[R-390] OT: Need advice regarding Blaupunkt tube radio

[Barry]

> "The B+ is indeed off ..." This implies that you at least have a VOM.
> How far off is it?

I have now incorporated a solid-state rectifier, good filter caps, and a dropping resistor.  B+ is right on now.

> "There's slight hum on the output ..." This implies that you haven't
> used an AC-coupled VOM to read the AC on the B+.

I've checked it with the 'scope and, as I recall, it was a fairly small ripple.  With the filter caps replaced now, though, I can assume it is in spec.  I need to measure it again to see.

> If the main filter caps need replacing, you will have many volts of
> AC on the B+. This will not necessarily translate to audible hum.

Oddly, there is still significant hum on the audio.  Bad tubes (filament problems?)?


> The B+ can be drawn down by a leaky coupling cap to the final audio
> tube. Is that tube's cathode at the correct voltage?

It's supposed to be at 6.2V and it's at 7.8V.  Not sure what would cause it to be higher than spec.

> If the set has
> a line fuse, is it the correct value or a copper bar? Are there signs
> of overheating around the last audio tube and its output transformer?

It's supposed to have a 600mA fuse and it had an 800mA fuse in it.  I knew it had a fuse but hadn't checked it for correct value.  I put a 600mA in and it works fine.  No noticeable headint around the tube or audio xfmr.  They both look great.

> You said the IF stages appear to be working. Can you hear a modulated
> IF signal at the speaker? Is it as loud as you would expect as you
> turn the volume control?

Yes, I can hear a modulated tone when coupled into the IF; however, I'm not sure what signal level would be appropriated.  If I crank the generator up enough (don't remember exactly the value but I think it was on the order of 1mV), then I get full audio.

> If you can read the oscillator grid bias, this implies that you have
> a high impedance meter. Do you see normal voltages at the screen and
> plate? If you can measure the AGC voltage with the signal generator
> driving the first IF, does it behave normally?

I checked the mixer's voltages (need to check the oscillator again).  It's voltages are very close to spec.  Plate voltage is a bit high (190 vs. 175), but everything else is good.

> Have you tried another tube for the osc/mixer?

That's what I'd planned.  I don't have another one but may order one as a last resort.  It works well on FM and it uses the mixer side of that tube in FM mode.  I'm still wondering if the oscillator is just weak.

> May your new year be filled with puzzles you can solve, as opposed
> to those that neither you nor any of us can't.
> 
> Bill Hawkins

Thanks again!

Barry - N4BUQ