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
Whilst awaiting the arrival of a replacement lens for my BL5000 I decided to fit the new belts I purchased from Martin at the weekend.....simple task. This accomplished I put the unit through its paces and everything is working fine, however.....during my tests I have discovered that when pressing the ON button the platter runs at 331/3 RPM and the tone arm glides nicely to the left and if no record is present the arm continues without dropping to the centre......but when its almost at the end of its travel the arm does stop and attempts to drop onto the spinning platter (nasty moment there), pressing OFF button lifts the arm back up, the speed goes to 45 RPM and the arm travels back to the rest position.
When a record is present everything works as it should.....I'm suspecting a fault on the differential stop circuit which is initiated by the switch MV. I will take measurements.
Craig
During the course of the above investigations I have noticed that the set doesn't always start in 33rpm mode......goes straight to 45, very annoying as I have had this fault on this set previously.....however I now have a fully functional BG4000 awaiting restoration that I can use to make comparisons. This is what I have found when measuring the start impulse of the suspect set, this measurement is at the collector of TR20 (S1)
And this is the trace on start up of the fully functional set......looks a lot like how it should with a decaying pulse. thing is the suspect one does work 50% of the time just not always.....I will look at the capacitor IC15.
Ok......did a little testing and discovered that the Start Pulse components are all good, checked the resistors too and everything checks out good.....
Isn't it the decaying pulse that looks wrong?
When power is applied TR20 is on (Collector low) for as long as it takes for C15 to charge. Then TR20 turns off (Collector high) and stays there until power off.
Martin
EDIT: The schematic you posted actually shows the scope trace.
Whats bothering me is that when the on switch is pressed on the performing set the start impulse rises to 6vdc and falls off to nothing, when the start impulse switch is pressed on the faulty set the the start impulse rises to 6vdc and stays there......see above scope readings. I dont understand why the 6vdc drops off on the good set and everything is working, I checked this at the 6vdc incomming terminals of the good set and I get the same rise then fall of the 6vdc .......whats going on?
Yes I see what you are saying.....the faulty set looks like it is giving the correct trace, and the good set with the decaying 6vdc looks suspect....but works perfectly...selects 33rpm everytime and also runs the tone arm to the centre of the turntable and back without dropping if no record is present...just as it should?
The problem must be elsewhere.The 45rpm button?
Yes......I'm sure the start impuse circuit is fine, I will make sure I'm measuring the voltages correctly on the working set......it cannot function without the 6vdc supply present.....so I think I'm missing a trick.
These old beauties are known to develop breaks to the PCB copper tracks.Particularly the outer ones.
Ok.....pretty sure I have the scope set up correctly now, this is the start pulse generated by the fully functional set....pulse fired of at 6.4vdc with a delay of 8ms....a per the manual....
Same scope settings measured at same place on the suspect set......pulse fired off at 7vdc with the same 8ms delay, often this delay is only 4ms which I suspect isn't long enough to reset the logic gates
So...just because I can I decided to remove the control panel from the functioning set......
And replace it with that of the suspect set...….
And to my relief the functioning set continues to do so...….I have been down the road of the control panel logic gates before and couldn't actually find anything wrong, but doing this has eliminated a chunk of the circuit, not solved the problem but I'm making some progress.
Continuing on this course, I feel like i'm cheating somehow, I have removed the main PCB from both sets and connected the board from the suspect set into the fully functional set and yes......the set continues to perform flawlessly! I will keep it on test while I install the fully functional set PCB into the suspect set.....will of course need to replace all the old caps at some point.
And sure enough when the PCB has been installed in the suspect set the problem persists...….but its narrowed down somewhat, I think I have noticed that on the occasions when 33rpm is selected I hear the reed relay click in and not when it starts up on 45rpm......so I'm off to check some voltages around the relays and coils.
GOT HIM...……….couldn't find anything wrong in the power supply board, all voltage levels as they should be and not falling off under load. At a bit of a loss decided to try the transport slide switch board, easy to exchange and a quick check.
When the board from the suspect set was replaced with that of the fully functioning set everything started to work fine! tried it several times and it fired off in 33rpm mode every time, arm runs in and out with no record present and stops in the correct place when it thinks one is......this is what I found on the slider switch.....the only contact that should remain closed until the slider moves over it is the one that sends a signal to tell the logic that a 17cm record is present and please switch to 45rpm......when testing this switch I am finding it's intermittent, sometimes closes after being operated more often it remains open circuit, should have found this earlier.
Intermittent faults are the worst. How will you know, when you fixed one?Excellent work!
Cheers Martin......and thank you for the support during yet another adventure into these beguiling machines
The work on the slider switches has cured both problems, the original fault of the arm stopping before it reaches the end of its travel when no record is present and also the issue of the start at 33rpm.....so I'm happy with all that, however....the mystery of the scope traces on start up remains, the top trace is the original fully functional set and the bottom is the now fully functional set. Bear in mind that the control logic board, main PCB and slider switch board within the original fully functional set have been taken from the previously goosed set! that only leaves the power supply/rectifier board? the pic is both sets turned on then off.
Moving on from this conundrum I need to do a little work on the original fully functional BG4000 output cable, the previous owner has replaced the 5 pin DIN plug with two RCA phono plugs, this has been achieved by chopping back the original cable justi inside the cabinet and soldering up the two new plugs to the cable and wrapping the whole thing with electrical tape...not ideal. However im struggling to get hold of some 2 core overall and individual screened cable to effect this repair (unfortunately Steve cannot provide me with a custom cable right now so I need to make one up myself). The question is can I use 4 core overall screened cable for this duty utilising the 2 additional cores as screens for the missing individual ones? Martin...I'm looking at you
Theoretically, you can do that.The result depends on the quality of the cable in general and the screening in particular.Another thing is channel separation; Two leads (left/right signal) running parallel for some distance will inevitably pick up fromeach other, when they aren't individually screened - and we do have quite some amplification after the cable.
I have been using a CAT6 screened cable for tape to amp, 5 pin DIN, and to my ears no difference to the original cable.
Collecting Vintage B&O is not a hobby, its a lifestyle.
Tape (Line) is also at a much higher signal level (and at a different impedance).Not at all as prone to pick up noise, and not at all the same amplification after the cable ("antenna").
Would an original cable that comes with like, BM 5000/BL 5000 be OK if used for BG (soldered to the BG) to amp
Why not use a good quality twin individual screened cable,and run a separate ground wire for the chassis along side it?
Enclose both cables in a length of that nice black or grey braided cotton sleeving?
It will look like one of those "audiophool" cables that are supposed to ensure that the electron's alway's travel in the right direction(beam me up Scotty?)but it would be a neat solution.
Regards,
Nick
When I redid the phono cable on one of my Beogram 4000 decks a few years ago I did it this way.I was recreating the cable to the way it original was from B&O. Not 100% perfect but it worked for me.
The inner wire is a Mogami W2806 which is a twisted pair of signal wires surrounded by a shield. The shield wire connects to the DIN5 pin 2 and to the phono L&R low (or minus) sides. That cable is then fit into a braided, metal shield that connects to the DIN plug casing and the chassis ground of the Beogram. I finished it by putting the cable inside a braided nylon covering. As I said, it works good for me.
I can't remember how much the braided, metal shield was per foot but the Mogami W2806 is $0.57 USD per foot. Not too bad.
Something like this:
https://shop.sommercable.com/en/Cable/Bulk-Cable-Instrument/Instrumentenkabel-SC-Sinus-Control-320-0252.html
Looks fancy and is quite large, so if it could be found in a smaller version.
I think that is overkill though. The later Beogram 4002/4004 phono cables just use twisted signal wires inside a shield.There is a Mogami W2447 cable with individual shields (on two wires). If he goes with the "sinus" control :-) it would be interesting to see.The cable I made on my first Beogram 4000 turntable project was pretty fat itself.
John
Finally made a decision on the cable and went for 4 core individual screen...….
Used the red and black cores for the signals and the individual screens from each of these to connect to the pin 2 of the DIN, the other two cores I cut back and used their individual screens for the ground connection of the plug...….
The overall diameter of the cable is smaller than the original so the rubber sheath isn't a snug fit, small cable tie takes care of that.
This is the deck end where the connection has been made...…..
Bit rough and ready but mechanically sound...……...
The other end with the insulation cut back and before the unwanted cores have been snipped away...
And finally connected in.....gave it a whirl with a beloab 5000 and a couple of S45's and it actually sounds spot on, no hum or untoward noises. I now have two 4000's in various states of restoration due to the swapping around of the various component parts, I need to consolidate the work to bring both up to the same condition....mostly on the original fully functional set.