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

[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.
>
>