Beocord 7000 voltage issues

[alf]

Hi All,

Just working on my BC7000 which has some voltage issues:

I can not find the 22VAC, which serves two connection ? All I am getting is 28VAC.

That brings me to the question if it is actually a transformer problem ?
Would like to know its typical VAC output voltages.

I am also missing the 12VdC supply for the drive which I should be see at
Plug16- pin4 ??

I have gone over the power supply board without finding any suspects.

Would really appreciate some help – thanks

ALF

[Die_Bogener]

There is a safety resistor R1 4,7 Ohms in the power supply for TR1… i have seen this defect already several times…
If there is a mechanical overload of the drive it will blow. Especially if the motor is bad… check the motor on a regulated supply. There are caps INSIDE the motor… at least in the BC5500 drive, 7000 i dont remember right now…

There is no 22V AC… it’s just 22V DC unregulated. And if in standby without load there can be more than 28V? Maybe, should be ok.

[alf]

Yes, the safety resistor was dead and has been replaced.

sorry, I meant of course 22VDC !!
Correct, the 27-28VDC are in Stand-By – ok, thats cleared up….when play is activated

i get just under 22VDC.

still missing the 12VDC for the drive as well as the 14.8VDC at P6-pin9 ??

i had the motor running on 12VDC with the benchtop power supply – no trouble.

difficult to imagine the motor is at fault.

i will look into the BC-435 and BC-136…other than those will cause a headscratch.

ALF

 

[Die_Bogener]

OK, safety resistor was dead…

Then the LM272 control of the motor can also have a problem.

[alf]

I look into it, but first I have to restore the 14.8VDC supply.

the L272M mot control could still be fine but without the supply I couldn’t test.

the BD 435 has to be ordered, unless I could use a BD437 as alternative ??

the same question is in regards to an alternative for the no longer available

L272M chip ?? Any suggestions ?

ALF

[Die_Bogener]

Ebay?

L272M N8 around 50 offers, especially in GB. ~ 5-8$

BD437 is ok, 45V instead 35V… will work.

[alf]

Right ?, just been there…nice shop.

good to know the BD437 would work – I can get that one even here.

the l272M are most likely chinese origin, sold in the UK, US and of course CHN

the order is out !

ALF

[alf]

Well, back on the BC7000:

sadly no change after replacing LM272 – that drawer motor worked fine before.

i replace the BD435 with a BD437 but what I can not get is the 12V supply for the capstan motor.

the fuse resistor has also been replaced !!

as said before, the motor works as been tested with my  benchtop VDC supply.

i begin to wonder if the transformer got a hit during the “accident” ? It would be good

to know what its typical VAC voltages are before hitting the rectifier diodes ?

other than that I am stepping in the dark right now without progress.

just such a shame that lovely deck doesn’t work.

anyone, anything ?!

ALF

[Dillen]

Can we have DC voltage readings on TR1 and TR2 B-C-E respectively, with the Beocord out of standby?

Martin

 

[alf]

Obtaining those voltages was  a bit tricky because the deck shuts back to standby

fairly quickly after PLAY was activated:

TR1:  B = 5.8 VDC,  C = 15.8 VDC,  E = 5.3 VDC

TR2: B = 15.8 VDC,  C = 5.9 VDC,  E = 16.1 VDC

are these voltages consistent with  proper functioning transformer  ?

ALF

[Dillen]

Check C9, TR3 and TR4 in that order.

Martin

[alf]

I checked all components in question off-board:

C9 = 21uF, TR3 and TR4 all hfe near 290.

I wonder if the protection kicks in as the deck shuts down shortly after play is activated because of a different fault elsewhere or too sensitive ??

ALF

[Dillen]

So it actually works for a brief moment?

Martin

[alf]

I would not call this working – when play is activated the display comes on for a few seconds and a very faint click (as something tries to engage) is heard…then came over, display out, nothing further……

as mentioned earlier, feels like a protection is kicking in ?

quite obviously something is shutting down the deck very quickly.

ALF

[alf]

I had another check at the transformer and its connection to board 3:

It secondary pins show AC voltages  of up to 40VAC when disconnected from board 3, but all those voltages seem to break down to levels of 5 VAC  and less the moment the transformer is connected to board 3 via P1.

so, is this transformer faulty or is it a plausible reaction because of a problem elsewhere?

bottom line is : the deck does not work

ALF

 

[Glitch]

alf,

I wouldn’t expect that the transformer is bad. It is normal for an unregulated power supply that the voltages be pulled down under load. Voltages lower than what is indicated on the schematic would imply that you have a short or some other similar fault on the board.

Do you have a bench power supply? If so, connecting the board to a bench supply is helpful for debugging.

A voltage sag from 40v to 5v seems to be more than a “sensitivity issue”. It sounds like a genuine fault and the protection circuitry is doing its job.

Glitch

[alf]

Well amongst many things I do not understand this is certainly amongst them:

the transformers secondary delivers AC voltage to the rectifier diodes….so how can that be this AC voltages can be lower before DC voltage is supplied to the rest of the circuit?

so, lets assume there are 40VAC delivered to the rectifier, turns into 5 VAC before getting rectified ?? Sorry, don’t get it.

the transformer’s fuse blew as well.

looks like I am missing something here, or do I ?

ALF

[alf]

As another check I disconnected P2 and P3, which are the supply lines to the deck.

so I must ask again:

what makes the transformer secondary AV voltages almost “evaporate” when

that transformer is connected to board 3 where its AC voltages are getting

rectified to DC ?  Which load exactly is there ?

just like to understand – thank you

ALF

[Glitch]

Apologies in advance if you already know all of this…

There is actually a lot going on inside a transformer from an electromagnetic point of view. A detailed explanation of exactly what happens is probably beyond the scope of this forum and would put most people to sleep. A simplified explanation is that the secondary windings have resistance, as the current output increases, the output voltage of the transformer decreases proportionally (i.e. V=I*R or more specifically V_out = V_noload – I_output*R_windings).

If you did a direct short of the transformer output, you would expect the voltage output to go to zero, right? Your circuit board is simply a load somewhere between a no-load and a direct short.

The rectifier doesn’t really change any of the above. All the rectifier does is switch what phase of the transformer’s A/C output is connected to the rectifier output at a given time. The current load on the transformer is still there.

There “could” be a problem with your transformer, but I believe that these kind of issues are rare. Transformers are generally robust and reliable devices that fail pretty decisively.

Have you tried to measure the current draw of your board? This can be done with a multimeter (be prepared to replace the fuse in the multimeter if the current draw is above the rating). A better option is to use a bench power supply with a current limit. Set the current limit to less than you expect the board to draw and increase slowly until you get a better idea of your situation. The bench power supply approach has the benefits of not having to deal with the protection circuitry kicking in and making taking readings difficult, as well as limiting the possibility of further damage to the board.

After you use the bench setup to find/fix any possible power related issues, it will be easier to debug the rest of the functionality.

Note:  I would keep following Martin’s advice as it will likely get you to a solution faster. My suggestions are in the spirit of trying to get more information so that it is easier for people to help.

Glitch

[alf]

Thank you very much for trying to help – much appreciated !

i really don’t know what the current draw is or should be in that circumstance.

the way I tested or checked was to isolate a possible transformer issue from the rest of

the deck by unplugging P2 & P3 which are supplying the rest of the deck, in other

words the only connection from the power supply to the main deck.

so, it is really only the power-pack (transformer) connected to board 3 which should

deliver the 5V, 14.8 V and 15.5V supplies for the deck but it doesn’t.

there one last thing I will try and report back.

ALF

all main components involved have been tested off-board.

[Author]

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