Kenwood TS-530S Hybrid arcing under driver tube

[KA2PTE]

I Pulled the RF board for some component replacement, and decided to clean the bandswitch wafers while it was apart. I have a TS-830S
in working order nearby and copied by eye the wafer positions with respect to the dot on the plastic wheels in the center. However when powered on
theres arcing under the RF driver tubes socket. When I move the band switch to AUX (then re apply power) , its not arcing anymore.

I heard one of the wafers has the B+ for the tube on it, so I am thinking maybe one of the 2 in the back were not aligned? Its hard to see inside between the boards to confirm. I had marked on the front panel where the arrow for the knob was pointing when I removed the shaft so that the final cage wafers would re-align right.

I also looked under the board to make sure no hardware was floating under the socket, looks ok.

I turned off the lights and the arc seems is from the tube socket foil right to chassis , which is almost a qtr inch gap. Looks like the plate only has 300Vdc on it, so am bewildered what can jump that distance...unless its RF ? The send switch is off, heaters are off.

I had re-soldered those foil pads for the tube socket and while I was in there replaced the (2) 100k half watt resistors nearby the tube, plus R38. R38 (470k) was ok but it was carbon and I hear they are noisier than metal film types.

I took the board out again for an inspection. No signs or marcs of arcing to the metal chassis frame so maybe its just arcing on the socket pins. There was some flux left on the pins from a flux pen I used so I cleaned that off good, and cleaned and defluxed the board some more but after reinstalling, still have the arcing. Someone else suggested to check the switch foil pads for cracks, as they crack easily. So far nothing stands out as being wrong.

Looking at the schematic for the 830S and figuring the RF board is about the
same , it seems the plate of the RF driver  is the common wiper of S1-7. So if thats
the point thats arcing, (except when the bandswitch its moved to the unallocated spot) still hard to see how
it can be arcing...

[M0VRF]

Fixed a Vox AC30 valve guitar amp many years ago and it was a cracked valve socket. The bias and HT were on adjacent pins.

Get a piece of perspex or glass, and place over the area then you can have a really close look at it with the lights off!

JohnB.

[WA2SQQ]

Regarding carbon being noise than metal film - in an audio circuit it might be a reason to change them, but in a RF circuit? What was the problem that required the board to be removed? I live by, “if it’s not broke don’t try to fix it” ....

[KA2PTE]

I will take a close look at the tube socket. At this age the radio is nearly 30 yrs old and anythings possible.


Was not sure if the noise generated by the carbon resistors could maybe become amplified harmonics in the RF chain
under certain conditions so while I was in there and I had the correct one in non carbon decided to replace it.

Original issue was poor rx on nearly all bands, and 7 mhz being the worst. I got a new L41, and a dual gate fet and
a D2 as a recommendation by a ham who services these alot and after installing them, figured I may as well try the bandswitch
overhaul as I hear it can cause the same kind of issues.

[KA2PTE]

I put the board under the magnifier again, and I saw some black stuff
between Pin7 (the plate)  to the common foil.
I scraped at it and now theres a tiny groove you can see in the pix if you
zoom in.

As I rubbed on the nearby ground foil it started to expose the copper
foil...there is a layer of some stuff coating lots of the boards in this
radio , probably niccotine as it was likely owned by an ex smoker. So maybe
its become conductive near this area of the tube and the voltage potential
ignited it, and ate up some of the board. It sems to have also stained the ground foil
as you can see in the pix. I thought it was flux but it looks like its embedded into that
green coating over the foil. It may have weakened it enough to get it to flake off near
the edge of the foil traces I guess.

[W1RKW]

I think you need to make sure that any residue of anything is removed and the area is absolutely clean.  Use some 90% or higher isopropyl alcohol and scrub that area with cotton swabs or a tooth brush. It needs to be squeaky clean.  Then look at the PC board material (close inspection) around those pins and board conductors and make sure the board material looks like the clean edge of the board and not discolored. Very possible there is some carbon tracking going on there.  If there is some carbon tracking you may have to remove the socket then look at the socket side for any discoloration as well. If discolored on the socket side you may have to do some grinding to rid the carbon. It's very possible you may end up with a hole there if there is carbon tracking is in the board material.  It may be tricky to clean that up if that is what is happening. Might have to grind some of that ground plane away from the "hot" pin, maybe even just break the connection and jumper it.

[KA2PTE]

Ok I cleaned up that pin and put a dab of
wood glue over it to further insulate it.

After re-installing, no more arc.

Aligning the bandswitch was not too bad. I have it running now and RX for
sure is greatly improved - !

Many signals over S9 where as before, it was rare to see a S9 on the meter.
L41 and a dual gate FET and D2 were all replaced as per the suggestion of KD7DNY.
That was before the arcing problems.

Other responses I got claim the niccotine is a "catalyst" that under high voltage will ignite
and catch the board burning, which is what happened here.