[R-390] deaf below 8MHz (R-390A)

Fri May 24 20:58:05 EDT 2019

Hi Andre, You might have a short in the rf/if B+ line causing f103 to blow
(I assume it's blown) and that short damaged 1 of the 2 rectifiers.  You
might have shorted something in the IF deck.  It's a really good idea to
bring up the power slowly with a variac after a recap to look for shorts.
You could uncable the IF deck power cable and try to find a short with your
ohm meter.  Same with the RF deck, uncable it.  Before that you could try
measuring f103 on the IF deck side, it should be above 25k ohms, I think.
Sometimes a short can happen in the connectors as the wiring is a little
tight in there.  Do as much testing with ohm meter before reapplying
power.  When you do, measure current through f103 CAREFULLY.  It's probably
very high.

Regards, Larry

On Fri, May 24, 2019 at 7:33 AM atfu <atfu at gmx.de> wrote:

> Further info:
>
> Before recapping AF and IF, E607, F102, F103 had good values.  Now I
> checked again. E607 dramatically down, F102 = 170 VDC, F103 = 0 VDC. I
> also recall that when I put back the recapped AF deck and switched the
> radio on, there was a louder than usual hum for 2 or 3 secs. But then
> everything looked normal. Well, perhaps ist wasn't really back to normal
> :-( If something in the AF module is playing up, that would explain, why
> E607 is down and the radio is deaf below 8: V207/6 gets its voltage
> directly from V606 via P119/J619, right?
>
> Best! A
>
> Am 24.05.2019 um 15:23 schrieb atfu:
> > Hi Larry, weekend = radio time ;-) So I studied the signal path and made
> > a plan (see below). As I went along, starting to follow your
> > suggestions, I got completely stumped at the results of A and Z (below).
> > At E209 I get a voltage of practically 0 and at E607 a reading well
> > below the required 150 VDC. (I used a good analog multimeter grounded to
> > the chassis.) That doesn't look good at all. The voltage at E607 was ok
> > before I changed the caps in AF and IF. What could have gone wrong??
> > (And how these findings relate to the "deaf below 8"-problem, I do not
> > even begin to understand.) Please be so kind as to help me on the right
> > track!
> >
> > Best!
> > Andre
> >
> >
> >
> > %<================================================
> >
> > Trouble ... fb >= 8 MHz > nil
> >
> > ... shooting (along signal path)
> >
> > (Signal < 8 Mhz gets routed by S206 thru Z201-Z203 to S207 into 1st
> > mixer/1st xtal, then via Z213-1,2,3 and S208 to 2nd mixer.)
> >
> > A. S207
> > Connection to 1st mixer ok?
> >      Probe testpoint E209. Should be ca. -4 to -6.8 VDC (in stand-by)
> > X    [Done: Is -0.3 V!]
> >
> > B. V202 (first mixer tube, 6C4)
> > Tube ok?
> >      Exchange with V203 or V204 (2nd resp. 3rd mixer).
> >      If now deaf above 8 MHz, then V202 faulty; otherwise ok.
> >
> > C. 1st xtal osc
> > ...
> > C1. P221/J221 ok?
> >      (How will I discover a fault?)
> >
> > C2. Crystal Y201 (17MHz).
> >      Contact ok?
> >      Take out, clean, put in. [Done. No difference]
> >      Working?
> >      (a) Exchange with Y412. (What effect to expect?)
> >      (b) See C4(c).
> >
> > C3. V207 (osc tube 6AK5)
> >      Tube ok?
> >      Exchange with V401 (2nd osc)
> >      If now deaf above 8 MHz, then V207 faulty; otherwise ok.
> >
> > C4. S208 (rear)
> >      Alignment and contact ok?
> >      (a) (Cold) Measure R from V207/6 to J208/D.
> >      Should be 56K (no FC7) below 8MHz, inf above.
> >      [Done: ok.]
> >      (b) (Hot) Observe voltage on V207/6 while rotating Mc-know up to and
> > incl. 8 MHz.
> >      (c) (Hot) Read out freq at V207. Pull shielding can and ind. couple
> a
> > wire to bulb.
> >      Should be 17MHz.
> >      [Tried with my cheapo China Counter but didn't get a coupling.]
> >      Contact J208/P108 ok?
> >      Pull and look/clean [Done: Looks ok]
> >      VDC at P108/?
> >      See (b) above.
> >
> > D. Z213-1,2,3 with C 283-1,2,3
> >      (How check?)
> >
> > E. S208 (front)
> >     Alignment and contact ok?
> >      Pull RF deck and have a look!
> >
> > F. Probe test point E210
> >      Should be -3 to -8 VDC (in stand-by).
> > X    [Done: 0 V!, irrespective of Mc-position]
> >
> > Z. Other tests made
> >      Power supply E607. Should be ca. 150 VDC.
> >      (Was 150 VDC before changing caps in AF and IF.)
> > X    [Done: Now 70 with MGC, 115 in stand-by!]
> >
> >
> > Am 21.05.2019 um 02:19 schrieb Larry H:
> >> Andre, OK - good progress, it's all about the process of elimination.
> >>
> >> Checking the alignment of the RF deck band switch can be checked a few
> >> different ways.  With power off, meter resistance from pin 6 V207 to pin
> >> D on J208 (see book on band switch alignment).  With power on - check
> >> voltage on pin 6 and rotate the megacycle knob from lowest through 8 mh
> >> and watch the voltage in each band.  Or - remove shield from v207 and
> >> wrap some turns of wire around it and feed to different rx, freq
> >> counter, or scope, looking for 17 mh, while moving MC knob from lowest
> >> to 8 mh.  No need to remove the RF deck, yet.
> >>
> >>
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