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Beomaster 8000 restoration #3

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ALF
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ALF posted on Tue, Jun 4 2019 9:38 AM

Hi All,

its sometimes really good to have a donor unit at hand.....

as today, the fm-signal on the recently restored BM8000 sounded distorted, no matter which station

and the volume of that signal was quite loud even at vol setting 1 ?!

after checking that other inputs are ok I decided to swapp over the fm-related boards #2 & 1 from the donor unit and everything sounded fine again....😁🙏

but,.....

where to look for the most likely problem source on those two boards ??

as I have decided to go for another restoration project I want those fm- boards to work again.

the tuning voltage was set correctly.....other trim pots on the tuner boards I won’t dare to touch đŸ˜ŗ

the donor unit is in far too very nice condition - so, its worth it to put in the work.

could someone throw some pointers in my direction.....pleeeease 🙄

ALF

 

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sonavor
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sonavor replied on Tue, Jun 11 2019 12:11 PM

It now sounds like your +-55VDC rails are not powering up. Check your +-15V supplies. Also check the two 5.6 ohm resistors on the startup board.

Dillen
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Dillen replied on Tue, Jun 11 2019 12:14 PM

Has it been wired for 230V now then?
Also the large transformer (not all can be rewired)?

Martin

ALF
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ALF replied on Tue, Jun 11 2019 1:04 PM

The +/-15V supplies are present and it sounds like the 55VDC rails are powering up, judging by the double click of the relays ?!

i will - to be sure - check again tomorrow.

and then there is still my special friend, the tone/filter board ! 🙄

ALF

ALF
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ALF replied on Tue, Jun 11 2019 1:09 PM

No, not while I had the unit.

i do recall the unit came without a power plug, something some foreign owners do  - it’s my best guess that it was most likely a US plug.

I am using a step-down transformer now, which works quite well.

ALF

ALF
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ALF replied on Tue, Jun 11 2019 1:12 PM

I am not sure whether the small transformer can be set for 230V operation ?

to check the big one I would have to pull it out, which would appear to be a rather BIG job

and then, how to rewire ?? Unless you can tell by the serial number ??

ALF

Dillen
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Dillen replied on Tue, Jun 11 2019 1:34 PM

Type 1903 is the 120V US version.
The transformers can not be rewired.

Despite soft-start circuit Beomaster 8000 has a very high rush-in current at power up.
Your step-down transformer must be able to handle this.

Martin

sonavor
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Also, you just reported that you hear the relays click....so do you have +-55VDC on the rails now?

ALF
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ALF replied on Wed, Jun 12 2019 4:03 AM

Sorry for the delayed response - need to sleep occasionally đŸĨē

yes and no.....I only have +/-51 VDC, measured on the big reservoir caps.

but still no sound, neither via FM nor TP2 ☚ī¸ But I can hear that static prickling noise when turning the Vol wheel ?!

mystifying....

ALF

sonavor
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ALF:

I can hear that static prickling noise when turning the Vol wheel ?!

mystifying....

ALF

The crackling noise when the volume changes is a symptom of the DC offset compensation circuit on the Filter & Tone Control board for the volume control device. Each channel has an adjustment trimmer there.  There is a procedure spelled out in the service manual for it.

That being the case, it sounds like the volume control to the output amplifiers is working if you are hearing the crackling noise.

That means the problem is with the source signal getting to that volume control circuit. I typically suspect cable connection problems first in this case. Always mark all of the cable connectors with their target connector designation using a Sharpee (black marker).  It will save you in the long run.  Make sure the cable plug that goes in the left, upper side of the Filter & Tone Control board is plugged in.

If the cables are all correct then try swapping out the preamp with one of your good ones. After that, the Filter & Tone Control board.

 

Dillen
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Dillen replied on Wed, Jun 12 2019 8:05 AM

Pre-amp loops installed?...

Martin

ALF
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ALF replied on Wed, Jun 12 2019 10:47 AM

 Hate to make the same mistake twice Martin, .....🙄

you mean the two metal jumpers on the left outside....yes, they are in !

ALF

ALF
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ALF replied on Wed, Jun 12 2019 11:03 AM

The crackling noise doesn’t worry me so much at the moment - yes, I know the procedure. The tone/filter board got even two new trimmers for that 😁

what is also unsettling is the fact that the signal strength meter for the FM isn’t showing any sign of life, MPX however comes on ?!

I checked the FM output at 2IC6 pin8 - should be getting +15VDC - and pin6 - should be getting -15VDC

I measure -14VDC at Pin8 and -6.4VDC at Pin6 although the +15V/-15V are getting to the board....meaning I have to take out

board #2&1 and check again.

in other words, one FM channel output - OP amp  is only getting half the required voltage supply ?!

I did check the cable connections and will do so again.

swapping board#3 did not achieve anything....same with board #4 ☚ī¸

Hope you are right with the cable connections 🙏🙏

ALF

sonavor
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sonavor replied on Wed, Jun 12 2019 10:20 PM

I would hold off messing with the FM boards for now. Work on getting Tape 1 & 2 source inputs functioning. Once you reach that point you can return to the FM board. 

Check over your source input box and wires. Someone could have mucked around with that. Verify your Tape source signals successfully reach the preamp board. Then continue from there.

ALF
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ALF replied on Thu, Jun 13 2019 12:51 PM

Well, some partial good news:

somehow I got the right channel working, using TP1 and 2.

getting some hiss on the left speaker but that’s it !

so far no findings on the input cage and preamp board. i have gone through the cables once again but everything is ok.

the switching between inputs is also working. I have a couple of AD7110 ICs of chinese make......which i have not tried out ?!

I believe both output amps are working, having adjusted the no-load current and DC-offset.

yes I know, sometimes one can not see the trees in a forrest !

for now I have to rest after spending half a day “messing around” on the beast.

ALF

ALF
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ALF replied on Fri, Jun 14 2019 10:47 AM

Ok,

saturday.....workday again !

couldn’t help to get back to my special friend the tone/filter board : game enough to change the vol attenuator chips AD4110KN one after another

And see........both channels have audible sound - hurray 🙏😁

so, another positive step in the right direction.....next to that awful crackling noise, which is quite audible when switching inputs and vol adjustments.

an initial “blind” adjustment of trimmer pots R146/246 delivered no relief and I may - as the SM suggests - disconnect R243/245 and R143/243 to extend the trimmer range.??

will be back 🤞

ALF

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