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 opened my BM 4000 and changed the 6 trimpods on the preamp board, before closing it again I checked the idler current on right and left channel, right channel was correct 80 mA, as adjusted 6 month ago, but left channel was down to 25 mA, I tried to adjust it but no reaction what so ever moving the trimpot, I started checking the component right to left channel, the only difference I found, measuring onboard without desoldering anything, was the 2 resistors, red and green arrows, red is 470 Ohms and green is 330 Ohms, onboard I measured red 350 Ohms and green 270 Ohm, right channel, blue arrows measures OK, I desoldered on leg of both resistors and measured again, measured OK. I desoldered the B leg of the MPSA 13, yellow arrow, right and left channel, from B to C and E, with diode tester, I got in the left channel 0.560 V to both legs, and right channel, B to C 0.650 V B to E 0.860 V
Connected everything again, and I get good sound in both channels, but what the fug is wrong here.
Here the preamp board with new trimmers
Collecting Vintage B&O is not a hobby, its a lifestyle.
Just to keep this one alive, help needed
Sören,
check MPSA13 C-E with diode tester - could be short which could be a reason for the strange ohms values while in-circuit.
A short C-E would parallel the 250R poti to the 330R and 470R resulting in the lower values you measured.
Gunther
Thank you Gunther, I didnt want to desolder the MPSA as the tracks on that board comes of very easy, and it seamed OK, will get to it this weekend, this week we are having birthdays to celebrate, Today I'm cooking for 25 people.
Vielen Dank Junge, Du bist Spitze
Changed the MPSA 13 now the left channel MJ 3001 get very hot, the 2501 less, right channel OK. Still checking, but help welcome
Edit, it was 2501 very hot, and 3001 less, C-E in 2501 was open circuit, changed it, but now again I cant adjust idle current in left channel, and no sound in left channel.
It most likely a transistor that is leaky or similar under load Soren, just got to take those measurements under load I think, and look for lower than normal volt drop etc...
Measuring components without power does not always find the culprit I'm afraid..
Olly
Thanks Step, that was my plan for today, but I was afraid doing anything under load as the MJ2501 got very hot, but now nothing seams to heat up, but I'm out of MJ2501 now.
Did some checks with power on, the 3001 is bad as well, I connected my DMM where I wanted to check, powered on, and then of, I'm afraid to leave it on for longer periods. Ordered new MJ 2501 and 3001, with luck they will be here next week.
Søren Mexico:Ordered new MJ 2501 and 3001, with luck they will be here next week
It's a pity you don't live closer
Got my Transistors 2 weeks ago, first I ordered with Futurlec, after a week I ask for a shipping date, and they told me "no stock" for 3001, their web shop showed 5 ready to ship when I ordered, I cancelled the order, and continued my search, but no such luck, most sites had either 2501 or 3001 not both.
Started looking for replacement and found NTE245 and 246, Newark could deliver to Mexico, no buying limit and to door with UPS, 5 days delivery. Bought some extra, stock is good if you are in Mexico.
Since one hour testing after adjusting no load current, nothing hot and the "bump sound" switching on and of is now nearly gone (normal is a little, like welcome, bump. Saludos now onto the Cubas.