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
Inspect again what is at the bottom of the deck. There could be component legs touching one another, etc. One never knows.
Jacques
Menahem Yachad: Another problem area is the pushbutton sensor under the keyboard. Check that both bulbs are working, that you get a circuit change whe pushing the left or right side of the plunger, and that the black wire hasn't come loose. See the little solder-blobs on that picture, around the line of solder joints? Blobs like that can cause momentary shorts and cause problems like you're having. It comes from resoldering with the wrong solder-iron tip. Did you change the cap in the CPU box? Did you put all caps back in the correct polarity?
Another problem area is the pushbutton sensor under the keyboard. Check that both bulbs are working, that you get a circuit change whe pushing the left or right side of the plunger, and that the black wire hasn't come loose.
See the little solder-blobs on that picture, around the line of solder joints? Blobs like that can cause momentary shorts and cause problems like you're having. It comes from resoldering with the wrong solder-iron tip.
Did you change the cap in the CPU box?
Did you put all caps back in the correct polarity?
I haven't checked the pushbutton sensor yet but I did replace the electrolytic capacitor in the mpu box. Right now though, I have been doing the power supply voltage testing without the mpu box and without the control panel connected. I wanted to verify the voltages first. I could probably re-attach the mpu box as I am seeing the +5 VDC supply voltage is working.
After the recap I went through and checked the capacitor polarity positions a couple of times. The caps are all in correctly.
I will double-check my transformer voltages now with what you measured.
Thanks,sonavor
Menahem Yachad: These are my measured voltages across the pins. 1-3 17.0VAC 2-4 10.7VAC 5-11 30.2VAC 6-12 no volts - it's the 39uF capacitor 7-10 17.0VAC 8-9 5.3VAC Just disconnect the black Xformer box from the PCB, connect it to AC, and measure the voltages across the pins. Because you're blowing a fuse, you need to build yourself a "Dim Bulb Tester" with 100W bulb. There's plenty of info on the web. It will save you blowing fuses, and disconnecting circuits, as you've done. You should have resoldered everything before reassembly - yes, the circuits are fragile, but for those joints you need a fine-pencil soldering tip. If you already have a high-quality Weller or Hakko solder station, you can buy the suitable tips. Check the big transistors mounted on the chassis. If you use a dim bulb tester, you can leave the BG8002 powered on for a while, and feel around for hot components - that's the cause.
These are my measured voltages across the pins.
1-3 17.0VAC
2-4 10.7VAC
5-11 30.2VAC
6-12 no volts - it's the 39uF capacitor
7-10 17.0VAC
8-9 5.3VAC
Just disconnect the black Xformer box from the PCB, connect it to AC, and measure the voltages across the pins.
Because you're blowing a fuse, you need to build yourself a "Dim Bulb Tester" with 100W bulb. There's plenty of info on the web.
It will save you blowing fuses, and disconnecting circuits, as you've done.
You should have resoldered everything before reassembly - yes, the circuits are fragile, but for those joints you need a fine-pencil soldering tip. If you already have a high-quality Weller or Hakko solder station, you can buy the suitable tips.
Check the big transistors mounted on the chassis. If you use a dim bulb tester, you can leave the BG8002 powered on for a while, and feel around for hot components - that's the cause.
I measure the following on my transformer -
1-3 17.6VAC
2-4 11.5VAC
5-11 31.5VAC
6-12 no volts - my transformer has a 27uF bipolar capacitor instead of the 39uF capacitor shown in the schematic. I measured the 27uF cap with my ESR meter and it measures good.
7-10 17.8VAC
8-9 0.795VAC
So the last one (pins 8 and 9) appear to have a problem.
Yes, I would say that pins 8-9 have a problem.
I have a local transformer manufacturer here, who can rewind the transformer for you, if you can't find another solution. He's now done 4 B&O transformers for me, perfectly. Send me a PM if that's your only option.
I am back to my transformer being okay. I pulled the transformer out of my working Beogram 8002 and I get the same measurements. The way I am measuring transformer output pins 8 & 9 must be different. If I measure across pins 8 & 9 with a DVM I measure 0.795 volts AC. If I measure each pin with a scope probe (at the same time) I can see that each pin is a sine wave of around 6.6 volts AC but they are in phase with each other. So measuring across the two pins will result in a value close to zero. Both of my transformers give the same result.
Tomorrow I should receive some new transistors to try out on the power supply to see if that is the cause of my +15 VDC rail issue.
I might be closing in on the +15 V rail problem and I am able to verify the -15 V works fine.
I decided to replace the TR18 (BC547B - NPN) transistor because its emitter connects to the anode of the D25 zener diode that controls the +15 V rail. I also re-soldered some PCB1 nodes that I didn't like. They didn't look good to me so I reworked them taking extra care. The result is the +15 V rail is now below +20 V. It still isn't steady. It will sit at +18 V for a while then drop down to +15 V and occasionally +12 V. So there is still something wrong. I did re-connect the PCB2 (MPU board). The control panel functions properly and when I press the Play button I get the -15 VDC at the D26 zener. That voltage is very steady so I believe that D26, TR19, TR20 and TR21 are all good. For the +15 V I am going to look at replacing the D25 zener anyway and maybe 0TR1.
-sonavor
I am still having a problem with the +15 VDC supply. I replaced the D25 15V zener with a different one and I switched out the 0TR1 PNP (TIP32) transistor. The anode side of the zener stays steady near 0 volts but the +15 volt side has now dropped and fluctuates around 7 and 8 volts. So that's not good. I checked C28 (4.7uF) and R77 (6.8k) and R78 (1k). Those components test good. Is something the +15 V supplies to pulling the voltage down. I am thinking I need to remove connections from the C28, D25 and 0TR1 collector node and see if I then get a nice +15 V. That will tell me the power supply section is all good as I still have a good +5 V and a good -15 V.
Sorry, nothing to read here
haha - thanks for the post anyway
I am just going to take my time and check out the components in the circuit path. There has to be something that got shorted, disconnected or went bad. Meanwhile I am itching to get started on a refurbish project on a pair of Beovox MS150.2 speakers I recently aquired. Unlike the MS150 speakers I repaired last year, this pair has the original stands and has all six of the speaker grills. Before that though, I almost have all of my parts for Frede's Classic Audio Tester project so I need to start putting that together (and start using it).
sonavor: haha - thanks for the post anyway I am just going to take my time and check out the components in the circuit path. There has to be something that got shorted, disconnected or went bad. Meanwhile I am itching to get started on a refurbish project on a pair of Beovox MS150.2 speakers I recently aquired. Unlike the MS150 speakers I repaired last year, this pair has the original stands and has all six of the speaker grills. Before that though, I almost have all of my parts for Frede's Classic Audio Tester project so I need to start putting that together (and start using it). -sonavor
Color me green with envy. You must have a sympathetic FS (or none at all) and lots and lots of space. Neither situation describes me.
Rich: Color me green with envy. You must have a sympathetic FS (or none at all) and lots and lots of space. Neither situation describes me.
I have been getting some flak about too many pieces of audio equipment entering the house. So I will have to start getting some backlog cleared out and going to therapy ...to try and slow down this disease. The bad part is B&O isn't the only audio brand disease I have. I will have extra space once some more kids move out of the house.
I haven't had time to get back to the 8002 the last couple of days. But I did get a cool thing in the mail today...an original copy of the Beogram 8002 service manual. I have been working off the PDF version from Beoworld. That works okay but I prefer to have both the digital and hard-copy of service manuals. The PDF version isn't completely clear on some fo the print in the schematic so the hard-copy clears that up. A big thanks to Leslie for supplying me with the hard-copy.-sonavor
sonavor: Rich: Color me green with envy. You must have a sympathetic FS (or none at all) and lots and lots of space. Neither situation describes me. I have been getting some flak about too many pieces of audio equipment entering the house. So I will have to start getting some backlog cleared out and going to therapy ...to try and slow down this disease. The bad part is B&O isn't the only audio brand disease I have. I will have extra space once some more kids move out of the house. -sonavor
My kids are 5 and 6, so I won't have that assist for quite some time. The hope is to move to a bigger place sooner rather than later - but still I doubt wifey will come around.
I have been trying oh so hard to resist the temptation of buying a Nak - there have been a few BX300 cassette decks on Ebay in the recent past that I've almost bid on.
Wifey is much more forgivable about record collecting, so that's what I've been concentrating on for the past year or so.
I can't figure out what goes on in the US with you guys.
Whatever I want, I buy, and I put in the living room whatever equipment I want. That includes a 7cu-ft subwoofer which I built myself.
I don't get any negative feedback from my wife at all.
Perhaps you should all marry Eastern girls!
G-d bless you all!
Menahem
7cu foot sub....have you checked your foundation for cracks recently?
I started to continue along the +15 V rail path and decided I didn't like working with the low res trace diagram from the PDF version of the service manual. So I scanned in that image from the new, hard-copy service manual. Then I changed all of the shading for the +15 V traces to red. This makes it much easier to work with (for my old eyes anyway). I can now quickly find some good places to disconnect the +15 V so I can look for something that is dragging down the voltage.
sonavor:This makes it much easier to work with (for my old eyes anyway
Good idea, I have the same problem
Collecting Vintage B&O is not a hobby, its a lifestyle.
No cracks in the foundation!
In reality, the sub volume is set at only about 20% - it is so powerful (936WRMS calibrated, Fs-22Hz) that only a small amount is needed to create the desired listening effect.
I guess that if I were to try to run it at foundation-shaking volume, I'd have to run out of the house from the pain in my ears.
The old adage - You can never have too much power!
Scratching the back of my head, I wish that I'd never sold my BeoVox 5702 20 years ago. I would have loved to have done a comparison between them and this sub - at normal listening levels, of course.
I apologize for the delay in postings (and progress) but I have other things competing for time. From the last post where I mapped out the +15 VDC rail voltage, my goal was to continue verifying the Beogram 8002 power. After checking the key transistors and zener diodes involved in the path I haven't found anything that measures bad. I went ahead and tried out some new components and the results were mostly the same. However, some measurements changed. That was throwing me off at first because I hadn't been able to really understand the circuit. Originally (before recapping), the 8002 would immediately start the turntable rotating when plugging the unit in to 120 VAC/60 Hz. The 8002 display would flicker at 33. After the recap, I got the standby button and all of the functions appeared to work correctly. I wish I had made all of the voltage measurements I am looking at now. While left on standby for a bit, the 8002 would start rotating again (but the display remained showing the standby dot). Eventually that condition blew out the fuse in the transformer. I re-soldered connections and tried it again. The results were a little different but the fuse still blew. Since then I have focused on the power and have tested with the motor connections disconnected. The transformer hasn't blown a fuse since. I started to test the power rail voltages with the PCB2 (MPU module) disconnected. I was able to verify the +5 VDC rail that way but I couldn't with the positive and negative 15 VDC. It made sense that I couldn't measure the -15 VDC with the MPU module off because the power on/off signal comes from there. I got confused at that point because I was seeing +15 VDC (for a short time) on the +15V rail. Later, I would only see +22 V there. The way I understand it now, I think the 8002 should only have positive and negative rail voltages when the power on signal from the MPU is present. So something was wrong when I saw a voltage on the +15V rail without the on signal. Maybe it was just because the MPU module was disconnected so there wasn't a real control signal. Either way, I am thinking the circuit for the rail voltages (+5, +15, -15) is probably okay but I need the power on/off signal back working to test it. The problem I now have with everything re-connected (except for the motor drive connectors) is the MPU module is not coming up in the right state or some connection problem with MPU module signals. The reason is that the 8002 is now starting up in a mode similar to before the recap. The display shows 33 all the time. I can switch it to 45, and then back to 33 using the speed control buttons but those are the only buttons that do anything. I can trace +5 VDC all the way to the MPU module and some of the module output pins also show +5 VDC. The power on/off signal (P6-4) measures +0.072 VDC. The attached picture shows the positive and negative 15 VDC circuit. Red text is the expected voltages and the black text show some measurements I made with the 8002 powered up. It seems like these voltages are probably correct according to what the MPU module thinks the state of the turntable is. The next step is to figure out why the 8002 won't start up in the expected standby mode state. Also, I was considering testing the power supply voltages one more time with the PCB2 (MPU module) disconnected and manually applying a voltage to turn the 15 VDC rails on.
I apologize for the delay in postings (and progress) but I have other things competing for time.
From the last post where I mapped out the +15 VDC rail voltage, my goal was to continue verifying the Beogram 8002 power. After checking the key transistors and zener diodes involved in the path I haven't found anything that measures bad. I went ahead and tried out some new components and the results were mostly the same. However, some measurements changed. That was throwing me off at first because I hadn't been able to really understand the circuit. Originally (before recapping), the 8002 would immediately start the turntable rotating when plugging the unit in to 120 VAC/60 Hz. The 8002 display would flicker at 33. After the recap, I got the standby button and all of the functions appeared to work correctly. I wish I had made all of the voltage measurements I am looking at now. While left on standby for a bit, the 8002 would start rotating again (but the display remained showing the standby dot). Eventually that condition blew out the fuse in the transformer. I re-soldered connections and tried it again. The results were a little different but the fuse still blew. Since then I have focused on the power and have tested with the motor connections disconnected. The transformer hasn't blown a fuse since.
I started to test the power rail voltages with the PCB2 (MPU module) disconnected. I was able to verify the +5 VDC rail that way but I couldn't with the positive and negative 15 VDC. It made sense that I couldn't measure the -15 VDC with the MPU module off because the power on/off signal comes from there. I got confused at that point because I was seeing +15 VDC (for a short time) on the +15V rail. Later, I would only see +22 V there. The way I understand it now, I think the 8002 should only have positive and negative rail voltages when the power on signal from the MPU is present. So something was wrong when I saw a voltage on the +15V rail without the on signal. Maybe it was just because the MPU module was disconnected so there wasn't a real control signal. Either way, I am thinking the circuit for the rail voltages (+5, +15, -15) is probably okay but I need the power on/off signal back working to test it.
The problem I now have with everything re-connected (except for the motor drive connectors) is the MPU module is not coming up in the right state or some connection problem with MPU module signals. The reason is that the 8002 is now starting up in a mode similar to before the recap. The display shows 33 all the time. I can switch it to 45, and then back to 33 using the speed control buttons but those are the only buttons that do anything.
I can trace +5 VDC all the way to the MPU module and some of the module output pins also show +5 VDC. The power on/off signal (P6-4) measures +0.072 VDC. The attached picture shows the positive and negative 15 VDC circuit. Red text is the expected voltages and the black text show some measurements I made with the 8002 powered up. It seems like these voltages are probably correct according to what the MPU module thinks the state of the turntable is. The next step is to figure out why the 8002 won't start up in the expected standby mode state.
Also, I was considering testing the power supply voltages one more time with the PCB2 (MPU module) disconnected and manually applying a voltage to turn the 15 VDC rails on.
Jon
If the fuse is blowing, use a 100W dim-bulb tester. When the dim bulb lights up, the short has occurred in the BG. The fuse will not blow, and the bulb takes the short's load. You can then feel around to a hot part, and that's your culprit.
I would like to check out the circuitry without the motor connected first because I can't get the 8002 to power up in standby mode. Are you saying that the motor connectors have to all be connected for that to work? Once I have that working I was planning on reconnecting the motor connectors and trying everything again (applying power through my dim bulb tester).
Connect everything the way it's supposed to be, and of course the dim-bulb tester. What happens, happens, and feel around for a hot part. The dim-bulb tester will prevent any other damage being caused, other than to the shorted part.
What a saga!
The worst part is that I'm sure you will end up finding a stupid fault, like a solder bridge at a connector!
A very educative thread.
I hope it will be something not connected somewhere. This turntable has come to life a couple of times during this project.
Keep it up John, we are following and learning
In looking at the Beogram 8002 15 VDC Power On/Off circuit picture I posted, the voltages I measured and showed (in black text) don't look right.
The Power On/Off control signal from the MPU Pin 38 looks like it would turn the positive and negative 15 VDC off by applying a +5 VDC signal to the TR21 base. That would take the voltage at the TR20 emitter to a near zero voltage leaving the voltage at the TR20 collector pretty near the same as -22.56 volts at the emitter of TR19. So the TR19 base and emitter have the same voltage and remains off. Note: I tested this out with some circuit simulation software.
TR19 off means no -15V and no +15V (because of TR18 turning off). So turning 0TR1 on enables +15V and turning TR19 on enables -15V. A low signal (less than 0.5V) on the Power On/Off line would disable TR21 and allow the +5V supply voltage through resistor R83 (560 ohms) to turn on TR20. TR20 then increases the voltage on the TR19 base to enable it so a voltage of around -22V appears on the -15V supply rail. The D26 15V zener holds that value to -15V. The negative voltage on the -15V rail causes the emitter voltage of TR18 to drop enough to pull the 0TR1 base down and enable the +15V supply rail (which is held at +15V by the D25 zener).
Since I am seeing a near zero voltage on my 8002 Power On/Off line going to the TR21 base, I should expect to see my 15V voltages. However, the voltage that turns on TR20 remains at +0.072V. Too low to all it to work. The +5 VDC supply voltage is present so it must be the circuit from TR23 that is keeping the voltage low. In simulating that circuit (Control for uC), I see that with TR6 turned on, I should see TR23 turn off and allow the +5V supply voltage to do its thing on TR20. Instead, I see +2.95V at the TR6 collector and TR23 turned on with +0.75V at the base. I believe I should see a near zero voltage at the TR6 collector and at the TR23 base (leaving it off).
I am thinking that maybe TR6 is not working correctly so I will replace it next.
Is my description of how the Power On/Off circuit function works correct? On the circuit marked "Control for uC", is the function of that circuit to turn off the 15V power supply if the MPU is not generating a strobe signal? Of course, if my description is correct, it raises another question as to why my 8002 starts up with the Power On/Off signal turned on. Isn't that signal controlled by the "Stop" button?
I squeezed in a little time this evening to work on the Beogram 8002 power circuit. I was right about TR6. When I started to remove it I found that the base lead was broken. There was enough of a stub to test it and the transistor was okay. So it wasn't a problem with the transistor being blown. I replaced TR6 with a new BC547B NPN transistor and now I have the positive and negative 15 VDC rail voltages again. That is what I should expect based on the other measurements I had made. The 8002 is powering up with the Power On/Off signal in the "On" position. That is a problem but with it on, I wasn't seeing the 15 volt rail voltages so I needed to take care of that first.
Now I have the +5, +15 and -15 power supply voltages working. I am back to getting the 8002 to power up in "Stop" (Off) mode. The MPU is outputting the uC control strobe voltage so I am fairly certain that uC IC1 is functioning. There must be a problem with a connection in and around the MPU though. When the 8002 powers up in the "On" state, the display only shows 33 (or 45) now and none of the control buttons (except 33 and 45) have any affect. I didn't resolder everything on the PCB2 module so maybe there is a bad connection there.
Keep going John, I understand now how lucky I have been with my Beos, never had one with failures like broken pegs or wires, not even bad solder points, I wonder what I will find in my BM 4400.
Thanks Søren. These projects certainly are a learning process.
I decided to run another test now that I know the rail voltages are working. Menahem had asked me to connect everything and apply power using my dim bulb tester. I put that off long enough so I reconnected the P3 and P4 connectors to PCB1 and flipped the switch on my Variac/Dim Bulb tester to bring the bulb into the circuit. When I turn on the AC power, the bulb flickered on the bright side. I ran the test again with P4 disconnected and the bulb stayed off. In both tests, the 8002 still powers on in the "On" state so there are two problems right now. I want to fix the power up problem first, then I will look at the circuits on the 8002 chassis that connect to P4.
Good job Jon, you're on your way!
Menahem Yachad: Good job Jon, you're on your way!
Thanks, I think I am on my way. I found some bench time today to go over the turntable. I last left off trying to figure out why the turntable was turning on in the "on" mode. While reviewing the PBC2 connections and my earlier re-soldering work there I realized something dumb I did. To make power supply voltage measurements on the chassis (OTR1, O1C1, OC1, OC2) I had disconnected the tonearm pulley and rotated the drive shaft where the tonearm was out to the left (over the platter area). Otherwise the tonearm was in the way of my test probes. So naturally if the turntable had power disconnected with the arm out there it would come back on detecting it had to return the arm to the start position. I re-attached the pulley to the tonearm drive and now I have a properly working control board. The Beogram 8002 comes on in "Stop" mode. All of the control buttons work. The control board and logic were actually working after I fixed the TR6 transistor on PCB1 but since the tonearm drive wasn't connected I couldn't tell. I am glad to get past the power and control logic problems.
That leaves me with what I hope is the last fault to trouble-shoot. I have still been testing the 8002 with everything connected except the PCB1 P4 connector. I know from the test a couple of days ago that the dim bulb tester shows a short in the turntable when I apply power with P4 connected. No fuse problems with everything but P4 connected. So my next step is to go through and check the components on the turntable chassis that P4 connects to. The main drive (OTR2, OTR3, OD1 and OD2) and the speed sensor (OPE2 device) are probably the most likely candidates. I will remove and check out the components for the drive first.
Here is an update to the Beogram 8002 schematic diagram that shows the voltages of the positive and negative 15 VDC power supply circuit for both On and Off states.
After leaving the Beogram 8002 on for a while I found another problem. The display starts illuminating a period symbol in four places and a small vertical bar in four places. These extra illuminated segments stay on during the standby (off) state and the play (on) state. All of the control logic appears to remain working correctly so this appears to just be a display problem. Since it doesn't start out that way I am thinking that maybe IC2 (SN 74247) 7-Segment Decoder might be malfunctioning after it gets warm.
Those voltages are good enough. You cannot expect the voltages to be spot-on, because the standard of precision regulation which we take for granted today, simply didn't exist then.
There may be a couple of out-of-spec carbon resistors which account for the discrepancies - You have replaced all the electrolytics, right?
To me, a vintage unit is AOK, if the voltages are close, the machine works as expected, and the scope waveforms are good.
Put some freeze-spray on that component to see if it is in fact the bad part.
Sonavor , I admire your persistence. Great thread .
Menahem Yachad: Those voltages are good enough. You cannot expect the voltages to be spot-on, because the standard of precision regulation which we take for granted today, simply didn't exist then.
Yes, I think the voltages in the power supply circuits are good. I just wanted to post an updated diagram with what I measure on mine with the On/Off buttons working. My previous diagram didn't show both On and Off states.
Tomorrow I check out the SN 74247, 7-Segment Decoder (IC2) on PCB2 and see if it is the source of the display problem. I hope it is and not the IC1 (MPU).
I decided to run a quick check on the drive circuit. Looking at the schematic, PCB1 P4-4 sends the signal to OTR2 and OTR3. So I de-soldered the wire from P4-4 at the OTR2 and OTR3 base node. Then, with P4 now connected to PCB1, I applied power to the 8002 and pushed the Play button. This test was with the 8002 AC plugged into my dim bulb/variac. The bulb never illuminated so it is possible OTR2 or OTR3 have a fault. Here is a picture of the test set up.
You don't need exactly the BD441 or 442 (4A 80V) if you need to replace them. Another BD with a higher Amp and voltage rating is fine (eg BD243 244). I like to use TO-220 types anytime there are power circuits - they have much better power dissipation capability and a larger Safe Operating Area.
Just pay attention that any replacement should be "Full-Jacket" version, without bare metal exposed, or you should lay down a silicon insulator sheet. Here, an insulated securing bolt is NOT required, because there is the copper pressure clip, which holds the transistors down.
And of course check the 1N4002 rectifiers also - very rare that they leak, but you never know.
I removed and tested OTR2, OTR3, OD1 and OD2 individually. They checked out okay out of the circuit. They could still fail in circuit but for now I soldered them back in place. I decided to recheck the 8002 with the P4-4 wire reconnected. There is still a problem with that part of the drive circuit. As soon as I press the Play button and the platter attempts to turn, the dim bulb light illuminates and the tonearm is forced back to the Stop position.
I guess the next thing to check is the output of the chassis portion of the drive circuit (P4-3). It connects to the brake circuit and that appears to be what is happening. The drive signal goes from PCB1 out P4 to the chassis part of the circuit, then comes back to PCB1. So there could be a problem on that portion back on PCB1 (possibly TR31, TR32, TR33, D39, D40). That part of PCB1 isn't exercised when P4 is disconnected.