Bang & Olufsen MX4000 CRT TV Repair - Page 1
Forum section: electronics / repair

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 14, 2023, 11:58:19 pm
content: Hi all,I've found this lovely set in a no operating condition and I'm trying to get it back to life.It's great looking and from what I've seen around it looks good with retro consoles:)The TV does not turn on and I'm missing the standby voltage as well.I have voltage on the main filter capacitor but I have no voltages right after the main switching transformer.It's a discrete transistor SMPS design as far as I can tell.I have about 167V at points labelled 1 and 2 but no voltages whatsoever on the secondary side of the transformer..I do not have voltages at points 5 and 7 of the primary..I would appreciate some help on getting this thing going..I'm the first one to open this so no previous work is performed and I see no visible damage/ leaked caps or anything out of ordinary.I attached part of the schematic my fault lies and here's a link for the full schematic (there is no SM for the MX4000 only for the MX7000 but power supply seems to be exactly the same; even component numbers are correct)https://www.dropbox.com/s/3y9bme6ui3fe4mg/MX7000.pdf?dl=0

author: james_s
author_rank: Super Contributor
timestamp: May 15, 2023, 12:06:41 am
content: Assuming you have 300V after the rectifier then the fault is most likely on the portion below the part that you captured, where the oscillator and feedback circuitry is located. If the oscillator isn't running and is not driving the chopper transistor TR1 then you are not going to have anything at all on the output.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 15, 2023, 12:13:23 am
content: Οkay that makes sense I will try to measure the transistors and passives there with a transistor tester to check if there's anything out of spec.I don't think I've seen anything higher frequency than DC on the TR1 when I was measuring it (my meter's bargraph seemed stable) so it's pretty possible there's something wrong with the oscillator circuit.I'll post back after I've done some tests!

author: james_s
author_rank: Super Contributor
timestamp: May 15, 2023, 12:17:30 am
content: First thing I would check is R14, it looks like that supplies power to the oscillator until the power supply starts up.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 16, 2023, 09:31:19 pm
content: I now noticed that I have 8.4V in the 8V standby rail.Ι have the same voltage on a point marked +15V.I'm not sure what's going on..I still don't get a stby light

author: Benta
author_rank: Super Contributor
timestamp: May 16, 2023, 10:30:04 pm
content: I'm probably dende, but I don't see your standby supply anywhere and I don't see R14 anywhere.Sorry, missed the Dropbox link.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 16, 2023, 10:45:43 pm
content: I found out that TR33 is shorted from B-E-C so total short in all directions.C100, C101 and C102 are fine..I need to replace TR33 of course but I don't see how I don't have a standby led if I have 8V on the 154V rail and also on the 8v standby line.I now need to check what may have failed alongside TR33 (asumming the flyback is still okay) before I order the parts..I doubt that it went on its own..edit#2 : I was impatient and powered it up after removing TR33 and I do in fact get a standby light!I didn't try to power it on of course just wanted to check if the HOR transistor was bringing down the standby circuit.I will try to source this transistor and post back..

author: james_s
author_rank: Super Contributor
timestamp: May 17, 2023, 12:12:26 am
content: Failure of the horizontal output transistor is fairly common in CRT displays, it is a highly stressed part and the horizontal circuit is a resonant tank with quite a lot of circulating power. I would suggest checking or just replacing C42 as it looks like that is the B+ filter, and also check carefully for cracked solder joints in the horizontal circuit.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 17, 2023, 12:18:36 pm
content: Well I replaced the HOT with BU2520DF and I tried powering on the set ; led went from red to orange but as soon as I heard high voltage it collapsed right after and TR33 is shot again.And I thought about getting a second one just in case..Now I have to go back to the shop.I also replaced C42.Apparently something else is also faulty; we are not done yet!edit: R130 reads short as well.Could it that be?It's a 1/8w resistor can it really kill a 45W transistor?C100 seems fine but I will replace it as well.edit2: R130 is not shorted and C100 wasn't in place when I replaced the transistor. So I powered it up without C100!I have taken it out to measure it and forgot to put it back..Does it make any sense TR33 to get shorted if this capacitor is missing?I don't see anything else to be faulty here except TR33..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: May 17, 2023, 06:23:56 pm
content: It was C100 that was making all this mess.I've put it back and then the 2nd transistor burned again.Then I removed C100 and measured it again at 1KHz and it was 2ohms!I tested another 220uF at the same frequency and it was just less than the stated value so I replaced C100 with a new one.I didn't have any other transistors so I've put a salvaged S2055N in there and it came to life and it's been awhile now that the TV functions as it should!

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2023, 06:56:22 pm
content: Nice job, it's always a great feeling hearing the HV come up after a successful repair. Yes if C100 is bad that can absolutely cause the HOT to fail. It's going to be exposed to high frequency and probably a good bit of ripple current so it's important to use a good quality low ESR part here.