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.
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.
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
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
Hello Michel:
would you mind to send the schematics to me ?
Thank you in advance!
Paco Mok
mc_wilson@21cn.com
would you mind to send the schematics to me ?
Thank you in advance!
Paco Mok
mc_wilson@21cn.com
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Hello Michel!
Would you mind to send the schematics to me ?
Thank you in advance!
AndreyD
gerasim46@yandex.ru
Would you mind to send the schematics to me ?
Thank you in advance!
AndreyD
gerasim46@yandex.ru
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
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
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
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
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