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Old 29th March 2011, 10:14 AM   #101
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Quote:
Originally Posted by simple simon View Post
I have found a schematic for a Sugden a48 circa 1986 and these show 2N5550 instead of the ZTX341 so it seems these are ok.

Simon.
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Old 30th March 2011, 03:07 PM   #102
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Originally Posted by simple simon View Post
Michel,

Many thanks for sending the schematics for the A28 which are the right ones for my sugden (ie not the a28II or A28B)
Michel,

I think the component diagram for A28 has an error on it. I believe the collector and base of Q10 are the wrong way round.

As I posted before by Q10 voltages are B 0.7V, C 0.0V, E 0.0V in contrast to the schematic values of B 0.7V, C 28.5V, E 0.0V but I am wondering if a) this might also be an error with the Q10 labeling or b) a fault upstream/downstream of Q10.

Simon.
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Old 30th March 2011, 09:35 PM   #103
fcaton is offline fcaton  France
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Hello,

Just a quick word to say that Michel's advice on my "buzz" was spot on. The capacitor read 10nF instead of 10uF, even though it looked new.
I replaced the capacitor (C48) with the nearest value I had on hand (22uF 100V) and the buzz disappeared .

I'll now revamp the amp with new switches and such.

Merci Michel!

François
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Old 9th April 2011, 07:17 PM   #104
Wizard is offline Wizard  China
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Hello Michel:
would you mind to send the schematics to me ?
Thank you in advance!

Paco Mok

mc_wilson@21cn.com
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Old 9th April 2011, 08:17 PM   #105
tvi is offline tvi  Australia
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Wouldn't mind a copy of the shematic also, if its not to much trouble.
captsnooze-at-iinet-dot-net-dot-au


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Old 11th April 2011, 09:31 PM   #106
Michel is offline Michel  Canada
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Hello Wizard and tvi,

I am on the road until next Monday.
As soon as I am back I will send you the schematics you need.

Regards,

Michel
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Old 7th May 2011, 10:29 PM   #107
fcaton is offline fcaton  France
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Hi everybody,

Some news:
I replaced all capacitors on my A28 with a few surprises: several caps were way below the required values and two completely dead: C41 per Michel's schematics (470uF 6.3V)

I also replaced the input switches with the reference given by Michel earlier in the thread and they fit perfectly

The voltages measured at the mid point look OK: 31.5 and 31.7 Volts, so I didn't touch the pots. As for the quiescent current I read about 12 to 34 mV on resistors R80 R81 on both channels which is apparently way too low. Before doing something stupid, could someone confirm that I should trim the pots until I read 100mV on those R80 and R81 resisots?

Thanks for your help!

François
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Old 8th May 2011, 04:16 AM   #108
Michel is offline Michel  Canada
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Salut François,

Glad you could repair your A28!

I am sure you have been successful mainly because you took the time to check all the electrolytic capacitors. I suspect many do not reach success because they dismiss the possibility of capacitors to lose most of their nominal value or they find the task too daunting. Many suppose that if it looks good it should not be that bad. What a mistake.
You are once again proof that it is absolutely necessary to verify them all if you want to repair it correctly.

I wish I could tell you that these days I don't see companies put capacitors that are way too tight for the job they have to do but unfortunately I still see it done in many brands and all kinds of products. Electrolytic capacitors are the number one plague that affects so many electronic devices, from switching power supplies to compact fluorescent lights. Always be extremely suspicious of them, especially if they are from obscure brands.

Sorry François but it felt good to vent some steam. Back to your question regarding bias adjustment. I do not know what value it should be set at. Sugden usually sets bias values by measuring the current drawn by the amplifier. You can do that by connecting a current meter on the respective fuse holders of the channel you want to adjust. To do so you evidently need to remove the fuse prior to connecting your probes.
On the A28B that value is set at 325 mA but I would never go that high on the A28 as the heat sinks are much smaller. If my memory is good the heat sinks should feel warm but not too warm. I did so many A28 years ago that I knew just by touching the heat sinks if it needed adjustment or not. Be aware that it takes a long while for the bias to stabilize so don't be in a hurry. Another way to solve this problem would be to contact Sugden and you could get lucky!

Salutations,

Michel
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Old 9th May 2011, 09:14 AM   #109
fcaton is offline fcaton  France
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Hi Michel,

I very much agree with your as most capacitors that were dead were the ones too tightly specified. For example the two 6.3V where specified as 10V on another schematic I have (see attached document) ! I am suprised that there was any sound at all out of my unit to begin with...

Since I had to test them, i.e. unsoldered them, I decided to replace everything: all electrolytics with Panasonic FC 105°C with one step higher voltage (i.e. 35V->50V etc..). The only ones that I had to stick with for the voltages are the 4700uF. Also, since the previous owner had some film capacitors changed (some were the original greens, some red , some yellow, unlike pictures of original units), I changed all films with Wima MKP4s. I think I'll also change the BC639/BC640 with the BD139/BD140.
The total cost was about 100 euros (including the relays and shipping) + several hours preparing the order and three evenings of unsoldering/soldering. If anyone is interested, I can email the spreadsheet I made (it's for an A28).

BTW, thanks for the tip on bias adjustement, Michel, because measuring from the resistors, then adjusting, was not safe at all. The radiator of my unit is dead cold.

When all is finished, and after a bit of burn-in I'll give my impression on my new A28...

Cheers,

François
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Old 21st May 2011, 08:45 AM   #110
AntG is offline AntG  Netherlands
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Hi Michel
Great to read this forum of Sudgen A28II users. I have just un-dusted my A28II from the closet. I remember how good this amp sounded when I bought it new all those years ago. It looks like the volume pot might need repair/replacement (crackle and odd behaviour at low volumes) If you still have the schematics for the A28II I would greatly appreciate it.

Best regards
AntG
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