that is strange, I was just happy that I could set the offset with one pot and the quiescent current with the other
I'm not sure about capacitor coupling since I can set the offset between 2V!!!! and 0mV but it's not so stable.
I'll try to attach a photo with my amp, I've seen there are a lot of versions of this amp.
Can anyone help me with a service manual sent to my username at yahoo ? com
I'm not sure about capacitor coupling since I can set the offset between 2V!!!! and 0mV but it's not so stable.
I'll try to attach a photo with my amp, I've seen there are a lot of versions of this amp.
Can anyone help me with a service manual sent to my username at yahoo ? com
here it is:
An externally hosted image should be here but it was not working when we last tested it.
Hi ,
i'll write in English so our readers understand. I have been reading your post with great interest since i'm the proud owner of two Sugden, one is an A28 original with cooler on back, and the second is an A28 II with the cooler radiator on back ,which has turned me down.i have repaired them, but not plugged it in yet(i will let you know in my next post).
Could you send me a pdf version of the circuit diagram of the two versions of this two ampli.
Many thanks,
Bruno.
Sorry for my translation
i'll write in English so our readers understand. I have been reading your post with great interest since i'm the proud owner of two Sugden, one is an A28 original with cooler on back, and the second is an A28 II with the cooler radiator on back ,which has turned me down.i have repaired them, but not plugged it in yet(i will let you know in my next post).
Could you send me a pdf version of the circuit diagram of the two versions of this two ampli.
Many thanks,
Bruno.
Sorry for my translation
Hi Michel,
thanks for your reponse and sorry for my mail:
broonob@yahoo.fr
early I have this schematics, I try to revive this two A28.
Can you show me where I connect my multimetr in this two different A28 and A28 II to set the bias?
Thanks for all
thanks for your reponse and sorry for my mail:
broonob@yahoo.fr
early I have this schematics, I try to revive this two A28.
Can you show me where I connect my multimetr in this two different A28 and A28 II to set the bias?
Thanks for all
fuzzy a28
Michel,
Many thanks for sending the schematics for the A28 which are the right ones for my sugden (ie not the a28II or A28B)
I have done (finally!) as you suggested and have replaced the three BC639 (Q12, Q13 & Q20 in the schematic) and one BC640 (Q15) or each channel. Alas the fuzziness is no different.
Where next might you suggest I look to solve the problem? Is there a way of testing the TIP35 & TIP36 transistors? Altering the voltage mid point and bias had no discernible effect on the fuzzing.
BTW I have stitched together the 4 x 2 scanned documents you sent me into one for the components and one for the schematic. Happy to send you copies but they are 5MB and 3MB pdfs respectively.
Cheers,
Simple Simon.
Michel,
Many thanks for sending the schematics for the A28 which are the right ones for my sugden (ie not the a28II or A28B)
I have done (finally!) as you suggested and have replaced the three BC639 (Q12, Q13 & Q20 in the schematic) and one BC640 (Q15) or each channel. Alas the fuzziness is no different.
Where next might you suggest I look to solve the problem? Is there a way of testing the TIP35 & TIP36 transistors? Altering the voltage mid point and bias had no discernible effect on the fuzzing.
BTW I have stitched together the 4 x 2 scanned documents you sent me into one for the components and one for the schematic. Happy to send you copies but they are 5MB and 3MB pdfs respectively.
Cheers,
Simple Simon.
Simple Simon,
Sorry go hear your amp is still fuzzing.
Is this noise permanent or intermittent?
Is it on one or both channels?
If on both, is the noise the same?
Has the bias been adjusted correctly?
When the bias is too low you can enter in class B and this way hear crossover distortion, which sounds like fuzzing.
Like I said before, what about the capacitors?
All the switches and relays too can generate some noise when they age.
This is especially true if their contacts are made of silver.
After some time oxide develops on the surface of silver plated contacts and you get a kind of diode that does some signal rectification.
Sugden used more than often Shadow switches which were extremely prone to that kind of problems. The only way to clean them properly is to remove the latch so you can completely remove the moving contacts from the switch. This way I could clean the inside with a small rotary brush that I had previously saturated with Caig DeoxIT D100L.
You have to be extremely careful when doing that as the contacts from the removed section are spring loaded and can easily fly away... Even after all that trouble some switches need to be replaced. The only permanent solution to all those switches problems is to replace them with ones that have gold plated contacts. A couple of years ago I found a C&K model F2UEEAU that fits the bill.
The Alps rotary switch that was used was no better that the shadow switches and suffered the same problems. They have been unavailable for a long time and the only way to replace it is to use a Lorlin and attach wires to it. Then again you will end up having to clean the Lorlins too as they too have silver plated contacts. They do exist with gold plated contacts by I have been unable to buy some as the minimum quantity required to place an order was too high for my needs.
The same goes for the speaker switches that Sugden used in the back of their amplifiers, especially the rocker ones.
As you can see, to keep the switches in shape you have to do a lot of preventive maintenance. It is not a bad idea to clean them all at least once a year to prevent deterioration.
I do not recall having ever seen a power transistor with a noise problem. They usually work, or not.
My email box can hold up to 10 MB so you should not have any surprise when you send your files. Thanks.
I have no more ideas at this moment regarding the source of the noise.
I hope I have given you some paths you can explore and hopefully put your finger on the problem.
If you do, please post a reply as it can help others stuck with the same problem.
Good luck!
Michel
Sorry go hear your amp is still fuzzing.
Is this noise permanent or intermittent?
Is it on one or both channels?
If on both, is the noise the same?
Has the bias been adjusted correctly?
When the bias is too low you can enter in class B and this way hear crossover distortion, which sounds like fuzzing.
Like I said before, what about the capacitors?
All the switches and relays too can generate some noise when they age.
This is especially true if their contacts are made of silver.
After some time oxide develops on the surface of silver plated contacts and you get a kind of diode that does some signal rectification.
Sugden used more than often Shadow switches which were extremely prone to that kind of problems. The only way to clean them properly is to remove the latch so you can completely remove the moving contacts from the switch. This way I could clean the inside with a small rotary brush that I had previously saturated with Caig DeoxIT D100L.
You have to be extremely careful when doing that as the contacts from the removed section are spring loaded and can easily fly away... Even after all that trouble some switches need to be replaced. The only permanent solution to all those switches problems is to replace them with ones that have gold plated contacts. A couple of years ago I found a C&K model F2UEEAU that fits the bill.
The Alps rotary switch that was used was no better that the shadow switches and suffered the same problems. They have been unavailable for a long time and the only way to replace it is to use a Lorlin and attach wires to it. Then again you will end up having to clean the Lorlins too as they too have silver plated contacts. They do exist with gold plated contacts by I have been unable to buy some as the minimum quantity required to place an order was too high for my needs.
The same goes for the speaker switches that Sugden used in the back of their amplifiers, especially the rocker ones.
As you can see, to keep the switches in shape you have to do a lot of preventive maintenance. It is not a bad idea to clean them all at least once a year to prevent deterioration.
I do not recall having ever seen a power transistor with a noise problem. They usually work, or not.
My email box can hold up to 10 MB so you should not have any surprise when you send your files. Thanks.
I have no more ideas at this moment regarding the source of the noise.
I hope I have given you some paths you can explore and hopefully put your finger on the problem.
If you do, please post a reply as it can help others stuck with the same problem.
Good luck!
Michel
Michel,
Thank you for your rapid reply. To answer your questions:
The noise is permanent.
The distortion is on the right channel but the left does not sound right but cant put my finger on it.
The RC distortion is not noticeable at very low volumes but rapidly increases with volume
I don't think it is bias/class B as a) I think I set the bias correctly as per earlier posts and b) does not crossover distortion reduce/less noticeable with volume?
I have yet to start testing capacitors as that is a rather daunting prospect. There are so many. Which ones to start with? Is the only way to desolder, charge up and time the voltage drop through a resistor? Not looking forward to that.
Re switches, I'm presuming you mean the source selector buttons on the front. Presumably a quick diagnostic would be to by-pass the switches with some wire and clips. Or is it the power/speaker relay switch?
I'll mail the collated diagrams.
many thanks for your help.
Simple Simon.
Hi,
well designed switch contacts, meet and slide (wipe) as they reach the final closed geometry.
This gentle wiping action keeps the silver (or any other metal) clean and needs to be operated fairly regularly to maintain the goodness of the contacts. Unfortunately this action eventually wears out the contacts, particularly if spark erosion is occurring due to switching off live signals.
Maintenance to help increase the life would amount to turning off the equipment, injecting a shot of switch cleaner (if the switch is sealed type then the cleaner cannot be injected) and operating the switch repeatedly (3 or 4 times), repeat this 2 or 3 times a year.
Give adequate time for the solvents to evaporate and disperse before you plug the equipment back into the mains.
I would suggest you look very carefully at all your electrolytic capacitors. Can you identify any that are bulging, or any that run warm, or any that are exuding chemicals?
Replace these as a matter of course. If you have some 105degC caps they are less likely to deteriorate, concentrate on the 85degC caps.
well designed switch contacts, meet and slide (wipe) as they reach the final closed geometry.
This gentle wiping action keeps the silver (or any other metal) clean and needs to be operated fairly regularly to maintain the goodness of the contacts. Unfortunately this action eventually wears out the contacts, particularly if spark erosion is occurring due to switching off live signals.
Maintenance to help increase the life would amount to turning off the equipment, injecting a shot of switch cleaner (if the switch is sealed type then the cleaner cannot be injected) and operating the switch repeatedly (3 or 4 times), repeat this 2 or 3 times a year.
Give adequate time for the solvents to evaporate and disperse before you plug the equipment back into the mains.
I would suggest you look very carefully at all your electrolytic capacitors. Can you identify any that are bulging, or any that run warm, or any that are exuding chemicals?
Replace these as a matter of course. If you have some 105degC caps they are less likely to deteriorate, concentrate on the 85degC caps.
Last edited:
Michel (or Andrew T even)
The diagrams that you kindly sent me (a28 not a28II or 28B) have lots of detailed voltages at various points. However my supply rail is 72V not the 57V on the schematics. Is there a nifty way to convert the voltages on the schematics to account for the different supply? What I had in mind was the schematics have the "mid" point set by VR3 at 29V (=57/2 +0.5). Could I subtract this from most of the other voltages and then add on my new mid point (72/2 +0.5) = 36.5V? I'm not expecting this to work for all noted voltages but say for Q9 to Q17?
Could you also tell me what the op-amp (IC 1) in the middle does? Is this likely to be a problem? Also just for interest what do Q10 & Q11 do? They are not in the signal path are they?
Many thanks,
Simon.
The diagrams that you kindly sent me (a28 not a28II or 28B) have lots of detailed voltages at various points. However my supply rail is 72V not the 57V on the schematics. Is there a nifty way to convert the voltages on the schematics to account for the different supply? What I had in mind was the schematics have the "mid" point set by VR3 at 29V (=57/2 +0.5). Could I subtract this from most of the other voltages and then add on my new mid point (72/2 +0.5) = 36.5V? I'm not expecting this to work for all noted voltages but say for Q9 to Q17?
Could you also tell me what the op-amp (IC 1) in the middle does? Is this likely to be a problem? Also just for interest what do Q10 & Q11 do? They are not in the signal path are they?
Many thanks,
Simon.
ZTX341 replacement
It seems that the Q10 transistors have gone as the BCE voltages are (0.0, 0.7, 0.0) and from Michel's figures should be (0.7, 28.5, 0.0). They are specified as ZTX341 though these seems to be no longer available. Any suggested alternatives? RS-uk has ZTX653 but these seem to be over specified (2A!) compared to the original (100mA) and a bit expensive.
Simon.
It seems that the Q10 transistors have gone as the BCE voltages are (0.0, 0.7, 0.0) and from Michel's figures should be (0.7, 28.5, 0.0). They are specified as ZTX341 though these seems to be no longer available. Any suggested alternatives? RS-uk has ZTX653 but these seem to be over specified (2A!) compared to the original (100mA) and a bit expensive.
Simon.
If the value of VCB/VCE from 100V isn't necessary, follow replacement type can be use:
http://www.datasheetcatalog.org/datasheet/philips/BC546.pdf
here the ZTX341 datasheet:
http://www.datasheetcatalog.org/datasheet/zetexsemiconductors/ztx341.pdf
Otherwise you must try to find a replacement type from the 2SC/2SD series or ask by "Diodes", the currently name of Ferranti/Zetex
== Diodes, Inc. ==
ZTX341 replacement
Many thanks tiefbassuebertr
I'll have a look. The Sugden I have has several BC548, and presumably BC546 were around at the same time so it makes me wonder if there was a reason BC546 were not used. My supply rail is 72V.
Hunting around RS-uk I found these.
2N5550 Datasheet pdf - SILICON GENERAL PURPOSE HIGH VOLTAGE TRANSISTORS - Micro Electronics
Any good do you think?
Simon.
Many thanks tiefbassuebertr
I'll have a look. The Sugden I have has several BC548, and presumably BC546 were around at the same time so it makes me wonder if there was a reason BC546 were not used. My supply rail is 72V.
Hunting around RS-uk I found these.
2N5550 Datasheet pdf - SILICON GENERAL PURPOSE HIGH VOLTAGE TRANSISTORS - Micro Electronics
Any good do you think?
Simon.
Hello everybody,
I hope you don't mind, but I also have a A28 which seems to have a similar (related?) problem:
there is a quite loud buzz (more like "trrrrrrrrrrrrrr") emanating from the power supply. It doesn't come through the speakers as it exists without anything plugged into the amplifier. It is also clearly audible even with music playing at normal levels.
The screws from the transformer seem tightly bolted. However, I don't hear the relay engaging when I power the amplifier. It looks like the relay is always "on". Could the buzz come from the relay?
Thanks for any help!
François
I hope you don't mind, but I also have a A28 which seems to have a similar (related?) problem:
there is a quite loud buzz (more like "trrrrrrrrrrrrrr") emanating from the power supply. It doesn't come through the speakers as it exists without anything plugged into the amplifier. It is also clearly audible even with music playing at normal levels.
The screws from the transformer seem tightly bolted. However, I don't hear the relay engaging when I power the amplifier. It looks like the relay is always "on". Could the buzz come from the relay?
Thanks for any help!
François