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Tram D201 low transmit and modulation


Active Member
Jan 19, 2017
I checked the voltages of the rf driver tube v700 i swapped out tubes with no change. pin 8 shows -.5v, it should be 165v im getting little or no reading on the other pins. only pin 2 is close to right -.6v it should be(-.8v). does anyone have any ideas. Robert
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R705 is reading 54k ohm it should be 47k ohm i have heard that some difference is okay but this looks like a lot
V600A pin 1 is -55v it should be 240v. V601B pin 1 is .0v it should be 240v. v602 is okay. In the audio tube V603 pin 3 and pin 4 are about 50volts too high from pin 3 355v and pin 4 390v
I put a new 47k ohm 2watt resistor for R705 but no change in the radio still low output low modulation
Do you have access to a tube tester?

Might be time to find out if a tube is contributing to the problem.

Or maybe more than one?

That's where we start with a D201 before we'll plug it in. Check the tubes, clean each socket as a tube is put back in after testing, and clean the controls.

Don't underestimate how much difference just one tuning slug that's adjusted wrong can make.

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I replaced all the tubes recently. but i will check them. I have a old tube tester its one that has a green eye a SECO i also have a signal generator voltohmyst wv-77e and a fluke multimeter but no oscilloscope . The manual said the transformers needed to be adjusted when i replace all the tubes. so i did :(
I grounded the s3c wafer 5th connection to short the alc and compression. to test transmit i pushed the mic button and the ground wire clip end that was grounded to the chassis snapped and sparked there are two white/red wires soldered to terminal 5. but im confused because the schematic shows a white/blue wire should be on #5 it is on terminal #9 s3c.
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I am missing a 40uf 450vdc capacitor. the can capacitor only has two 40uf 450vdc caps in it. Two leads are hooked up to one. So, i ordered a 40uf 450v x 40uf 450v x 40uf 450v. can capacitor that should put it back right.
The 3-section C5 is wired with two sections in parallel, making it a two-section part, 80uf and 40uf.

We adopted the habit of using a 500-Volt part with two 100 uf sections. The voltage on C5 can spike up to 470 Volts or more when the tubes are cold. Once they warm up and begin to draw B+ current it falls to the 410-Volt range. I'm convinced the 500-Volt part will last longer.

Back in the day, a 450-Volt multi-section filter would have a "surge" rating above the so-called "working" 450-Volt rating. A look at the B+ on C5 with a 'scope shows a sawtooth-shaped 120-Hz ripple waveform. A DC voltmeter will average out the peaks and valleys of this ripple, riding on the 400-plus Volts DC. If the peak value of that ripple waveform pushes past 450 Volts, that's an unhealthy stress on the capacitor. A DC meter reading is only part of the picture.

Rather than try to find the right white/blue wire on S3C, finding that wire at B43 on the audio board is safer. Simply pulling that wire from that hole in the pc board will disable modulation limiting. If that's what's cutting your mike audio, pulling that wire would make a difference.

Too easy to find the wrong lug on S3.

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I checked the value of the can capacitor in the radio it is a 100uf 500vdc x 100uf 500vdc so it is okay
it took me a minute but i found the number 43 on the audio board . and the white/blue wire was right next to it.
i swapped out one tube at a time starting with v600 then to v700s then to three, four, and five hundred. When i swapped out v602 and turned it on the meter on the tram went all the way to the right side. I went on swapping out and checking then when i swapped out v401, the meter went back to zero. other than that not much improvement.

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