Because of hum I need to replace this toroidal transformer:
btw its a MK3 model
on the transformer it says:
18/92
nuvotem talema etc
9276-p1s2
talema said its a custom transformer for cambridge systems... no response from audiolab
Theres a schematic here but its basically gibberish to me:
http://www.diyaudio.com/forums/attachment.php?s=&postid=651296&stamp=1117220989
I dont know much about electronics so no idea what to replace it with.
I have not been able to eleminate hum (vibrational) moving it or tightening the bolt.
thanks
btw its a MK3 model
on the transformer it says:
18/92
nuvotem talema etc
9276-p1s2
talema said its a custom transformer for cambridge systems... no response from audiolab
Theres a schematic here but its basically gibberish to me:
http://www.diyaudio.com/forums/attachment.php?s=&postid=651296&stamp=1117220989
I dont know much about electronics so no idea what to replace it with.
I have not been able to eleminate hum (vibrational) moving it or tightening the bolt.
thanks
Hi,
it appears that the supply is a single voltage for the whole power amp. You must check to see if the pre-amp stages are separately powered from other windings in the transformer of if they draw their power from the power amp stage and are regulated down to a suitable DC voltage.
+-44 Vdc comes from about 31 or 32Vac.
But, that was in the old days of 240Vac transformers.
You may find that the replacement is rated for 230Vac and a 230:30Vac transformer will produce 31.4Vac from a 240Vac supply. Just lucky it happens to be a standard value, easily obtained.
Anyone able to confirm?
As an aside,
mechanical hum can be produced when the transformer is fed with DC from the AC supply. Does the noise go away at some times of the day?
I think there is insufficient room inside the case to add a DC blocking circuit to feed cleaner AC to the transformer.
it appears that the supply is a single voltage for the whole power amp. You must check to see if the pre-amp stages are separately powered from other windings in the transformer of if they draw their power from the power amp stage and are regulated down to a suitable DC voltage.
+-44 Vdc comes from about 31 or 32Vac.
But, that was in the old days of 240Vac transformers.
You may find that the replacement is rated for 230Vac and a 230:30Vac transformer will produce 31.4Vac from a 240Vac supply. Just lucky it happens to be a standard value, easily obtained.
Anyone able to confirm?
As an aside,
mechanical hum can be produced when the transformer is fed with DC from the AC supply. Does the noise go away at some times of the day?
I think there is insufficient room inside the case to add a DC blocking circuit to feed cleaner AC to the transformer.
Hi,
All I can tell you is theres 6 wires to the transformer:
2 from the switch
a black and a yellow which dissappear into the circuitboard
a red and orange which are connected at the same point to the 2 big cap's.
Havn't noticed the hum go at any time in the day - this amp has been hidden away so I can't hear it
. Used to hum at my old house as well.
Thanks for your help.
All I can tell you is theres 6 wires to the transformer:
2 from the switch
a black and a yellow which dissappear into the circuitboard
a red and orange which are connected at the same point to the 2 big cap's.
Havn't noticed the hum go at any time in the day - this amp has been hidden away so I can't hear it
. Used to hum at my old house as well.Thanks for your help.
Hi,
the six wires gives a good indication.
Let's just check to be sure.
The two wires from the switch sound like the live and neutral feeding the primary. Can you confirm?
The four wires appear to be a dual secondary.
Two wires connected indicate that the secondaries are converted to centre tapped.
That leaves two wires feeding the rectifiers/diodes. Can you confirm that two diodes are connected to each wire where they meet the circuit board. The two diodes from each wire should then have a pair of common points and these common points should connect to the two other pins of the smoothing caps. Can you confirm?
If all this checks out then you have a dual secondary transformer.
Easy to buy a replacement.
Do you have a multimeter? or borrow one? or splash out and buy a cheap one?
the six wires gives a good indication.
Let's just check to be sure.
The two wires from the switch sound like the live and neutral feeding the primary. Can you confirm?
The four wires appear to be a dual secondary.
Two wires connected indicate that the secondaries are converted to centre tapped.
That leaves two wires feeding the rectifiers/diodes. Can you confirm that two diodes are connected to each wire where they meet the circuit board. The two diodes from each wire should then have a pair of common points and these common points should connect to the two other pins of the smoothing caps. Can you confirm?
If all this checks out then you have a dual secondary transformer.
Easy to buy a replacement.
Do you have a multimeter? or borrow one? or splash out and buy a cheap one?
Yes
The two wires connected to the pcb run into a tall black thing marked D901, I expect this is a diode.. a duel one perhaps, as it has two exit points as well, which are connected like u said to the remaining points on the caps.
Additionally - they connect through a grey thing marked C901... about 1cm tall.
I do have a multimeter.
Thanks again.
Hi,
there's something wrong with the measurements compared to the schematic.
The amp produces about 60W into 8r. That needs 31Vpk at the output.
The schematic shows +-44Vdc (=88Vdc) with just quiescent current flowing. These numbers are believable.
Your 69Vac should give 96Vdc. Which is far too high.
Your 91.4Vdc disagrees with both the lower schematic voltages and the higher AC voltage. I do not understand, unless the mains is a bit high at the time of measurement (mains @ +3.8% gives input = 249Vac resulting in 91.4Vdc).
Can you remeasure without shorting the caps? and confirm the mains voltage CAREFULLY.
there's something wrong with the measurements compared to the schematic.
The amp produces about 60W into 8r. That needs 31Vpk at the output.
The schematic shows +-44Vdc (=88Vdc) with just quiescent current flowing. These numbers are believable.
Your 69Vac should give 96Vdc. Which is far too high.
Your 91.4Vdc disagrees with both the lower schematic voltages and the higher AC voltage. I do not understand, unless the mains is a bit high at the time of measurement (mains @ +3.8% gives input = 249Vac resulting in 91.4Vdc).
Can you remeasure without shorting the caps? and confirm the mains voltage CAREFULLY.
mains measured 235v
Sorry but the schematic was just from the forums here, posted on assumption that the diff between mk2 and mk3 models wouldn't be of any significance.. perhaps I'm wrong. It was said not to be a mk1 schematic. So I've gone and measured both amps:
MK3 68.7 / 90.5
MK2 63.5 / 84.8 (manual says 50w for this guy)
I hope those numbers make more sense :\
Sorry but the schematic was just from the forums here, posted on assumption that the diff between mk2 and mk3 models wouldn't be of any significance.. perhaps I'm wrong. It was said not to be a mk1 schematic. So I've gone and measured both amps:
MK3 68.7 / 90.5
MK2 63.5 / 84.8 (manual says 50w for this guy)
I hope those numbers make more sense :\
Hi,
I still can't make sense of the measurements.
I'm guessing that distortion of the mains incoming waveform is confusing the AC measurement and giving false readings.
However, I have a proposal.
A 230:30-0-30Vac transformer will give 86 to 87Vdc from 230Vac
or 88 to 89Vdc from 235Vac
or 90 to 91Vdc from 240Vac.
These are in the ballpark for your DC measurements and also close to the schematic of 88Vdc.
Try to get any one of the following that fits the space in the case.
200VA for 8ohm speakers or 350VA to 400VA for 4ohm speakers.
The bigger option may not fit, so try for a 300VA to fit.
230 to 30-0 & 30-0
or
230 to 30-0-30
or
115+115 to 30-0-30
or
115+115 to 30-0 & 30-0
If you are offered an electrostatic screen and/or guass band at a sensible price then take the offer. Otherwise standard is OK.
I think your Mk3 original is a 240:32Vac but you are very unlikely to find this off the shelf. The Mk2 may be 240:30Vac
I still can't make sense of the measurements.
I'm guessing that distortion of the mains incoming waveform is confusing the AC measurement and giving false readings.
However, I have a proposal.
A 230:30-0-30Vac transformer will give 86 to 87Vdc from 230Vac
or 88 to 89Vdc from 235Vac
or 90 to 91Vdc from 240Vac.
These are in the ballpark for your DC measurements and also close to the schematic of 88Vdc.
Try to get any one of the following that fits the space in the case.
200VA for 8ohm speakers or 350VA to 400VA for 4ohm speakers.
The bigger option may not fit, so try for a 300VA to fit.
230 to 30-0 & 30-0
or
230 to 30-0-30
or
115+115 to 30-0-30
or
115+115 to 30-0 & 30-0
If you are offered an electrostatic screen and/or guass band at a sensible price then take the offer. Otherwise standard is OK.
I think your Mk3 original is a 240:32Vac but you are very unlikely to find this off the shelf. The Mk2 may be 240:30Vac
try to confirm the problem is definitely the transformer.
Unbolt it and temporarily sit it on a something to isolate vibrations from the case, say a towel folded 'till it's a couple of inches thick. I'd expect it to be noticeably quieter since normally the case will probably be acting like the soundboard of a guitar and mechanically amplifying the sound.
A full check would involve putting a few feet extension cables on all the wires to the transformer and placing it well away from the case so it will be obvious which the sound was coming from.
Assuming it is the transformer, rather than go through all the cost and hassle of trying to replace a custom part I would try to fix it.
Could you impregnate it with 'Transformer Varnish' for example, or get a transformer maker to do it for you?
BB
Unbolt it and temporarily sit it on a something to isolate vibrations from the case, say a towel folded 'till it's a couple of inches thick. I'd expect it to be noticeably quieter since normally the case will probably be acting like the soundboard of a guitar and mechanically amplifying the sound.
A full check would involve putting a few feet extension cables on all the wires to the transformer and placing it well away from the case so it will be obvious which the sound was coming from.
Assuming it is the transformer, rather than go through all the cost and hassle of trying to replace a custom part I would try to fix it.
Could you impregnate it with 'Transformer Varnish' for example, or get a transformer maker to do it for you?
BB
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.