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'm working on my 3rd Beomaster 4000 (i'm giving each of my kids one...). This one had a dead power switch and when i replaced it, it came back to life - sort of.
Everything works - sort of. For example, the FM will bring in a station or two, but if i push the AFC button down, it will untune a station. I can't get a stereo signal on anything, the red indicators won't light and the tuning meter won't budge. But i can get a few stations.
On the amp side of things, everything sounds very faint and thin. I can turn the volume way up and i don't get much out of the speakers. This is true for both channels and seems to be independent of the inputs (tape or FM).
It has all the old orange capacitors but they all test pretty well with my ESR meter. Maybe all it needs is recapping but i do find it strange that it is so weak in both channels. I would have expected one to be better than the other if it was an issue in the amp.
i have a nice, steady, 70V on the main power rail so the power supply seems fine.
Looking for suggestions as to any likely suspects that i can start with.
Thanks for any suggestions.
I've managed to fix the FM section to work again (apart from the tuning meter is always at zero), and now i'm moving into the amp section.
Any idea why both channels would be weak? By this, i mean i can turn the volume up to 6 or 7 and i just don't get much sound out of a set of speakers. This is now happening on all inputs, so the amp sections seems the likely culprit. Weird that it is the same in both channels.
Hi Cray
Have you checked the idle emiter voltage at each chanel for 12mV?. I think this is a good indication of the power amp side of things, there are lots of threads identifying how to do this.
Good luck...Craig
Thanks Craig- Yes, i've checked (and adjusted) the idle emitter voltage to 12mV on left and right. Also adjusted the left and right no-load idle current to 80mA.
The whole thing just sounds weak with very little bass. Both channels. Seems like a bad transistor but it is strange that both channels behave this way.
I'm replacing all the electrolytic capacitors as i did find 3 or 4 with bad ESR readings, but i doubt that is the issue. Just getting my mouser order together.
Sounds like the problem is common to both channels, therefore you need to check out the common components....I would start by looking at the volume and balance potentiometers.....maybe a slider issue or dry solder joint
Hope this helps
What Craig said - and power supply voltages/ripple.
Martin
No luck on either the volume slider or the balance slider. I looked at all solder joints, cleaned with deoxit. No change.
The power supply has a small sawtooth ripple, but only about 400mV (see picture of waveform measured where the 70V enters the amplifier board). Power supply makes a lot of sense, but this doesn't look like the culprit.
The front speaker and ambio selector switch is another common point as is the loudness selector, its unlikely that both 5000uf output caps have failed but probably worth a look...lots of wires on each of those capacitors.
I finally found the problem. By tracing a 1000Mhz signal through the amplifier, i found that it went "kaput" right after the 4.7uf 63V capacitors which sit in the signal path on the way off the amp board heading to the volume and balance logic.
Both capacitors where totally shot (they were the only purple or maroon colored capacitors on the board, the rest being orange). I replaced them with some higher voltage ones i had sitting around, and presto - things are now sounding great and i have some confidence about replacing the rest of the electrolytic caps..
Next problems i have are with the AFC FM logic (pressing the AFC key takes a station out of tune vs. "locking it in"), and an intermittent signal strength meter. I'll probably have to start a new thread on those problems.
Thanks for help everyone.
Jeff