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

[Bill Hawkins]

Hi Barry,

Not much mail today, so here's a thought or two -

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

"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+.

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.

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? 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?

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?

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?

Have you tried another tube for the osc/mixer?

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


-----Original Message-----
From: Barry
Sent: Sunday, January 02, 2011 4:26 PM

Hi Ross,

I did remove the cover from the first IF transformer, mainly because the
lower ferrite core was difficult to turn.  Thinking it was made more like
some of the transformers I've worked on (e.g. R390A), I thought I might be
just twisting the insides; however, once I removed the can, it was apparent
that wasn't the case.  These are made quire well and I wasn't twisting the
windings after all.

If I'm not mistaken, the caps are not the "sandwich" type like I've seen
before, but they are very small diameter, quite long tubular caps that run
vertically inside two of the corners.  I don't know if these are as prone to
the silver disease as the sandwich styles.

The B+ is indeed off and I assume I do need to replace the main filter caps.
There's slight hum on the output, but I don't hear it at low levels so the
AC on the B+ isn't very bad.

Hmmm, still not sure where to go from here.  There are some other
electrolytics that I figure should be replaced; however, other than that,
shotgunning the caps doesn't seem to be a need here.  This model used very
high quality components, at least for the day.

Thanks again,

Barry - N4BUQ


 On Sun 02/01/11  3:26 PM , bavarianradio at comcast.net sent:
> Hi Barry, I have restored many Blaupunkt tube sets. The most common
failure
> in these is the 460 kHz transformers. I have rebuilt many of them. Try
> tapping one while feeding your 460 kHz generator into the ECH81 grid and
> see if they act intermittently. Even though you seem to be getting a peak,
> there could still be a problem. Rebuilding the transformers requires
> removal from the chassis or circuit board, removing the aluminium can,
> removing the "sandwich" type mica capacitors and installing new silver
> micas on the bottom, 150pf for the input and 250pf for the output. 
> Otherwise, have you recapped the entire radio yet?? also check for proper
> B+ voltage as well.  Good Luck, Ross