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Old 17th Dec 2011, 1:35 am   #1
yestertech
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Default Horrid intermittent on Transistor Black Box 1004

I have a "rush job" on to refurbish one of these ahead of the festivities, but this unit is having none of it !
Having found a defective O/P transistor, VT7 and replaced it with a new AD149, I was expecting all would burst into life. However, all I had was a very faint distorted output and incorrect mid-point voltages at the output cap. At around 15 volts rail, I stopped increasing the mains input in case some expensive smoke appeared.
Whilst checking for other problems, there was a 'pop' and the unit burst into life.
Here it remained, whilst I checked the voltages again. I was able to adjust R13 to obtain a reasonable mid point at C12. I advanced the variac to full and re-adjusted the mid point volts and all was well until I increased the volume. There was another pop and the output went back to faint and distorted. At this point I unsoldered and checked VT5 and VT4 on the PEAK ATLAS and these showed fine. ( VT6 was OK and VT7 is new )
Having resoldered them and powered up again slowly via the variac, the full output returned - until I applied full mains, whereupon, you guessed it, another pop and faint and distorted output again.
I ran over all the resistors with a meter - nothing untoward found. I then blitzed all the electrolytics having already changed the main reservoir cap.
On powering up, normal sound again for a while ( hooray ? ) ... until 'pop' and low and distorted output again.
By this time, the air was rather blue ( empty house thankfully !! )
The fault does not respond to any flexing, poking or wiggling, so I am wondering if it's possible to have an intermittent transistor which fails beyond a certain voltage ? VT3 seems possible as the voltages down the resistor string R18 to R20 are much lower when the fault occurs.
Being DC coupled of course, it could lie elsewhere
By the time I measure on the low side of R19 the voltage is only 1.8v, as opposed to 12.3 when working properly.
The rail volts also falls from 24 to 19 when the fault happens

To make matters worse, the unit is full of NKT germanium jobs, and one of them is not listed at all in my TOWERS book.
Hence I can't just change all the likely suspects
Having had an horrendous intermittent on my recent QUAD 303 repair I was hoping this one would be quick - WRONG !! - How can something so simple be such a PITA ?

Any thoughts most welcome, as I have very little time left to sort this one

Andy
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Old 17th Dec 2011, 2:16 am   #2
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Default Re: Horrid intermittent on Transistor Black Box 1004

I know it's stating the obvious, but this sounds like a dry joint.
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Old 17th Dec 2011, 9:41 am   #3
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Default Re: Horrid intermittent on Transistor Black Box 1004

Seconded! They can be very well hidden indeed.
With luck, the fault will stay on long enough to narrow it down with voltages.
Hairdryer and freezer?
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Old 18th Dec 2011, 11:09 pm   #4
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Default Re: Horrid intermittent on Transistor Black Box 1004

Thanks Gents, but sadly it would appear not !!
The fault is now solid and I CANNOT figure out what's going on with this ( expletive deleted ) Unit
So, as 'tis the season to be jolly, I'll offer £10 for a couple of festive beers if anyone can provide any tips which lead to a fix for this dreadful thing !!

I have now :
1. reflowed and checked all joints.
2. "bleep tested" all point to point connections on the copper side.
3. had all the transistors out and checked them on the Peak Atlas - predictably all fine.
4. Checked the mustard cap C9 and the OA5 diode
5. Rechecked all resistor values

I'm 90% certain this is a component fault - The thing is pulling 70-100ma quiescent current, depending on the setting of the variac up to about half mains.This pulls the normal rail volts of 24 down to about 19 v at full mains, so current is flowing around something. There is still a low distorted output.
There appears to be a low resistance path from collector of VT3 to ground ( around 170 ohms ) which doesn't seem right.
There is little or no bias voltage at the base of VT2 - In turn there is very little voltage at the base of VT3
I very much suspect the transistors may be breaking down under voltage, but as I have no substitutes this is hard to prove.
I am tempted to replace the lot with an IC, but even this would take too much time.
In the next 24 hours I will have to abandon this one, which is a shame, but other things are pressing.


Andy

PS If anyone knows any silicon transistors which may make good replacements for this lot, I'd be glad to know...

Last edited by yestertech; 18th Dec 2011 at 11:14 pm. Reason: PS
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Old 19th Dec 2011, 1:24 am   #5
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Default Re: Horrid intermittent on Transistor Black Box 1004

Just a thought.... have you looked at the output with an oscilloscope to make sure that it is clean and the unit is not oscillating? It would certainly explain the current drain.
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Old 19th Dec 2011, 9:01 am   #6
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Default Re: Horrid intermittent on Transistor Black Box 1004

If the fault is "solid" it should be easy (ha ) to locate.

Do have a circuit ?
The clues will be there all along. Measuring base-emitter volt drops is often very revealing.
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Old 19th Dec 2011, 8:26 pm   #7
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Default Re: Horrid intermittent on Transistor Black Box 1004

Andy,

The low resistance path from VT3 collector to ground may be your meter reading through VT4 base-collector and R22. Try connecting the meter the other way round. If the reading increases, then all is probably well there.

For the main problem, have you checked for a short from VT7 collector to the heatsink?

Paula
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Old 19th Dec 2011, 9:03 pm   #8
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Default Re: Horrid intermittent on Transistor Black Box 1004

This could even be a resistor playing up.

David
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Old 20th Dec 2011, 1:02 am   #9
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Default Re: Horrid intermittent on Transistor Black Box 1004

Can I suggest a couple of (hopefully not obvious) things to try then...

Firstly, check the base of VT2, as R13 could actually be at fault itself, this will directly affect the biasing of the whole output arrangement.

A failed C9 would also put the whole lot out of whack, as would a high resistance R17.

C11 would be the main contender if you hadn't already gone over the electrolytics.

GD1 going O/C would cause some major problems, I'd expect more quiescent power draw and low distorted sound. Could you measure the voltage across this during normal and failed operation? (should be fairly straightforward as the voltage should be consistent over the full supply rail).

Hope this helps.

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Old 20th Dec 2011, 4:57 pm   #10
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Default Re: Horrid intermittent on Transistor Black Box 1004

I have "moved " the pre Xmas deadline for this, so at least I can take more time to sort it out, not that that's any nearer !
With thanks for the above suggestions, sadly none of these check out.
There are no shorts or other problems I can detect with the O/P pair.
R13 seems to change DC resistance when adjusted, although it does not read 25K when at max. I think this may be due to the presence of the transistor.
C9 checks out OK as does R17.
C11 is already replaced of course
There is voltage on both sides of GD1, and the tester correctly IDs it as a diode, so I think that's OK
Normal vs failed is no longer possible, as the fault is solid.
having got the scope out yesterday to check an earlier suggestion about oscillation, I can confirm there is none. What is interesting is that with 1KHz applied via C7 to the base of VT2, the O/P is lop-sided, suggesting that the signal alone is pushing VT2 into conduction.
The base voltage is also extremely low, around .23 v with R13 at max. The collector of VT2 also appears high. I swapped VT3 with VT2 and it's the same. No clean signal achievable at the collector of VT2 and low forward bias volts.
When I removed VT2 the base voltage is then much more in the normal range adjustable via the pot. I can't state absolute voltages as I have to leave the variac about half-way up.
The quiescent current at full voltage is around 220 mA !!! and I fear frying something.
I am more and more inclined to think I have bad transistors in this unit, despite the fact they check OK on the PEAK ATLAS. I think they are failing under "real" conditions. Could these germaniums be suffering from some kind of "whiskers" I wonder.

What's also telling is that during fault finding I had one of the legs break clean off VT1 - My data books gave a sub as AC107. Having put this in, VT1 pre amp stage is the only thing working properly. A nice clean sine wave at the collector and sensible voltages on base and collector. The packaging for all the NKT 22x's is the same apart from VT4, which is as well as I can't find this one in any data books

Andy

PS I have now established I have AC128, given as a sub for NKT 224 - I will try one of these in VT2 position and note the difference.
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Old 20th Dec 2011, 8:59 pm   #11
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Default Re: Horrid intermittent on Transistor Black Box 1004

Andy,

These feedback circuits are always a problem to sort out. You could try breaking the feedback loop by disconnecting R27 from VT7's collector. If you connect R27 to the centre of a potential divider made of two 470 ohm or so resistors connected in series across the supply, the voltages around VT2 ought to return to near normal. It may then be easier to establish what the problem really is.

Paula
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Old 20th Dec 2011, 10:01 pm   #12
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Default Re: Horrid intermittent on Transistor Black Box 1004

I'm pretty much out of my depth here, but keeping it simple, I noticed this bit in post #1:

Quote:
having already changed the main reservoir cap
Do you think your replacement cap could be faulty?
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Old 20th Dec 2011, 10:52 pm   #13
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Default Re: Horrid intermittent on Transistor Black Box 1004

In your first post, you mention changing VT7 which was faulty but not VT6 which is the other 'half' of the output pair. I would always advise changing both even if only one measures faulty (it was always a requirement at Philips). Admittedly sometimes you can get away with it but generally it's better to change the pair. Excess current through one or the other could cause problems and I have had intermittent faults caused by output transistors being faulty. The problem with all of these sorts of output stages is that because they are DC coupled, a problem in one area can affect the voltages all over the circuit. An AC127 for VT4 and an AC128 for VT5 will be fine as drivers. I think that's what we used as subs anyway. Make sure that one of your speakers is not going O/C (they're in parallel to give 7.5 ohms). Will give reduced output (though not distorted).


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Last edited by Sideband; 20th Dec 2011 at 10:58 pm.
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Old 21st Dec 2011, 12:47 am   #14
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Default Re: Horrid intermittent on Transistor Black Box 1004

Thanks all for further thoughts, much appreciated.
Annoyingly, after at first sight looking like the AC128 subbed for VT2 was working properly I changed the remaining NKT224 's for AC128s. Slightly more gain but still very distorted HO HUM
As to other suggestions, I checked the new reservoir on the ESR meter prior to install and that was good.
Rich, I did worry about both output devices not being matched. A day or so back I tacked in another AD149 in lieu of VT6 but it made no difference. I will probably install this one permanently anyway for good measure.
Also, ref the speakers, I am using a test unit ( 6x 4 ish elliptical ) on the bench rather than the BB internal speakers. This was working OK to begin with, so I guess I should check its not gone short, although I doubt this would affect the DC conditions so badly the other side of the OP cap.
Paula, I will try this trick to shift the DC conditions.
What's really bugging me is where the base potential is going to on VT2. It still seems really low. The pot ( R13 ) at full CCW measures slightly low at around 16K, but is difficult to remove. In any case, it should still be possible to set it for the correct bias point.
I replaced the 100k resistor on the top end of the base divider just as a precaution.
and I still don't understand what's pulling that 220ma quiescent current, if as it looks, VT2 is NOT conducting.
In the time I've spent on this damn unit, I could have re-built it as a valve version and still had time to enjoy a mince pie or 6

Andy
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Old 21st Dec 2011, 1:07 am   #15
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Default Re: Horrid intermittent on Transistor Black Box 1004

Could there be any damage to the PCB itself, a short or partial short or insulation breakdown between tracks, or even a hairline crack somewhere?
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Old 21st Dec 2011, 9:38 am   #16
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Default Re: Horrid intermittent on Transistor Black Box 1004

Andy,

VT2's base potential will be less than would be expected if its base current is higher than it should be.

In this circuit, the emitter voltage is mainly set by the potential divider consisting of R14+R15 and R27, dividing down what should be the mid rail voltage from VT7's collector. That would give an emitter voltage around 0.8V. The base voltage could be expected to be around 0.2V higher, at 1V or so.

With the fault, the mid rail voltage is very low, so the emitter voltage is now determined mainly by the emitter current flowing through R14+R15. The design emitter current seems to be about 650uA, which would only give an emitter voltage of 0.1 volts and a base voltage of around 0.3V. This isn't too far off what you're measuring.

Paula
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Old 21st Dec 2011, 11:27 am   #17
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Default Re: Horrid intermittent on Transistor Black Box 1004

Is there a schematic for this somewhere ?

If you haven't put a scope probe to this yet then that's where I would start. Its possible you have an oscillation - this would lead to all sorts of strange phenomenon !

dc
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Old 15th Jan 2012, 9:59 pm   #18
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Default Re: Horrid intermittent on Transistor Black Box 1004

With thanks again to all contributors on this thread - Having taken up this evil machine again to clear my queue, I can now report success.

The problem turned out to be particularly onerous; an intermittently failing transistor, in the shape of VT4. This was the only device for which I had no immediate sub, typically.

As stated earlier in the thread I had previously tested this and it was working perfectly (I have the bench notes to prove it). It now tests effectively as back to back diodes. So at some point after replacing VT6, it had started working out of the blue, failed, worked again after cap changes and then finally failed 'solid' all the time leading me to believe that the problem was in either the board/soldering or one of the passive components.

For good measure I re-installed the original VT2 and 3 and then reset the quiescent current which is now a healthy 30 mA. Soak testing at high volumes has proved it to be a reasonable performer, although the supply rail 'sags' rather at high volumes. I'm guessing looking at the scope and RMS output volts about 4-5 watts. Does anyone know for sure?

It sounds quite good on my small Tannoy bench monitor, but pretty dire on the original Pye units in the box. There again, it's intended use if for 78s so probably quite passable !

Bench clear - NEXT !

Andy
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Old 15th Jan 2012, 11:22 pm   #19
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Default Re: Horrid intermittent on Transistor Black Box 1004

Quote:
Originally Posted by setsappeal View Post
an intermittently failing transistor, in the shape of VT4. This was the only device for which I had no immediate sub, typically.
Did you use an AC127? I'm sure we did at Philips. The original NKT transistors became obsolete so Philips/Mullard were used instead.



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Old 16th Jan 2012, 10:24 am   #20
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Default Re: Horrid intermittent on Transistor Black Box 1004

Hi Rich
Yes an AC127 did the trick for VT4 , so the AC128 which was used to replace VT5 was left in circuit also .

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