Odd DAC90A fault cured
 

Morning all,

The latest DAC90A on the bench was an ivory one bought from eBay.
It was described as working and indeed it was to some degree.

As it was filthy and had a damaged loudspeaker grille it was duly stripped down for a full clean and inspection.
This revealed that someone had carried out the odd capacitor replacement and had fitted a non Celestion loudspeaker.
I decided to treat it to the usual work, replacing all the waxed capacitors, doing the UL41 pin 4 mod, tuning gang grommets and any wildly out of spec resistors. Also, the crumbly wiring around the dropper section was replaced too.
As a matter of course with these, I remove and check the dial lamps both for continuity and rating as I have been caught out in the past with a flasher bulb being fitted to one holder on another set (that had me scratching my head a bit)
Anyway, all work done, power up time came and I was rewarded with dial lamps so dim that they barely lit the filaments. Weird I thought as I had already changed the shunt resistor for a 180ohm and with the standard spec lamps this results in a pleasantly warm glow from the scale. Voltage checks revealed hardly any volts (1 or 2 hundred milivolts at best) across the lamps so I set about rechecking all my work just in case and measuring heater chain voltages in case there was an unexpected high resistance or leak to chassis but all appeared well here with al voltages correct apart from V1 heaters which were quite low and the valve envelope only warm to the touch but the radio did actually work which was even more puzzling.
I had noted that the original dial lamps were 6.5volt ones at 300mA and there was the clue which I had missed. I did some research on here and found a post that said you could use 6volt lamps in parallel if you couldn't get the originals and indeed someone in the past had rewired the lamp holders to be in parallel rather than series ! I simply didn't spot this as all the wiring looked to be original in that area. Re-wiring them to be in series restored normal dial lamp operation and V1 heater voltage.

Just goes to show that even on a set that is very familiar, the phantom can still trip you up. (said phantom had also had a twiddle of the aerial and oscillator trimmers but had left the I.F. transformers alone)

Rob

Re: Odd DAC90A fault cured
 

Nice catch, well done.

Re: Odd DAC90A fault cured
 

I can see why people like having little pieces of paper inside saying what they'd changed. It seems like the Twiddler had done a good enough job for you to think the wiring was original, and presumably it provided some benefit relative to the lamps available when it was in use. Well done. It's always the bits that look fine that are the last to be checked...

Re: Odd DAC90A fault cured
 

Hi Rob.
Nice repair.
I have also a Bush DAC90A in cream cabinet. Mine did not have the original loudspeaker, but it does work O.K,
Also, I may check the lamp shunt resistor on my radio as the bulbs are not as bright as they could be,
Cheers
Mike

Re: Odd DAC90A fault cured
 

Thanks for sharing.
Sometimes these things are not obvious at all, as it might appear that the first repairer had just resoldered some lampholder tags that had heat dodgy joints.

Re: Odd DAC90A fault cured
 

Hi. Another item to remember is that the wiring leading from the ballast resistor unit will no doubt need replacing as the sleeving will have crumbled with the proximity of the heat dissipated from the resistor.
Cheers
Mike