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

[Larry H]

Andre, I'm quite surprised that there is 0 volts on f103 and it is not
open.  It should be a 1/8 amp fuse, so you should check to see that it is
not larger.  Yes, it points to a short - possibly C606B in the audio deck.
It sounds like it already damaged the power supply, so be careful that all
shorts are fixed before powering it up again.  The resistance to gnd at
f103 should be 20k or more.  If it's less, you could try uncabling the IF
and RF decks.  If that cap is shorted, then F102 should have blown.  Verify
it is 1/4 amp.

Regards, Larry

On Sat, May 25, 2019 at 12:28 AM atfu <atfu at gmx.de> wrote:

> Hi Larry. Thanks agn. The fuses F102 and F103 are _not_ blown. Can that
> be compatible with there being a short in B+? I'll be away until Sunday
> and then will resume testing. Unfortunately don't have a variac.
>
> Best! A
>
> Am 25.05.2019 um 02:58 schrieb Larry H:
> > 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 <mailto: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!]
> >      >
> >      >
> >
>