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Sugden P28
Sugden P28
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Old 9th March 2009, 09:47 PM   #31
simple simon is offline simple simon  United Kingdom
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Michel,

Many thanks for responding. I had hoped you were still on the scene.

My heat sinks are at the back (well one big heat sink) and I have a black case with white lettering. I sent it in for repair once and all they replaced were two resistors which I thought was a bit odd. It did not last very long.

Thanks again,

Simple Simon.
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Old 13th March 2009, 12:42 AM   #32
chaoshusky is offline chaoshusky  United Kingdom
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Default Greetings

This seems like the right place to ask...

I have been given a Sugden A28 amplifier to look at/repair, but am not having much joy as of yet.

Do you happen to have the schematic for this? If it helps, it has a relay on the speaker output, and is also confusing me as one of the ground leads for a speaker output appears to be connected to the ground for the relay coil. I now have it switching on, and the 2.5A fuse on the circuit hasn't blown again, but there is no sound and the relay does not click at all. The other speaker output ground is connected through a big resistor. There is another resistor of the same size and value...but nothing is connected to this! Is that where the aforementioned ground for one of the speaker outputs should connect to? All this is under the AC input and rectifier diodes, right next to the headphone socket. Sorry if my description doesn't help much!

Someone has previously repaired this, and like the owner says, it looked like someone has tried to solder it with a hot poker! I've replaced the really bad dodgy re-wiring to fix broken traces so it's much neater with a shorter electrical path, one problem being on the balance pot.

Any help would be much appreciated. Please send the schematic to chaoshusky@gmail.com if you happen to still have it!

Kind regards,
Lee
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Old 14th March 2009, 03:10 AM   #33
chaoshusky is offline chaoshusky  United Kingdom
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Default Bump!

As i'm a new poster i cannot mail Michael with my question, and hope this makes the thread seeable again. Sorry if this annoys anyone! But i'm really quite stuck without the schematic that i can't seem to find anywhere!

Much thanks in advance,
Lee
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Old 16th March 2009, 12:53 PM   #34
chaoshusky is offline chaoshusky  United Kingdom
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Default Can nobody help me?

Please help! I've looked everywhere and i'm absolutely desperate now! Any help will be vastly appreciated, and i will always return favours.

Thanks and regards,
Lee
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Old 22nd March 2009, 08:31 PM   #35
Michel is offline Michel  Canada
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Hello Lee,

Here is my same question, once again!
A28, A28II (heatsink in the back) or A28B (heatsinks on the sides)?
You are requesting a schematic for an A28 but you mention the presence of a speaker relay!!!???
There is no speaker relay in an A28 or A28II, unless the unit has been modified...
Is it possible for you to send me some pictures of the inside of the unit?

Regards,

Michel
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Old 22nd March 2009, 08:59 PM   #36
destroyer X is offline destroyer X  Brazil
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Default BC639/640 are the same transistors as BD139/140


They have the same "chip" inside..difference is the case and the power dissipation, as the BC639/640 have small case and do not have the metal back that helps heat transference.

They are direct substitution and will work guaranteed, having some advantage to BD139/140 because increasing in heat dissipation, will work cooler and this will keep specifications more stable without thermal drift.

Of course i can be wrong, the informations i have is that they are absolutelly the same transistor.

regards,

Carlos
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Old 22nd March 2009, 09:02 PM   #37
mgm31 is offline mgm31  France
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So sorry, I forgot....

I think that's an old version (mkI ?) with radiator on the back.

Michel, I sent you a mail with hires pictures.

Forum is limiting the size of the picture, so I include only lowres picture here.

The relay is sitting just near the phones plug, I can hear it's click a few seconds after power on, I suppose it to be a mute, perhaps a protection relay.....

Thanks for the help

Thierry
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Old 23rd March 2009, 05:51 AM   #38
Michel is offline Michel  Canada
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Apologies Lee,

The post, with accompanying image, from mgm31 gave my memory, a needed jolt!
The A28 (heatsink in the back) does have a speaker relay.
I don't know how I could have forgotten that since there is a recurrent problem with this relay, or not exactly the relay...

If you look closely at the image, you see two twisted wires (red and brown) beside the power transformer that are going to the main pcb. Right between where the wires are soldered, there is a small electrolytic capacitor. If you look closely, you will see that this capacitor is not the same color as the other ones, or should I say, not the same color anymore!! Too much heat as that effect on capacitors external jackets. They usually get a darker "tan" and sometimes "grow" short pants! This particular one has waaay too much on its plate. It is used in the circuit that operates the speaker relay and when it is not working properly, this is the first place where you should look. Always replace it with a capacitor of the highest voltage rating possible, as space permits. 105º C is mandatory!

The specifications for the BC639/640 and BD139/140, as destroyer X writes, are indeed very similar. You could easily replace the two drivers and bias transistors of each channel with them. It is made even easier by the fact that they have the same pinout.

I usually never do that kind of switch as I have a lot of respect for the choice of parts the designers made to build their products. Especially if the part is in the signal path!! I usually request their approval to do it and I will only do it if I can no longer find the original part or in the case of a part that is breaking too frequently. You have to understand that I do that because I feel I am professionally bound to the manufacturers the company I work for represents. I believe that even a technician working in a repair shop should follow that "code" when repairing a customer's defective unit, but that's another story...

Best regards all,

Michel
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Old 23rd March 2009, 07:22 AM   #39
mgm31 is offline mgm31  France
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After a night rest, I still wonder why I have only positive voltage on power transistors (respective to ground taken on the negative speaker plug).

Does this amp don't have positive / negative branch ? Are the 4700uF/80V capacitors on the output signal path ?
Strange....
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Old 23rd March 2009, 05:50 PM   #40
Michel is offline Michel  Canada
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mgm31,

The A28, A28II and A28B are all coupling to their speaker through 10 mF capacitors (the 4.7 mF are used in the power supply section). The reason for that output capacitor is that the power supply used in the design is positive only. That is the reason why you do not find any negative voltages relative to ground.

Regards,

Michel
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