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Author Topic: Drake TR-4C troubles!  (Read 4276 times)
KC2VDM
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Posts: 145




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« on: November 10, 2013, 07:03:55 AM »

I'm lost as to what could be wrong with my Drake. It's been working fine for a few months, with the worst trouble being the dial lights not coming on now and then (Which I figured out is just a bad connection in the lights housing). But Anyway, I turned the radio on to show a friend of mine how it sounded, I looked away to mess with something else, and he says he sees smoke. I look, and its coming from the bottom of the rig. I shut everything off and unplug the power supply from the wall.

Underneath there was a burned resistor in the PA cage. It's mounted to the wall and the transformer can T-10. I found out its 470 ohms and marked as R10 on the schematic (I may be wrong, I couldn't read the part number on the schematic, but the value I could). Also the color code matched a 470 ohm. So I replaced it, looked for any other burnt components, and turned the radio on and... nothing.

What does work: Tube filaments light, Dial lights light, and there's no smoke.

What doesn't work:

No receive
No Transmit
No S-meter reading (even with turning the RF gain counterclockwise)
No Plate current reading
No sound at all from receiver
No crystal calibrator signal

I'm don't even know where to begin. Help!

-Alex
KC2VDM

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AC5UP
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Posts: 3875




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« Reply #1 on: November 10, 2013, 08:08:05 AM »

Congratulations! The 250 vdc B+ voltage supply has joined the choir of the silent. As you'll note from the AC-4 squidmatic if D5, D6 or R3 were open there would be no +250 vdc. Which means no +245 or +150 regulated and unregulated DC volts (see lower left corner of the TR-4C squidmatic). This would fit the symptoms you describe and shut down everything except the heaters and pilot lights.



Mandatory Disclaimer: There are voltages inside the radio and AC-4 power supply capable of promoting you to client status at a local funeral home. NEVER assume it's safe to touch anything unless you're sure the power cord is unplugged. When testing voltages keep one hand in your pocket, work only on a well lit, stable and reasonably clean work surface. You do not want the radio rolling onto your arm and sharing a few Joules of EMF unless it's a necessary part of the learning experience.



The 470 Ohm resistor in question, is this a 1 or 2 watt item? Damned if I can find an R-10 on the squidmatic I have ( http://elektrotanya.com/drake_tr-4c_sm.pdf/download.html  look for the words 'Get Manual' in a small font under the red cover pic) - but - I do see an R40 just a bit east of the 12BY7 that's 470 Ohms and in the right place. If that's the dude, you should first figure out why it blew. Prime suspects would be a full or partial short to ground through a bypass condenser like C60, C153, C213, etc. and there are a plethora of small value C's on that line.

Best guess is one of them is shorted and that took out R40 about the same time the +250 vdc source in the AC-4 went toes up. Troubleshoot the radio first then the AC-4 as a separate unit.

Once you're convinced you checked everything in the TR-4C that needed to be checked or replaced and you have normal output on the AC-4, plug them together and see how it plays.

NOTE: If R40 is indeed the 470 Ohm part that smoked you can confirm my hunch with a quick continuity check. One side of the resistor should read approximately 27 Ohms (through R192) to pin 11 of the 12 pin Jones power supply jack. The other side of R40 should be 470 Ohms higher to pin 11.
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K8AXW
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Posts: 3860




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« Reply #2 on: November 10, 2013, 04:14:14 PM »

Quote
Mandatory Disclaimer: There are voltages inside the radio and AC-4 power supply capable of promoting you to client status at a local funeral home. NEVER assume it's safe to touch anything unless you're sure the power cord is unplugged. When testing voltages keep one hand in your pocket, work only on a well lit, stable and reasonably clean work surface. You do not want the radio rolling onto your arm and sharing a few Joules of EMF unless it's a necessary part of the learning experience.

I have to add, "Never take a few rams of "Ole Stumpblower" to steady your nerves before getting into a piece of electronic gear!
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KC2VDM
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Posts: 145




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« Reply #3 on: November 12, 2013, 03:25:21 PM »

Thank's for the help AC5UP! I've been probing around a bit more, and am following your advice. The one side of R40 (That was it, by the way, not an R10) reads 2 ohms, and the other side reads 464 ohms. (to pin 11). I checked a couple of capacitors (C153 and 154) and they were good. I didn't have time at the moment to go over any more. I'm still working my way through the rats nest. Though I have my eye set on checking C145A next.

I noticed something though, one terminal of T-10 isn't connected to anything. I'll have to figure out which connection it is later on tonight, but it looks like it was that way from the factory. Yet every terminal on the schematic has a connection? Why would it be left out on the rig but not the drawing?

KC2VDM
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AC5UP
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« Reply #4 on: November 12, 2013, 05:10:19 PM »

Why would it be left out on the rig but not the drawing?

There's a chance that at some point in the model run the original part became backordered and a slightly different part subbed.

If the lack of one part shuts down an assembly line that can get real expensive real fast...  Wink
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WA1RNE
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« Reply #5 on: November 12, 2013, 06:28:41 PM »


 
As a suggestion, if none of the capacitors are shorted, start looking for low resistance paths from R40 at the T9/T10 side to ground.

An easy but unobvious check is to pull the 12BY7A driver to eliminate the possibility of a suppressor grid to screen short. After pulling the tube, check the resistance from the T9/T10 side of R40 to ground. It should be many megohms or open as everything in the driver output stage is AC coupled.

If there's still a short (or the 2 ohms you saw before) check the tube socket for carbon tracking or any other metallic debris that could create a short to ground.

After that, unsolder R40 where it connects to T9/T10 and start eliminating circuit paths in the driver circuit one by one, either by close inspection or temporarily opening paths.

If you're afraid of forgetting where wires connect, take a few photos of the area before you start.

WA1RNE
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