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

[atfu]

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!]
>      >
>      >
>