Gary, this is an 11-tube (I'm guessing that C188 is in the 13-tuber). with a solid state BFO.? What I'm seeing in the schematic isn't exactly what is on the radio but the photo in Garey's service manual shows what I have (serial number 6393).? I don't see anything jumping out at me between my paper schematic and the one in Garey's CD, aside from a slight value change for C168.? If I am interpreting the dating of the schematics correctly, Garey's is later. (mine is 919664054 and the CD is 1026665400 and my C168 agrees with the CD.
Something very curious, though, has happened.? The whole voltage changed when I'd short the switch contacts -- presumably by virtue of being tied to ground via R116.? Of course, when I just short out the switch contacts, then AM mode won't work properly.? So I tried the obvious: I took a test lead and attached a 1k resistor to the far end and then touched the other end of the resistor to the chassis.? When I do this, everything works as intended!
I am tempted, at this point, to solder in a 1k resistor from the point where the junction of T11, C163 and R124 join, to the chassis but I'd sure like to be able to understand why.
73,
Steve Wedge, W1ES/4
Time flies like an arrow.? Fruit flies like a banana.
Sent with secure email.
------- Original Message -------
On Friday, June 24th, 2022 at 3:01 PM, wb6ogd <garywinblad@...> wrote:
Steve,Could it just be a bad switch contact?
Measure for voltage at T11 at C188.
73,
Gary?
WB6OGD?
On Jun 24, 2022, at 11:28 AM, Steve Wedge, W1ES/4 via groups.io <w1es@...> wrote:
?
Welcome to my bench.? Today, we will look at demonic possession of a BFO.
Recap: BFO doesn't work with switch in any of the BFO positions.? Notice short note when switching from BFO modes to AM, right at make & break of rotary switch contacts? AM works normally.
Replaced Q6, subbed C166, C163, and even T11.? Removed both screws holding down the BFO board and they're clean on the underside.? Continuity to the chassis is good.? R124, 141 are good. Checked C168, 166 and 163 with cap meter and Sprague TO-6
The only way that I can get the BFO to come on and oscillate is by shorting the switch contact that feeds the AM detector's voltage divider to the other contacts.? When I do that, the set works normally with expected AGC behaviour in each position.? All points that should be at ground/chassis potential are where they should be.? The germanium diodes all measure around 0.230 V.? T11 adjusts the frequency and it's in the range (a scope probe pulls the frequency a couple of kHz).
C162 appears to be a base bypass.? Unfortunately for me I don't have another .05 ceramic cap at the moment but did try a .047 film cap, with no change. The switch has been cleaned.
What am I missing?? It's probably something dumb but I am just not seeing it!
TNX & 73,
Steve Wedge, W1ES/4
Time flies like an arrow.? Fruit flies like a banana.
Sent with secure email.
