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Old 12th March 2008, 08:16 AM   #21
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Default Sugden P28 and A28B

Michel,

Why do you refer A28B schematic for fixing P28? What is the background connection between these two models? P28 seems the successor of A28, correct?!

I'll check those 5 parts value with your suggestion after receiving the schematic.

Many thanks for the reminding
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Old 12th March 2008, 06:14 PM   #22
Michel is offline Michel  Canada
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Location: L'Assomption, QC
Julian,

The A28 series, including the A28B, are integrated amplifiers.
The P28 series, including what they should have called the P28B, the one you have, are amplifiers.
The P28B is an amplifier I saw only once. I do not think Sugden made that many of them. It was practically an A28B without the control section so the amplifier circuits are exactly the same.

Regarding capacitor replacements, all I can tell you, be very careful.
You can easily ruin the tonal balance you already seem to like.
Sugden uses a lot of Rubycon capacitors and have the bad habit of using the low 85° C series and use them near their rated voltage value. I strongly suggest you use 105° C if you replace any.

Good luck!

Michel
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Old 13th March 2008, 12:10 PM   #23
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Michel,

I don't know how to send my appreciation on your schematic and the advise on capacitors. You're really Mr.Right on Sugden AMP.

I'll be careful on those capcitors as my first step, then enjoy measurement with my peer by your schematic.

Best regards
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Old 15th March 2008, 01:12 PM   #24
mrprytz is offline mrprytz  Norway
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I got a P28, back mounted heatsink. I had some resistors that got a little bit to hot I think, they went black. A capasitator that looked like it was 6 months pregnant, and this fussy noise. No, after changing all the bad parts, the noise is cone. But there is DC present.


Can someone please help me, by sending me some schematics. I don’t want it to die.
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Old 17th March 2008, 03:29 PM   #25
Michel is offline Michel  Canada
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mrprytz,

Unless the output capacitors are defective, because the P28 couples to the speakers through 10 mF capacitors, you should never have any DC at the output.
Give me your email address and I will send you the P28 schematic.

Regards,

Michel
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Old 19th March 2008, 04:53 PM   #26
mrprytz is offline mrprytz  Norway
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Michel,

Yeah, that would be great. Can it really just be that the output cap's? I'm not used to tings being that easy in real life. That **** only happens on Mac Gyver. hehe. If you would be so nice to send the schematics to mrprytz@hotmail.com. And if you got more hints on what else can wrong, in am more than happy to read hear from you.

From reading your post in this thread, I have learned that you know this amp pretty well. You rock.

Thanks a lot.
Jørgen
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Old 4th December 2008, 03:53 PM   #27
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Default sugden A28 II and P28

Hello all,
very interested to read your posts : I have a pair of these amps which have been wonderful and are now sick. Being in Switzerland, it's a bit far to Yorkshire for Sugden to fix.

Please may I ask - Michael please would you send me the schematics? mine have the heat sink on the rear. Or anyone else send them, who has received them in the past.

My email is DAVJUNK@HOTMAIL.COM

Thank you. I appreciate your help

David
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Old 5th December 2008, 02:58 PM   #28
zlang is offline zlang  Denmark
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Hi

I have 2 A28 amplifiers which is not working properly

Could anyone please send me a schematic ?? (peter.langgaard.jensen@gmail.com)

Thanks in advance

i'm looking forward to listen to these great amplifiers when i get them fixed :-)

/Peter
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Old 7th March 2009, 04:05 PM   #29
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Default another fuzzy Sugden a28

I too have a Sugden a28 which slowly went fuzzy and then went silent. I think it is a very early a28b but I am not sure.

Before I found this thread I have spend many many hours trying to reverse engineer the circuit diagram. If some one could put me out of my misery and post one (to simon@REMOVETHISBITmcbrown.plus.com) I would be very grateful. I could then fix the amp and see how close I got to getting the circuit right.

Many thanks,

Simple simon.
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Old 7th March 2009, 06:58 PM   #30
Michel is offline Michel  Canada
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Simple simon,

A28 and A28II have heatsinks in the back, A28B are on the sides.
Is your unit black or grey? White or gold front panel lettering?
I will send the right schematic after confirmation.

I believe you have an A28 or A28II because they usually are the ones with the fuzzing problem.
If it is the case, replace all the BC639 (4) and BC640 (2) near the output transistors.
I remember 6 total but it could be 8.
Sugden should have used bigger transistors as the ones there have more than a full plate.
In the long run they overheat and deteriorate (get fuzzy) until death takes them.
BD139 and BD140 are excellent replacement candidates.

If your unit is particularly old, grey casing or black with white lettering, make sure you verify all the electrolytic capacitors in the amplifier section as they are often way below their nominal value. As a rule of thumb, this should always be the case with old equipment as capacitors usually have around 2000 hours of life at their rated value. It means that in an amplifier like the A28 which generally runs pretty hot, the capacitors will begin to deteriorate seriously after a couple years. Imagine when the unit is left on all the time...

I have been servicing hi-fi equipment for the last 27 years and have replaced zillions of capacitors in more equipment designed or built in the UK than any other place. Is it a UK way of thought?
They usually have a tendency to choose the lowest voltage and temperature ratings possible. Go figure. So, when you replace, always use a capacitor with a higher voltage rating and 105° C temperature rating, as space permits. Do not use generic brands as they are of extremely doubtful origin and there are lots and lots of crappy capacitors out there. Stay with Panasonic, Nichicon, United Chemi-Con, Siemens/Epcos/Sikorel, Vishay, Rubycon and more. This list is not complete and there are websites dedicated to capacitor classification that can give you much more information.
Like: http://www.badcaps.net/

One other thing, always replace defective parts in stereo when you upgrade and/or repair with something else than the original part. I often see repairs made with an upgraded or simply different part/brand done only on one channel. This is a nonsense as it could affect the tonal balance of the system. I see this done by many servicing centers and do not consider this way of doing things, very professional...

Best regards,

Michel,
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