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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Sun 08, 2012 4:10 pm 
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Work got quite busy right after Christmas. I finally found time to get back to this project this morning. First I installed a 27K/2W Q-killing resistor across the driver plate RFC, per the schematic and Knight mod instructions. No change, the 250KHz oscillation could still be made to appear around the max setting of the driver tuning cap. But a clean 30 watt signal on 40 meters could also be obtained by just slightly detuning that driver cap. Output typically measures about 30 watts on the dummy load.

Next I changed out that non standard 2.5 mH PA plate choke. I had a Miller 5mH choke which was also a 3-pie type, but has an iron core rather than the phenolic core of the originals. Current rating is 160mA, should be adequate here. Again, no help and no noticeable changes in the tuning behavior.

I also found two used 6DQ6B GE tubes in my junk box. Both tested low on emission on my Hickok 538A. Minimum Gm is supposed to be 3800. These read 1600 and 2200. My original Sylvania 6DQ6B is hot, OTOH. It measures about 8000 Gm. I may try one of the old duds in the Tx anyway, just for grins. May also get one or two spares eventually.

My other options seem to be to continue changing out bypass caps, all disks, one at a time on the various HV points in the Tx strip- also to get the B+ distribution right with the correct value dropping resistor(s). I know for sure one is lower than the correct value, due to questionable mods.

It will be interesting to see if a low emission PA tube kills the 250KHz oscillation without totally killing the 40m output.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60).......Solved
PostPosted: Jan Wed 11, 2012 2:39 pm 
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A breakthrough this morning- on the advice of 2 or 3 of you guys, I finally got around to measuring R6, the PA grid leak resistor. Bingo! I've found the culprit. R6 was way high out of range at about 193K ohms, should be 47K. Well, I have literally hundreds of this value 1/2W carbon comps, NOS. Popped a new one in and now I see no LF oscillation at any setting of the Driver Tune cap.

Thanks for all the great help, guys! I'm not the best troubleshooter. Eventually I fumble my way through. Still plenty to do before I'm finished with this radio, but I think it will be all downhill from here.



The grid leak resistor is at the far end of a short piece of shielded wire coming from the PA, next to L5 and C30,
the Driver Tune cap. The original resistor is seen high out of spec on the DMM.

ImageImage

A clean looking sinusoid at ~7010KHz, shown at .05us/div, 0.5us/div, and 5us/div.

ImageImage

Image

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Last edited by Clutter on Jan Thu 12, 2012 1:54 pm, edited 1 time in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Thu 12, 2012 12:21 am 
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It's nice to see a thread come to a happy ending. (Or maybe there's more?)Great job!


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Thu 12, 2012 2:56 am 
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And I bet you're really happy that you didn't accept the way it was working and kept at it until you
had the problem nailed and fixed! That is always a good feeling, congrats!

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Fri 13, 2012 3:25 pm 
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Thanks, guys-

So many of you had good suggestions. I'll continue on a while with this project, but it becomes simply a restoration now, and is therefore a lot easier to deal with. Will update this thread with progress reports from time to time.

The radio is now air-worthy, so I'll proceed to get it working with a VFO and set up some relay T/R switching, then it can debut on 40CW some morning. I may build in a little Curtis keyer PCB (reed relay switching) and a T/R relay, and swap in a 1/4in stereo jack for a keyer paddle. Everything will be 'no holes' style so the radio can be quickly restored to stock later if desired. It will have a couple of miniature coaxes passing through vent holes in the cabinet back with inline jacks for Rx Ant and Rx Mute.

Now I plan to turn to my next tube project- restoring an old Lysco 600 transmitter given to me by a retired local ham last year. That's more of a 50's design, with a 6AG7/807 tube lineup. I think he'd be happy to hear it on 40m again.

Between finding this great forum and also finding a tube tester through these pages, my interest in tube gear has been reawakened. I cut my teeth on tube gear in the mid-60's, and all my early homebrew projects used tubes.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Feb Thu 02, 2012 9:41 am 
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Clutter wrote:
hbr nut wrote:
Clutter wrote:
Something is far enough off nominal values that the output peaks with the Drive Tune cap at full unmesh. I know that can't be right. I'd really expect the normal setting to be somewhere near half mesh. So far I have only tried the radio on 40m. BTW I hear this old novice Tx can do maybe 50 watts out on a fresh set of tubes.


I've worked on a number of these transmitters and they all have this issue. I'm not sure what was going on at Knight when they came up with this set up. The solution is to put a capacitor in series with the grid tuning capacitor to reduce the overall capacitance. I don't remember what value I used, but I imagine around 25 pF is a good starting place.

Expect about 35 watts output on 80 thru 15 meters, about 25 watts output on 10 meters and forget about 6 meters.

Darrell


Thanks, Darrell-

This makes a heck of a lot of sense. I recorded Drive Tune Settings vs. band for 80 through 10m. They were 3, 1, 3, 1.5, 2 respectively, and that's on a dial marked from 0 to 10. So that says- measure C30 full mesh and insert a series cap about equal to that. I'll do it. Great suggestion!


I've had a little more time to think about this. I think they engineered these radios so that there were no settings which could resonate the driver plate on a harmonic on any band. Ideally, there should be only one peak per band. Should you move the correct resonance to mid scale or higher, possibly you'd see a second peak appear at a lower setting of the Drive Tune knob. I think I'll leave well enough alone.

Meanwhile, I got hold of some smaller, modern HV electrolytic caps, so I'm going to make up a string of them on perf board to replace the PS output cap C29.

My efforts now are focused on building up a booster amp to get enough output from my VFO to drive the 6HF8 stage. Once I have that plus a keyer setup capable of handling cathode keying voltages, it will be time to put the little transmitter on the air.


David

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Fri 15, 2013 6:32 am 
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Just an update for the group-

I have so many irons in the fire I still haven't gotten around to cosmetically finishing my T-60 yet. It still has the case rust and a few other such blemishes, but OTOH having gotten some good help to find the problem, this little radio straightened right up and works as designed once again. Recently I put it on 40m in the evenings and have worked S59N and 9A5M without difficulty. Output is around 30 watts to a 70ft half wave vertical wire fed with an LC tank. The coax is connected to a link winding. I can't even find my J-38 right now. I modified a HB keyer with Curtis 8044 that I had on hand, adding relay output, and it's a lot easier to run it with that keyer anyway. That straight key tires me out.

The original three electrolytics including the yellow one seen topside in this view have all been replaced with modern parts.

Image

I'd like to get a few critical on-air reports on the CW note, but it sounds pretty good in my receiver. Cathode keying generally sounds pretty nice.

This working 40m VFO project needs to be completed and then this will be a truly usable setup on the bands. Running with crystals is a real pain. I have a nice, compact cabinet for it that's just the right size. Snug, just enough room for various additional electronics in the back. A little sheet metal work, a simple amplifier circuit to bring the output voltage swing up enough to drive the grid of a vacuum tube- and I'm good to go... I'll get it done one of these days soon, since that VFO is also needed for a couple other projects also underway.

Image

David

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Fri 15, 2013 4:35 pm 
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Does it happen with the crystal removed?

Have you tried 20 or 15 yet?

Once the soldering and wiring is straightened out and if the problem is still there I would try oscillation snubbers at the grid, screen and plate pins, A 10-22 Ohm carbon right at the grid, a 100 Ohm at the screen with a .01 disc bypass right close to either side. A GDO with diode mode and tuned up from as low as it goes thru VHF might help pinpoint a problem.

I do not care for RFC-1 and RFC-2, try a 3300 Ohm or so carbon resistor across them. Ive had lots of trouble with LF parasitics in the past with similar.

Carl


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Sat 16, 2013 3:10 am 
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Sounds like some good suggestions. Have only worked on 40m so far. I'll try 20 and 15 with a dummy load right away to see if anything obvious pops up. Wish I owned a spectrum analyzer. Maybe my old Heath dipper has a diode mode. I'll look into that. Resistive snubbers right near the tube electrodes sounds instinctively like they might be a good idea. FWIW the radio has a 'standard' plate parasitic suppressor with small inductor wound around a carbon comp.

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I do not care for RFC-1 and RFC-2, try a 3300 Ohm or so carbon resistor across them. Ive had lots of trouble with LF parasitics in the past with similar.


My radio had the 27K 2W carbon comp across RFC2. This was a mod from Allied Radio at some point. Early models didn't have the resistor. There is no swamping resistor across RFC1, as I remember. I'll look into that.

I haven't tried it without crystal. I think the answer would be no- zero output with no crystal. I'll try to remember next session. It shows no output on the SWR meter set for FS at 30W, as I remember. But them we'd be talking about an LF oscillation and the meter is way less sensitive as frequency decreases.

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Last edited by Clutter on Mar Sat 16, 2013 10:43 am, edited 3 times in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Sat 16, 2013 10:24 am 
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I took a look on 20 and 15m and it's not pretty. While 40 looks clean on my Tek 465B, 20m and even more so 15m have some sort of modulation on the waveform. Haven't had a lot of time to think about what I saw. It didn't look anything like the relaxation oscillation I had before. Rather, it looks like the amplitude of the 'sinusoid' decreases slightly every other cycle- in a very regular way. Like full amplitude, ~85 percent, full amplitude, repeating every other cycle. Weird. I'll get a scope photo later and put it on here. Anyway, I suspect I'm fairly clean on 40m but I wouldn't put this TX on 20 or 15 right now. BTW that wasn't my plan anyway. I figure I would have used the radio mainly for occasional ragchews on 40m only- FWIW.

I'm going to wring this thing out much better now before I let it see much more air time. Will do a little more measuring of cap and especially resistor values. Will also plan to add the suppression resistors/caps that Carl mentioned. I'll try to get that done soon, before putting the radio aside for a while once again.

It broke my heart to see how badly this radio had been abused by a previous owner. In addition to front panel and chassis damage, someone had coveted the audio components in it. Not only did they swipe the 12AX7 and 6DR7 tubes, but they also removed every single component and wire related to the AM screen modulator. All they left were the two original 9-pin miniature tube sockets, and the MIC phono jack on the front panel. Yikes! It was not quite a basket case, but it was pretty sad when I got it. It'll see better use now, and will finish out its days in the hands of a ham who started out as a Novice on 40 CW with one of these. :wink:

I didn't buy the radio as any sort of collector item, just wanted to put it on the air out of sentimentality- and I will still do that. It's a low priority right now. I have so many other projects in various stages of construction. Right now I have begun doing the sheet metal work for the VFO. That will take me a while to do it the right way- and I can use that VFO almost immediately in 3 different projects going on right now.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Mon 18, 2013 8:07 pm 
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Clutter wrote:
I took a look on 20 and 15m and it's not pretty. While 40 looks clean on my Tek 465B, 20m and even more so 15m have some sort of modulation on the waveform. Haven't had a lot of time to think about what I saw. It didn't look anything like the relaxation oscillation I had before. Rather, it looks like the amplitude of the 'sinusoid' decreases slightly every other cycle- in a very regular way. Like full amplitude, ~85 percent, full amplitude, repeating every other cycle. Weird. I'll get a scope photo later and put it on here.
Just wondering if you see the "every other cycle" thing on 20, but an "every third cycle thing" on 15?

Curtis Eickerman

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Thu 21, 2013 12:39 am 
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Actually, no- not as I recall. Clearly, I need to get back to this and dig into it now. I'll say this for now- the 20m waveform was aberrant just as I've described, but the 15m display was a bit more complex. I'll take some photos and post them soon.

I have too many pots on the fire. Have been merrily drilling and filing away on the shielding panels for my VFO enclosure, the past few nights. It's a bit tedious, but is generally moving along. I'll get back to the T-60 later tonight.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Thu 21, 2013 3:49 pm 
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OK, I got a few photos of the waveforms last night. Let me say up front that the measured PERIODS of all these waveforms imply the fundamental frequencies are correct for the appropriate band settings, i.e. I do get 7, 14, or 21MHz as appropriate. I believe the settings for all photos were 50ns/div and 20V/div.

20 meters with Driver Tuning adjusted carefully, 'spot on'. It looks clean.
Image

20m with Drive Tune mis-adjusted. Depending on which side the cap
is set, either some of each alternating positive or negative peak is
'missing' (but not both at the same time). This is seen with only
slight mis-tuning of the knob. The setting for a 'perfect'
sinusoid is very touchy. Note that this differs greatly from 40m
where the waveform remains quite clean with any mis-tuning to
either side of the peak.
Image

15 meters- the scope has trouble syncing to this waveform.
Image

15m- Mistuned (Driver Tuning quite high, like half mesh).
A variety of different waveforms can be seen as the Driver
Tuning is rotated. At all times the fundamental remains 21MHz,
apparently.
Image

I don't think I'm seeing a low freq relaxation oscillation at work here as before, but perhaps it's happening just the same. It now looks to be nothing like what I saw when the PA grid leak resistance was way too high.

My next move is to add some resistive suppression here and there, as Carl suggested, and see what that does...

Maybe I AM seeing an "every third cycle thing" on 15, Curtis. It's kind of punching me in the eye, isn't it? (last photo)

David

P.S. Here is a photo showing where I'm sampling the waveform. 10x probe across the dummy load, which is four NHG-200 non- inductive resistors in parallel on a stamped type TO-3 heatsink. Any potential measurement errors with this setup? I figure the high impedance of the probe is not affecting the impedance the transmitter sees- much(?) I know, I ought to have a proper pickoff circuit in a shielded box...

Image

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Last edited by Clutter on Feb Wed 17, 2016 12:52 pm, edited 1 time in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Wed 27, 2013 5:07 am 
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Hoo boy-

The more I look at the decrepit wiring in this radio... I keep finding things. There were two 'new' wires someone added in replacement of the kit originals. These are the wires from the tank circuit down to the band switch under chassis. The tap between the 6m and 10m sections of the tank coil and the tap at the beginning (80m) end of the tank coil had done some sort of funky melting where the insulation of the two respective wires had begun to merge. It almost looks like some sort of RF welding of the plastic insulation had taken place. It took considerable effort to separate the two. Much tougher than splitting zip cord. They had not yet shorted, however.

I'd throw out the reminder that I had originally found a couple of puddles of wax under the location of the original plate RFC (since replaced). I never saw that in the other two T-60's I have owned. I guess that does speak of a history of instability with this particular PA.

Then the one that really takes the cake- there is a 10pF disk cap across the grid pin of the 6DQ6B PA. Looks original. One end is grounded. Well, originally, that ground would have consisted of a short ~3/8in of bare wire from the tube pin over to a ground lug quite close by. Some tinkerer has rerouted this connection over to the Accessory socket and then back over to that ground lug the 'long way' to pick up a ground. I mean, we're talking about an extra 5in of unnecessary lead inductance. I bet *that* is doing wonders for stability. Heh!

I have my work cut out for me. It hurts to look at the state of this radio under chassis. I can see why, once I had it running clean on 40m, I just wanted to button it up and forget about it- and pretend it was somehow right. It's not pretty. Many bad solder joints, many irrational wiring mods. Some person or persons have been into this radio who have no idea what they are doing. On those days when I'm feeling clueless, I just remind myself that these guys make me look like Einstein. Siiighhhh...

I'm working up to adding the snubbers at the PA grid and screen. Will keep looking at 40/20/15 as I phase in these changes. BTW I see I have not yet changed the output electrolytic in the CRC filter. I now have some modern miniature parts so I'll get that done first of all- but had already tried known good bathtub caps in parallel at that point and noted no change in the the LF oscillation at that time. Nonetheless, I'll do that first thing...

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Wed 27, 2013 5:25 am 
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While the snubbers are a good idea I'd suggest fixing the faults you just listed first then seeing how the signal looks.

Ground loops can do strange things which is why I always make sure that if possible new components are grounded to the same point as the original components were.


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Fri 29, 2013 1:03 am 
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Earlier today I corrected the grounding problem with the 10pF cap across the PA grid. Verified that it indeed measures 10pF out of circuit. I also added a 22 ohm 1/2W carbon comp in series with the shielded lead, right at the grid pin. I also replaced the PS output cap with a series string of modern electrolytics with 105K RN-60 type equalizing resistors. Old value was 20uF 600V, new string is 90uF 750V. I also replaced the green 6m/10m tap wire and the orange 80m tap wire going between the tank coil and the band switch under chassis.

None of these changes has made any observable difference in the 40/20/15m output waveforms. I'll keep at it. There is still plenty more to check.

I'm wondering if I might be causing any measuring artifacts by improper scope lead setup, perhaps some sort of a ground loop? Will think about some alternative ways of making the measurement. But it still looks marginal on 20m and the waveform is just funky on 15m. Still looks very nice on 40m.

In addition to adding snubbing in the screen lead to the PA, I need to re-verify that the screen voltage is right. Believe it was, though. Do indeed need to take a slow and very thorough look at the wiring sequence per the manual instructions, since such extensive 'shotgun style' wiring changes have been seen under chassis.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Fri 29, 2013 1:28 am 
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Clutter wrote:
Maybe I AM seeing an "every third cycle thing" on 15, Curtis. It's kind of punching me in the eye, isn't it? (last photo)
I kind of wondered.

I knew the Heathkit DX-60 design did something that I thought the T-60 might also do.

For the DX-60, the HG-10 VFO for 20, 15, and 10 was actually always operating in the 7 MHz range and depended on the driver stage being tuned to a multiple of the oscillator (i.e., 2X, 3X, 4X) to drive the final at either 14, 21, or 28 MHz.

I believe that is why you are seeing the every other cycle thing on 20 (X2 frequency multiplcation), and every 3rd cycle on 15 (3X frequency multiplcation). Essentially the oscillator kicks the driver in the seat of the pants and it then rings at the appropriate harmonic for an extra cycle, two cycles, or three cycles until the next kick in the pants.

I am suspecting that might be completely normal for the point where you are monitoring the signal.

At the output there will be additional suppression of all except the desired signal by the final stage tuned circuit match into the antenna.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Mar Fri 29, 2013 9:26 am 
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Hmmm-
Well that's not as dire as what I imagined you were getting at, Curtis. Anyway, it's sure not a pretty waveform, not by a long shot. Kind of looks like it is 'ringing down' on 15m. And that is the PA output I'm looking at. The 10x scope probe is clipped across the 50 ohm dummy load on the PA. The PA load is open construction, not fully shielded. (See the photo in a previous post, above) May be a measurement error with the scope, maybe some sort of ground loop or unwanted coupling to the scope electronics, etc. I need a better signal pickoff.

On 20m, with real touchy driver tuning, you can get the waveform to look like a fairly decent sinusoid. Wish I had a spectrum analyzer. I'm going to build up a little W7ZOI AD8307- based power meter this spring. Parts are on the way. Maybe I can cobble together a workable 'measurement receiver' with some bandpass filtering. This little radio will pretty much only see light duty on 40m for occasional ragchewing or a little DXing. BTW I'll be building a fully- shielded 40dB tap in connection with that power meter project. Probably better than what I'm working with right now.

It would be easy enough to cobble together some additional bandpass filtering to follow the PA output for some further testing. Might be that would produce something more resembling a clean sinusoid on 15m. FWIW the relative settings on the Drive Tune, PA Tune, and PA Loading all look about right on each respective band. I had previously confirmed a correct grid dipper dip on every HF band in both the driver and the PA circuits, with appropriate settings evident on the respective variable caps. The period of each PA waveform I see is right for each respective band.

"Completely normal?" If so, that's music to my ears. It's certainly a doubler or tripler setup just as the DX-60 was. In time, I'll find the opportunity to see it on a spectrum analyzer. That will tell all.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Nov Thu 26, 2015 2:41 pm 
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I have both admiration and sympathy for those working to stabilize and behavior-sanitize the Knight T-60, the electrical design of which is a variation of the design used in the Hallicrafters HT-40. It's a electrical design with built-in problems--its final is non neutralized, and high-L driver-to-final tuning/coupling design only tunes through an in-band peak by luck--but at least the mechanical design of the HT-40 is RF-literate. From an RF engineering standpoint, the T-60's signal flow v mechanical flow is crazy. Output-network and driver-to-final tuned circuit switching are combined in one compact bandswitch, with the switch many inches away from both parts of the circuit it serves. Side-by-side solenoidal coils in that bandswitch can and do intercouple to weird effect. The crystal socket is many inches away from its associated oscillator/driver tube, requiring that both leads to the crystal cross the cable between the final and its grid tuned circuit. The oscillator driver tube--unshielded until modifications were made after many T-60s were in the field--is only about 3 inches away from the output-network coil, making the "RF gozinta" couple directly to the RF "gozouta."

It's as if an early sketch of a layout done by an audio guy was put into production by mistake: The rig's AF tubes start right at the front panel and head straight to the final amplifier in a nice, tight line, whereas the oscillator/driver tube is far from the front panel, which is to say "everything it should be close to"!

I just picked up a beat up, obviously dropped T-60 on Ebay for $30, and rebuilding it in an RF-literate way--and adding 30 meters somehow--will be the second big clutch of tasks. The first task cluster will be to make it work like it did out of the box so I can feel all the pain inflicted by its bad design.

To that point, however, I do have some suggestions that may help those trying to kill squegging and low-frequency parasitics in their T-60s (and any other transmitter that may harbor them). The first thing to try--and leave in place even if it doesn't work--is to add a 100-ohm carbon comp or carbon film resistor between the 6DQ6's screen terminal and the outside world. That's right: Neither the screen feed wire nor the screen bypass capacitor should connect directly to the screen terminal. The reason this change is necessary is that beam power tubes, especially beam power tubes designed for use as TV horizontal-sweep amplifiers, are prone to oscillations caused by the screen exhibiting negative resistance when the plate voltage dips below the screen voltage (as will be the case when the tube is putting out anything like design-maximum power as an RF amplifier). That negative resistance can cause oscillation if a suitable LC or RC circuit is connected to it. Adding a low positive resistance in series with the negative resistance works against that tendency. (I recommend also adding a 470-ohm resistor between the screen bypass and the screen power-feed line, as this will help the bypass do its job.)

I present an evolving writeup about this effect; web-search for "David Newkirk" and "negative resistance" and you'll be able to jump right to it. Screen and/or plate negative resistance is why the 807 drove RF folk nuts for decades with pops and clicks on CW and splatter and distortion on AM. There's even a reference to killing 40-meters-destroying key clicks from a T-60... :-)

Returning to the T-60, two more things:

Because the T-60's 6DQ6 final is not neutralized and because the set's mechanical design is so RF-illiterate, the final may operate as a locked oscillator--locked, that is, to the signal from the crystal oscillator. (If you can pull out the crystal and still see RF power output, there's your proof. Wrote George Grammer, W1DF, in his review of the T-60 in May 1962 QST: "The 7-Mc. oscillation was only observed when the amplifier was lightly loaded and crystal was pulled out, but on 3.5 Mc. the crystal had a hard time getting control under any condition of final-amplifier loading.") This is the cause of the hysteresis (output peak occurring at different TUNE/LOAD settings when tuning upward and tuning downward) and output-power jumps users of the T-60 (and the HT-40, BTW) experience. Allied's later modifications included additional shielding and adding a 27-k resistor across the 6HF8 pentode's plate RF choke, and these may help, but I'd go with the fix described by Grammer: Solder a 22-k resistor across L2, and solder a 4.7-k resistor across L1.

If the 100-ohm 6DQ6-screen stopper resistor doesn't do away with the set's low-frequency parasitics, leave that fix in place--really--and then also try adding 220- or 470-ohm resistors in series with one or both of the 5-mH chokes associated with the 6HF8 pentode driver. The 6HF8 pentode is a high-transconductance tube, and installing same-value RF chokes in the grid and plate of such a tube sets the stage for tuned-plate, tuned-grid oscillation. A few hundred ohms in series with either or both chokes should sufficiently reduce their Q such that oscillation cannot occur. The effect of those resistors won't be much at dc, and therefore they should not affect the oscillator/drivers' ability to oscillate and drive. :-) Adding just such resistors cured just such an LF oscillation in the 12HL7 driver stage in a homemade transmitter of mine some years back.

Best regards,

Dave
amateur radio W9BRD
radio pages: web-search for "David Newkirk" and "Enjoying Radio"


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Mon 18, 2016 12:53 am 
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Joined: Nov Wed 09, 2011 4:07 am
Posts: 662
Location: MD-DC 20855
Wow- Great information, Dave. I had overlooked this thread for a
good while, and just found your comments. Rereading through all
three pages in this thread, I say "well done!" to Ancient_Hacker,
who pretty much nailed it when he said:

"A 200KHz oscillation is about right if you have a poor screen
bypass capacitor, or a drifted-high grid resistor."

(14 Dec 2011, Page 2)

Everyone was so very helpful while I struggled with this funky
old radio, which I did mainly out of sentimentality- a T-60 was
my novice transmitter in 1963. It was a fun experience overall,
however. Thanks, all! And thanks to Dave for some enlightening
analysis on the shaky design and layout of that radio.

It is working today and I use it occasionally on 40 meter CW.
That VFO is still in the 'unfinished projects' category; I imagine
I will finish it pretty soon. It's something I really seem to need-
often!

David

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