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Beomaster 1000 model 2318 repair project

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AntonvZ
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AntonvZ posted on Wed, Feb 19 2020 4:00 PM

Hi all, my first post on this forum.

You guys are doing a sterling job and thanks for all the wisdom shared, it has already helped me a lot.

The reason for me wanting to restore/repair the old lady is that it is of huge sentimental value, it was bought the year I was born and has been in my family ever since. It survived my student years (running a CD player through the tape input) but eventually got stored when the whole AV revolution started and has only been used sporadically over the last 15 years.

I have read most threads relating to BM 1000 but not finding anything that specifically relates to my issues.  The model 2318 seems a bit light on information - even my included circuit diagram is for a model 2316/2317.

I recently started venturing back into vinyl and thought that the BM 1000 with my (still working and recapped) Beovox 1200's would make records sound the way I remember them ie kinda warm and cozy. 

That is when the problems started. The BM 1000 would not switch on.  Opened it up and discovered the large 3000uF 50V cap had leaked out a copious amount of gunk, I replaced that and she lit up just fine and worked for a short while (couple of days) although the left channel for QII was dead.

At the time that issue was limited to the QII input only, QI and tape and FM worked fine so that is a preamp issue I will have to address eventually.

I also replaced the two 3000uF output caps because I had a lot of distortion on the left channel at medium volume and up.

Unfortunately the following issues have now appeared. They are different for each channel and will list accordingly. My electronic knowledge is limited and at this point my testing equipment is limited to a multimeter. However I can solder well enough.

Left channel only hums softly. Like a transformer hum. The hum is not affected by volume, when on a single speaker the hum continues even when the balance is switched completely to the right channel. Some basic investigation revealed the following : the voltage at point J over resistor 678 is measuring at 350mV iso 24mV.  Temperature after 5 minutes is 55C. Turning the potentiometer has no significant effect.

Right Channel very soft and tinny sound, no bass whatsover. The voltage at point J over resistor 754 is 681mV and that temperature jumps up to 122 C within 5 minutes.  Turning the potentiometer has no significant effect.

The 30V reading at point J measured chassis side of resistor 678 starts at about 32V and then slowly falls to 29.85V but is not stable at the point, it fluctuates all the time.

I desoldered the potentiometer for 30V adjustment, cleaned and replaced it - made no difference. I did test it before replacing and it did give me 0-1076 ohm.

I have checked all three fuses, they are fine.

After 5 minutes the bottom of the unit (where the transistors are) and the heat sink are hot enough to raise nasty blisters.

Some pointers as to where I need to start troubleshooting would be greatly appreciated - I really would love to restore the BM 1000 to working order.

Thanks in advance

Anton

 

 

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Dillen
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Answered (Verified) Dillen replied on Wed, Jul 29 2020 3:49 PM
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Grissa,

What are your concerns exactly?

One thing to keep in mind is, that Beomaster 1000 has positive ground.

Martin

Grissa
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Answered (Verified) Grissa RTFM [@Lee5] replied on Wed, Jul 29 2020 5:39 PM
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Thanks for your intervention.

If you can see on the photos ... The positive of the capacitor there is the black which comes out of 6.3v of transformers and orange, purple, 2 black, brown and a black white, gray ... They come from speaker and in capacitor negative a red that beeps if I touch the emitter of transistor 2n3205 next to it

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Dillen
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Dillen replied on Wed, Feb 26 2020 3:55 PM

The output rail is the center "lead" running through the push-pull stages of the ampilfier.
In your Beomaster it will be the negative pins of the large output series capacitors, left and right respectively.
round -15V DC will be right.
Do a few DC voltage measurements around the output stage and driver transistors and compare the working and the bad channel.
If you find something that differs let us know.

In some cases it's possible to diagnose transistors in circuit, but the best thing is always to take them out.

"There is a 15mV reading between chassis and power capacitor negative - is this leakage normal?"
- Assuming you mean the power (filter) capacitor POSITIVE, because this Beomaster has positive chassis.
It's so little, it could be your test leads or meter.
No worries here.

Martin

AntonvZ
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Dillen:
DC voltage measurements around the output stage and driver transistors

Apologies Martin, I did muddle up the filter capacitor positive and negative. Positive has the 15mV reading, Negative is 39.6V.

Output rail DC measurements at output series capacitor negative pin.

Left channel : 0.75V 

Right channel : 13.8V

Output stage transistors

Left channel: 0.756V

Right channel : 13.82V

Other measurements:

400uF x 40w (Replaced with 440uF x 65w) capacitor on negative pin 28.18V

Left and Right channel speaker outputs at pins bounces around 3.95-4.12VAC

All readings taken with volume on nil and balance centered, no input selected.

Dillen
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Dillen replied on Thu, Feb 27 2020 2:59 PM

Points to a problem with one of the output stage transistors or, perhaps most likely, one of the driver transistors.
Can you do some measurements around the driver transistors?
Surely something in that area is wrong.

Martin

AntonvZ
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Dillen:
Can you do some measurements around the driver transistors?

Hi Martin

I've done measurements around the driver transistors and may have found the culprit.

It was a process of trail and error between left (not working) and right (working). All measurements were taken with transistors on board and not desoldered so readings like resistance are probably skewed. I started with continuity testing until I stumbled across one transistor (U2734) on the left channel (that gave perfect continuity) and then moved to resistance and voltage measurements. Below is my measurement comparison left to right.

Clearly this transistor has issues like a dead short or something, there is very little resistance compared to right channel and high negative voltage between pins E-B compared to the right channel.

As for a replacement I'm at a bit of a loss, seems like this U2734 transistor came from some esoteric/exotic/boutique B&O nirvana. Can't even get a spec on it but saw that BC141 is listed as similar. Would this work as replacement? I assume that whatever goes in there has to be changed on the right hand channel as well. Spec at bottom of this post.

 

BC141

Silicon NPN Transistor


similar to U2734, see note
TO-39
Uce/Ucb: 60/100V
Ic: 1A
β (Ic/Ib): 63-250
N: 3.7W
F: ≥50MHz
Tmax: 175°C

AntonvZ
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Update

The left channel has sound again ! Lets have a Party !!!

First my thanks to Martin, without his help I'm very doubtful I would have found the problem so quickly.

It was indeed the left channel transistor U2734  (hard to find specs on) that led me to a similar transistor BC141 (see previous post for spec of BC141)

While reading up on that discovered "paired" transistors and figured that the paired match for U2734 would be U2735 and closest match to that is BC161.

Was overjoyed to find stock of both at my local in Cape Town although the sales assistant did smile and said that stock had been lying there for years.

Replaced U2734 with BC141-16 (you have to desolder the heatsink with the capacitor) and fired her up and had sound but very distorted.

Next replaced U2735 with BC161-16 and magic - both channels were working.

This also cured the huge heat buildup on the output transistors and the BM is running a lot cooler now. As in warm to touch, not scalding hot.

So the old girl is now recapped on most of the electrolytic capacitors and firing on both cylinders. Yaay.

Will address the next set of issues in another post.

AntonvZ
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So now for the next set of issues.

1. Stable voltage is now sitting at 40.8V iso 30V. Adjusting the trimmer has no effect whatsover and this is a new one.

2. My original problem. The RIGHT preamp channel (QII) is dead.

  • electrolytics caps in that area have now been replaced
  • it is not the cable (2 RCA to DIN) as it works fine with two other audio sources
  • it is not the turntable either as that works well with my Technics amp
  • tape and QI work fine with other audio sources, the dead right channel is only on QII.
  • the pins on the QII din input are configured correctly 
  • there is continuity from the pins to the PCB
  • there is 0.65V power supply to both 10u/64V caps
  • doing left to right voltage voltage checks are giving approximately the same values left to right.

Anyone else every had this problem?

Thanks in advance.

Anton.

Craig
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Craig replied on Tue, Mar 3 2020 6:08 AM

Very well done!

craig

AntonvZ
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Dillen:
Points to a problem with one of the output stage transistors or, perhaps most likely, one of the driver transistors.

Hi Martin

Still having a problem with getting 30V stable. Adjusting trimmer has no effect.

Done the following measurements and cant figure out where exactly the problem is.

Power filter capacitor 43.8V

Power transistor 2N5034-  Base red wire 39.8V, collector (think brown wire) 42.6V, emitter (think orange wire) 42.98V - as far as I can read the circuit diagram i should be getting 30V on the emitter.

Rectifier - orange wire 43V, brown wire 1 20.7V, brown wire wire 2 20.05V, 4th pin assume ground 15mV 

400u/40V capacitor - 40.02V

Stereo bulb 39V going in 9V coming out.

Point J/K 39.9V

Is this the 2N5034 power transistor causing the above, if so can i replace with 2N3055 (which i have) as it seems as if it was used on some models as per below snippet from B&O repair manual.

Dillen
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Dillen replied on Wed, Mar 4 2020 11:43 AM

"Power transistor 2N5034-  Base red wire 39.8V, collector (think brown wire) 42.6V, emitter (think orange wire) 42.98V"
- It's an NPN transistor. With the base voltage lower than the emitter, this transistor should be closed.

" as far as I can read the circuit diagram i should be getting 30V on the emitter.""
- No, You should have 30V on the collector. Make sure you get the pins right and measure the right place.
But in this case it looks wrong any way around, so I suggest you replace the 2N5034
You can use a 2N3055 if you don't have a 2N5034 (https://beoparts.com/2014/12/28/beomaster-1001-1200-transistor-2n5034-8320103/).
You will have to adjust the voltage regardless.
Also check the two zeners and the BC154.

"Stereo bulb 39V going in 9V coming out."
- With 30V across a 18-19V lamp, it must be lighting up the whole village?

Martin

AntonvZ
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Dillen:
You can use a 2N3055 if you don't have a 2N5034 .

You will have to adjust the voltage regardless.

Hi Martin

Thanks as always.

Swopped out the 2N5034 with a 2N3055.

Update and Edit, just to set the record straight. My Beomaster 1000 version 2318 came with 2N3055 transistors and not 2N5034 as indicated in the circuit diagram supplied with the set. I only noticed this weekend while cleaning up that the one I had removed was in fact a 2N3055. 

Readings now 43V on the emitter, 39.4V on the collector and 43.6V on the base.

Still can't adjust voltage at trimmer.

Will next look at the BC154 and the zener diodes.

Dillen:
"Stereo bulb 39V going in 9V coming out."
- With 30V across a 18-19V lamp, it must be lighting up the whole village?

Indeed, a whole new feature, the way it brightens and dims depending on the volume setting is escpecially entertaining. Wink

Regards

Anton

manfy
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manfy replied on Sat, Mar 7 2020 7:46 AM

AntonvZ:

Dillen:
"Stereo bulb 39V going in 9V coming out."
- With 30V across a 18-19V lamp, it must be lighting up the whole village?

Indeed, a whole new feature, the way it brightens and dims depending on the volume setting is escpecially entertaining. Wink

That is a good sign that your 2N5034 (now 2N3055) doesn't regulate at all. The volume setting changes the current drawn and the 30V supply goes down accordingly.

Check the Zener diode 908. You should normally read 21V on the cathode - in your case it should be 30.4V since you get 39.4V on the collector of 2N3055. If the diode is ok, check transistor BC154 and if that one is ok check the BC140.

Beyond that, there's nothing in that circuit that gets easily damaged.

 

Rgds,

manfy

 

AntonvZ
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manfy:
Check the Zener diode 908. You should normally read 21V on the cathode - in your case it should be 30.4V since you get 39.4V on the collector of 2N3055. If the diode is ok, check transistor BC154 and if that one is ok check the BC140.

Hi Manfy

This is the most frustrating thing ever. 

Cathode of Zener diode 908 reads 40.6V (I have already replaced this with a 9.1V zener diode)

BC154 C=41.2V E=41.2V B=39.2v

BC140 C=41.5V E=41.5V B=40.8V (I have already replaced this with a BC141 transistor, closest match)

Trimmer Leg connecting to 470 ohm resistor #905 =39V, Leg connecting to 2.2K ohm reads 15.64V. Single leg side reads 39.2V and adjusts to 15.64V if you turn it.

My experience with electronics limited, would appreciate you thoughts on the above.

Regards

Anton

Dillen
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Is the trimmer slider ("single pin side") connected to the BC154 base at all?
Is the emitter resistor (910 / 3,3K) OK?

Martin 

manfy
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manfy replied on Sun, Mar 8 2020 7:37 AM

AntonvZ:

Cathode of Zener diode 908 reads 40.6V (I have already replaced this with a 9.1V zener diode)

BC154 C=41.2V E=41.2V B=39.2v

BC140 C=41.5V E=41.5V B=40.8V (I have already replaced this with a BC141 transistor, closest match)

Trimmer Leg connecting to 470 ohm resistor #905 =39V, Leg connecting to 2.2K ohm reads 15.64V. Single leg side reads 39.2V and adjusts to 15.64V if you turn it.

Thanks! That's helpful.
At first look, I'd say you mounted the new zener the wrong way around, i.e. in forward direction. Crying No worries, it's no biggy. The 3.3k resistor in series should limit the current to just 13mA, which shouldn't be a problem for the diode.

First checks (with power cord disconnected); multimeter set for continuity check:
- check connections as suggested by Martin above
- check if cathode of zener is connected to emitter of BC154 -> if not, zener is mounted with wrong polarity!!
- check zener with multimeter in diode test or resistance test mode in forward AND reverse direction.It could be dead (=shortened out) and if so replace it!

BC141 voltages look good, except collector voltage, which should be 30V regulated; but that's ok for now.
Trimmer voltages look good, i.e. trimmer is working. Set output voltage of trimmer to approx 21.5V (that's the nominal value that should give you roughly 30V once the power supply circuit is fixed).

More later.

rgds, manfy.

 

 

manfy
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manfy replied on Sun, Mar 8 2020 7:53 AM

Just to make sure we're talking about the very same thing:

You have a BM1000 type 2318. Martin said that it uses the same board and design as 2317. So, I copied the power supply from my 2317 service manual and added the nominal voltages in blue. All my posts and suggestions are based on this specific circuitry below.

 

Some important notes about the voltages given:

- the ground symbol (which normally represents 0V and often connected to the chassis) is positive in this design!!
- if you connect the black probe of your multimeter to the chassis and measure the voltages with the red probe, you will actually get negatve values on your multimeter!
- the voltages given are measurement values referenced to positive ground (= red probe of multimeter connected to chassis, black probe connected to test point)

Do NOT forget about this! It's an important fact for:
- polarity of electrolytic capacitors, diodes, transistors
- differential voltages measured directly between transistor terminals
- calculations made for the purpose of assessing behaviour of the circuit or individual branches in it
- and probably some other things I can't think of right now...

[edit: reworded and reformatted for clarity]

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