Audiolab 8000a transformer replacement

[adi44]

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/attac...amp=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

[AndrewT]

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.

[adi44]

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.

[adi44]

I should probably add, i have 2 of these amps, the other (an mk2 model) doesn't hum at all so i put it down to dodgy transformer

[AndrewT]

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?

[adi44]

The two wires from the switch sound like the live and neutral feeding the primary. Can you confirm?

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.

[AndrewT]

Hi,
start measuring.

Set to AC V and measure the voltage across the two ~~ connections on the rectifier.

Set to DC V and measure the voltage across the + & - connections on the rectifier.

I wish I had asked for this earlier!

[adi44]

69 V across the ~ ~ (inside 2 contacts).
91.4 V across the DC contacts.

ps, I learnt not to try measure voltage across a capacitor in the process

[AndrewT]

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.

[adi44]

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 :\