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
Hi everyone !
I am in the middle of restoring a BM4400:
So far all electrolytics have been replaced incl the two big ones.
Both output trimmers have been replaced with 25-turn new ones
All output amp TIPs have been replaced with TIP142/147 and The BD-165 with BD-139
And YES, they were all conected the correct way - even with some of the short brown wires crossed :-)
All TO-92 transistors in the power-amp section have been replaced together with D101,102, 103, 104
IC3 together with TR15-16 and D22 in the fault switch have also been replaced.
TR1 and TR2 have been replaced
A long list of resistors - as suggested by Martin and Menahem - has been replaced as well.
Turning on the unit via, the DBT shows as follows:
Without P11/12 connected the DBT briefly illuminates, goes out, overload bulb does not illuminate, RL1 does not engage.
With P11 or P12 or both connected the DBT briefly illuminates, the RL1 engages, DBT goes out.
What am I missing here ?
The PCB has been inspected with a strong magnifier for cracks, but 'negative' - no cracks to be visible !!
Is it likely that the RL1 is faulty ??
Replacing so many components was not only easier but in my opinion sensible, as desoldering, testing and
putting them back in would have reintroduced worn-out parts.
Is it worth taking out the RL1 ??
Any help/suggestions is more than welcome :-)
ALF
Alf
It may be worth taking a look at the thermal protection device R40 (PTC) as I understand this device it will increase its resistance as the heat sinks warm up, if it gets too hot it will remove the bias from IC3 allowing it to switch and therefore energise R1. similarly DC on the base of TR16 will switch TR16 and in turn TR15...which is always biased will then hold TR16 ON, again removing the bias from IC3 and energising the relay R1.
does any of that make any sense to anyone but me?
maybe worth a look at thermal protection device anyway and see what its resistance is ;¬)
Craig
Any DC on the output rails?
Martin
What Craig says is quite right about the function of the Overload protection but what makes me scratchin my head is when connecting P11 or P12 or both makes the protection active so there must be something wrong after P11 and P12.
Both are connecting the output of each power amp to the speaker switches and the headphone socket. The overload protection is directly connected to the outputs of the power amps via R47 and R48. So if P11 and P12 are not connected everything is fine, if one or both are connected the fault switches gets active.
I would check if there's no short between P11 and ground and also P12 and ground and no voltage coming from P11 / P12 due to a short somewhere.
Ralph-Marcus
Ralph
That makes sense......the protection relay thermistor is in the 15v supply and therefore would not be affected by the 35v DC which is only present when P11 and/or P12 are plugged into the board, therefore a fault in the thermistor would be present all the time. And as you say P11 and P12 come together at the speaker/headphone socket.......maybe disconnecting P11/3 and P12/3 in turn and see if the issue goes away?
Disconnected pin3 of P12 - no change
Overload bulb does not come on and RL1 engages prompt
I did however not disconnect pin3 of P11
Dillen: Any DC on the output rails? Martin
VDC present at the two big Caps as per SM
USTAB +/- 34.8 VDC
With P11 and P12 plugs disconnected I can measureat at the pins on tbe board:
P11: Pin1 1.92 VDC, Pin3 1.43VDC, Pin4. 0.94VDC
P12: Pin1 1.96VDC, Pin3. 1.44VDC, Pin4 1.09VDC
Craig: Alf It may be worth taking a look at the thermal protection device R40 (PTC) as I understand this device it will increase its resistance as the heat sinks warm up, if it gets too hot it will remove the bias from IC3 allowing it to switch and therefore energise R1. similarly DC on the base of TR16 will switch TR16 and in turn TR15...which is always biased will then hold TR16 ON, again removing the bias from IC3 and energising the relay R1. does any of that make any sense to anyone but me? maybe worth a look at thermal protection device anyway and see what its resistance is ;¬) Craig
Craig,
The R40 resistance measures around 57 Ohm
Should be 40 ohms but I don't think it matters, its not where the problem lies.....or the relay would be held in irrespective of P11/P12 plugged in
ALF: Dillen: Any DC on the output rails? Martin VDC present at the two big Caps as per SM USTAB +/- 34.8 VDC With P11 and P12 plugs disconnected I can measureat at the pins on tbe board: P11: Pin1 1.92 VDC, Pin3 1.43VDC, Pin4. 0.94VDC P12: Pin1 1.96VDC, Pin3. 1.44VDC, Pin4 1.09VDC ALF
looks like these measurements are taken from the pins on your board.....try disconnecting both P11 and P12, power up making sure the relay is not engaged and try measuring P3 on the socket of both P11 and P12. and see if your DC is coming in that way as Ralph advises.......
Sorry Alf......it should be 50 ohms
Did just that:
Pin 1+4 on both plugs P11/12 shows about +/- 34.5 VDC
Pin 3 on both Plugs P11/12 shows a jumpy reading between 1 to 20mV
RL1 did - as before with both plugs disconnected - not engage !
Hi ralph,
Not quite sure I follow a 100%
Would it be fair to say there is a common problem if not perhaps the same after all ?
The only thing I found so far was a way-out-of spec R186 with 17ohm instead of 4.7ohm -
I replaced both, R186 and R286 but this obviously isn't the problem !!
After checking the wiring as thorough as possible - nothing suspicious !
Of course I could start allover again replacing all TO-92 transistors in the power-amp section
Yet I am not convinced this will solve the issue ?!!
With disconnected P11 and P12 i can measure the +/-35 vdc and pin3 on each plug and also the three pins on the board
Because the RL1 is not engaged, but what does that tell us ??
Trouble is I can not verify any voltages in the power-amp with an engaged RL1 !!
So, where to go from here ?
I think where Ralph is heading on this one is that you have a fault common to both channels, or possibly you have a separate fault on each channel, the output rails come together at the speaker output/headphone socket circuits and both pins 3 of P11 and P12 connect at this point also, so if you have a short or some other issue here it will affect both P11 and P12 which are common at this point. However as you have measured the voltage at these sockets with the 35vdc present at the sockets for pin 1 and 3, and R1 not energised, socket 3 does not have 35vdc present therefore your not tripping R1 through any fault at the speaker output/headphone socket circuits.
So your fault appears to lie in the output amp, when repairing my unit recently I also found when replacing all the output amp components it still wouldn't work, Martind advice was "Well, repairing these beauties is often a headache"
If the darlingtons and their emitter resistors are all OK, the problem is most likely around TR112/113/115.Could also be a cracked solder joint or another PCB break.
I would leave one channel isolated and concentrate on the other, check polarity's of your transistors and diodes, did you take pictures of the inside before you started......I found it very usefull because the circuit diagram is not always correct.....
Hi Craig,
Wouldn't it be great to go back to the days where we just changed a blown fuse and everything is fine again ?!
Not even thinking about why it blew ?!
Well, I change a full set of TO-92 transistors - can't loose from where the project stands, can I ?!
Replaced
TR112/113 with BC182B
TR114 with BC237B
TR115 with MPSH 55
TR116 with BC238B
TR117 withBC 558B
TR119/120 with MPSA 06
Still no improvement !
In my earlier post I said there is DC voltage at both plugs P11 and P12 on pin 4 and 1 of about +/-34.5 VDC but only about 3 mv dc at both pin3 (center pin) - witwith RL1 not engaged.
Again, there no visible cracks on the board or any cracked joints !
The whole problem is a mystery to me - naturally I did check the transistors I replaced of correct polarity as well as the diodes......
Also measured the PTC R40, which showed around high 50ies Ohm - the moment I turn on the receiver its resistance jumps immediately to about 310 Ohm !
That is sadly all the news I have at the moment - waiting for devine intervention:-):-)
Hi Alf,
nearly four years ago Soren Mexico did a similiar job on a Beomaster 4400 having also problems with the Fault Protection circuit and finally managed to fix it: Sorens Thread
Maybe you should read some of the posts ( okay, maybe pages of posts ) in this thread. It might be helpful to see how he managed to isolate the final failure.
Hi ralp,
I did ideed red through this and other related posts to find pointers, expanations and suggestions......but I am also aware how difficult
it is to come up with a long-distance diagnose and even a solution of a particular problem.
Discovering an ivisible crack on a solder-joint or pcb track requires some magical skills, experience and luck - some here seem to have those attributes ?!?
Considering the work and hours spent on that receiver I would be lieing not to have come close to throwing in the famous towel !!
There isn't much left in the power-amp section that would make sense to me to replace ?!
I keep asking myself whether something somehow makes contact with the chassis that shouldn't - but I can't see it ?
I keep on it for a little while longer - with or without support.
I have, with admiration, been following your thread Alf, dont give up now, take a break for a couple of days and start again, my luck finding the failure was comparing right and left channel again and again, sorry I cant help you more, but go on you are very near completion.
Soren
Collecting Vintage B&O is not a hobby, its a lifestyle.
Hello Søren,
Thank you for your encouraging words - yes, it has been a struggle so far and I have come a long way looking back at this monster!
I also have been checking, replacing, more checking more replacing but not yet ''home'' !!
Yesterday I started double checking the TO-92 transistors in the power-amp-section for correct 'polarity' and yes, i did discover some errors
I have made which was at last some devlopment and encouragement to carry on - the devil must hide somewhere. :-)
cap C132 turned out to be the wrong way around as well as 5TR120/119/219 - all TIPs checked out ok and after reading about your
22/22k resistor it showed me again mistakes can be made soooooo easily.
Now off to 'work' for some more checking - I am still convinced it will be just a little thing causing all that stress and failure !
Keep you posted !!
A brief up-date :
After detecting a couple of wrongly installed transistors in the power-amp section I turned to one of 'martins list of recommendations' and repaced D105/205 and TR11,12,13 from the overload circuit !
Despite RL1 being engaged the overload light now stays on ???
It is getting more and more ''interesting'' and I wonder what comes up next on the way ?
Only a BG4000 or BM8000 could be more nerv-wrecking !
Back to work unless someone stops me with a constructive suggestion:-):-)
Small success:
Broken track discovered at TR13 - now Overload light works again as it should :-)
Small details Alf, small details progress go on, I love it
Its amazing the things you find if you step back and take a breath, its very easy to lose concentration for a moment and these components are very small....I installed 2 diodes the wrong way around and it was a couple of days later that I noticed I had the polarity wrong on both of them......fortunately it was prior to powering up....keep at it Alf
It is also another reason to take good, detailed pictures and post them of your work. A number of times I have studied my pictures and realized that I did something wrong. When I'm relaxing at my desk and reviewing things, instead of hunkered over the workbench, the brain processes the information differently. Just the act of posting pictures and writing up the description makes your brain review your work in a different way that is often helpful. It also helps to have other sets of eyes look at the work. I remember one time I had installed a capacitor incorrectly on a Beogram 8000. A Beoworld member spotted it and let me know...and I fixed it. That's a great thing about the forum.
-sonavor
As Sonavor I often go back to my threads and pics away from the workbench, sometimes I just leave it all for a couple of days, and come back with new ideas and a fresh view, as Craig I have several times inverted diodes or transistors in my repairs, at times forgot to solder a leg or two. A couple of times broke tracks desoldering component, and had a long time finding it again
Hello sonavor & søren,
thank you for your replies - shortly i will upload a couple of pics.
i still keep thinking about that thermistor - unusual symbol used in the circuit diagram - and
the protection relay RL1.
can we really rule-out both components ? Sure, it has been explained before that a faulty thermistor
should trigger the faul switch at all times, but........?!?
also, is the RL1 so reliable not to react perhaps over- sensitive ?
i took off the two tuner related boards to check again for possible cracks of board 5, but negative .
again, just thinking aloud !
enjoy the pictures 😩
work impressions - enjoy
more work impressions
spotted the devil ?
he is hiding somewhere !
not pretty, but it is what it is - a mess !
more of the same
if you would like to see pictures of a special location - please feel free to tell me
in the end it could only be beneficial !
cheers
hello once again,
the pic I uploaded shows two of the old TIP141 darlingtons on the left side - also visible are the plastic washers
to isolate the TIP from the heatsink
on the right side two of the new ones installed without the plastic washer as I believe they are unneccessary due to integrated
plastic housing !!
am I correct with that assumption - a thought came up that the newly installed TIPs without plastic washers may cause the short
triggering the fault switch and RL1 ??
still have not yet discovered any other reason for the persisting problem !
let me know if those washers are still required ?!
The plastic mounting washer/grommet on the original transistors prevent the metal mounting screw from touching the metal plate of the transistor. On the new version of the transistors it looks like the plastic case has the plastic fit through the hole of the metal so it has a built in isolator for the mounting screw. The new ones don't need the washer/grommet.
thanks John,
that is what I thought but then......simple oversights and the damage is done.
being so involved, one often doesn't see the trees in the forrest and I keep checking now for some time
with very little luck !
No problem and keep posting pictures with your questions like that. It makes it much easier to see what your question is when we can view the problem.
Hi ALF,
I noticed in your picture P1010148 that you may have crossed over the C&E leads from the board to the TIP147 - 0IC202.
The manual shows the layout from the bottom view which has ECB from left to right. When you flip it over to get the top view perspective you will have BCE from left to right. You have B going to B, C going to E and E going to C???? You may need to double check this as the manual may be wrong but also the printed boards are often wrong. If the manual is correct then the leads should not be crossed over. I don't have a BM4400 so I can't do a physical check for you. Maybe someone else can?
Regards
Lonnie
hi lonnie,
thanks for that - incredible eye for detail !!!
this goes for each channel - I had two crossed for each side :-((
question is would those TIPs been damaged by the cross-wiring as my fault-switch
still activates the RL1 after I corrected the connections ??
You may just want to confirm with your multimeter on the output device leads that you have continuity between:
Emitter of 0IC100 and Collector of 0IC101
Collector of 0IC102 and Emmitter of 0IC103
and repeat for other channel.
That should confirm that you have them wired up correctly.
I'd say you may have shorted one or a couple which would allow DC at the speaker which would trigger the fault signal.
Best way to check is disconnect the leads and test them.