward, fast forward, 45 and 33).On these 5503 type Beogram 4002 turntables, it appears the line voltage is fixed for 117 VAC. I don't seen any switches for adjustment.My Beogram still has a lot of work left. Both of my type 5503 units smelled like they had been stored in a basement. The next step for me is to start going through the adjustment/alignment steps. For the most part I have been following Menahem's nice PDF document on his BG4002 restoration so I don't really have anything new to add. I don't want to hijack this thread so I'll start a new thread for the work I am doing on my two Beograms.
Sonavor, so you are reading 45 volts in the off position? Just want to confirm.
Yes, a little over 45 VDC in the off position and as low as 9 VDC during play mode at 45 RPM. I see voltages from 9 VDC to a little over 13 VDC during the Beogram 4002 operation. The highest voltage is at rest. In my restoration of this turntable I followed the lead of Menahem's nice Beogram 4002 restoration guide on his Condor Audio web site and raised the four large wattage resistors away from touching the circuit board (on that main board). I ran my Beogram through some paces tonight. I am going through the adjustments specified in the service manual. The first couple - adjusting the platter for height and spacing took me an hour and a half. There was so much adjust-recheck-adjust to get the platter set right. I hope I can get through the rest of the adjustments this weekend. I already know that the control switch for when the tonearm reaches its farthest point needs to be calibrated (calibration is just adjusting the position of the board the switch sits on). My Beogram also has a badly corroded Din connector so I am going to replace that connector along with the cable. I'll do that last though.
Thanks for the reply on that voltage question. So mine is no abnormal. Good to hear what you have been doing since it parallels my work, only you seem to know what you are doing technically. ;0)
i can fix what I can find is broken and solder pretty well. I have no idea how the boards on mine got broken by the previous owner. I found not only the crack around the screw hole on the main board but the upper board that is in front of the tone arm also had a crack in it and two broken traces. I will be checking some other components then for faults and look at Menahem's restoration guide. By the way, I see two capacitors in your photograph on the bottom of your board, mine does not have those. Is this a modification by you or were those there all the time?
The two capacitors were originally tantalums and I replaced them with Wima polyester capacitors. The larger one is 4.7uF and I discovered today that it is too big (physically) to go on that side of the board so I moved it to the component side. I'll post some pictures once I start my thread.
Both of those capacitors are not documented in the service manual but both of my type 5503 BG4002 decks have them. So I am guessing they are from some update B&O put out. I just don't have the documentation on them. I have a couple of audio components I bought that have some broken boards. You never know what is inside these things until you get them.
Progress on my BG 4002, I think. I found that one of the main problems was the control panel had the plastic piece underneath the 45 button come loose and was keeping the 45 always on and lit and keeping the sensor arm from working correctly it seems. Now the sensor arm recoconizes whether there is a record on the platter and it sets down at the 33 position. But neither the 33 or 45 indicator lights will light when you push start and the arm is in motion. If you push either button they light but not when start is pressed. What would cause this fault? I think I might be close to having this fixed if anyone can help here.
When you press the speed selector buttons, they immediately apply a low to the lamps so they come on. The lights illuminate during operation of the turntable (like when you press Start) through control logic that latch the low side of the lamps. That's a simplified explanation. Since you know your lamps are not burned out, there must be something out in that latching circuit.
Here is the speed selection circuit.
Thanks very much for the reply Sonavor. I will check as many of the components as I can in the selection circuit. Is there some particular place I might check for voltage when I press start to see if I can isolate the problem some?
john
I've never traced through that part of the circuit but it appears like the 1TR3 (45 RPM) transistor turns off when the 33 RPM button is pressed. That causes the 1TR2 (33 RPM) transistor to turn on and latch that selection. The 45 RPM button does the opposite. It sounds like that part is functioning on your Beogram, right?
The part that isn't functioning is when that speed selection comes from the Beogram (not you manually selecting the speeds via the buttons). As I said, I haven't looked at that part before but it looks like it starts in the part labeled Speed Selector on the schematic. So you would need to monitor those control lines to see what is going on. Is the problem only that the speed indicator lamps don't illuminate...but the turntable does switch to the correct speed? Or does the turntable just stay in one speed when it is left to the turntable to decide?
When you push start without a record it appears the speed stays at 33 across the platter and then back to stop. It is running at 33 when a record is in place. I just realized when I tested it this time that if you push the 45 button it only stays at the 45 speed as long as the button is depressed. So bacsically it always defaults to the 33 speed.
So it won't latch and hold the 45 RPM speed. You might need to check the components around and including 1TR3 then. I would think the 33 RPM lamp should come on during normal play then. But you say it never lights up unless you press the 33 RPM button? If that is the case then there could be a problem with any of that speed selector circuit and the turntable just defaults to 33 RPM by design. When you press and hold the 45 RPM button, does the turntable speed actually turn at 45 RPM? When you get the 33 RPM lamp to illuminate, do you have to hold it down for it to stay lit? It makes sense that holding the buttons down would illuminate the lamps as it grounds the lamps so they have to light (as long as the bulbs are not burned out).You need to measure the voltages at the base node of 1TR2 and 1TR3 when the Beogram is going through its operation.
My thanks again Sonavor. The platter motor was pretty noisy so I have pulled the pulley off to try and lubricate the motor with a little oil dripped on the spindle. I am giving the oil a little time to penetrate before I put it back on and try checking the speek again. I will check in a few hours and get back with what I have found out. Thanks, again.
As long as the 45 speed button is pushed the table will spin at 45 it seems. Otherwise 33 and no indicator lights when on is pushed. Measurements from the base of 1TR3 when it is on is 0V. Measurement for 1TR2 is 20V. I have replaced 1TR3 so I know that the transistor is not bad.
Here are what the Beogram 4002 - 5503 speed selection voltages measure on my turntable. Note that my Beogram has a burned out 33 RPM lamp.
I measured three states of voltages at each test node: Turntable Off, Turntable On at 33 RPM and Turntable On at 45 RPM.Plus the DC voltage rail for the 33 and 45 indicator lamps (Off and On)
You should see similar voltages for the states I have shown.
Here is an update to the picture. I added text to show where the control line is for the automatic 45 RPM speed switching. I didn't measure the 1TR2 base voltage with the platter on for the automatic 45 RPM speed change. I'll see if I can measure that.
This version of the picture shows the 45 RPM auto-detect speed change. The 1TR2 PNP gets turned on by a short, negative voltage signal (-1.8 VDC on my Beogram). That takes turns off the base of 1TR3 and turns on the base of 1TR4 which latches the speed select to 45 RPM.
Hello Sonavor,
I really appreciate you sticking with me on this, and all the work you are doing with these measurements and schematics. Hopefully it will be of benefit to others as well. I will make the measurements tomorrow and try to post my results back to you. I am not sure if I will be able to do the 45 measurements since the 45 button needs to be pushed down for it to run at that speed. Just to be sure I should have the common lead grounded to the chassis when I take these reading with the red V probe at the points shown?
Also I have a question about the lamp that is on the main board which in the 5503 schematic is labeled IL1. Since it is under the board when it is mounted to the chasis what purpose does it serve? I have noticed that no matter what button I push on the control panel it never lights. I see no light sensitive diodes near by either. A puzzle to me.
John Francis:Also I have a question about the lamp that is on the main board which in the 5503 schematic is labeled IL1. Since it is under the board when it is mounted to the chasis what purpose does it serve? I have noticed that no matter what button I push on the control panel it never lights. I see no light sensitive diodes near by either. A puzzle to me.
It is part of the stabilizing circuit of the Wien bridge oscillator that creates the waverform to drive the AC platter motor. Hewlett-Packard was founded around products using this circuit. The bulb is used for the thermal properties of the filament, and sees very little current so it doesn't actually light up. Good trivia to know
Thanks for the information Mika. I also wondered about that bulb.
John Francis: I am not sure if I will be able to do the 45 measurements since the 45 button needs to be pushed down for it to run at that speed. Just to be sure I should have the common lead grounded to the chassis when I take these reading with the red V probe at the points shown? John
I am not sure if I will be able to do the 45 measurements since the 45 button needs to be pushed down for it to run at that speed. Just to be sure I should have the common lead grounded to the chassis when I take these reading with the red V probe at the points shown?
I just checked the turntable to verify what is going on and to correctly answer your questions. So this is what I have found.
1. When you press start the 33 lamp lights but immediately goes off when you release the button. The arm travels across and if there is an LP in place it drops as it should. Without a LP it continues to travel at the 33 speed and does not switch to 45, then returns and shuts off. If you put a 45 on the platter it finds the 45 and drops as it should but no light is on in the control panel and the speed is 33. As long as the 33 or 45 buttons are being pushed down they illuminate and 45 will run at 45 if the button is held down for 45.
2. and 3. You understand correctly what is happening there.
I have not yet made the measurements at the points you did but will hopefully get to that, Christmas eve chauffering of my wife looms near. I hope you have a happy holiday and will try to post again later today.