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've picked up a beomaster 900K which has a few issues, the main one being the tuner being inoperative on all bands. The unit powers up fine and power supply voltages all seem good, other issues being that it sounds very "bassy" unless turned to a minimum. Phono and tape inputs work if a little "hissy" unless the tape input is set high.
The archive forum of this site suggests faulty AF116 transistors as a common fault for the tuner so I'll be taking a closer look at these in the next few days. The reason of posting is to ask if there is a recap kit available anywhere to save me identifying all the electrolytics and then ordering probably a lot more than I'll need.
Welcome to Beoworld !
Your Beomaster will need two fresh IF transistors.In the vast majority of these Beomasters, the capacitors will still be fine and I don't have a kit for this modelfor the same reason.If, after replacing the IF transistors, the sound is still muddy, you can take a look at the capacitors related to the tonecontrolbut I see no need to replace the whole lot unless your Beomaster is in a very bad general state.
Martin
Update - replaced the two transistors and radio now working nicely on all bands. I still need to check the bass/treble circuits and have realised that if I turn the treble right down there is a crackle and the right speaker gets significantly louder albeit at worse sound quality so still some work to do.
One question - not having seen a user manual I assume the meter on the left of the display is a signal strength meter - it moves when tuning MW but seems to stay at 5 all the time for FM - not what I would expect.
Thanks for the welcome and for the archive - saved me significant time!
The signal meter should also work on FM.It may sound strange but the meter will stay at max deflection if the IF signal is too weak.The reason for this is that it reacts negatively (measuring "noise") and is at rest at max deflection.If the noise level is not high enough, the meter reading will not fall.
Either there is still some fault in the IF circuit or the type of IF transistors mounted may not be suitable (or one of them is bad).What type did you use ?
Replaced with AF126, the actual tuner tunes in very well even with the built-in aerial. The meter moves much more on the other bands.
AF126, though often seen listed as a direct replacement, in my experience, is not particularly good withfrequencies this high.In other words, I don't really like this transistor.Try with f.e. AF109r. I have used this type myself for years with good results.If you fit it upside down (or at the solder side of the board) with the indicator pinfacing towards the rear of the Beomaster, the pinout is correct.
So the radio has been working nicely for the last couple of weeks, only in the last day it seems to have lost some of its ability to find weaker stations. STill gets the local stations fine but stations it previously found easily are now a bit of a struggle. Also noticed that the "stereo" light is now flickering in time with the music rather than solid on.
All no-signal voltages all look acceptable so any pointers gratefully accepted!
The stereo decoder circuit, slightly depending on the exact version of decoder, is quite sensitiveto the IF signal quality. If the IF is weak, the stereo decoder will not sense the MPX signal or be running "on the edge"which it often as the flickering stereo indicator lamp, you mentioned.
As said above, with transistors rated for max 75MHz pushed to 88-108 MHz, I think, I would worryabout them first.
Must be some kind of misunderstanding here.Aren't we talking IF transistors?
Yes these are the IF - so ~10Mhz should be their working area? I am at risk of showing my ignorance here though! I have the schematic but need to sit down and figure out the various parts such as oscillator, filters etc to be able to debug it properly, I do have a scope to use so will have a further look around.
With a 10,7Mhz IF the AF126 should be OK.
Guess what's waiting for repair in my pile of 'when I get the time...'.a 900 with a bad IF transistor!
OK, I am not totally familiar with FM detector circuits but it looks very much like this uses a ratio detector, and the sudden loss of sensitivity would point at an issue with either the diodes or capacitor(s) in the ratio circuitry, which would be d8/d9 and the associated local capacitors. I will test these this week and see if I can find an issue. Any pointers in the meantime would be gratefully accepted!
Hi Dillen (and everyone else)!
I'm new on the forum and found this discussion about B&O 900K problems. I have the same issues as explained here, but I don't know what to replace with the AF126 or AF109r.
Is it all four caps saying "AD139 / 332" on the backside?
Is there any particular components on the circuit diagram that explains what to solder, and how pins are connected?
I have the manual, found a circuit diagram and know some basics in electronics.
Check this image: https://www.instagram.com/p/BNje5qyg8W8/
Thanks,
Calle, Sweden
Calle,
Welcome to Beoworld!
Don't get me wrong but from the wording of your post, I can tell that the job of repairing the IF circuits is not for you.The "caps" at the backside are not caps, they are transistors and they are part of the stereo amplifier output stages.The IF transistors are AF116 or AF117 and are found on the main circuitboard - not particularly easy to access.I admire people with an urge to Do It Yourself, but I have an even greater respect for people who stop when they get into too deep waters.I think you (and the Beomaster) will be best off leaving the job to someone with the skills, experience, tools and instruments needed.
Depending what transistors you fit, their bias may have to be corrected, typically done by changing resistors to suit.
Tanks for your reply Martin!Yes, you are certainly right about letting someone else do the magic. I just find it very exciting to have a look into the world of electronics!
For what I understand when I look at the circuitboard, printed on board AF126/1 and AF126/2 are the IF transistors TR3, TR4, (both with printed AF116 on them), on my 900K due to the servicechart 1725 (Beomaster 900K, 900M, 900RG), located almost straight in from the extra speaker outlets, close to the edge of the circuitboard and one other in between of the metal boxes.
What would be the easiest/best transistors to replace them with?My idea is to get the spare parts since I found this forum, and let someone who works with electronics to repair this for me.
/Calle