Toroid Transformer - faulty?

[onpiopah]

I am cutting my teeth on a Cambridge A1 Mk3. It works but only gives out a tiny amount of sound on both the channels.

The Toroidal transformer specs state '230v 120VAC 30vac x 2' yet when i place a meter across, both outputs purple/grey and red/yellow they only produce 21 ac volts.

Is that right

 

[davidsrsb]

Transformers fail by the primary going open, which then means zero out.

 

[onpiopah]

Thnaks for your help, would I be right in saying that if they are producing an equal voltage from both outputs they are more than likely working and it is only if there is no output then they are faulty.

 

[davidsrsb]

Yes, faults make them get very hot and then an internal thermal fuse blows open.
The A1 is 30W, so I would expect dc rails of about 30V. This means 21V ac is about right.
Is the tape switch stuck?

 

[onpiopah]

Excuse my nieveity, but if the transformer is only providing 21v how does this end up as 30V as the rail voltage?

 

[wintermute]

Hi onpiopah, from your initial question, if the transformer is rated at 30VAC output and it is only providing 21V ac then something is wrong.

davidsrsb is talking about rectified voltage. If you rectify 21V AC you multiply by 1.414 to get the rough DC volts, which comes out at around 29.6V You also need to factor in transformer regulation which would take it over 30V (the transformer provides a higher than rated voltage when it is not running at the full rated current).

In your case however it seems you are getting no where near the voltage you expect. Is the 21V ac you are seeing with the transformer secondaries loaded or unloaded?

Tony.

 

[Mooly]

Don't count on voltages printed in service manuals as being correct. They should be but can sometimes be in error.

If you are only getting very low sound then you need to look elsewhere. A scope would quickly check where things are going amiss. Does this amp use the notorious electronic input selector ? Can't just recall the IC number from memory but they are failure prone.

 

[davidsrsb]

The A1 service manual shows 28Vdc rails, about right for the rated power. It is based on the TDA1514 power amp chip, which has absolute maximum of +/-30Vdc

 

[onpiopah]

Thanks, I have read around ther 1.414 multiplier and understand why now.