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Old 18th February 2013, 12:21 PM   #141
korben69 is offline korben69  France
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Join Date: Nov 2006
Location: Lyon
Hi

I've done some investigations and finally found a faulty cap, the one within red circle.
When I touch it while playing the hum appears, so I think it needs to be replaced.
I haven't check his solder pads, but replacing after more than 30 years seems to be a good idea.
Do you have some refs to suggest me ?
The original value is 4700uF/80V & the other upper pair is 10000uF/35V rated.

Regards

Phil
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Old 18th February 2013, 08:32 PM   #142
Michel is offline Michel  Canada
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Phil,

I find it highly doubtful a capacitor of this type could create the problem you are describing. A bad solder joint is more likely to be the cause.
Replacing the capacitors would definitely be a good idea since it looks like none of them have ever been replaced. I usually use ECO-S2AP472DA and ECO-SH1A103DA for the large ones. Do not forget the small ones as they need attention too. Look at the capacitor between the two right side blue trimmers. The external jacket shrunk because it has been exposed to too much heat. I am pretty sure the other capacitor beside the relay is compromised too has they usually are the first one's to die. Be sure to verify the one just between the twisted brown and red wire, my memory tells me this one too should be replaced. All the other ones should not be in too bad a shape but if you have the time and patience, you should verify them all. If you have a constant low level hum, it is a sign that some of them lost much of their capacity.

Regards,

Michel
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Old 19th February 2013, 03:56 AM   #143
DennyG is offline DennyG  Australia
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Default Replacing caps

Hi Phil,

If you decide to replace the caps on the A28, as I did with great results for sound quality , then the pdf's listed below may help (the information in italics was copied from another thread). Note that the Elna replacement caps were generally larger than the originals but they all fit within the cabinet. I would not have purchased the Nichicons as they measured significantly lower than nominal. Use those recommended by Michel.

The link to the other thread is given in a post of mine earlier in this thread.

The cap list for the replacements I used in the Sugden A28 is is available here:
home.exetel.com.au/justlucky/SugdenA28/Sugden_A28_Cap_replacement_list.pdf
Also indicated are the voltage ratings and dimensions for the original caps along with some of the markings.

There is no indication in the list as to the cap function. If you need that information I loaded the circuit diagram and pcb layout to here:
home.exetel.com.au/justlucky/SugdenA28/Sugden_A28_circuit.pdf
home.exetel.com.au/justlucky/SugdenA28/Sugden_A28_PCB_layout.pdf

These were kindly made available by Michel as gif's and I've turned them into PDF's.
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Old 19th February 2013, 12:03 PM   #144
korben69 is offline korben69  France
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Hi Michel & DennyG

Your'e absolutely right Michel : bad solder join confirmed, trouble solved, no more hum.
As shown on first pic, one 4700uF/80V was concerned, checked the others big cans : Ok.
Then checked DC output and noticed it to be low enough to fire the amp.
Works fine BUT only on Left chanel, Right one is mute or plays at very very very low level.
More to investigate... but where to start from ?

I like Pana TSUP series, good quality caps at budget price.
Michel stated 100V vs 80V, sounds to be a wise advice.
I'll replace all the caps, more than 30 years of duty is a very good lifecycle

Second pic shows the relay cap to be replaced.
This one is 10uF/100V rated. Unusual value to me.

Thanks DennyG for your informations, great work done
I'll have a carefull look at your caps cookbook before my next order !

Many thanks for your attention, share, advices and kindness.
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Last edited by korben69; 19th February 2013 at 12:07 PM.
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Old 19th February 2013, 01:06 PM   #145
Michel is offline Michel  Canada
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Phil,

Glad to hear you found your bad solder joint and that the hum is gone.

Now for the low level signal on the right channel, the first things I suggest you do are:

Try playing the amp at high volume to see if it changes anything
Try tapping on the relay while the music is playing
Cycle repeatedly the speaker/headphone switch (if there is one)
Cycle repeatedly a 1/4" jack in the headphone plug (if no speaker/headphone switch)
Cycle repeatedly the front panel switches

All those tests are much easier to perform with a high frequency tone (±5 kHz) while an oscilloscope monitors the output signals of the amplifier, if you have access to this equipment.

Let me know the results and if unsuccessful we'll advance from there.

Michel
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Old 23rd February 2013, 04:12 PM   #146
korben69 is offline korben69  France
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Hi Michel

Right Level signal fixed : bad solder on speaker connector...
I've to check all the front/rear panel connexions.

No more hum, the amp sounds dead quiet
I've replaced the guenuine "poor" power cord and added an IEC socket.
First pic : left/middle = old wires - right = new copper wire
Second : IEC replaced, fits fine.
Third : detail of IEC with X2/Y2 caps and Earth coil.

I've found a freeware for frequency testing : Logiciel Tone Generator - Générateur de tonalités de test et de fréquences sonores

I've unbolted and cleaned power transistor/pads and replace thermal paste (both sides, artic silver 5).
I've noticed before this operation that right side of the haetsink was warm, bad contact.
Now temp is correct for all the transistor on both heatsinks.

More to come with caps replacement, hope it will be soon.
Regards

Phil
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Last edited by korben69; 23rd February 2013 at 04:37 PM.
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Old 26th February 2013, 07:26 AM   #147
egelundvej25 is offline egelundvej25  Denmark
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Hi Gentleman.

I just got me an old A48iii - that has some problems. Left channel sounds really bad when turning up the volume above 9 o’clock and right channel blows the fuse. I was trying to adjust the bias and midpoint voltage for left channel – but became unsure – where do you measure the midpoint voltage? And what is the reference?

Regards, Michael.
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Old 26th February 2013, 04:05 PM   #148
Michel is offline Michel  Canada
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Hi Michael,

You should never try to adjust biases and midpoints on a defective amplifier.
This should only be done when you know the amplifier is working properly.

The bias should be set to read no more than 300mA through each fuse holders and should be readjusted over a period of an hour to let the circuit reach its operating temperature.

I never touch the midpoint controls but if you need to I guess the best way to do it is to input a 1kHz sine and monitor on an oscilloscope to make sure that when you raise the volume to maximum level, both crests are reached at the same place on both sides.

Does the right channel blows a fuse when you raise the volume or when you power on the unit?

Check the emitter resistors on the left channel.

What colour are the transistor insulators?
Transparent or milky white?

I will wait for your answers and advance from there.

Regards,

Michel
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Old 26th February 2013, 06:12 PM   #149
egelundvej25 is offline egelundvej25  Denmark
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Hi Michel.

I have taking a closer look:
Right channel - power transistor is shorted between B & C (the fuse blows at power on)
Left channel - bad solder on one of the power transistors.

I have ordred new power transistors - BD693 & 640 and the big capacitors- I will let you know how it turns out.

Thanks for your help so fare - Michael.
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Old 26th February 2013, 06:30 PM   #150
Michel is offline Michel  Canada
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Michael,

The power transistors are not BD693 and BD640, they are 2SB776 and 2SD896.
If you are talking about the drivers, they are BC639 and BC640.
What about the insulators?
I am asking that because you can replace all the transistors in the world but, if you have the wrong insulators, you will have to do it until the end of times...

Michel
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