Beomaster 5000/Beocenter 7700

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I am nearly a geezer but new here and minimally experienced with audio gear.
I'm hoping my questions aren't too elementary:

All my B&O trimpots are old skeleton-type with no clear markings to indicate their specs, and they all need to be cleaned. Q: Deoxit D5 or Faderlube? 5% spray or 100% solution with needle applicator?

The Beomaster 500 has no FM reception-- white noise with only occasional blips of obscured reception, as if there were no antenna. The last B&O tech in the Seattle area ( crazy, since it's mostly Danish- and Norwegian-heritage people here) who looked at it for a week or two and said, " probably the whole FM (mumble mumble) needs to be replaced, and I'm not going to do this work anymore. " Then he gave me a parts-only 5000 (sans the FM IF PCB, of course), a several functional cartridges and a couple needed Soundsmith stylus work (!), and 5 turntables in various states of repair. I should have asked for his caps and ribbons and resistors and transistors, too.

If somebody could give me the first few steps to diagnose and fix the FM, Occam's Razor style, I would really appreciate the consideration.
Great site-- friendly, helpful and fascinating.
 
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Welcome to diyAudio :)

Skeleton presets can get flaky with age, not just the wiper/track interface but also poor bonding/loose 'rivets' where the track connects to the end pins. Replacement would be the normal option tbh. If you do attempt cleaning then its really only of any use if the preset is turned a few times end to end... which means it needs resetting as per the service manual... and so could be an involved job requiring test equipment and expertise.

RF faults are often difficult to diagnose without extensive testing and again, having the necessary gear to do meaningful measurements. That said you might get lucky... if it really is a case of 'low RF gain' then its possible a front end transistor could have suffered in some way but replacement may be problematic as the device types are special and the circuit may well have been aligned around the actual device used. Other common issues could be a small disc ceramic cap that has gone leaky somewhere in the front end.

As ever, a service manual and at least basic testing of supplies and static DC levels around the RF stages is in order.

Probably not what you wanted to hear :( but you have to be realistic in these things.
 
may be the front end fet or the power supply to the FM section.
there are multiple resistor trimpots in that thing that may need replacement. mark each position and then rotate left and right to possibly restore contact then test the tuner. The caps are likely good enough to allow operation.
those chips likely are running 12v on the Vcc pin, test that as well.
if it comes to life , replace all electrolytic caps and trimpots
 
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Thanks-- the no-load/idle current trimpots are the problem with the 7700 right now-- I have enough equipment to set those and have done so a few times, but one side in particular won't maintain the 10mV and the unit shuts down as it's supposed to in that case.
Looking at the schematic, I'm assuming these are 250kOhm rated and that sealed trimpots might be a better bet? Bourns cermet for example?
The Hi Fi Manuals site has both service and full schematic manuals for anything you can think of-- even the most obscure B&O stuff, so that has been a pleasant surprise.
In short-- thanks-- it is exactly what I wanted to hear! ( Though I still want to try Deoxit D5 or Fader Lube-- if i knew which is best in this situation!)
 
Thank you. I'm following all of it except 'chips running 12V on Vcc pin'. I'm assuming that means to measure the voltage at the Vcc pin (assuming it's labeled so I can find it)?

Lastly, what is a good source for better-than-radio-shack-chinese caps?
And thanks again. Beginner's learning curve is the best part of curiosity and discovery, right?
 
As others have replied, the trimpots needs to be replaced. Don´t power up the 7700 before you have replaced them. It shuts off because the idle current runs off. Occationally it shuts off too late. Then you´ll have a lot more replacements :).
The 5000´s in general suffers from bad processors in the "tin can" on the front panel. Som times it can do wonders just to resolder all the screen/can connections on the whole board, but it´s no guarantee. Good luck with replacements ;-)
 
For noisy skeleton trimpots in the past I've gently applied pressure to the wiper to make good contact with the carbon track,and then measured the value from wiper to both ends of the pot. Sprayed with Deoxit, rotated from end to end a few times then set them to exactly where they were using the previously recorded resistance measurements. This assumes the end rivets are intact and the carbon track is not open circuit (and that no one has been adjusting them arbitrarily). B&O equipment is usually very well made and designed with access for repair in mind. Good luck.
 
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