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Beomaster 4000

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Krolroger
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Krolroger posted on Tue, Nov 6 2018 4:34 PM

I probably paid slightly over the odds for this non runner but it's in nice cosmetic condition with rosewood veneer.  It seems to be an earlyish version of this unit with only one idle current trimmer per channel in the output stage.  No RCA sockets either.

It's clean internally too, though the switches needed a good blast of Kontakt 60.

The right hand channel was defective (practically no volume) and is now partly sorted, due in part to a defective TR40.  But I still have a lot of hiss over the audio which I think is down to TR39, a lockfit BC159B.

My question is whether I can use BC559B as a substitute in this position?  Lockfits don't seem to have the best reputation.

Also, there is some evidence of plasticiser migration as some of the internal wiring is sticky.  I don't think the physical integrity of the wires is compromised.  Is there any fix for this: such as cleaning with ipa or meths then some sort of clear coating?

Thanks all,

Simon


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Dillen
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Yes, you can use BC559B - I suggest replacing in both channels.
And there is only one idle current trimmer in each channel in all Beomaster 4000s.

Martin

solderon29
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The'4000  is( obviously )a development of  the '3000,but with a different output stage,which employ's Darlington transistors,and sound's argueably better too?

You don't seem to have replaced all the caps on the output stage pcb(yet?)Those yellow jobbies need to come out,even if they read ok!And the presets of course.

You'll have the same problems with the function select switch,and you will have noticed that the radio section is the same as that used in the '3000.You have plenty of experience with that area of course!!!

Interestingly,the '4000 has an "ambio/surround sound function too,where if you connect an extra pair of speakers,you can achieve an ersatz surround effect,that's quite good.

A nice receiver,well worth some tlc!

Good luck with it Simon.

Nick

 

Krolroger
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Thanks, Martin, BC559B nailed the problem.

I took out the bias setting Darlingtons TR43 and TR54 in order to remount them.  They are SPS 5418.

I can't find a data sheet for them so I can't tell if they've gone low gain though they seem to test ok on my Peak tester.

Should I replace them with MPSA 13 as a matter of course (which show an hFE of approximately 10 times as much)?

Cheers,

Simon

Krolroger
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Hi Nick,

Work in progress!  Replacing trimmers/caps as we speak..

Dillen
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Don't worry too much about hfe for those transistors. Unless they are proven faulty, I would gladly leave them alone.
High Hfe is not better than low. Particularly when we talk about amplifiers like this one.
The risk of self-oscillation rises as you fit "to eager" components like higher-Hfe transistors and lower-ESR capacitors.
Try aiming for something close to what's fitted originally.

And I agree on replacing the remaining capacitors on the board - the tantals too.
- And replace tantals with tantals.

Martin

Krolroger
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Hi all,

Work progresses on the amplifier board but a couple of things have come up.

The service manual states that from set number 177100, the emitter resistors were changed to 0,33 ohm and the (external to the board) 8,2K/8,2K ohm voltage dividers have been changed to a lower value.

Well, my set is numbered 177418 and there is no evidence of any change.  Curious.

I remounted the original TR43 and TR54 as per Martin's advice (fiddly job).  The original TO-3 transistors on the back panel seem to have reasonably fresh (not dried out) thermal paste on them, so I left them alone.  Heat transference from them into the back panel seems fine so I'm not overly concerned.

I needed to set the idle current so I thought I would measure the voltage drop over a single emitter resistor.  The prescribed current is 80mA and the emitter resistor is 0,15 ohm.  I should therefore measure 12mV across it.

I set the idle current to 12mv each side but was alarmed to see the output transistors get uncomfortably hot in a short period.  I then reverted to the method advised in the manual and reset the idle current to 80mA, down from an indicated 100mA or so.

However, I now see only 9,5mV over the resistors but nothing is getting too hot.  I probably need to whip them out and check they haven't drifted in value.

This got me thinking about the process of inserting a cheapish (or even not so cheap) Chinese DMM into the circuit - a Uni-t 61e in my case. Its shunt resistance is 10.4 ohms on the mA range which seems on the high side and must make a small difference to the measured value.  Measuring an emitter resistor is clearly a tidier way of doing this if you know the exact value of the component.

Or maybe the way to go would be to place a precision 0,5 ohm resistor into the circuit and measure across that.

Anyway, it's ok for now but I will revisit in due course.

Onto the tants..

Regards,


Krolroger
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Evening all, and a Happy New Year.

The Beomaster 4000 is coming along nicely - if rather slowly.

I've now replaced all electrolytic caps including tants (bar a couple in hard to reach places) and installed multi-turn trimmers on the amplifier board, and a number of failed transistors as described earlier.  Tants for the most part were in spec, but were showing elevated ESR compared to replacements.

I remounted the power transistors with new mica/paste and have done quite a lot of reflowing.

On FM it sounds great over headphones.

But I am getting quite a lot of hum on Phono with an open input with the volume slider at 50%. (Just at the moment, I have nothing to plug into it).  Tape 1 and 2 input exhibit no hum, just some background noise which is slightly worse on Tape 2.

Is this to be expected, and will it quieten down with what shielding is provided by the top and bottom covers when in place and a Beogram plugged in?  Or should I be looking elsewhere, like signal ground connections?

 

 

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