Bush DAC90A problems.

[ms660]

Originally Posted by cernem1alt

On the DAC90A T1 transformer which terminals are the primary and which the secondary?

The primary winding connections are the two tags that the capacitor is connected across, the secondary winding connections which should connect to the loudspeaker are the other two tags.

For future reference for sound output transformers the secondary winding for the loudspeaker will read very low resistance when measured on a test meter, usually below 1 Ohm, the primary winding will measure several hundred Ohms, that's assuming both windings are intact.

EDIT: Post crossed.

Lawrence.

[Boater Sam]

If you can see the end of the winding where they join to the tags, the thicker wires are the secondary to the speaker.

[cernem1alt]

Thank you all

[cernem1alt]

This Bush DAC90A was in a bad way damp, dropped and possibly fiddled with
T1 O.P trans past redemption OT, .C22 Mains RF end blown off, set dead obviously

Bush DAC90A work done: new N35A002F OP trans wired 1-4 primary D-B secondary, new bulbs all new caps., lamp limiter test looks good. Switch on MW no problems but strong station 5 live big hum on audio knocks audio out. Talksport perfect, LW band nothing just big hum on light program where Radio 4 is.

Having said that I am enjoying the problem solving it has been a great learning curve with a bit more to do so any ideas welcome my guess is either around the AGC or valves,

Any ideas most welcome.

thanks Mike M1ALT

[Boater Sam]

Is it hum or interference? If it is "tunable" then look for something creating interference in your locality, wall warts, computers, set top boxes, TVs etc.

If it is mains hum will be there if there is no aerial signal and all across both the bands. Turn the radio round till you find a null for the frame aerial, try standing it on end for a while, is it still humming? Does the volume control change the hum?

[ms660]

The description sounds like modulation hum.

Lawrence.

[cernem1alt]

It's set borne as I have my other DAC90A working fine in the same local seems to be on very strong signals and all of LW band

[ukcol]

You state that C22 had the end blown off and that you replaced ALL capacitors.
Make sure you have replaced C22 with the correct value (0.1uF) and it is fitted correctly. Use a class X type for safety.

If C22 is low value, open circuit or not fitted you are likely to get the modulation hum mentioned by Lawrence above.

Also check the frame aerial as if there is a problem there you could be picking more signal up from the mains wiring than from the aerial - this too can cause modulation hum.

[PaulR]

The set will originally have had a capacitor across the mains (C22 on the Trader sheet) to reduce mains borne interference. This has usually either destroyed itself or been removed. If the hum appears when a station is tuned try replacing it with a 0.01uf CLASS X capacitor. Make sure it is Class X as these are specifically intended to be used across the mains. The usual advice is to remove this capacitor and not replace it, but I have found that it is useful in reducing modulation hum.

Edit - same thoughts as Colin!

[cernem1alt]

C22, I replaced with a 0.01uf CLASS X first off, I forgot to mention above that chassis is neutral this is a late gold dial DAC90A it looks like that was done at the factory. However poke around the loop aerial green and yellow wire eaten away replaced and a squirt in strategic places with servisol (other switch cleaners are available) and Radio 4 LW no problems 5 live MW no problems, it could be solved.

[crusher19860138]

It could have been the contacts on the wavechange switch that you have cleaned up, also changing C22 has helped, and replacing the aerial cable, well done!

[cernem1alt]

The old wax C24 0.01uF Mains RF by pass capacitor was soldered to the mains pin (not the pin to switch) and then to the chassis was this correct? I don't think so, but I need some clarification.

Thanks Mike

[Station X]

Several threads relating to the same radio merged.

[ms660]

Originally Posted by cernem1alt

The old wax C24 0.01uF Mains RF by pass capacitor was soldered to the mains pin (not the pin to switch) and then to the chassis was this correct? I don't think so, but I need some clarification.

Thanks Mike

I've only got Trader sheet copies, according to them it's connected to the mains input pin and chassis in the DAC90 but not in the DAC90a.

Lawrence.

[ms660]

Thread titles...merged...same radio...I must be losing the plot....I thought the DAC90 and the DAC90A were different radios.

My apologies and please excuse me if I've got that wrong.

Lawrence.

[cernem1alt]

My fault I posted to many separate questions about two different radios and quite rightly so the moderator tidied things up. However they do tie in that the C24 0.01uF Mains RF by pass capacitor I am dealing with in my DAC90 was wired to the mains input pin and chassis but this is not the case in the DAC90a I just checked, so given the DAC90 is a different radio I presume that wiring to be correct it seems so now I have downloaded the service diagram, in a way I have answered my own question.

[ms660]

Just so you're aware, with C24 connected as per the Trader sheet and the mains neutral connected to the pin that connects to the on/off switch and the mains live to the other pin, the chassis will be at live potential with respect to earth when switched off at the receiver if the receivers mains cable is still connected to the mains, in that case the maximum current via C24 that can flow to earth will be approx. 7.5mA.

Lawrence.