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

[atfu]

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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