Static level drops 1 S-unit when touching radio?

[Cutlass327]

Was doing some rewiring in my milk crate setup here, and accidentally had a 13.2v hot wire fall and touch the chrome faceplate of the Galaxy 939. I now have 3 S-units of static (had around 1 before) and when I touch anything on it the static drops 1 s-unit. Could I have damaged something?? It seems more sensitive to my computer monitors now (Acer wide screens). Did it toast a noise filter cap or something?

 

[loosecannon]

first place to check is your bypass capacitors.
these will be small ceramic disc caps that go from PC board ground to chassis ground, and are usually found around the edges of the PC board.
if you look at the schematic over on CBT you will find them all in a row in the lower right corner.

i believe there is also one electrolytic cap in these chassis that has its positive lead going to chassis ground, but i can't remember which one it is right now.

i will look at the schematic, but maybe someone else might know which cap im referring to.
LC

EDIT: found it. it's C80 a 22uf electrolytic cap. in the 959 board its on the right side of the PC board towards the middle near the voltage reg. it might be in a different spot in a 939.

 

[Cutlass327]

Thank you. That was what I was fearing. Guess I'm going to have to open it up and peek inside again.

 

[nomadradio]

I would check the connection inside the PL259 connectors on your coax jumpers. Just wiggle, or gently twist the coax where it enters the back of the PL259 and see if you hear any difference while doing it. I'll guess that the actual surge current wasn't through any of the radio's DC-powered circuits. The radio's chassis and case have no DC connection to the power supply. But your coax should lead away from the radio to a ground connection. This means the fault current should have been through the shield of your coax and jumpers/ wattmeter. And whatever else was in line with the antenna coax.

I suspect the surge current from the DC power supply has compromised the connection inside one or maybe more than one PL259 connector. This would explain why adding your body capacitance to the radio's case changes the noise level.

A poor ground connection through the coax connectors would be the logical explanation. A surge of excess DC current can do that.

A continuity test on each coax jumper may or may not reveal this. Swapping a suspect coax jumper for one that hasn't been exposed to surge current might settle the question if it makes this symptom go away.

We wear out coax jumpers used on the bench here as a regular thing. Repeated connection/disconnection to a half-dozen or more radios a day tends to wear out the ground connection inside the PL259. When a radio is connected to the dummy load, any increase in receiver noise that happens when a jumper is flexed indicates one that has a worn-out ground connnection inside the PL259.

Happens a lot, just from wear and tear, even if the shield braid is securely soldered to the PL259. And if the braid was just "folded over" and jammed into the plug, that surge almost certainly oxidized the surfaces where the braid makes contact with the inside of the plug's body.

73

 

[loosecannon]

Damn the man is good isn't he.

Cutlass nomad is most likely pointing you in a better direction than i was and im only putting the most likely in there in hopes that i get lucky lol.

I know i have popped that electrolytic before with carelessnes though.
Just cant remember exactly what I did.
LC

 

[OldTech03]

Also, check your Starlite faceplate these can cause noise especially if any filters associated with them are damaged since the el circuits develop an AC signal to light them up. As a matter of fact, if you look at the schems I think most of the power routing goes thought that driver board so any caps close to or on that board could cause an issue. Just something to check along with the info by the gang.

 

[Cutlass327]

Also, check your Starlite faceplate these can cause noise especially if any filters associated with them are damaged since the el circuits develop an AC signal to light them up. As a matter of fact, if you look at the schems I think most of the power routing goes thought that driver board so any caps close to or on that board could cause an issue. Just something to check along with the info by the gang.

That is unplugged. It was unplugged when I got the radio, so I plugged it in. It was never plugged in more than 1 minute since I have owned it - the whine was too much. I don't need it anyway - I just turn the desk lamp on if I cannot see the controls!

 

[Cutlass327]

OK. I just when over the coax connections. Felt the 259 on the rear of the radio shift a little as I twisted it tighter. I'll have to check again after skip dies off a little, meter is bouncing from skip coming in 2 channels down on Ch20AM. I wanted to keep on the channel I discovered it on, CH21, but not today.... but from what it looks like so far, it has improved!

 

[OldTech03]

If there is noise being generated by an issue inside the radio simply unhook the antenna and see what noise you have and if you happen to have a dumb load use it and do the same touching tests with the volume turned up. If it's internal it should show up without an antenna.

 

[Cutlass327]

The noise level stepped up a little when I fired up the computer monitors. Tightened that coax, I'm back down to S-1.