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Beogram 4002 (5521) restoration woes

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notwist
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notwist posted on Sat, Sep 5 2020 7:24 AM

Hi guys, 

I recently inherited two Beogram 4002 turntables with five additional brand new EN20 cartridges. Seeing that I love my B&O collection I decided to restore them as both were defective. The first Beogram actually was a succes on the first go; it was basically a transistor burnout, some failing caps and two resistors failing. It needs some alignment but I wanted to do this when the second one was done. Easier to do two alignments at the same time. 

That second one is proving to be more of a problem. It had three of the metal can transistors burned in such a way that i had to restore the tracks. I recapped the entire unit. I replaced most of the old 1w resistors that weren't in spec anymore, the other 2 seem OK. I did not touch any of the existing film capacitors. Now the machine at least doing 'something' besides being dead. I changed the belts using Dillen's replica's. 

When I power this machine on, either nothing happens or the carriage will start. If it starts it will move over the platter and then return to the base. Sometimes I can press the start button and it will actually work. Most of the time nothing happens. So. 

- Startup isn't working as it should (it starts by itself sometimes)
- Start button doesn't work; nothing happens (besides very intermittently see #1)
- 33rpm button: when depressed runs the motor but stops when released
- 45rpm button: nothing happens
- When the tonearm is working it starts at 20% of the record instead of the start. 
- When it does start it produces good sound at the correct speed
- The lamps all seem to be working (sensor, tonearm, selector etc). Perhaps not at the right intensity?

As far as I can tell from my other Beogram, the expected behaviour is that, when plugged in, nothing happens. When you press the start key it will scan the record once, then return. Then you can press start again and it will actually start playing. 

I have read through dozens of posts on this brilliant forum and it strikes me that these problems have all been encountered multiple times in one way or another. But because there are so many posts on these subjects what i'm hindered by is that same multitude of posts. 

Who can set me off in the right direction? Which problem (or interconnected problems) should I tackle first? This machine can be daunting at first. I don't want to introduce more problems of course. 

Many thanks for your help and guidance! Hope to restore this second one in pristine condition as well ;-). 

Regards, 

Bert

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sonavor
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If your position scale looks good, which I would expect it to, then the issue is with the sensor. That first bar for the 30cm position is thinner than the next bar.

Have you tried a 7 inch single record?  If not then I would suspect it wouldn't get detected either.  I wouldn't be surprised if the run-out groove detection was failing as well. 

What is the voltage on the collector of 4IC1 when the black mark is over the sensor and what is the voltage when the clear part is over the sensor?

-sonavor

sonavor
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Suggested by Craig

One other thing to check.  Did you replace the 1uF electrolytic capacitor 1C33 ?  If not then you should do so.  I always replace all of the old electrolytic and tantalum capacitors if they are still the original ones.

-sonavor

notwist
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Hi Sonavor, 

This is not my first hassle with B&O and European made equipment. So as a precaution I updated all capacitors including several of the film capacitors known to be rather shoddy quality. This includes the tantalums. Exchanged with mostly film and mlcc and the larger reservoirs with electrolytics (vishay). 

notwist
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Hi Sonavor, 

I made two photos of the bar, in my opinion this is OK. No scratches but perhaps I'm missing something here. On first glance it seems the same as the one on my other Beogram 4002. Which suddenly works 100% and doesn't do the 'scan once' misschief anymore. But based on my experience with this one I would do a revision on that one as well. But first the problematic one. 

The black sensor has roughly 2mm of space between it and the ruler (scale). It would be difficult to minimise that gap and it seems to be roughly the same on my other Beo 4002. 

Bert

notwist
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Hi Sonavor, 

it is too busy here, need to put out fires all the time. So what I did in the meantime is to exchange the revised main PCB with the untouched PCB in the working Beogram. 

The result is: the revised PCB works in that Beogram. The carriage moves to the correct lowering point and plays well. 

So, it seems that the problem is not originating from the main PCB. At least not as far as I can see from the circuit. If it were down to an issue on that PCB it would have migrated to the other Beogram. Which it did not. 

Question is, now what?

sonavor
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Then it sounds like your issue is narrowed down to the position sensor and lamp. Ideally you would monitor that signal with an oscilloscope to make sure the voltages are in proper operating range and not working with voltages that are at the edge of a limit. But since your second board works in the other Beogram then that puts the sensing problem on the sensor, lamp and position scale. I would still look at how far the position scale bar is from the slit in sensor housing. Try to put it as close as possible without touching each other.

-sonavor

notwist
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Hi Sonavor, 

before heading home I positioned the sensor as close to the scales as possible, this had no effect. I don't know if it's at all possible to upload a movie here, that would show you the behaviour (easier than text). I don't have single unfortunately so I can't reliably test that feature with regards to the sensing array. 

You are right, measuring voltages would be smart. I don't think I have to hook it up to my oscilloscope for that, this is all rather basic work that can be measured with my workbench multimeter. The question is what voltages are we looking for? Which components are vital to the operation and must operate within a certain bandwidth? And under which circumstances? Is there a way to test the sensor to see if it is malfunctioning? What are the options to replace it (if any). 

I don't usually do turntables and this design is somewhat eccentric to say the least. That is why you have deep knowledge and I do not, such knowledge can only come from a load of time spent with this design. For me, even though I am technically skilled, it is essentially a pandora's box with regards to specific features and how they cooperate. 

Again, many thanks for your help and advise. 

sonavor
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There are of course the sensor setting voltages per the service manual.
After that though, you need to see what gets picked up on the main board from that sensor.
I usually measure the collector of 1TR17 and the node of 1D41, 1R82, 1C33, 1D42 if checking the performance of the Run-Off stop signal.
If none of the functions from the position sensor board were working then I would suspect the wiring between the boards but you are seeing the 25cm position get detected. I would still be interested if the 17cm position will detect.

Since the board in question works in your other Beogram 4002 unit though I would have to suspect your position sensor and/or lamp.
It isn't producing the necessary stimulus on that narrow 30cm mark on the position scale. It waits until it gets the large marking of the 25cm position.

I think you have exhausted all of the possible mechanical parts of the puzzle.

When you try the board from the working Beogram in this Beogram it too fails to detect the 30cm position, correct?

-sonavor

notwist
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Hi Sonavor, 

I have indeed transplanted the unrevised board to the revised Beogram and it is exhibiting the same behaviour. Thus I would conclude that the sensor is the culprit and perhaps it's direct surrounding components. The lamps in both machines appear to have about the same brightness (the lamp in the arm). 

I will try to get a 17cm (single) from somewhere, if lucky someone can bring it to the lab tomorrow. Will check this out immediately to see if that works. 

But the real question in my mind is, if the sensor is broken can it be replaced? Either with a DIY part or another type of sensor. I'm not sure about this at all because I have no clue how to test the sensor in an objective method. Shouldn't these give an ohms reading? 

Regards, 

Bert

notwist
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I have removed the housing to get a better look at the components in question. It appears to be an infrared diode and a photosensitive diode. Not entirely sure because the service manual I am using is showing different parts (lamp or led, not infrared). Perhaps I am wrong about the IR. 

In the end this setup (a light source and a light 'gatherer') is really common. So the question is what components can we use to replace the old ones. If necessary of course. I can desolder them and double check some parameters. 

Maybe i'm going about this the wrong way, if so then please excuse my inexperience with this machine. I fully understand that in respects to the precise workings I am way out of my league. Then again I restore and repair much more complex machinery so perhaps this is why I am simply trying to exchange the parts where the fault appears to be coming from. 

I will check the power supply rails once again as wel. If the light source doesn't get enough juice it will likely produce a weak light (or IR) to bounce off from. 

Regards, 

Bert

sonavor
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I have yet to run into a case where the sensor was bad although it seems a possibility.  Maybe Martin (Dillen here on the forum) can answer that.
I always change the light source. I have been using a Cree C503B-AAN-CY0B0251 for my replacements. They have given me good results so far.  After the installation I check and adjust 1R88 to set the proper voltage then check the signals that result from that sensor.

-sonavor

notwist
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Hi Sonavor,

your article is quite clear, thank you very much. The light source that is in this machine gives out very little in the way of light at the moment. Not near the level of brightness as in your article. So let me order this replacement and we'll try it out with the alignment adjustments. It will take a day for the lamps to get to the lab so I will report back on Thursday. 

I believe that we should have a compatible sensor in 'the pantry' as well, let me check inventory. 

Again, many thanks for the help. It is too nice of a turntable to just let it be spare parts. 

Kind regards, 

Bert

sonavor
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notwist:

It is too nice of a turntable to just let it be spare parts. 

Kind regards, 

Bert

That is for certain.  

I can say from my experience that these Beogram 400x turntables are very rugged. The only thing that would prevent getting one back running properly again would be some extraordinary event like someone taking a sledgehammer to one. Probably a little over dramatic there but the point is that none of the parts in the Beogram 4002 are parts that cannot be be repaired or sourced.

-sonavor

notwist
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Hi Sonavor, 

so the replacement LED arrived in the lab. I replaced the LED / IR that was in the machine. With the new LED (as per your part number) in there, the whole system is going completely haywire. 

When I power up, there is light. The motor starts spinning on itself, 33/3 rpm is lit up. Pushing automatic just moves the tonearm over the record completely. If I stop it will return to it's 'off' position but the motor won't stop. Just keeps spinning as if it wants to move even further back. 

I think that my machine is not using a regular LED. When I test the component I desoldered, it measures much different (even though it registers as a diode) then the new one. I think that my machine is using an IR diode which explains why there was no light to speak of. 

Of course I can measure voltages again on the LED (it's about 2.1v) which is also quite a load higher than your post (which has it as 0.7v). My schema says it should be about 1.1v so it's already way too high. I can check to see if I can get it to back down using the trimmer. In my schema 4IC1 is determined as 'srd 212' which I can't find anything on. However, 4D1 is registered as OP160 which is a infrared emitting diode, unobtanium unfortunately, 

It seems an incompatibility has arisen.
Otherwise I am a bit stumped. 

Regards, 

Bert

notwist
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Hi Sonavor, 

replacing the new LED with the old IR diode (tested, working) does not change the haywire behaviour. Essentially it seems that it is now really out of whack. 

Tried what happened when I press 'start' with all components back in there. The arm moves across the platter (record on, spinning by hand) and then returns to the start position. It does not lower anywhere. When in the start position, the motor of the assembly does not stop, keeps running. The motor for the platter also keeps running. Also the 33 rpm is still lit. 

Basically it seems that it is now immediately starting in manual mode? Or something like this? Why? I only desoldered that one LED and touched no other components. Did I overload the sensor thereby breaking it? Would this whack the machine in such a manner? 

Voltage on the LED / IR is down from 1.9v (LED) to 1.1v (IR). Seems to be OK. I can still change that voltage using the trimmer, not that it helps in this situation. 

I find this completely odd. 

Kind regards, 

Bert

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