Sugden A21 series 1

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richardcooper2k
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Sugden A21 series 1

Post by richardcooper2k » Fri Feb 19, 2016 4:04 pm

Hello, it's been a while. Hope you are all well.

A friend of mine has asked if i can take a look at his 1968 sugden class A amplifier.
I have taken it out of it's wooden sleeve and things don't look very straight forward (to me)
The boards don't have component numbers on them for a start.
And i understand there are some matched components in the circuit.
I've not been able to find much info on the internet, just this:
http://www.audiomisc.co.uk/JES/A21/A21.html

The fault is that there is only sound on one channel. The other one just hums. On the hum channel one of the external power transistors gets very hot and a 0.7 ohm power resistor glows red after being on a short while. The local hifi shop service department wont touch it. I phoned Sugden today and Patrick said if they can repair it, it would cost £100 - £180. Which seems fair. He also said he would e-mail me the schematic.

So i am wondering whether to have a bash myself. The amp has been repaired previously, most electrolytics on the power board have been replaced and one of the external output transistors is a 2N3773 while the rest are (i assume origional) BDY20. The replaced transistor is on the failed channel.

The sound on the channel that does work i thought sounded very nice

What'dya recon ? Any hand holding available ?
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Mike H
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Re: Sugden A21 series 1

Post by Mike H » Fri Feb 19, 2016 5:03 pm

On the channel that doesn't work I'd make a wild guess that the cold power transistor has packed up, gone short circuit so the supply Voltage is all across the remaining one, the one that gets hot, also is what is making the resistor glow red. Is this resistor one of the R19 / R22 ? The low value suggests it most likely.


It's an interesting design, an O/P stage DC biased to half supply Volts by a resistor divider chain, then the AC is put in by capacitor coupling.

My one with the germanium O/P trannies is virtually identical in principle, with the difference that the AC is coupled to the O/P stage by transformer.
 
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Re: Sugden A21 series 1

Post by Mike H » Fri Feb 19, 2016 5:12 pm

PS: but also check all the caps around the output stage in case one of them has a short circuit.
 
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Nick
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Re: Sugden A21 series 1

Post by Nick » Fri Feb 19, 2016 6:01 pm

I would agree with Mike. I have fixed one of these in the past, it was the output devices that had failed.
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Re: Sugden A21 series 1

Post by richardcooper2k » Fri Feb 19, 2016 7:10 pm

Thanks for the responses. Much appreciated

I forgot to mention the owner said the signal (music) faded away gradually over time rather than stopping all of a sudden. I wondered if that would be more indicative of a capacitor ?

Anyhow, if i can figure out how to get the darn thing apart i had figured it might be worth replacing all the transistors on the faulty channel (just on the power board). And replace the caps. The originals seem to be available from Cricklewood (transistors) i think
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Mike H
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Re: Sugden A21 series 1

Post by Mike H » Sat Feb 20, 2016 12:36 am

Could very well be a leaky capacitor. But a failing o/p transistor could have same symptoms, as its gain gets less and less.

Check also those diodes are OK, if one went open circuit then that half of the output transistors will be biased full on.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Sat Feb 20, 2016 11:34 am

So i managed to get the thing apart to gain some sort of access to the power board. Initial investigations revealed to things on the failed channel.
1) an electrolytic capacitor with a dry joint at one end (it just pulled straight out)
2) what i think is a diode which was only attached at one end. The other was pointing straight up so had been left this way by whoever had been into this amp previously. The other channel (working) has the diode connected at both ends and is in parallel with a resistor

One thing that is confusing me is that i can't seem to match what is on the board with the circuit diagram linked in an earlier post. It is tricky because there is no printed information on the boards, only holes and connections. But even the component counts and values don't seem to match up, regardless of topology. For example, as far as i can see, there are no diodes in parallel with resistors. There is a trimpot on each board, none i can see in the schematic. The power resistors are 0.7 ohm on the boards and 1 ohm in the schematic. There are no series diodes on the boards that i can see. And couldn't match up the dry joint capacitor to any position in the schematic when i tried.
So perhaps it wasn't originally the circuit in the link, or someone has later been tinkering with the topology or maybe i have just got it all wrong.

One thing is that the amp was working to the owners satisfaction before it died on one channel. So maybe the first thing is to remedy the dry joint and see if that cures it
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Mike H
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Re: Sugden A21 series 1

Post by Mike H » Sat Feb 20, 2016 5:16 pm

 
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Re: Sugden A21 series 1

Post by richardcooper2k » Sun Feb 21, 2016 11:45 pm

I've been having a look and it seems it is a hybrid of the 2 circuits. Whether it was made on the cusp of change from the series 1 to the series 2 or maybe Pete Hartley modded it when he was doing repairs. I am going to try and sketch out the schematic so i can see it myself and post it here.
My friend said he did ask Pete to defeat some of the pre amp controls which he did but apparently it wasn't as simple as just disconnecting them.
Pete used to run a shop on London road in Sheffield but sadly died in January last year: http://www.hartleyelectricmusic.co.uk/about-us.html
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Re: Sugden A21 series 1

Post by Mike H » Mon Feb 22, 2016 10:58 am

Tracing the circuit is what I would do. Would like to see that. :D

PCB's with no legends on was quite common in the old days. When I was a student (around 1972 - 74) I often used to buy old boards to get the components off them, all were plain SRBP.
 
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Re: Sugden A21 series 1

Post by richardcooper2k » Mon Feb 22, 2016 11:51 pm

Ok, so i have traced the circuit and it is most similar to the one Mike linked to for the A21 series 2:

http://www.audiomisc.co.uk/JES/A21series2/circuit.html

I will list the differences:
R5 is 4k7 instead of 1k2
R12 is 5k6 instead of 56k
R16 is 100 instead of 200
R20 is 0.7 instead of 0.6
The replacement electrolytic capacitors are generally rounded up eg 400 replaced by 470uF, 80 replaced by 100uF, 4000 replaced by 4700uF, the 2 80uF in parallel replaced by a single 220uF. The exception is on the left (working) channel C10 (400) is replaced by 100uF whereas it is 470 on the right (not working)
The output transistors (VT4 and 6)are BDY20 instead of BDY38. One of the BDY20s has been replaced by a 2N3773 (VT6)
There is a diode (i think that is what it is) in parallel across R13 (only connected at one end on the right (not working) channel)
There is a 3k9 resistor connected across the 2 terminals of the trimpot that are not earth
There is a series RC network connected from the collector of VT6 to earth, the same as in the series 1 circuit. ie a 0.1uf cap in series with 4 paralleled 10 ohm resistors
All electrolytics have previously been replaced apart from the 4 large 2500uF ones (2 of which are the main power supply caps and the other 2 seem to couple the output (C15 on right channel).

Is it worth taking any measurements or shall i just get a full set of transistors for the right channel and solder them in ?
Should i replace the big 2500uF 40V caps as well (this would mean different mountings as modern ones will be much smaller) ?
And i guess it would be worth changing R20 0.7 ohm power resistor as this was the one getting red hot.
What i don't quite understand is if R20 was passing so much current and VT4 got too hot to touch, how was it getting to earth ?
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Nick
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Re: Sugden A21 series 1

Post by Nick » Tue Feb 23, 2016 8:28 am

What i don't quite understand is if R20 was passing so much current and VT4 got too hot to touch, how was it getting to earth ?
I would guess vt6 being short circuit.
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Re: Sugden A21 series 1

Post by richardcooper2k » Tue Feb 23, 2016 10:46 am

Ah, yes. That would make sense to me now. VT6 is open circuit - low resistance - so not much heat generated at that point
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Re: Sugden A21 series 1

Post by richardcooper2k » Tue Feb 23, 2016 11:04 am

VT6 is the one that has been replaced previously (it's now 2N3773 rather than BDY20). I wonder if C12 has gone short circuit. But you would pick that up when setting the quiescent current as you are looking for 0.8 colts across R19 which is parallel to it. Could be intermittent.
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Nick
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Re: Sugden A21 series 1

Post by Nick » Tue Feb 23, 2016 1:32 pm

If vt6 is a short circuit, then vt4 could still be passing the correct current (likely, as the 1A fuse has not blown, so the voltage across R19 would still be correct. But now the resistor under vt4 has got half the supply voltage across it at the normal current, hence the excess dissipation.
Resistance isn't futile it's V / I.

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