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
Follow up on an archived thread:
My Beomaster 3400 is blowing both primary fuses F1 and F2. Mains voltage is 220V (and yes, the voltage selector is properly set). Does anyone have a clue of what might be failing?
Although probably unrelated, the D8 rectifier has failed in the past. Testing the four 1N4007 diodes the official B&O repair service here in Portugal soldered together showed one of them was gone and I replaced the whole thing with a W01M. Also, the F3 fuse (between the secondary and the rectifier) stays unharmed.
Many thanks
Zé Loff
Separate the transformer secondary and run the Beomaster with just the transformer torule out any shorts in the transformer itself.
Martin
Sorry for the late answer, but I haven't been able to find the time to work on this...
Both fuses remain OK while running the isolated transformer, as Martin suggested.
I then reconnected the B80 C7500/5000 bridge rectifier, the TRIAC, the two large capacitors and the shunt between the two secondary coils but left the tuner (?) circuit - i.e. the F3 fuse and the D8 bridge rectifier - disconnected. Both fuses blew again.
I also took the opportunity to test the B80 rectifier, and it appears to be functioning properly.
(I apologize for any mistakes in parts and/or circuit names, but I don't have the schematics, so I'm mostly guessing...)
Any hints, anyone?
That means the mains transformer is most likely fine.
Hmm.. maybe a blown output stage, then.Next diagnose step will be the secondary (DC) side of the B80 rectifier.Put an ohmmeter across the + and - pins (Beomaster unpowered) and let us knowthe reading.
I have a manual of sorts for this one. I'll see if I can get it loaded for you.
Peter
If the rectifier is connected to the capacitors, resistance grows in time.
If the rectifier is disconnected from the capacitors (but connected to the transformer), there is infinite resistance between the B80 + and - pins if measuring with the correct polarity and some resistance if measuring the other way round. This latter resistance varies depending on the scale setting on my multimeter.
When the rectifier is disconnected from the capacitors and everything else is connected to the transformer (i.e. the rectifier, the TRIAC, the coil shunt and the tuner circuit) the fuses don't blow when the unit is powered on. And (most importantly, I think) there is zero resistance between the two capacitor pins where the rectifier is connected to.
Clear case of a shorted output stage.This is where we normally stop the remote diagnosis because things start to get complicated fast from this point on andparts, experience and instruments will be needed.But basically, the next step will be to identify the affected channel(s).In front of the cooling fin, on the circuit board are four amplifiers, they are four identical circuits and each circuit ends with two largetransistors mounted on the cooling fin.Each amplifier also has two large emitter resistors at the board, usually grey or red and about 20mm long , sittingparallel physically (but not electrically).Can you tell if one or more of the resistors looked toasted ?Or if one of more transistors have shorted (ohmmeter across pins) ?
Peter: I have a manual of sorts for this one. I'll see if I can get it loaded for you.
That would be great... At least I'd know a bit better what I am looking at...
Dillen: But basically, the next step will be to identify the affected channel(s). (...) Can you tell if one or more of the resistors looked toasted ?Or if one of more transistors have shorted (ohmmeter across pins) ?
But basically, the next step will be to identify the affected channel(s).
(...)
Can you tell if one or more of the resistors looked toasted ?Or if one of more transistors have shorted (ohmmeter across pins) ?
None of the resistors look toasted. I had already (visually) checked the whole unit, and there are no parts showing obvious signs of failure. Nothing looks toasted and no caps have bursted.
Instead of disassembling all the 8 transistors I disconnected the power inputs of all four amplifiers and then reconnected them sequentially, testing (i.e. seeing if the fuses blew) between each reconnection. With this, I found out that the rear/right amplifier is the culprit.
I need to get some thermal grease to reattach the transistors to the heatsink after testing them (I can try to just unsolder them and test them in-place, but it looks tricky), so I'll only be able to get back to this in a couple of days.
Many thanks for all the help. I'll post the results of my findings as soon as I can.
Zé
Good job so far.In most cases, there will be more burned components besides the output stage transistors.Drivers etc. and even the trimmer can sometimes go bad. You will need to check practically allcomponents in that amplifier channel.
Hi Peter, I have recently purchased a Beomaster 3400 and am in country New South Wales Australia.
The item was damaged in transit and I am wanting to get it restored.
No one will look at it for me without some sort of manual.
Help?
"No one will look at it for me without some sort of manual."
Bunch of cowards
It's my all-time favorite - had it for 38 years, full-restoration in 2001, and I adore it.
If you have problems getting it back to A1 condition, contact me off-line.
Menahem.