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Beomaster 8000 start-up issues

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ALF
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ALF posted on Wed, Oct 10 2018 9:43 AM

greetings All 😁

after I finished a total recap job on my MB8000 the time to test that monster finally arrived......😢

the Standby dot showed, turning it on only fired up the display ?

no idle current could be adjusted as there is no power/current present on the output boards ..... ?

checking the start-up circuit I unearthed two dead resistors: R1 and R2, the 5.6 Ohm/5watt types !

as I received the BM as a ‘not working’ unit I can not say anything about its history....all power darlingtons

on the heatsinks were dead with the exception of 2.

while I did the recap job I noticed most boards looked untouched, no obvious signs of previous repair attempts.

the question is: what was the reason for 7R1/2 to fail ?

Both are critical (fuse-like function) and I feel somewhat hesitant to simply replace both to prevent any further damage ?!

as usual, betting my life on a recap job with a warranty is not an option but I took a long time to make sure 

silly mistakes to be avoided at all cost 🙄

has anyone had a similar issue with the star-up circuit and if so what was the cause ?

missing segments on the DPs are a minor issue for now ! As always, your comments are appreciated and invited.

ALF

PS: removing the left channel output board only took a lifetime under utmost caution 😡 gaining experience is such an uplifting feeling 🤙💪😁

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ALF
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ALF replied on Thu, Oct 11 2018 1:02 PM

Ok,

the two fuse resistors 7R1 and 7R2 are replaced but still no starting-up, only a short lighting up on two displays followed by relay clicking and everything is off again.

on to the fault switch (TR 15/16/17) : all transistors tested ok, which tells me the fault switch should be doing what its supposed to do.

now on to finding what could possibly causing the short ??

one suspicion:

some or one of the power darlingtons mounted is making contact with the heatsinks, shifted off the mica foil while being installed ? - don’t you hate that gooey white paste 😬

I also noticed the 330nF capacitor (as per SM) on the starting-up board is infact a 0.1uF cap ?

still hoping for some pointers and ideas as this monster is quite complex....!

thanks

ALF

sonavor
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Hi Alf,

It is too difficult without actually seeing what you are doing and really looking at it myself to provide you with anything concrete.  However, in this type of scenario you should get down to basics and start simple.  Since the fault is resulting in problems with the start up circuit you need to start with power coming on. 

You have +5 VDC since the standby LED comes on when you plug the Beomaster in.  When you press a function to use some audio source is when the main power tries to engage. I suggest removing the power connectors from the boards (output amplifiers, preamp, tuner, filter & tone control) then turning the Beomaster on. If the Beomaster comes on okay like that then it will indicate the problem is power routing to one of the boards and you can one-by-one locate the culprit. If the Beomaster still doesn't come on then you can focus on the power supply and start up circuit side of things.

John

 

sonavor
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Hi Alf,

Also....regarding installing the left output amplifier assembly, I was able to capture the sequence on this Beomaster 8000 I am working on.  If the transistor spring clips are mounted correctly they shouldn't shift as the board is being manoeuvred into position. It is still not the easiest of installations (or removals) but it is per the service manual and does work.

John

ALF
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ALF replied on Sun, Oct 14 2018 9:49 AM

Hello John,

my apologies for the delayed response......right, sadly I have to agree with what you pointed out....I also knew this would be a very

big challenge the BM8000....I looked at it as a kind of tutorial, loosing fear of opening the monster is nice.

I should have done exactly what you suggested right from the start of trouble.

well, I disconnected power from the 2 output amps (red and black wires) and the 15V supply, similar to the filter/tone control and the preamplifier.

this way I can switch on the BM out of standby ...no trouble.

in the meantime replaced the the 2 fuse-resistors R1 and R2 on the the starting-up board and replaced

the no-load adjustment trimmers on both output amps.

reconnecting the preamp and tone control/filter boards still caused no problems.

on to the first output amp board - next to the power supply: still no problem bringing the BM out of standby, however the no-load current adjustment

is extremely touchy and seem to drift over time, which caused some concern 🙄😡😬 and also checked the board’s two BF type transistors...all good.

that leaves the second output amp - next to the preamp board as the main suspect ?! I could not detect any cracked joints, all BF transistors tested fine....

this is where I stumbled over the first roadblock !

I thoroughly checked the connections of the power darlingtons on the heatsinks.....nothing to see, unless one or two make contact outside the mica insulator...which I could not see.

I scanned an archived post of yours from 2013? to see if there are similar problems that may have occured to your BM8000.....but wasn’t the case.

BTW, those 3 voltage regulators connected to the PSU are all fine at 4.95V, 15V and -14.94V.

apart from replacing the old no-load trimmers I noticed that both wire-wound resistors on the “trouble board” were practically “naked”....due to excessive heat ?? Both were replaced as well.

this is where the projct stands at the moment.....question remains, is that second output amp board really responsible for triggering the fault switch 

or is it fair to think of that big second rectifier next to the associatd reservoir caps ??

ALF

 

sonavor
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Hi Alf,

Which output amp are you calling the first one?  The right channel?  Is the left channel output amplifier the one you are having issues with?  The output amplifiers do connect to the fault circuit. From your description above did you hook up and try the left channel amp again?  While your suspected output amplifier is disconnected what do you measure on the diode bridge that supplies its ±55 VDC?

What is the history of the output amplifier board?  Did it ever work for you? Does it have a lot of burned areas on the board?  

Due to the trouble with working on the amplifier board while it is install and the trouble removing/installing it is why I prefer to test my reworked amplifier assemblies outside the Beomaster by themselves. It does require a bigger investment in test equipment but like to know those assemblies are working before reinstalling them. 

John

ALF
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ALF replied on Mon, Oct 15 2018 4:23 AM

Hi John,

well, I am aware testing the output amp board outside while not being installed would certainly help but with pensioner budget restraints

there is not a lot of money left for additional testing equipment 😢😬

I did reconnect the “good” output amp again while the trouble side remained unplugged and as said, tried to adjust the no-load current to 18mV....

it is getting there after a while but very touchy to adjust ??

on the “good” side I got about 49VDC at the + terminal of 30D1, same at the +\- terminals of the big reservoir caps.

checking the “trouble” side delivered a jumping array of mV at the + terminal of 30D2, dito at the big reservoir caps........

i will check the diodes of the rectifier.

to clarify: looking from the front of the BM....

the “good” ouput amp is the right one, next to the power supply board.

the “trouble” output amp is the left one, next to the preamp board.

ALF

sonavor
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Hi Alf,

Well of course the faulty amp would have to be the left channel output amp which is the most difficult to remove. I understand about budgeting test equipment. Like woodworking there are ways to get by with less tools but having the right tools make jobs so much easier.  I was just saying the reason I do it is because I do quite a lot of them and it saves me a lot of trouble doing it that way.  You can still solve this without testing the boards externally. Did you replace the 100Ω trimmer resistors for the no-load current adjustment?  What does the +15 VDC look like to the output amplifier boards.

John

ALF
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ALF replied on Mon, Oct 15 2018 7:52 AM

Yes John,

it had to be the difficult one !😬😬 ...... one doesn’t like those cheap quick repairs !

naturally it would be easier to have an external test-facility .....  no intentions though to pull out that board one more time too many !

just checked: 15V is present, both 100 Ohm  trimmers for the no-load current adjustment have been replaced.

I wonder if I would have to unsolder the big rectifier as it has connections to the 10K uF reservoirs in order to test safely without getting zapped ??

sadly I do not know anything about the BM history, except I bought it as a non-working unit with most power darlingtons at the heatsinks missing.

two brand new sets have been installed in the meantime.

The BM is in quite good condition otherwise, minus the usual missing segments in the DP’s which is the least of my worries for now.

one of the questions I have : is the big rectifier damaged as I am getting 35 and 39 VAC on its AC terminals ?!?  and basically nothing on

its VDC terminals except a few mV ?!?

ALF

sonavor
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Hi Alf,

On the bridge rectifier for the ±55 VDC rails I get around 76 VAC, 60 Hz across the AC power in terminals from the transformer.  On the output terminals of the bridge rectifier (which goes to the reservoir capacitors) I get 105 VDC. You should be measuring across your AC terminals and not each terminal to ground independently. So it looks like you have over 70 VAC there. Measure the voltage across the output terminals (DC voltage).  It sounds as if the power in is probably good. I would suspect the left channel output amplifier board. It also sounds as if the right channel board also has some problems.

John

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