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DaveC 13th September 2004 09:17 PM

Sugden P28
I have a pair of Sugden P28 Amplifiers, one of which is very sick and the other slightly so.
Anyone have schematics and/or know which preset does what and what how they should be set?

Hoping someone can help


AndrewT 13th September 2004 10:52 PM

Hi DaveC,
Do you mean P128 or P28? I have just reverse engineered the circuit of my 2 pairs of P128, because Sugden claim they have no service information on these amps. If this is what you want then how do I upload a paper circuit to the web? regards Andrew T.

DaveC 14th September 2004 06:10 PM

Hi Andrew

Thanks for the reply.

Unfortunately mine are P28's

I have a C128 preamp but couldn't afford the matching P128's at the time so went for the cheaper P28's


AndrewT 14th September 2004 07:35 PM

Hello Dave, Can I contact you off forum to discuss your options? regards Andrew T.

Michel 5th November 2004 07:02 AM

Sugden P28 and P128
Hi daveC,
I think I do have the schematics for both amplifiers.
Let me know if you are interested and I will look for them.
What is wrong with your P28 anyway?
The first things that go wrong after awhile in most of the Sugden products are the capacitors in the power amplifier section.
Sugden has always had the extremely bad habit of using capacitors that are too close to the working voltage. They do tend too to put resistors , transistors or zener diodes that radiates heat close to them. When replacing them always use 105 C long life capacitors. The other things that go wrong after the capacitors are the BC639 and BC640 drivers which have a little too much on their plate.
They usually do not die but instead develop a fuzzy sound. When I have this problem I find it more efficient to replace all 4 driver and both bias transistors. Another thing you have to look for is the 2SB776 and 2SB896 power transistor insulators, if they look like real mica they are fine but if they look on the thin side and have a milky color replace them as soon as you can because they do tend with time to cause a short circuit. The high value preset is to adjust the voltage at mid point and the low value is to adjust the bias which should be set at around 300 mA measured through the supply fuse. Let it sit and readjust periodically for at least an hour.
If after that you do not find the problem let me know and I will try to give you a hand.

pillhead 9th March 2006 03:59 PM

sugden P28
Hi Michel,

i'll write in English so our readers understand(anyway,i'm not too used to writing French canadian,tabernacle!).
i've been reading your post with great interest since i'm the proud owner of one of these machines,which has turned me down.i have repaired it, but not plugged it in yet(i will let you know in my next post).
To make a long story short,could you send me a pdf version of the circuit diagram?

Many thanks,

Michel 9th March 2006 05:35 PM

Bonjour Philippe,
Are the heat sinks on each side or at the back of the amplifier ?
Send me your email address so I can send the schematics.

AndrewT 10th March 2006 07:09 AM

Hi Pill,
wire up a light bulb in series with the mains input to the amplifier.
Then switch on.

If all is OK the bulb will flash briefly.

If your repair has gone wrong, then the bulb will come on and stay either dim or bright.
It might stop the amp blowing up.

rwa 19th April 2006 01:27 PM

Hi guys
can some one send me the scematics of both versions of the p 28. I've got one of each. One with the heatsinks on the back and one with the heatsink on the side.
And both are out of order.

Make me smile again


King 20th August 2006 07:59 PM

Need Help on My P28
Hi Michel of Quebec, I have a P28, and the left channel occasionally cut in and out, but not always! Any reason?
I read a reply from you to another mamber, seems to me you might have the answer.
Thanks in advance!

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