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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now obsolete (only available as ‘used’ on ebay) so I looked up BDX33a/34a as replacements only to find that they have been replaced by BDX33c/34c which seem to have slightly different electrical characteristics  but purchased them anyway as there seemed no other option.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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Pete
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Answered (Verified) Pete replied on Sat, Jun 13 2020 11:45 AM
Verified by Pete

Today marks a very special occasion for me. The B&O my dear uncle gave me many years ago sings again!Lets have a Party !!!Lets have a Party !!!Lets have a Party !!!

Since lock down here in the UK, I have for the first time ever, built a circuit board from a diagram, tested it and it works. 

The voltages manfy illustrated tested symmetrical on both left and right channels (all be it slightly lower by a few volts in places due to power supply on throwing out 45v) and the only issue had was when the board was clamped into place with the spring clips onto the heatsink. It seems a 50 year old board does not like my soldering technique (or lack of it) as the new BDX’s strained at their feet and lifted welds to the remaining tracking on the pcb… but after a quick repair with a bridging wire… hey presto! the amp produced its warm tones once more.

Having restored all the other components to function correctly, all potentiometers cleaned, belts and clutches cleaned and tightened, and no load current adjusted to spec it’s time to close the lid (hide the train wreck) and clean the outside. The only thing about this amp/receiver that isn’t working is one LED light on the first FM slider.. I will leave that off in memory of my uncle. He would have found that amusing (-:

Thanks go out to everyone who helped along the way. It’s been an enjoyable (sometimes frustrating) learning curve... and a special thank you to Dillen who sold me an original amp PCB which arrived the day I repaired to old one. IronicLaughing

Removal of components and soldering new into place will never be the sameWhistle and 40 watt tungsten bulbs are a godsendCool

Beocord 1200 and Beomaster 1700 type 1701

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Pete
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Pete replied on Wed, Apr 22 2020 5:06 PM

Received an assortment of  "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.

Is that minimum resistance towards R211 or R213 ?

Apologies if this seems obvious but I haven't a clueCrying

Beocord 1200 and Beomaster 1700 type 1701

Pete
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Pete replied on Fri, Apr 24 2020 5:08 PM

Well well well it looks like B&O printed incorrect capacitor polarity markings on this PCB. (c108/208 '5000uf')

I took the originals off (4700uf) to replace them and foolishly trusted the markings on the PCB !  I've only just discovered that I replaced those two biggies on the board the wrong way round! - The evidence is in the old leads on the 4700uf caps which i removed and in the circuit diagram, maybe thats why everything got a little hot ?

I'll find out later when I turn this thing on... probably for the last time before I ceremonially smash it to pieces with a large rock Unsure

Beocord 1200 and Beomaster 1700 type 1701

manfy
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manfy replied on Fri, Apr 24 2020 6:37 PM

Pete:

Well well well it looks like B&O printed incorrect polarity markings on this PCB.

Yes, good catch. The screenprint is wrong.
Negative must be connected to the coils L100/L200. The layout drawing in the service manual does show the correct polarity.

Pete:

Received an assortment of  "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.

Crying I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.

If I were you, I'd measure the setting of the old trimmers and use that as the base setting for the new trimmers at first power-up.

Pete
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Pete replied on Fri, Apr 24 2020 8:58 PM

manfy

Thank you!

Of the two 1k VR's removed; one is fried and the other is reading 1.1k with 0.6K towards R113 0.5k towards R111 so I'll set the values and see what happens, one channel powered up at a time.

Below is what the service manual says regarding components near IC100

Thanks again

 

Beocord 1200 and Beomaster 1700 type 1701

Dillen
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Dillen replied on Sat, Apr 25 2020 7:02 AM

manfy:

Crying I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.

Turning the trimmer towards R113 would take the idle to max as it would drive IC100 off.
To start with minimum idle current IC100 must be on = trimmer slider moved towards R111.
IC100 is clamping the two bases of the output transistors, and it's an NPN type.

Martin   

manfy
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manfy replied on Sat, Apr 25 2020 7:18 AM

Thanks for this, but naah, I still can't see intuitively how IC100 sets the idle current of the output transistors. It balances the base voltage of the output transistors and I guess by doing so it indirectly sets their working points and idle currents.
Anyway, never mind. The circuitry does work in principle or else it wouldn't have been realized that way.

The function of TR102 is very clear with this description and thinking of your initial description of the problem, I'd carefully look at that transistor. If this one is shot, i.e. shorted from C to E it certainly would explain why your output resistors R121/122 go up in smoke immediately after power-on.
Please note that the description in the service manual is just a conceptual explanation! If you look at the circuit diagram you see that 21R1 is fed by +50V and not +34V!! And that in turn would nicely explain why IC100 got shot and why all those resistors got fried.

In your first post you asked about TE1088 vs BDX33C. Don't worry about it. The service manual shows that B&O used 3 alternatives for that darlington: BDX33A, TEO1088 and FJ3001.
So, BDX33C is perfectly fine. The only difference between A, B, C, D type is max Vce of 60, 80, 100, 120V respectively.

Do you have a DC power supply that goes up to 50V?? If so, the output amp can be tested nicely on its own: Leave input and output disconnected and slowly increase the supply voltage while watching the current drawn. This way you can measure various points on your circuit at a lower voltage that is non-lethal to the components. This allows you to assess what's wrong without sending additional components into nirvana.

--------------

Cross-posted with Dillen. He's a professional with hands-on experience in B&O equipment, so his word can be trusted! Smile

 

Pete
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Pete replied on Sat, Apr 25 2020 1:11 PM

Do you have a DC power supply that goes up to 50V?? ...

No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet.. 

Not sure what to try nextEmbarrassed

Beocord 1200 and Beomaster 1700 type 1701

manfy
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manfy replied on Sun, Apr 26 2020 10:29 AM

Pete:
No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet.. 

Believe it or not, that's actually a good thing to see such a high current at such a low voltage: It shows that you have a major problem with one amp and that is usually quite easy to find - as opposed to intermittend problems for instance.
Your 3-15V supply is sufficient for that.

First to your caps C108/208: You said the speakers were disconnected during first powerup. That's good. The caps were "almost free floating" except for R106 and R107 that connected them to GND. That makes good 5 kiloohm, so with a bit of luck you didn't seriously damage those caps. (I trust you measured their capacitance ok when you reassembled them the right way, right?!).
Since the speakers were not connected, those caps mounted wrongly were certainly not the reason for the excess current.

So, it appears that one channel is working fine and the other has this overcurrent problem. That makes things simple because you can compare measurements from one channel to the other and if any value is substantially off, you can focus on the components in that path.
If the current is sufficiently low at 3V then you can measure and compare voltages. Usually you'd start around the transistors because they are most likely to fail.
Alternatively you could just compare resistance values at various test points without any need for a power supply. Such a high current suggests that something is shorted somewhere in the faulty amp.

 

 

 

Pete
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Pete replied on Sun, Apr 26 2020 11:39 AM

thanks for your reply manfy

I'm somewhat confused at the high current at such a low voltage given that the B&O power supply throws out 50v and I also think both channels are now affected after previous tinkering. Originally one channel worked but then I replaced everything in the hope of a simple balanced result - with the capacitors the wrong way round and perhaps the board shorting somewhere on the case as I didn't nip off the component leads enough below the board - so stupid me!

I'll be removing one leg of most of the components to test their values over the next few days (when it's Lightning/raining outside!) and I have spent a further £12 for all Transistors/IC's on eBay (arriving Friday) - not a bad price compared to my first attempt via 'mouser' in the USA (-: So will write a result of resistances/uf's before attempting to replace anything out of kilter.

'Lockdown' here in the UK is good for these sort of things WhistleWink

Beocord 1200 and Beomaster 1700 type 1701

manfy
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manfy replied on Mon, Apr 27 2020 7:42 AM

Pete:

I'll be removing one leg of most of the components to test their values over the next few days ...

Hmm Yes, that's one option, I guess....but I'd strongly advise against that!! Lots of things can go wrong with all that soldering, not to mention the mechanical stress on the components...

If you must, remove all the transistors and test them out of circuit with a multimeter -- or a transistor tester if available. Carefully compare their values between left and right channel.
Once all the transistors are out, you can easily test resistance values on the board without danger of interference by other components.
The only other components on that board that are likely to fail are the e-caps. Did you recap the board before first power-up? There's no point in messing with the resistors - they never fail except for overheating or mechanical failure.

PS: If you're unfamiliar with transistor testing with a multimeter, research it on the internet. You should find plenty of tutorials or uTubes on that!

 

Pete
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Pete replied on Sat, May 2 2020 1:47 AM

The enduring learning curve to enthusiastically repair one channel of a shorted Beomaster 1700 amp continues..

Step:

1.Google (other search engines are available) to find similar issues/solutions

2. Find a service manual/circuit diagram/schematics et ‘al

2.Forums are your new friend

3.Try to explain everything

4.Ask the right questions 

5.Don’t expect all the answers

6.Hope for questions…

7.Photograph everything before, during and after ‘tinkering’

8.Remove components from a PCB with a sucker or wick to avoid damage to PCB - see photosOMG

9.Research any discontinued component electrical characteristics to find valid alternatives

10.Test component values / replace if necessary

11.Learn the difference between PNP / NPN and understand that not everything is the same in component pin layoutEmbarrassed ie BC556b is PNP yet has a different Base/Collector/Emitter configuration/orientation to others!

12.Don’t trust PCB screen print layouts or schematics, compare every component to other circuits and industry standards. eg Capacitor polarityUnsure

13.Buy new components based on above

14.Send out white flag

15.Wait

16.Turn on… 

= Success / Failure


This weekend all the defective components will be replaced to the PCB and every one checked for connection to their correct place on the PCB according to the circuit diagram. Check over many times and check again then to power up - (power supply checked!)

Wish me luck - see you on the other side

The ‘Life is too short’ option is:

HiFi 2.0 Channel Power Amplifier Board HiFi Stereo Audio Amp DIY KIT MJ15024 and hide it in the B&O case (-:


Photos illustrate chaos, frustration and incompetence, mildly mixed with enthusiasm and hope (-:


board without TR/IC's

PCB connections drawn based on circuit diagram due to soldering train wreck!

what a messHope this helps someone out there...

Beocord 1200 and Beomaster 1700 type 1701

fkatze
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Maybe its me but it looks a bit like some of the traces are missing. and lifted.  

 

Honestly don't know what to make of it.  Is that supposed to be the desoldered or soldered state?  I can't see how any of that could be working.

 

Pete:
PCB connections drawn based on circuit diagram due to soldering train wreck!

Pete
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Pete replied on Sat, May 2 2020 6:35 PM

lots of tracking had lifted so had to work out what goes where before putting transistors back. What u see is the board without transistors. Work in progress - sort of 😇

Beocord 1200 and Beomaster 1700 type 1701

Dillen
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If you ask me, then what you see is a board beyond rescue.
Your soldering iron is way to hot.

Martin

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