ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
I am fixing an old 4000 and have managed to get the amplifier healthy. Now i'm working on the FM section.
The tuner works, brings in stations, etc. after changing the zener diode 138. As an aside, i've seen this listed as a 22V zener diode on some schematics, and as a 27V on others. Does it matter or are there subtle changes between versions of the 4000 that would say go one way or the other?
The AFC doesn't work as expected. You tune a station, press in AFC to "hold" the frequency. But when I press the AFC button, the station is pulled out of tune. In fact, sometimes it pulls in a neighboring station. All the capacitors in this section seem to be tantalum, which i wouldn't expect to go bad. On the other hand, Dillon included a bunch of tantalums in my 3000 rebuild project so maybe that is the issue with this old 4000.
Looking for advice as to where to start looking here.
A 2nd issue i see is that the signal strength meter works only sometimes. When it is not working, it just sits at zero even on the strongest stations.
As always, thanks in advance.
Jeff
Beomaster 4000 was produced in several versions.From the outside you can tell, that some will only tune up to 104MHz while others tune all the way to 108MHz.
Yes, the tuning voltage does matter - but not in relation to the AFC problem.
If the AFC pulls in a nearby station instead of the one you have tuned, it may be because that station isconsiderably stronger and at a very close frequency.
If the tuner pulls away from the station (towards "nothing") when AFC is activated, it usually points to a problem in thedetector - or the IF stage just in front of it.Loss of symmetry.
S-meter - put a voltmeter across it when it doesn't deflect. If there's voltage but no movement then the meter is bad.
And yes, tantals can go bad.It's my experience that they are almost always found either all fine or all bad/marginal.
Martin
Thanks Martin-
I fooled with the secondary detector screw - the one in the silver box with just two adjustments peeking through. With some minor fiddling, the AFC is now working. That adjustment seems to move the center around.
Fooling with the S-meter adjustment pots got that working too, although i can only adjust strong stations to show about half of signal strength.
And i'm guessing that the Zener i installed with the wrong voltage is the source of my tuning issues - stations are off on the dial and the pre-sets by a decent amount. I'll find a 27V diode and see if it fixes this.
Without exception, all tantalums show "marginal" (they yellow band on my ESR meter). But that is a similar reading to what i see on the brand new ones i got in your kit to rebuild my Beomaster 3000. So i'll assume they are good for now.
Just a final follow-up... I changed out Zener diode 138 to the correct voltage (27V in my case). Tuning meter now works. Stations appear on the dial about where they are supposed to. In short, this fixed the rest of the issues. So for folks repairing the power section of their FM circuitry in their Beomaster 4000, make sure you double-check the voltage of Zener 138 and match what is there (i had the wrong schematic). Also, make sure you smear a good amount of heat sink goo on this diode and screw it to the silver box. I had it floating in air for a while for testing and sure enough, the stations drifted out of tune just as others have described. With it screwed down, it has held a station steady for 4 hours now.
The little clear pre-set station covers that Martin makes for this receiver are great, by the way. I bought a couple of these and they look original and they snapped right in. Now that this is all fixed again, the stations under that cover stay nice and "in tune".
Great - now on to the next!