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Beomaster 1700 (type 1701) revisited - Tech Help needed

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Pete
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Pete posted on Mon, Apr 20 2020 2:52 PM

 

Hello from a new member and long time happy lurker (-:

Our Beomaster 1700 (type 1701) has been in various lofts for 20years. Spurned on by the CV19 lockdown here in the UK I’ve been trying to repair the amplifier board. The loudspeaker cable on the left channel was shorted and fried a few components.

I’ve read as much as I can find on the web (manuals etc) yet found your forum the best place going forward. There are a few issues that I hope members can help me with:

As suggested in previous posts i replaced all burnt-out parts (resistors, a variable resistor, a diode and all transistors) and electrolytic capacitors all with original tolerances. nb, the 0.39ohm 1W wire-wound resistors were only available as ceramic 5W at the time. 

The Heat-Sinked TE 1088 and TE 1089 are now obsolete (only available as ‘used’ on ebay) so I looked up BDX33a/34a as replacements only to find that they have been replaced by BDX33c/34c which seem to have slightly different electrical characteristics  but purchased them anyway as there seemed no other option.

In the manual, the VR1k resistor sets the 'no load' current. I set this resistors' trim as close to the original one that i took off the board. Disconnected Speakers and input connector from pre-amp stage and turned the mains supply on….

Within two seconds the replacement 0.39 Ohm 5W ceramic wire-wound resistors glowed (located between the emitters on BDX 33c/34c.  There was a burning smell from those and the 27 Ohm 1/8W resistor connected to MPS A05 and MPS A55.

Maybe a few things caused this

1. The variable resistor 1K value needs to be changed

2. The Ceramic 0.39 ohm 5w resistors were the wrong type.

3. The replacement BDX33c/34c are so different that a few other resistors values need to be changed.

4. Having no audio signal load to the board may have also caused the ‘thermal run-away’

I have now ordered lots of 0.39 ohm 2w wire-wound resistors (original spec), a supply of various fixed resistors and variable resistors - further £20 spent (-:

Q; How can I check the 12mV (maybe it’s different now?) across one of the 0.39 Ohm resistors without blowing everything again?

Q; Is there a formula to work out alternative values of resistors and mV value in this amplifier stage - due to change in output transistors?

Q; How can i test transistors without an oscilloscope or wave generator, i only have a multimeter with hFE pnp/npn holes?

Q: Is there anything above that I’ve overlooked?

The B&O was a gift from my uncle in the late 80’s - My uncle died last year, he loved his music and various B&O through the years and it would be lovely to get this up and running as a tribute and for it to take pride of place in the home office.

I haven’t done much in electronics since the 90’s so If I can’t repair this amplifier board I’ll consider trying to find a power amp stage that can run on a single 50 volt supply and hide it in the case! I really hope this isn’t my only option.

I hope to attach images:

Output stage diagram for one channel - The ‘Red’ squares indicate parts that burnt-out when speaker cable was shorted many many years ago

Photo of board as it is now) while waiting for more replacement parts to arrive after 1st attempt - R212(vR) - R214 - R221 - R222 - also showing some of the old components removed from original shorting mishap circa 1990's

Electrical Characteristics of BDX 33a/34a (B&O’s suggestion for replacement to TE1088/9)

Electrical Characteristics of BDX 33c/34c (interwebs suggestion for replacement to BDX 33a/34a) 

 

 Any help would be warmly received - Thank you in advancee.

  

 

 

 

 

Beocord 1200 and Beomaster 1700 type 1701

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fkatze
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fkatze replied on Tue, May 19 2020 4:45 AM

Pete:
Yes, I'm quite mad! 

 

...as long as you know Laughing

 

You are pretty far in now, and it is too late to scan that original PCB...

I wonder if you wouldn't have been better off going for something like an LM1875 (in a pair) solution if the goal is just to get it to make sound again?  Fewer components needed and probably an easier solution overall.

Pete
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Pete replied on Tue, May 19 2020 11:52 AM

fkatze:
You are pretty far in now

Oh yes way in deep over my head <gasp> Embarrassed

I've scanned the PCB layout from the manual received via snail-mail so as to check all tracks against circuit diagram - all seems ok - (notes on diagram)

Checked a few resistances on the connectors to the Power amp inside the case (Power Amp PCB not connected)

Signal GND to Chasis = 90 k Ohms

Signal GND to GND on Power Supply = 0.5 Ohms

I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board? 

I'll construct the new board with that 'option' over the next week - just makes it a little harder with two separate sources for GND

all good fun Erm..Laughing 

 

Beocord 1200 and Beomaster 1700 type 1701

manfy
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manfy replied on Tue, May 19 2020 8:01 PM

Pete:

Signal GND to Chasis = 90 k Ohms

Signal GND to GND on Power Supply = 0.5 Ohms

I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board? 

0.5 ohms between both ground planes at the power supply is normal because that's where they are centrally connected. 90k at the board is possible because of various components that link the power path and signal path.

Those 2 holes between signal GND and power GND look like a precautionary design measure. At the time of board design they were not sure if power GND/signal GND separation was "worth it". But ultimately they found that the separation was benefitial in terms of noise rejection, so they kept the planes separated, which forced them to run the extra wires from power supply to amplifier.

When you construct the new boards, you should retain that separation, or else you'd create a ground loop when connecting the preamp and that might cause a hum or other noise. For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.

In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!

Pete
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Pete replied on Tue, May 19 2020 9:40 PM

manfy:

<snip> For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.

In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!

Very valid points, will be taken on board - literally - Thank you (-:

 

Beocord 1200 and Beomaster 1700 type 1701

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