dac90a - to replace or leave alone??

[Shaman]

Hi, Rob service sheet here, check the capacitors. Then check the voltage readings on the valve pins, be carefull live chassis. the smoothing Capacitors are c20 c21 32uf and 16uf on this sheet...regards....Alan.

http://www.radiomanuals.info/information.../s1161.pdf

[tiv48]

here is a dac 90a that will never need that cap changed
http://www.facebook.com/?ref=home#!/photo.php?fbid=182661611760772&set=o.143081272396522&type=1&theater

[Bushbaby]

Hi All,
I hope you all had a great day yesterday and a relaxed one today.
I tried to be all nostalgic yesterday and listen to the this DAC90a in the kitchen. I couldn't beleive it - mains hum worse than ever with very little radio station reception at all!! What are the chances that the UL41 has gone down since the last time that I used it??

Thanks

Rob

[Tedzed1]

Hello Rob, is pin 4 on the UL41 connected to anything? It is only used as a connection point, and serves no other purpose,
I had this problem with a DAC10, just snipped the connection and left the wires connected but suspended below the valve base,
it greatly reduced the hum.

[Bushbaby]

Hi Paul,
Thanks for your reply.
I'll need to check.

Rob

[Yorkie]

Easiest way to check Rob is by substituting the valve for a known good one. Easier said than done if you don't have one of course, but whereas UL41 valves aren't noted for being long-lived, conversely, the smoothers in DAC90As weren't noted for being major suspects of hum, and rarely need changing. I had a DAC90A in which I changed the smoothers for a new BVWS new manufacture unit, which made not a jot of difference, then did what I should have done first - took a UL41 out of another DAC90A which didn't hum, and that cured the problem, so I kept the old smoother, used it in another radio and it worked fine.

[Bushbaby]

Hi David,
I trust (as with all fellow forum members) you've had a good Christmas.
I do have other UL41s so I'll replace. The interesting thing with these valves is that they will test as good on my Avo yet have hum?

Best Regards

Rob

[Yorkie]

A valve tester won't necessary detect issues which may cause hum. I think in the UL41, (and of course other valves), that the hum is induced from the heater to the cathode. A valve tester will test for heater/cathode shorts or leakage and the valve may test good in that respect, but that doesn't mean that it will not hum in use. The other tests a valve tester conducts are of course heater continuity, and emission, which again, may be fine, but the proof is in the hearing, which is the most critical test of all!

[Steve_P]

Are you sure it is the valve? How loud is the hum, does it only occur on station and does the volume control vary it?

Some sets like this do hum a bit in use, don't forget. Transformer laminations and hum pickup through wiring.

Cheers,

Steve P.

[Bushbaby]

Hi Steve
The hum is loud and seems to drown out just about everything!

Regards

Rob