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BG8000 fuse issues

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ALF
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ALF posted on Mon, May 1 2017 7:02 AM

greetings all,

a new arrival, a BG8000, which had a partial re-cap job done by the previous owner.

I noticed C24 was put in the wrong way as its positive should connect to the F1 ?!

anyhow, I corrected that, completed the re-cap job incl C27 - see pictures - and now F1 is constantly blowing

even with my DBT ?

I checked D16 to D19, all measured ok.

did I overlook something ? any ideas or suggestions are most appreciated :-))

cheers

ALF


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ALF
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hi john,

I reinstalled that flimsy film tachodisc for now to get some signal pictures.

the chassis mounted on/off switch seems to work fine, sees about 5.04V which goes down to 10mV and about 550Ohm to 0.5Ohm.

I have attached a picture taken from the speed sensor under the platter and was wondering whether this is due to the ageing tachodisc ?

is the signal on your BG8000 the same ?

ALF


ALF
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this second picture shows the signal on P2-pin4 or 6 coming from the IR sender diode op240 at the spindle

the double lines are from an unsteady hand of mine while taking the shot!

ALF


sonavor
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Hi Alf,

I had a Beogram 8000 on my bench for an adjustment last week but didn't see your request about the signal you wanted me to compare. I will try to remember when I start on another Beogram 8000 turntable. I'm not sure yet when that will be yet. I might be opening one of mine up soon to change the muting relay. When I do I will take some pictures of those signals on an oscilloscope. 

Regards,
John

ALF
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ALF replied on Sat, Aug 5 2017 11:16 AM

Hi John,

not so much a "request", more a favour 🙄

In the meantime I borrowed a metal tachodisc - also bought one from Martin - but sadly that did not fix things.....

the carriage still moves over just a bit after 'Play' is activated but does not reach the drop down point and returns straight back to its resting position.

only after a couple of tries I may get lucky and everything works as it should ?????

i begin to wonder if there is a fault in the uP control circuit - board 7 ? Suspect being the cemos IC ?

I also see 4 dots on the display, not just the center stand-by dot ?

anyhow, the investigation seems ongoing .

regards

ALF

sonavor
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I have seen 4 dots on the display before. It was due to bad connections inside the metal box that houses the microcomputer. The 4.7nF capacitors for the display had problems (bad capacitors and bad connections). Those capacitors are tricky because there are solder connections on both sides of the board. I reference that problem here and here. I can't really help with the operation logic problem you are having without being able to test it myself. There are too many variables at play there. From your earlier reports of the Beogram functioning though I would be inclined more to suspect things that interface with the microcomputer rather than the microcomputer itself. 

John

Dillen
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Tonearm stops a couple of milimeters short of the lead-in groove;

- Wrong (to tight and/or to stiff) servo belt.
- Dirt or wrong lubricant(s) on the threaded shaft/bushing.
- Too low motor voltage at slow forward (small electronics mod. needed).
- Wrongly adjusted search (button-) optos.
- Bad LDR(s) in the operation panel.
Any combination of the above.

Martin

ALF
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ALF replied on Tue, Aug 8 2017 7:35 AM

Hi John,

you are most likely spot on with your diagnose - I have ordered some Murata Ceramic Caps and will probably throw all of these old trouble makers out.

not an exercise I am looking forward to perform - I keep an eye on the actual display decoder chip SN74247N as well !

all control panel adjustments are pefect as required.

there is also the uP control board I will revisit and check - the servo belt seems fine, the servo motor voltage oscillates between 0.96 - 1.26 VDC while

playing the record.

as described before, after a lenghty break it can work properly but after a few tries playing up again - just like something that has not been reset in a memory ?

it is not that the arm stops a few mm before the drop-down point, if it gets to that it works fine. The problem is somewhere else when it only travels one or two cm towards the center and swiftly returns for no aparent reason ?

lots to do, if there is development I will post again !

at this point many thanks for your patience, help and pointers 🤗

ALF

ALF
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ALF replied on Wed, Aug 23 2017 3:18 PM

what happened In the meantime ?

new metal tachodisc from Martin has been fitted plus new servo belt.

in the processor box  I changed the white old uP socket to a tulip-type / gold plated contacts and the C28 cap

plus reflew a number of suspicious solder joints.

I have not yet got those 4.7nF caps for the PCB 2 and those wheird dots on the display still appear!

Now, here is the strange thing:

pressing 'Play' may or may not work, carriage moves 1-2cm and returns most time straight away to its resting position.....

a uP initiated reset as Martin suggested ?!

if however I press 'turn' and then 'play' everything  falls into place, arm moves into position, lowers and plays.

so far I tested this procedure more than a dozen times and it worked every time ????

I could live with that just to play records, but something is still wrong....! i simply have no further ideas where and what else to check ?!

suggestions are always welcome 😳

ALF

 

Dillen
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Dillen replied on Wed, Aug 23 2017 5:00 PM

Weak power supply - in some way or another.
Cannot handle both the (quite tough) startup of the eddycurrent coils and the movement of the carriage.
If voltage is unstabile or not adequate, the CPU will be reset.

Martin 

ALF
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ALF replied on Thu, Aug 24 2017 4:37 AM

Thanks Martin,

that would explain the strange behaviour.

as said, going with 'turn' first and 'Play' to follow everything works fine !

i feel quite a bit wobbly to tackle that 'unstable power supply' issue.

are you pointing to a potential problem in those 'motor coils' drawing too much current or the  two darlingtons BD441&442 ? 

I am aware as it stands it is not properly fixed but with the 'detour procedure' I get the table to play records 🙄

ALF

ALF
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ALF replied on Fri, Aug 25 2017 1:09 PM

A final remark to that perhaps "strange" procedure-detour

when I think about playing records I actually use the record cleaning function 'Turn' fairly regularly

as it seems silly not to clean/wipe before starting......in that sense it really does not matter, does it ?!

the BG 8000 now plays records again after weeks of blood sweat and tears......plus some work 😂

Time to 🍷 and enjoy the  📻

ALF

sonavor
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Hi Alf,

I finally ran those Beogram 8000 measurements you asked me for. I posted them here.

Regards,
John

ALF
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ALF replied on Wed, Sep 20 2017 10:01 AM

Hello John,

thank you very much indeed - they look very similar to mine 😉

I can now operate the BG 8000 just as normal and have put the cap replacement in the uP box on hold

despite still seeing the 4 dots on the display.....right now I am just happy the table plays records without playing up 🙄.!!

one thing however is a bit annoying which is a slight hum that I can't seem to get rid off completely.

one of these things that can have so many causes......

a cable from Steve at sounds heavenly already made a big difference.

cheers

ALF

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