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Beomaster 6000 type 2252 volume control

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Craig
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Craig Posted: Mon, May 25 2020 11:23 AM

Gentlemen...

My Beomaster 6000 has developed a problem with the volume control. Manual volume operation from Min to Max is fine, the step volume controls from 0 to 30 work fine, but from 30 to 60 don't work. This is the same on the preset volume slider....great from 0 to 30 but nothing over 30......the motor sounds a little rough also.

I have recapped this in the past, though it has laid dormant for a while, I don't remember these issues being present at that time.....but hey who knows, any ideas anyone before I start to pull it all apart?

Craig

Keith
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Keith replied on Tue, May 26 2020 11:05 AM

Have you tried using a remote control, might give you an idea if it’s the key pad or ic chip.

 

Keith.

Craig
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Keith no I don't have a remote for this unit, I seem to recall using Peters remote for his BM8000 on this machine and I think it worked fine, however I cant be sure.....non the less I have started to take a look inside, always a bit exciting.....starting by removing the dial drive cord and tape, im a bit nervous about breaking the tape

 


Craig
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PCB6 looks favourite as a staring point so with reference to the diagram below (click it to view as it saved too big) I have found I'm getting the correct voltages for each of the volume select pushbuttons (1 to 15v) when each is depressed measuring at the cathode of 6D8. when doing this I can balance the drive motor 5M1 back to null by moving the volume potentiometers when depressing buttons 0 to 30 only, above 30 the motor drives and can be stopped by rotating the volume potentiometers but cannot be balanced i.e. rotating the pot beyond the point at which the motor stops turning does not reverse the motor as it does onoperation of buttons 0 to 30.....I think the positive here is that I'm getting the correct voltages out of 6IC6 when pressing each preset button.

 


Craig
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Craig replied on Tue, May 26 2020 4:00 PM

Next I have checked the voltages I'm getting at the cathode of 6D5 when I select RESET and move the slider from 0 to 60 and I measure the correct voltages 1 to 15 respective to the 0 to 60 volume levels.....interestingly I can also only bring the drive motor back to null at preset levels 0 to 30 using this control method......when using the manual raise and lower volume switches everything is fine. I need to look harder...…...

Keith
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Keith replied on Thu, May 28 2020 8:22 AM

Looking at the manual, 1-5 6pcb shows the volume chip ic6 which shows pins 5,6 and that run the top end. Just wondering if the has gone faulty.

 

Dillen
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Dillen replied on Thu, May 28 2020 8:51 AM

The volume control board is known to develop lots of cracked solder joints, particularly where connectors are soldered in, and the trimmers oxidize.

Martin

Craig
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Craig replied on Thu, May 28 2020 8:59 AM

Keith

Looking at the circuit diagram I can see that the top end comes from those pins too, thing is when I measure the voltage at the cathode of 6D8 for the steps 0 to 60 I do get the prescribed readings 1 to 15v, this suggests to me that the signals to 6 IC6 are good. When I run the manual volume control up and down the motor drives up and down correctly too, this suggests  6 IC5 is functioning ok too......I have tested the diodes 6D4, 6D5 and 6D8 and all are good....I have also removed and tested all the trimmer on PCB6, the problem is the same when trying to adjust the preset volume also, at the same point.....I think I need to check the slave volume potentiometer as it is commom to both volume control methods.....it should be noted that the actual volume of the unit goes up and down with the manual volume control buttons, it looks like the 5R6 pot has three variable resistors, one for the volume of each channel and one for the slave volume which is common to both step volume and preset volume functions...so three in total, anyone known these to fall over in the past?

Craig

 

 

Craig
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Ok.....spent an afternoon going over the function of 6 IC6 and using the truth table below confirmed the IC is working exactly as it should...all inputs and outputs respond to the functions identified, so a little progress

 


Craig
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Craig replied on Thu, May 28 2020 3:16 PM

Martin

Thanks for that I will go over all the soldered joints and reflow them, I have replaced all the trimmers and removed them yesterday to check them out too. Going by what I have proved with the 6 IC6 functions all checking out from the control panel pushbuttons I think the CPU is good. Looking at  6 IC5 I can see it has been replaced in the past, whats bothering me is that everything works fine at volume steps 0 to 30 then it goes to rat@hit.....

Craig

Craig
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Craig replied on Fri, May 29 2020 5:06 PM

Ok.....unplugged the connector from PCB5 and measured the resistance across 5R6 slave volume potentiometer and its measuring out at 83k ohm, should be 100k ohm however I think its fine.

Moving on im happy with  6IC6 following the checks I made yesterday so I have looked at 6IC5 today, measuring the voltage at the cathode of 6D5 and setting the volume select pushbutton 20 im getting 7v and the motor 5M1 drives, I then manually rotate the volume potentiometers up along with the slave volume pot until I get 7v at the cathode of 6D4.....these equal volages at pins 7&9 and pins 6&8 at 6IC5 stop the motor 5M1 at what im assuming is the 20 position on the volume scale, rocking the potentiometers back and forth makes the motor drive up and down.....great, just as it should. When I do this having selected the 30 pushbutton I get the same results but with a 8.8v measurement at the cathodes and pins.....however when I go above 30 to 40 I get 10v at the 6D5 anode and I can rotate the pots to give an equal 10v at the cathode of 6D4, therefore equal voltages at pins 7&9 and 6&8 of 6IC5 however the motor continues to drive and wont stop until I drop the voltage at 6D4 back to 8.8v, also I cant make the motor run up and down at this voltage level at the cathode of 6D5, this is clearly wrong and as I know 6IC5 was replaced in the past im happy to lay the blame at its feet......its also a different part number from the original (LM1877 which doesn't look massively different) however I have ordered a replacement LM378 from littlediode and when it comes I will fit it and go from there.....

Craig
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Whilst awaiting delivery of the replacement IC the volume drive belt and indicator cord have been replaced, I have got a replacement belt on its way from Martin also, as its age is unknown and its undergoing surgery the belt may as well be changed out too.

 


Craig
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Feeling industrious I have removed the suspect IC in anticipation of the arrival of the new one...….

 


Craig
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New IC turned up today...…..took a little longer than I thought it would, but these are strange times and it seems most things take a little longer and will do so for a while im thinking

 


Craig
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Soldered him in and cleaned up the solder side of the board.

 


Craig
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Dropped the PCB back into the set and plugged in all the connectors.....a bit nervously because although I felt quite sure this was the issue one is never fully sure that there isn't something else underlying the fault...……...

 


Craig
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Powered up and tested....all good! the preset volume settings all work exactly as they should, I'm very pleased with it.....however, and there is always an however, the belt is slipping ever so slightly.....but annoyingly none the less, this said there is one on its way from the ever resourceful Mr Olsen which will be fitted on arrival....the noise I mistook for a noisy motor is in fact the rubber belt slipping and making a disturbing groaning sound....so all is well ;¬)

Craig

 

 


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