I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps.

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Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away.

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Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference.

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I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another?

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Thoughts?

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Tom

W3TA

Emanuele,

?RE:? Assuming each of these should be two different coils, it seems one of those two coils is split in two. Yes, now it is clear

??????and both L3 and L6 are similar looking ?too

Guys, agreed for L3? , but the connections to L6 ?don’t have a source wire shown and don’t have the same switch profile for grounding so the L6 area is a big part of the mystery.

The switch profiles and pointers [whatever each is called properly] seems clear enough on the drawing, so that is not inquestion?

The S-40A, changes? to get to the S-40B ?might offer some clues.? Is there an S-40 manual? What does it show.

Another thought; it is easy to add ,subtract, and change caps and resistors, but changing a big stock of custom band-switches might rattle the cage of the bean counters, and won’t be easily accepted, or might be a lot of work [what do they stock?, and does halli build switches from various ?wafer sets?].

Even those custom coils might be hard to change.

Note that L3 and L6 don’t have the same part number.

Emanuele, good to see you out of hibernation!

Are you portraying L3 ?? ?so? one coil is in the middle and the other is split with half on each side???. So lots of mutual inductance between the upper and lower coils shown on the schematic ??

Did you see my image showing S1A rear with rotated rotor to show it in each of the 4 positions? And what do you think?

It is unclear to me about what your images are showing… so far, will try again.

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Richard, you apparently have skipped over a number of posts, I previously posted an image with S1A front? rotated to each band separately. It showed ?that the signaw goes in one of the coils and udes mutual conductance to send the signal out the other coil. It was necessary [for me] to show the grounding wafer/wiper? In each position to see what happens. Maynard seems to go along with my proposal. Then Nick asked about the mixer tanks area. We cannot find a signal path from V1 to V2 for band 1, and likely band 2 [I forget now]. We can see a path for the S-40B but it [C62] is not in the S-85 drawing or parts list. So I/we are stuck at that point. We need somebody to open an S-85 and look to see if there happens to be a C62 or something I am using the manual on Bama, there may well be others?????? Help.

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Maynard, maybe I said /showed that stuff poorly. Hopefully C27c is solid as a rock, but C29 will develop a good signal voltage up to 1.6? Mcs.

My red lines were only intended to show the path to the plate? …. And there is no signal path going to the right in band 1.

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RE:
The B+ line at the bottom of R26 and C29 is heavily filtered. ??Yes ??It ought
to be B+ DC voltage only with all AF and RF signals that might be
present filtered to ground. Yes ??And C27C, 10 uF, will do that. Yes ???

My problem too? with the S-85 drawing on BAMA and looking at band 1? there is nothing complex? to look at.

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Somebody will want to know the model no ?of the Skybuddy?there is more than 1?

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Here is what I see for the S-85 mixer tanks for Band 1 only? … a simple situation.. but how can any signal get away from the RF amp plate circuit ans to the next grid???

Sorry I overdrew the bare switch. ?????The Band 2 ?connection is more involved so forget it for now.

Is there any answer ??