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 have done a lamp and capacitor replacement on a Beomaster 1900 and everything works quite well. I have read a few archived posts about idle trimmers failing and leading to the failure of the power transistors. My question is should I replace the idle current trimmers before they fail as preventive maintenance? I found a section in the manual for a reference to no-signal current adjustment, is this the same as idle current adjustment? If so according to the manual they are 2R249 and 2R349 and are 250 Ohm trimmers. It says to measure either the collector current of 2IC200 and 2IC300 or -12mV across across the emitter resistors, R256 and R356. The resistors are a bit harder to get to than the power transistors. But could someone tell me how to measure the 25 mA to the collector? I assume the red voltmeter probe would go to the collector, where does the black lead go? Also with the trimmers do I start with all the way clockwise or counterclockwise, or does it matter? Or have I just shown how little I know with asking these questions. I can follow directions but know very little technically. Could use your input.
John
I would change them, you never know what happens when you start adjusting on them, the collector current is measured with an Ampere meter, and you will have to desolder the C leg on the transistors and put the amp meter in series with the c legs and where you desoldered. I always check the no signal current (idle current) on the resistors, bass, treble and balance in 0, volume in 0, Check the manual for if TAPE or FM should be activated, I dont know the 1900 well.the trimmer are so easy to change and so cheap, that it IMO is not worth not to do it, (the last sentence came out funny, hope you understand it.)
Good job do far John, congrats
Collecting Vintage B&O is not a hobby, its a lifestyle.
If in doubt, replace the trimmers.
Start with the trimmer at its center setting, under normal circumstances this will not be too far away from being correct.Check the idle current (same as no-signal current) with a (milli-)voltmeter across an emitter resistor.You can get to the emitter resistors solder joints from below.Adjust to a reading of 12mV if the manual states this figure.Adjust with no speakers attached and no amplifier input (volume at zero).Adjust 30 seconds after power on and check again after 2 and 5 minutes.
Start with the channel that has the highest initial idle current.
Martin
Thanks, Soren and Martin. I will probably do this as soon as I have the trimmers.
I wanted to post the final result of the idle current trimmer replacement. I got the 250 ohm trimmers and with the help of my 17 year old daughter have completed the project. I set the trimmers to the mid position as Martin suggested, and while I held the voltmeter on the emitter resistor solder leads on the back side of the board, she turned the trimmer till it read the correct voltage. For those who may try this at some point a hair in turning can make a difference so go very slow with your adjustment. After 10 minutes we rechecked and adjusted as necessary. Both trimmers were done this way.
My thanks again to Soren and Martin.
jfrancis: I got the 250 ohm trimmers and with the help of my 17 year old daughter have completed the project
Sometimes you need 4 hands, I have tried to get my FS to help me, but she is afraid of anything with electric power to it, normally she starts the dish washer and run away .
Now we need some pics of the "another one saved" item,, Congrats John, and for Jeff (Piaf), you may try to explain why it is a good idea to change caps and trimmers, and not let it die softly, (joke, sorry Jeff).
Hello Soren,
Here is the photo you asked for, not a studio shot and does not do this beauty justice. If I had not been able to fix this myself I would have sent this away to get repaired it is too nice to let die.
Thanks for the feedback and help along the way.
Just beautiful, I may go and buy one some day, I love my BM 2400, my son in DK has a 1900.
Søren Mexico: Now we need some pics of the "another one saved" item,, Congrats John, and for Jeff (Piaf), you may try to explain why it is a good idea to change caps and trimmers, and not let it die softly, (joke, sorry Jeff).
Hey Søren,
I SAW that! Have a heart! It is not my fault that that numbskull used old parts and cleaned the trimmers.
On the other hand look at all the fun I have had and all the DRAMA! Truly how many people have experienced an amplifier with all the “personality” changes mine has provided? And don’t forget the lovely aroma that hastened my purchase of a really good fire extinguisher.
Entertainment like this is hard to find, unfortunately not quite hard enough. But what the heck, I still have my sense of humor and with this beast, I needed it!
Jeff
Beogram 4000, Beogram 4002, Beogram 4004, Beogram 8000, Beogram 8002, Beogram 1602. Beogram 4500 CD player, B&O CDX player, Beocord 4500, Beocord 5000 T4716, Beocord 5000 T4716, Beocord 5000 T4716, Beocord 8004, Beocord 9000, Beomaster 1000, Beomaster 1600, Beomaster 2400.2, Beomaster 2400.2, Beomaster 4400, Beomaster 4500, Beolab 5000, Beomaster 5000, BeoCenter 9000. BeoSound Century, S-45.2, S-45.2, S-75, S-75, M-75, M-100, MC 120.2 speakers; B&O Illuminated Sign (with crown & red logo). B&O grey & black Illuminated Sign, B&O black Plexiglas dealer sign, B&O ash tray, B&O (Orrefors) dealer award vase, B&O Beotime Clock. Navy blue B&O baseball cap, B&O T-shirt X2, B&O black ball point pen, B&O Retail Management Binder
I just did this adjustment, as i discovered that it was running warm and found it almost impossible as I didnt dare to hold both probs with one hand on the measureing points on the PCB.Did this as serial process, measured - then adjusted, and voltage rushed dramaticly in voltage. either direction, sometimes up to scary 1.5 volt with just tiny movementThis was scary.But I found a trick.Unpluggd the amp from power, then measured ohm with both probs in one hand, and adjusted with the screwdriver until I got the minimal resistance, which was something 150 ohm or something. - close to center as dillen said, but the adjusting is incredible sensitive.Then powered up amp and had about 15 mV on both measuring point, good enough, 100 times better then 1.5 volt.Now it is running cool
My re-capped M75 are my precious diamonds.
No signal current is the same as idling current, volume on "0", no speakers connected. and change these trimmers, I normally notice where the old trimmers are positioned and set the new ones in the same way, But Christians advise is maybe better, then you dont start out too high. Old and maybe damaged trimmers are hard to adjust, newer a lot easier and the trimmers may cost you 50 cents a piece, there are more expensive ones out there, but the same quality as B&O used is cheap.