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32V-3 Power Supply Problem




 
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wa2fxm
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« on: January 01, 2018, 01:05:47 PM »

I've got an intermittent power supply problem with the 32V-3 that now seems to be permanent. At power on in CW the LV is fine at 260V on the front panel meter. But switching to CAL the voltage drops to 0V. No popping fuses, no smoke, just 0V on the meter. Switching the HV on in CW, both LV and HV sputter and bounce around erratically. The symptoms showed up a few months ago as I was working on the rig and then suddenly disappeared. I was able to tune it up on 75 meters and made a decent contact last week. I was ready to try again a few days ago and suddenly the symptoms are back. If I'm reading the schematic correctly, switching to CAL just sends the B+ up to the RF section. I'm not seeing anything that looks like an obvious shorted component. And it's acting more like an open than a shorted B+. Can anyone give me a hand with this one?

Mark WA2FXM
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KD6VXI
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« Reply #1 on: January 01, 2018, 01:54:02 PM »

Cold solder joint would be my first guess.  Broken or crudded up wafer on a switch?


--Shane
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wa2fxm
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« Reply #2 on: January 04, 2018, 08:46:08 AM »

I mispoke Shane. It acts like an open in the sense that there are no blown fuses when the voltage goes to 0V. The LV Meter reads before the switch. I'm measuring the B+ right at the choke filter output. Switch to CAL and the B+ just sinks to 0V. With the HV switch thrown in CW the B+ takes a different path through the relay. Here the B+ sputters, varying a few volts around +25V. I'm also seeing -11V on the plate of the 3rd Multiplier tube in CW so it seems to me something is going on in the RF section, but I'm not sure that I completely understand the whole LV power supply design to be able to figure this out. The -75V Bias line is connected to the center tap of the LV transformer which I've never seen before in a full-wave rectifier circuit. The bottoms of the filter capacitors are sitting at -75V. I'm not sure what I should be seeing now for normal operation.

Mark WA2FXM
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WD8BIL
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« Reply #3 on: January 04, 2018, 10:18:46 AM »

Have you cleaned and burnished the HV Relay contacts?
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WZ1M
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« Reply #4 on: January 05, 2018, 03:04:54 AM »

The LV power transformer is a major problem with the 32V series of transmitters. I have rewound, I bet, 20 of these in the last 10 years.
Regards,
TRS
Gary
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W3RSW
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Rick & "Roosevelt"


« Reply #5 on: January 05, 2018, 10:29:26 AM »

Iím with Buddley. Check the physical build of the notorious 48 volt relay, K-301, first.  Not just the contacts, but are the small, delicate ceramic posts cracked, split or even there anymore?

I had similar problem in 2002.  The HV post was half gone. My fix was cutting off the tip of a small electric wire splice type of twist-loc and filing off to suit long dimension. Then drilled a out a cup in end to fit the relay arm nub then cemented to arm with epoxy. Been working ever since. My theory was that short of ceramic, the plastic in a wire twist-loc would handle more than 240 vac.

This was done with hemostats holding the twist-loc tip flat on desk, etc. Those ceramic posts are pretty small.  K-301 should have been a larger, beefier relay. Not sure why Collins used such, but overall compactness of entire rig in its day might be partial explanation.
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RICK  *W3RSW*
wa2fxm
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« Reply #6 on: January 06, 2018, 05:49:05 PM »

Thank you gents for steering me in the direction of the relay. I thought the sputtering sound was coming from the VR-75 tube but it was indeed the K301 relay right underneath the tube that is arcing. But it's not the contacts that are the problem. The whole armature is shuddering back and forth. I'm only measuring +26V across the coil where I assume it should be 48V to activate the relay. So in CW the LV is fine at 260V. Switch to CAL and it drops to 0V. Turn on the HV in CW (without the interlock engaged) and LV drops so that there is only 26V across the relay coil, not enough to totally fire the relay which bounces back and forth and the B+ contacts arcing with a very nice deep green color. The only thing in common that gets switched by the CAL switch and K301 is the B+ going to the RF section. I'm still thinking there is something in the RF section that is pulling the B+ down.

Mark WA2FXM


* relay.jpg (1934.75 KB, 2988x5312 - viewed 68 times.)
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wa2fxm
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« Reply #7 on: January 07, 2018, 12:12:51 PM »

UPDATE: I pulled P302 off the connector. This disables the B+ going to the RF section. Measured at the LV filter output I get +295V in CW AND in CAL. With the HV switch on a full +280V, enough to fully activate the relay. No shuddering and no arcing. So I guess I'll be digging into the dreaded RF section of this rig to look for something at least partially shorted? I did get that puff of smoke drifting up from under the plate tuning coil awhile back. Anyone have any experience in tearing the RF section apart? Should be no problem right?  Undecided

Mark WA2FXM
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W3RSW
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Rick & "Roosevelt"


« Reply #8 on: January 08, 2018, 08:42:53 AM »

Odds are youíll find a screen bypass cap leaking if the voltage suckdown is on all bands,
If only on a couple of bands, etc., itíll be a variable ceramic cap or padding cap.

I had the second case once.
Specifically C-117, the pad for 80/40/20 on the plate of the first multiplier, v-102
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RICK  *W3RSW*
wa2fxm
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« Reply #9 on: January 09, 2018, 10:30:08 AM »

Great minds must think alike Rick. I agree on the bypass caps. I'm guessing C106 at the Buffer Amp or C120 at the 2nd Multiplier. The B+ line to the RF section is a virtual short. I'm measuring 0.2Ω at J102 pin6 to ground (on all 5 settings of the bandswitch). I see about a dozen possible caps but those two are the only ones directly tied to the B+ line. All the others are behind resistors. So the question is, how do you get to them? Do you start pulling components out to clear space and dig down to them, or do you disassemble the RF compartment from the rig?


* rfsection.jpg (2432.98 KB, 5312x2988 - viewed 54 times.)
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W3RSW
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Rick & "Roosevelt"


« Reply #10 on: January 09, 2018, 12:58:04 PM »

Depends on how many layers down.  Grin

Several people have clipped wires as close to defective body as possible, leaving a pigtail to lift out to be more accesable, then soldering in the replacement with spaghetti insulation. Replacement doesnít normally have to be exactly in original location if the RF shielding isnít critical. If your going to replace many wholesale on general principals, then removal of entire RF section is about it.

Small clippers, hemostats, etc. are helpful. Consider that large irons made those original joints; a 100 watt Weller gun with relatively long, small tip will provide enough fast heat to keep the joint local these days. A 40 watt pencil tip may be plenty for just adding new to pigtails.

If at all possible identify the bad cap first, by changing out one suspect at a time if no other recourse.
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RICK  *W3RSW*
wa2fxm
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« Reply #11 on: January 12, 2018, 11:06:30 AM »

Well it's not C120. I clipped it out but still see a short on the B+ line. That's the only capacitor that I can get at with the narrow dikes.

So, it looks like removing the RF section is the next step. This looks like a bit of a nightmare. Anyone who has gone this route and can offer some guidance please chime in now.

Mark -WA2FXM
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W3RSW
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Rick & "Roosevelt"


« Reply #12 on: January 12, 2018, 06:19:45 PM »

As you know, C-120 is a 0.01uf/300v screen bypass for 2nd multiplier.
Good first guess.  Many of these caps seem to be barely voltage rated.
Surely there're other (a lot of micas in LV bypass service)
choices that you can get at without total teardown.

You mentioned a puff of smoke coming up under the final tune coil.
In your pix of RF assby. side, I see a tan mica with possible overheat scorch
just to the right of the first ceramic switch wafer under the 4D32.
My Collins manual photo doesn't identify it; just a few components are noted for orientation, but I wonder.  It could be just a shadow, etc.

By the way, I had to change out the screen cap. for the 4D32 even though it comes via dropping resistor from HV.  Changed out the original 1000pf/1kv with a 1000pf/5kv.  Howard Mills traded me this rig and said that possibly someone had modified it for low output to run into an outboard high power amplifier/modulator. No, the bad screen cap. was simply sucking down output power.  He now say's he routinely changes out the 4D32 screen cap. in every
32V that comes his way.

Back to problem: Try hard to isolate some caps before tearing down RF section completely. I haven't had to pull it so can't help with experience.

And did you try all band positions to see if the suck-down is on all bands?

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RICK  *W3RSW*
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