Sugden A21 series 1

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Mike H
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Re: Sugden A21 series 1

Post by Mike H » Tue Feb 23, 2016 6:10 pm

At this point I would unsolder all the o/p trannies and test them.

If the drivers are OK the circuit might even start working and stabilize at half supply Volts.


I've saved off the schematic and I was going to put the component values on it including what Richard posted but not had time yet.

Might be able to sim in LTspice.
 
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Re: Sugden A21 series 1

Post by Mike H » Tue Feb 23, 2016 6:26 pm

richardcooper2k wrote:VT6 is the one that has been replaced previously (it's now 2N3773 rather than BDY20).
A quick Google suggests 2N3055 is a better match to BDY20.

But obviously someone's been having a go at trying to repair in here.
 
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Re: Sugden A21 series 1

Post by pre65 » Tue Feb 23, 2016 6:51 pm

If the power transistors are different now, would it not pay to change them all ?
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Re: Sugden A21 series 1

Post by richardcooper2k » Tue Feb 23, 2016 7:07 pm

I think the fitted BDY20 s are very similar to the BDY38 s in the schematic. They just have a higher voltage rating so i think they should be fine. BDY20 s are £3.60 from Cricklewood so i'll probably stick with those as i think they are probably the origionals

If it is possible to sim the circuit it would be interesting to see what difference the differences from the series 2 circuit make
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Re: Sugden A21 series 1

Post by Mike H » Thu Feb 25, 2016 12:32 am

Good old Cricklewood to the rescue again. :D

There ya go then.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Thu Feb 25, 2016 8:52 pm

Hmm. So i disconnected the output transistors on the failed channel.
I used the diode tester on my multimeter.
Both tested ok from base to emitter and base to collector both directions
Both tested ok from emitter to collector both directions
So either the problem only shows up under load or it is elsewhere

What to check next ?

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Re: Sugden A21 series 1

Post by Nick » Thu Feb 25, 2016 10:00 pm

The driver maybe?
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Re: Sugden A21 series 1

Post by richardcooper2k » Thu Feb 25, 2016 11:20 pm

Tried same thing with one of the drivers VT3.
As i was removing it i noticed it was 2N3440 instead of BDY52 like the other 3 drivers. So looks like it has previously been replaced
Tested it in the same way and it seemed fine
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Re: Sugden A21 series 1

Post by Mike H » Thu Feb 25, 2016 11:34 pm

richardcooper2k wrote: Both tested ok from emitter to collector both directions
What was this exactly? Should be high resistance in both directions. Only exception being if some bias is put through base to emitter. But diode test is not the best for that.

Diode test base to emitter and base to collector, OK, where positive is on the base (NPN device). In reverse, should be infinite resistance.

To test collector to emitter, best done using Ohms range WITH some bias current applied to the base (I often use fingers placed between collector and base), works best with an analogue moving needle type multimeter on a high resistance range.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Thu Feb 25, 2016 11:49 pm

Same again with the other driver VT5
Seems OK
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Re: Sugden A21 series 1

Post by richardcooper2k » Thu Feb 25, 2016 11:52 pm

And yeah by tested OK i meant i i got "OL" on tester (over limit)
Unless it was in a direction it was supposed to behave like a diode
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Re: Sugden A21 series 1

Post by Mike H » Fri Feb 26, 2016 12:02 am

OK then!

Altho 'OL' could also mean the transistor is 'blown' open circuit, so a biased test will prove it's still working.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Fri Feb 26, 2016 12:09 am

Tried all 4 both ways collector-emitter on 2M resistance scale - all OL
I tried dabbing my finger across the base and collector while i was doing it but it didn't seem to have any effect (on all 4)
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Re: Sugden A21 series 1

Post by Mike H » Fri Feb 26, 2016 12:17 am

You are using the resistance range aren't you? Won't work in diode test.

Also these trannies wil be low gain so fingers might not be enough, may need a bias resistor.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Fri Feb 26, 2016 12:23 am

yep 2 meg resistance range

so connect a resistor from base to collector ?
What sort of value ?
And what am i looking for (send me a link if it's easier - i tried google but didn't find anything yet)
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