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K
Kachina
Guest
So here's an update. I replaced the mode switch bakelite wafers with ceramic wafers. This was EXTREMELY tedious and I hope I never have to do that again.
Unfortunately I am still experiencing most of the same problems. The overmodulated audio seems to have gotten slightly better.
Even though I have tested the transmitter with 2 mics, I'm going to try another as soon as I finish wiring an adapter to allow me to use the several mics I have wired for my 4 pin ft-101.
The transmitter also still transmits full carrier on both upper and lower sideband with no modulation.
Another problem I just realized I have while looking at the signal coming out of the transmitter is that it is always transmitting on the same frequency no matter what channel is selected. It seems that no matter what channel crystal is selected, it always transmits somewhere around 27mhz.
I'm thinking all the transformer cans are completely out of wack, but there also might be some circuitry changes I haven't detected yet.
Unfortunately I am still experiencing most of the same problems. The overmodulated audio seems to have gotten slightly better.
Even though I have tested the transmitter with 2 mics, I'm going to try another as soon as I finish wiring an adapter to allow me to use the several mics I have wired for my 4 pin ft-101.
The transmitter also still transmits full carrier on both upper and lower sideband with no modulation.
Another problem I just realized I have while looking at the signal coming out of the transmitter is that it is always transmitting on the same frequency no matter what channel is selected. It seems that no matter what channel crystal is selected, it always transmits somewhere around 27mhz.
I'm thinking all the transformer cans are completely out of wack, but there also might be some circuitry changes I haven't detected yet.
I gotta name this problem "Mark 3 syndrome". The tuning slugs have been tweaked by someone puzzled by a transmitter with no output. Just randomly twist tuning slugs to see if a result magically appears as if by magic.
A video with the step-by-step description of aligning this transmitter would help. Just as soon as someone gets a "round tuit".
This transmitter mixes three internal frequencies together to produce the channel frequency. If any one of the three shuts down, no output.
The starting point for me is to tune in each crystal oscillator with a shortwave/ham receiver. Each one has one or more tuned circuit downstream from that crystal. Peak each one for a peak on the receiver's S-meter. If any of them won't peak, it points to the circuit with a fault.
Once all three crystals are present and accounted for, the mixer circuits are next. The 5.0485 MHz AM offset crystal gets mixed with the 16 MHz channel crystals. The sum of those two frequencies on channel 1 AM is 21.3185 MHz. The only one I have memorized. Once you have that, the 21 MHz gets mixed with the 5.6465 carrier crystal for AM, both feeding into the 6CB6 tube V3. This is where you first get the 27 MHz sum of those two frequencies. T4 and T5 get peaked next. By now you should have a wattmeter reading to show the peak.
But this is a scenario we see more and more often. A transmitter won't show power and a tweaknician cranks every tuning slug to a random position nowhere near where it belongs.
This can become a real slog if one or more of the IF/RF transformers goes bad. Seeing a proper peak for each one (or not) is your only clue if this was the cause of the original pre-tweak fault.
Oh, and about that "extremely tedious" part. Now you know why we tend to hot-wire the radio for AM only. Much cheaper.
73
K
Kachina
Guest
Thanks for the help. Just to clarify, I have full output on AM, and on sideband I have full carrier with no modulation when I key up (in fact when I key up on LSB or USB the carrier is about a watt or so more). However, it is always transmitting around 27mhz and the frequency and output doesn't change no matter what channel crystal is selected. You probably already understand all this, I just want to make sure I'm making sense when I describe the problem.
As far as tuning in the transformer cans, here are some stupid questions:
1. Are the oscillator and mixer circuits always running even when not keyed up as long as the power is on?
2. If so, does that mean for T1, a 5mhz IF transformer connected to output of V1 when in SSB, but bypassed in AM, I would:
A. Switch to LSB (5.648mhz crystal) and then tune in either on a receiver or spectrum analyzer (I have a TinySA) to 5.648 and try to peak the signal by adjusting the the slugs? Does this cause a problem when the frequency changes to 5.645 in USB mode or are they close enough to not matter?
3. Then for T2, switch to AM mode and peak 5.6465? etc?
Am I on the right track? Is there a probe I should be using for an antenna near the can or will it put out enough signal where just having the receiver/TinySA close to the transmitter will be good enough?
Sorry for the novice questions, I've never had to align transformer cans before beyond just peaking for max receive on receivers or max output on transmitters. In fact I was thinking about unsoldering the connections to the cans and then feeding the input side with the right frequency with my waveform generator and peaking it that way, but that seems like a ton of work and overkill.
K
Kachina
Guest
Here's another update.
So I decided to use the TinySA I got to see if I could monitor the signals coming out of the oscillators and track down where the issue was. Full disclosure, this is my first time using the TinySA for something like this so I'm not sure if what I'm seeing is a problem, but it seems like it is. First I wanted to make sure that I am getting the 5.0485 Mhz out of V1 (12AX7). This was the reading I got when measuring from in the input of T6 (5 mhz IF transformer):
That first spike is the the 5.0485 mhz. There seem to be harmonics at about every 5mhz or so. I figured this shouldn't be a problem because that's what I thought the 5mhz IF can was there to filter out. However, when I measure from the other side, this is what I get:
As you can see, the spikes are still there. Adjusting the slug in the can did nothing to null out these signals. I'm not sure if the TinySA is picking these up from other parts of the radio or if this is a problem. i decided to check the output of V6 on channel 1 to see if it was mixing the 5.0485 and making the 21.3185 as you described, however this is what I'm seeing:
A very tiny spike almost in the noise around 10.1 Mhz.
Am I missing something, or is something very wrong?
So I decided to use the TinySA I got to see if I could monitor the signals coming out of the oscillators and track down where the issue was. Full disclosure, this is my first time using the TinySA for something like this so I'm not sure if what I'm seeing is a problem, but it seems like it is. First I wanted to make sure that I am getting the 5.0485 Mhz out of V1 (12AX7). This was the reading I got when measuring from in the input of T6 (5 mhz IF transformer):
That first spike is the the 5.0485 mhz. There seem to be harmonics at about every 5mhz or so. I figured this shouldn't be a problem because that's what I thought the 5mhz IF can was there to filter out. However, when I measure from the other side, this is what I get:
As you can see, the spikes are still there. Adjusting the slug in the can did nothing to null out these signals. I'm not sure if the TinySA is picking these up from other parts of the radio or if this is a problem. i decided to check the output of V6 on channel 1 to see if it was mixing the 5.0485 and making the 21.3185 as you described, however this is what I'm seeing:
A very tiny spike almost in the noise around 10.1 Mhz.
Am I missing something, or is something very wrong?
K
Kachina
Guest
I also just want to add that if anyone has a photos of the underside of the original version of the Mark 3 - the first version without the AGC etc, unmolested, that would be a life saver.
K
Kachina
Guest
Thanks for this! Tremendous help. I already found some bizarre differences I have fixed. One thing I noticed I don't have this 5w resistor, do you know the value?
That's not original. Original would be a 4.7 ohm carbon comp resistor. Someone replaced it.
K
Kachina
Guest
OK, I'm assuming it's R64, connecting the AC coming in from the transformer to the center leg of the rectifier? If so I'm glad it happened to stick out in your picture because I just checked and the resistor I have in place was shorted out - which explains some of the voltages running too high.
It is. We adopted the habit of using a 2-Watt part to replace it. A high-mileage radio will have all the color bands burned to black and brown.
73