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
greetings all,
yes, I have been working quietly away.....on my BM4400, which caused lots of head-scratching on the way.
finally got to the stage where the fault switch no longer triggers the safety relais, but the overload lamp
wont go out and one channel is lower in output volume.
I adjusted the idle current to 35mA with a complete set of new output darlingtons, no trouble here but still can not achieve equal volume in both channels ??!😡
I can trace the audio signal up to the “entrance” of the power amp section and it is fine on both channels.
traced the signal up to TR212 &213 E its fine, on B however there are trouble at TR213 ??
Shouln’t I get the same audio on both TRs E and B ?
checked practically all transistors, diodes, caps and resistors in the trouble channel and still hit a roadblock.
does anyone have any bright ideas of where else to look/check ?
Pleeeeaaase 😫🙏
ALF
Alf
Did you replace C238.....are you comparing the signal levels you are measuring on your scope between both channels for a comparison, and is anything getting hot i.e. output transistors? sounds like you are making progress.
Craig,
C238 has been replaced as a security measure at least twice on that journey 🤓
I used a audio signal tracer, DMM and my scope........
thank you for your input
C238 is indeed crucial as it partially de-couples the negative feedback loop.It's polarity is noteable too,in that the positive terminal goes to ground.
The two transistors Tr212/213 form a differential pair and are again crucial for the operation and stability of the output stage.
Can you compare the voltages around these two,with the voltages around Tr112/113 in 'tother channel?
I would expect all the voltages in the output stage to be out of kilter if these two devices were faulty.
If you replace Tr 212/213,you must use a gain matched pair.
Regards,
Nick
Check the JFET in the muting circuit. I have seen these go leaky and occasionally even short D-S.2N5458 I think.Check also the solder joints for the multiplugs in the tonecontrol/preamp area. They often crack.
Martin
Hi Nick,
good to hear from you 🤓
there is still a small percentage of mistrust towards transistor tester.....
all TO-92 transistors in that sick section have been replaced and I must admit not really caring much for the gain values!
checked TR212/213 again in that respect ...and yes, their gain value did not exactly match. I replaced both with an almost perfect matching pair.
same trouble 😠
again, both emitter volages are a match, base however a shows different voltages.
the voltages around that pair do differ from the pair in the healthy channel - we are talking mV though.
regards and big thank you for caring 😁
Hello Martin,
thanks for the pointer...the muting circuit was indeed under suspicion, TR108 & 208 did check out ok, but as a precaution
I replaced both with a J107 type as the original as per SM are no longer available.
No change in the problem 😩
are multi plugs in the tone control/preamp area had their joints reflown, no trouble from here !
obviously something else is not playing ball !!
regards
Are the general voltages around the output stage ok?
I don't have the circuit description for this amplifier,but I wonder if it's significant that the overload lamp is continuously on with your's?The sensor's for this stage are diodes d100/101,that appear to monitor the balance of your two suspects Tr212/213?
Are those diodes special types,or something like 1n4148 which are favourites with B&O designers!
Have you changed those two?
there would be two pairs of diodes monitoring - two in each channel: D100/200 = IN 4448 and D101/201 = IN 4148
I will revisit those and check again.
Also, I did replace both pairs TR112/113 and TR212/213 with near perfect matched pairs after your suggestion.
there is almost nothing left to be replaced in the sick channel but it is obvious I must have missed the culprit.....
the call for help is still alive 🙄😫
Hi Alf!
You didn't say anything about the emitter resistors (0.18 Ohm) did you change them as well?
Hi Jean,
both emitter resistors are brand new and idle current is set to 35mV as per SM
When you have checked and double checked all the components in the circuit, and you clearly have, then the problem must lie elsewhere.....cracked trace or bad connection on a plug in connector/jumper wire........I have found a number of these, one in particular was a hairline crack that I only spotted through a magnifying lens. You just have to keep plugging away.....wouldn't be any fun if it was easy ;¬)
OK, not having the board in front of me, it hard to see what can go wrong but did you check/replace C153/253?
These are located on the solder side of the board. Cap was good but I had a broken trace on that spot when someone tried to lift the capacitor i guess.
interresting thought, but NO....no trouble there!
just coming back to your question about the ‘general voltages around the output stage’:
which ones are you particularly thinking of ?
so far I compared quite a few voltages - also, he Overload bulb came still on after disabling D100/101 & 200/201 ???
only went out after taking out R35 🙄
I do not quite believe a broken track is responsible for the problem, after inspecting the board well and truly very closely!
so, there is still plenty of room for new ideas 🙏🙄😁
Alf,
My last recommendation would be to check both MPSA06, can't remember exactly the transistor numbers but they are just before the big output ones.
HFE value should be above 100 even if checking good. It happened to me where one measured 87 and the other 126. Min HFE for this transistor is 100.
You may want to go back and make sure that all your transistors meet the min spec at least.
All fine, measuring 230 plus 😁
thanks for trying anyway.
Again,
i revisited the new output darlingtons and their voltages....all fine and matching the “good channel”
replaced some way out of spec lttle ceramic caps, mostly 10nF and 22nF ones.
rechecked the speaker terminals, no loose wires, nothing visible that is touching where it shouldn’t.....
but still, there is some 15mV DC on pin 3 in opposition to around 2mV DC on pin 3 of the good channel?
seems like this is now the big road block ?? Or not seing the trees in the forrest, unless we are in parts of the amazon forrest !
again, invitations for more ideas are still most welcome.
I was where you are back in 2012, couldnt find the failure, but with too many hours on it to give up, Chartz and Martin gave me the idea to change all components in the poweramp, both channels, it was a big job but doable. Martin provided the components for free, and most of the forum helped me at some point, the BM 4400 has been working without problems until a couple of months ago where the bass potentiometer gave in.
Collecting Vintage B&O is not a hobby, its a lifestyle.
The output darlingtons have been changed 3 times, all TO-92 transistors in the right channel have been replaced twice, most resistors, all diodes, 99% of all caps have been replaced plus the trimmer pots......
did I miss anything ???
i send you mine if you send me yours and I would be very happy to replace your bass pot .....how’s that ?
🙄😁😁
I am trying to remember my problems back in 2012, dug out the drawings and are hoping to get my tired gray cells to work. more tomorrow
Jacques
Thanks to both of you......
now, to the “ pre-drivers “......could you be a bit more specific ?
I understand you are talking about the TO-92 typ transistors in the poweramp section like TR212 to TR220 ?
ALF,
I think that Jacques is referring to the MPSA06 we discussed earlier.
However, if you replaced all the parts into the faulty output stage or changed some and checked others, I would think that one component is changing its characteristics under stress; it might be a transistor, a resistor or capacitor. The fact that it's new, does not mean much... not to me at least...
From now on, let's do it like pros, OK?!
Here are my recommendations:
- Erase the black board and start over. Write down all your measurements and NEVER assume!
- Check your work 3 times!
- Transistors: new or old, write down HFE value for each. Make sure they meet their minimum specs. If they work in pairs within the same channel, they have to be within 10% of each other. From left to right channel say 20%.
- Resistors: unsolder one end and measure. They have to be within the tolerance specified. If too much troubles, measure across the resistor and compare with the good channel.
- Capacitors: If you can check ESR, do it but it has to meet the given specified capacitance value.
- Test the BALANCE control linear potentiometer. Measure the total resistance of each track and also the resistance across the potentiometer where in the mid position you should have about the same value for both channels.
Secondo:
- Set both (Left, right) TAPE 2 input trimmers about the same position, say right in the middle.
- Feed a 1Khz, 0.5V signal into TAPE 2 and scope the signal right from the preamp board. Yes, start from there. Remember, a small difference right there will be huge at the end! Again, don't assume!
- Follow the signal within the tone control circuit WITH and WITHOUT the LINEAR switch activated!
Thirdo
Send us pictures! We might be able to spot something you don't see. Help us to help you. We have tons of pictures taken from our own restorations and all of us together, we might be able to nail down the problem with you.
Get back to us!
Charles
Reading your post from "Wanted" forum, not even a faint sound of thanks for guiding you to a potentially defective BALANCE potentiometer ... it made me happy.
I have a contact in the UK who could possibly help.
Good luck!
Jean,
a faulty balance potentiometer was always on my list of potential suspects, but never on top of the list.
I usually check every transistor with my tester but if it fails under stress, that is not easy to detect .
every semi conductor is different and there is always an element of trust to be applied at the end.
I value every input and even stating the obvious does help sometimes.......
Is your UK contact forum member Craig ? If yes, we have been in contact.
again, thanks for taking the take to respond.....I truly hope it is the damn balance pot.
ALF: Jean, a faulty balance potentiometer was always on my list of potential suspects, but never on top of the list. I usually check every transistor with my tester but if it fails under stress, that is not easy to detect . every semi conductor is different and there is always an element of trust to be applied at the end. I value every input and even stating the obvious does help sometimes....... Is your UK contact forum member Craig ? If yes, we have been in contact. again, thanks for taking the take to respond.....I truly hope it is the damn balance pot. ALF
ALF, I'm sorry to read that you took my words seriously because I wasn't; I took care to use the appropriate emojis to demonstrate that but I have no control how you take it. it's yours.
I do not think my advices are more or less important than others, here we share without expecting anything in return, that's fine and the way it should be.
No, my contact is not Craig.
Let's see if we can rescue your machine now
Well,
I aquired a balance pot replacement - its in the mail....patience .....🙄
one contact in the pot was damaged, but that is unfortunately not the happy end finish....
there is a problem somehwere else.
also, a new audio signal tracer is expected any day now to arrive and will hopefully shed more light on the signal?
Will be back 😕 in the meantime there is coffee or tea
Hi,
While repairing my Beomaster 4400 I found a strange fault. The AFC switch did not work. The switch is soldered to a printed board in the front.
The printed board was OK, no cracks, but the connections were dead. Obviously the copper lines on the board were separated from the copper collars around the holes. No fault could be seen but the resistances were high. I soldered separate wires to the AFC switch and it started to work.
Maybe this is a well known problem with thin printed boards, but this was first such problem that occurred to me. However, the wires on the printed boards are easy to check with ohmmeter. Just thought that similar faults can also explain other strange problems.
Olli