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Old 28th May 2011, 06:12 AM   #111
DennyG is offline DennyG  Australia
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Default A28 schematic

I'm using my recently dusted off A28 to drive a pair of Celestion Ditton 66's with updated crossovers. The A28 seems to drive them better than the two other amps I've tried so far.

My A28 has some of the issues discussed in this thread and therefore it would be of great help if Michel could send me the servicing information. The model I have looks like to one in the picture in post #37 by mgm31 ie black front panel with white lettering, grey case and with the heatsink on the back.

Thanks for your help in advance!

DG
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Old 10th August 2011, 03:49 PM   #112
Michel is offline Michel  Canada
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Hello Denis,

You probably do not need to replace the volume potentiometer as a cleaning usually corrects the problem.
For this cleaning to be effective you need to use a quality product like DeoxIT®, #D5S-6 from Caig.
If your potentiometer is an Alps Blue Velvet you have holes on the right side giving you access.
If your potentiometer is an Alps Black Velvet you need to lift the film pressed over the connections to be able to reach the slots and gain access.
I don't remember if the Black Velvet was mounted upside down and if it is the case it makes the task of cleaning it easy to accomplish.
If it is soldered to the main pcb I think you will have to unsolder it to be able to clean it.

The schematic you requested is on its way!

Best regards,

Michel
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Old 6th September 2011, 11:36 AM   #113
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Bonjour Michel,

A friend brought me this amp to be fixed. Board is numbered P463 002 and looks alike picture in #37. Problem is around the 7.5v zener and the 650ohms glass resistor wich are melt down. PC board trace has suffer. I was pleased to read on your forum that you could provide schematics for this amp. This could help a lot.

Many thanks in advance

Marc 22
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Old 16th October 2011, 08:00 AM   #114
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Smile SUGDEN A28II

Hi Everyone.

This is my first post in this forum.It's great to know that so may people are still using the Sugden A28. I acquired a Sugden A28 II (heat radiator at back) from a reseller.It's working fine and driving my dyna Audience 52.I don't have much technical knowledge.

I came to know that the amp runs at class A and class AB also.My query is that how should I know if the amp is running in class A mode or class AB mode.

Another query is that the transistors in the amp are marked as follows.

B776 D896 D896 B776
D 6H D 7D5 D 5J D 6H


Are these original transistors being used in this amp.

I would also request Mr Michel to forward the schematics of the amp at only_pallab@yahoo.co.in.


Will be grateful for any help from the forum members.

Thanks
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Old 3rd November 2011, 12:21 PM   #115
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Hello Michel!
Would you mind to send the schematics to me ?
Thank you in advance!

AndreyD

gerasim46@yandex.ru
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Old 21st February 2012, 02:37 PM   #116
IG81 is offline IG81  Canada
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Good ol' problematic Sugden's!

Michel,

You sent me the A28B schematics for my P28 (heatsinks on the side) a few years ago, along with some helpful advice, thanks again. I replaced the parts marked by an * on the schematic but goofed on that capacitor that's in parallel with the four 5k11 resistor and used a 47nF (!), the amp was badly oscillating and I shelved it for a few years. Fast forward to yesterday, when I felt like taking a stab at this again. I immediately noticed that the supposedly 47pF capacitor seemed way to large and replaced it with a 470pF I had on hand. I forget what was the original value on the board, but the plan indicates 1nF; you suggested 47pF in you email to me. The 470pF seems to work, as it took care of the huge oscillation problem, it was like a ~2Hz instability everywhere.

The amp still has a couple of issues. While I'm able to set quiescent current to 320mA as per the schematic, the 32V "mid-point" adjustment via the 220k trimpot does not want to go above ~23V, on both channels too; strange as I remembered these to work allright. There is also a buzzing sound comming out of the amp; probing the output clearly shows ~1.5V peak-to-peak of ~330kHz sine wave. I don't claim to be able to hear that high , but the buzz might be related still. This gets superimposed on any input signal too. I can also measure the same signal of smaller amplitute at the inputs.

I've not yet run out of ideas for what to verify and where to check, I'm more posting this as general info and I'll post solutions, hopefully, if I find any. But if anyone has some tips, they certainly are welcome.

I think this is a great little amplifier and it sounds very good IMO; too bad about the reliability. I think part of the problem on the "B" version is that the heatsinks run horizontal fins, greatly reducing air circulation and making for hotter output devices. Many smaller TO-92 devices on the board also feel quite hot to the touch; perhaps some clip-on heat-dissipators could help them a bit.

IG
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Old 21st February 2012, 03:40 PM   #117
Michel is offline Michel  Canada
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newbeenew and andreyd,

Sorry but I did not receive any update when you posted your requests.
Do you still need the schematics?

newbeenew,

The first watts are in class A but as soon as more power is required there is a transition to class AB. So, if you are not a "loud" listener, much of what you hear should be in class A.
The 2SB776 and 2SD896 are the original transistors used in the A28.

IG,

I am confused!! You say you decided to use a 470 pF instead of the recommended 47 pF saying it "seemed" too large?!?! Then you say that this 470 pF capacitor took care of the oscillation problem but then later in your post you say you measured 1.5 VPP at 330 kHz?!?! Since when a 47 pF capacitor got larger than a 470 pF???

The buzz you are hearing is directly related to that oscillation.

If you do not install the 47 pF capacitor, as you have been instructed to, you will never get rid of the oscillation problems.
I really see no point in trying to make the bias and midpoint adjustments or installing clip-on-heat-dissipators or trying to find new ideas..., if you do no get rid of the oscillation first.

Enough said...

Regards all,

Michel
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Old 21st February 2012, 03:47 PM   #118
IG81 is offline IG81  Canada
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Quote:
Originally Posted by Michel View Post
newbeenew and andreyd,


IG,

I am confused!! You say you decided to use a 470 pF instead of the recommended 47 pF saying it "seemed" too large?!?! Then you say that this 470 pF capacitor took care of the oscillation problem but then later in your post you say you measured 1.5 VPP at 330 kHz?!?! Since when a 47 pF capacitor got larger than a 470 pF???

The buzz you are hearing is directly related to that oscillation.

If you do not install the 47 pF capacitor, as you have been instructed to, you will never get rid of the oscillation problems.
I really see no point in trying to make the bias and midpoint adjustments or installing clip-on-heat-dissipators or trying to find new ideas..., if you do no get rid of the oscillation first.

Enough said...

Regards all,

Michel
Hi Michel,

Good to see you're still watching this thread.

What I meant was that a couple of years ago, I somehow goofed and used a 47 nano-farad cap, I don't know how this happened. Yesterday I repalced it with a 470 pico-farad, the smallest I had on-hands and it solved the major part of the problem, which was the whole amp oscillating at 1 or 2 Hertz, which I could see on the series-lightbulb, slowly flashing. This is gone now and the amp "works", other than the super-high frequency oscillation and mid-point voltage problem. Of course I will get some real 47 pico-farad caps on my next parts order to have the proper value. Only after that wil I be able to really see if there's something else wrong.

IG
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Old 22nd February 2012, 03:08 AM   #119
IG81 is offline IG81  Canada
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47pF seems to have done the trick. I can adjust mid-point and the oscillation and buzz are gone. Bias stable at ~310mA. Thanks again!

IG
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Old 22nd February 2012, 04:52 AM   #120
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Michele thanks for answering. I still require the schematics. Also please let me know what type of bookshelf speakers are best suited for such a amplifier. I am currently using Dyna Audience 52.
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