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Beomaster 4400 power-amp problems or ?????

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ALF
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ALF posted on Tue, Feb 7 2017 9:46 AM

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

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Dillen
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Any DC on the output rails?

Martin

Lonnie
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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

 


Craig
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Alf

As lonnie quite rightly points out....the printed boards and circuit diagrams can contain some inconsistencies and therefore its always best to replace components in the same orientation as they came out, after all they worked  that way for forty years or so, have a look at the picture of one of mine, it shows from left to right OIC201/OIC202/OTR200/OIC203.....unfortunately OIC200 is out of shot, however the important thing to note is the clearly visable crossover of connections on OIC202.


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Craig
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Craig replied on Tue, Feb 7 2017 1:37 PM

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

Dillen
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Any DC on the output rails?

Martin

RaMaBo
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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

Craig
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Craig replied on Tue, Feb 7 2017 5:40 PM

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?

ALF
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ALF replied on Wed, Feb 8 2017 7:04 AM

Disconnected pin3 of P12 - no change

Overload bulb does not come on and RL1 engages prompt

I did however not disconnect pin3 of P11

ALF

ALF
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ALF replied on Wed, Feb 8 2017 7:16 AM

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

 

 

ALF
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ALF replied on Wed, Feb 8 2017 7:19 AM

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

ALF

Craig
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Craig replied on Wed, Feb 8 2017 9:17 AM

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

Craig
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Craig replied on Wed, Feb 8 2017 11:52 AM

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

 

 

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.......

Craig
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Craig replied on Wed, Feb 8 2017 1:15 PM

Sorry Alf......it should be 50 ohms

ALF
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ALF replied on Wed, Feb 8 2017 2:20 PM

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 !

ALF

ALF
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ALF replied on Thu, Feb 9 2017 9:32 AM

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 ?

ALF

Craig
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Craig replied on Thu, Feb 9 2017 12:46 PM

Alf

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..... 

ALF
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ALF replied on Fri, Feb 10 2017 9:20 AM

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:-):-)

ALF 

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