[TMC] PAL 350-500 restoration notes

Wed Jan 5 11:49:39 EST 2022

Great job Chris!

73 & Happy New Year

George - W1LSB

> On Jan 4, 2022, at 7:08 PM, Chris Bolkan <bolkyboats at gmail.com> wrote:
> 
> Hi TMC lovers!
> 
> I bought a couple of TMC PAL 350-500's back to life and here are the
> biggest things I found:
> 
> First, I have to state my qualifications for writing this up. None
> whatsoever! I have been communicating with John Poulton who has been a
> fantastic resource and become a friend. I read his entire writeup as I was
> doing these. It was incredibly helpful. These notes simply supplement what
> he wrote up. I am relatively new to ham radio and a complete newby to
> tetrode amplifiers. A lot may be just plain wrong, but if I had known this
> stuff when I started it would have helped me a lot.
> 
> 
> 
> *1)      **PAL 350 and 500 power output capabilities-*
> 
> The big HV power transformer for both the PAL 350 and PAL 500 is the same
> part number. I do not believe that other than changing the final tube tubes
> and overload trip point in the power supply there is any difference between
> the PAL 350 and 500 potential output power. In fact, the PAL 350c switched
> to 4CX350’s but kept the same power rating until they made the PAL 500.
> 
> 
> 
> *2)      **To Key or not to Key-*
> 
> I have come to the conclusion based on looking at the circuitry of all of
> the 350-500 variants that these amps are not intended to be “keyed” with
> every push-to-talk button push or telegraph keystroke. I believe that they
> were intended to be operated turned on and biased on continuously. Any
> “keying” was most likely done in the signal provided by the exciter. This
> did not make sense to me at first because of my experience with
> noncommercial amateur radio amplifiers, but as I have worked on these it
> now does. They are rugged and underrated enough to be biased on constantly.
> In addition, the amplifier operated in this configuration will not produce
> any key clicks.
> 
> 
> 
> *3)      **I tried Bias Keying anyway-*
> 
> I did rewire one power supply to “bias key” the finals. It was very simple
> to do and works quite well functionally. I do not know if keying the amp on
> and off with bias produces key clicks that are objectionable or not, but it
> greatly reduces standby power consumption, power relay clicks (more on this
> later) and is elegantly simple. The schematic for keying in this manner is
> in the (
> https://www.tmchistory.org/tmc_manuals/manuals_db/pal-500/tm_pal-500_5_66.pdf)
> PAL 500 manual on John’s TMC history site. I know TMC sent out some power
> supplies and amps with bias keying configuration (I have one), but they
> also sent out a change notice to go back to the original wiring (mine was
> converted back to original but evidence of bias keying was there)
> 
> I suspect that the bias keying idea came along with the advent of the PAL
> 500, but it ended up not compatible with the fleet of existing units in the
> field or it induced eventual screen grid resistor burnout, or maybe the
> amps produced key clicks. Who knows? I did increase the screen grid
> resistor wattage per John’s recommendation. I like this mod and plan to
> keep it unless actual transmission on-off clicks are objectionable.
> 
> 
> 
> *4)      **Testing output tubes-*
> 
> Unknown condition final tubes presented a big problem for me. The best way
> I found to “test” them at home without a tester is to first ohm out the
> filament, then ohm out between the elements of the tube (better if you can
> hipot or megger test between elements). If they test good to that point,
> they are *probably* safe to put into the amp.  Fortunately, the power
> supply does a very good job of protecting things (as I found out many
> times). Before plugging the tubes into the amp, I preset the tube bias
> voltage by measuring and setting it with a VOM on the center pin of the 4CX
> tube sockets with the plates switch on but HV switch off*. I found that
> -60V is a good starting point. Then plug the tubes in, let the filaments
> warm up good and turn on HV. If they are in reasonable condition you might
> see around 150mA of plate current.  I first tried setting the voltage to
> the tube data sheet’s -55V recommendation, but that consistently drew more
> current than I was comfortable with for initial tube testing. Assuming you
> don’t get crazy low or crazy high plate current the tube(s) are most likely
> functional. If you do get crazy low bias current or the HV breaker pops
> from a short, the offending tube is probably weak or bad.
> 
> *You might want to ground the HV while setting the bias voltage as current
> lighting the HV off lightbulb still energizes the HV circuit with a couple
> hundred volts even when it is switched off. I got bit good once and after
> that always ground the HV when the HV switch is turned off if I am going to
> be messing around in there.
> 
> 
> 
> *5)      **Setting the bias-*
> 
> At this point you can turn both bias pots full CCW and set quiescent
> current per TMC setup procedure. It looks like TMC wanted about 70ma of
> bias current per 4cx250 tube. There is about 20ma of bias current flow
> without the tubes which is why they want the first tube set to 90 and the
> second to 160. That had me stumped until I figured it out. The same offset
> applies to 4CX350 tubes, but the final bias current is higher.
> 
> 
> 
> *6)      **Neutralization. *
> 
> I went down this rabbit hole and spent a bunch of time trying different
> things that led to different settings on the neutralization cap but nothing
> really made a difference in the end. I tried setting it by minimizing
> leakage signal through the finals using a signal generator feeding a signal
> into the driver and looking with a scope on the output terminal, and tried
> setting it to 1V per the procedure in the manual using a high frequency
> high impedance detector probe circuit I found on the internet. The two
> procedures led to different settings on the neutralization cap, but neither
> setting seemed to produce instability. So, I’m at a loss on this.
> Fortunately, it just seems to work. If anyone can provide good input on
> this, I will update this, or just send the procedure to John.
> 
> 
> 
> *7)      **Dirty/contaminated relay contacts*
> 
> I found one problem to be particularly difficult to diagnose and common to
> both of my units, which means it might be common to others. Fortunately, it
> ended up being an easy fix. I would see varying amounts of drive to the
> finals for a given drive from the signal generator. Sometimes there would
> be great Drive. Sometimes less. And sometimes I could watch the drive start
> out high and slowly drop to a lower level. Once in a while it dropped to
> nothing. Sometimes turning on and off the plate switch would bring it back
> to full or partial drive and then it might stay or slowly drop off or
> slowly increase. It was very hard to troubleshoot. I did not know if it was
> in the circuit of the 6CL6, 6146, ADLC, feedback or what. I disconnected
> the ALDC circuit from the driver stage. Problem was still there. Then one
> time the drive dropped out, I moved the scope probe from the output of the
> 6146 to the output of the 6CL6 and it was still out, so that narrowed it to
> the circuitry around the 6CL6. I would frequently check voltages. The +150,
> -150 and -155 bias were always present no matter how much drive it was
> putting out. The bias on the 6CL6 was rock solid and was always the same
> voltage generated in the power supply. What I finally discovered was that
> the -150Vsupply voltage, while always present, was changing within a range
> of about 5V. It would drop to -145 or so and the driver would drop out. At
> -150 it was full drive. In between the drive was in between. It turns out
> relay K102 in the power supply* had something weird with the contacts. It
> always produced something between 0 and 5 volts drop in the contacts.
> Enough to change the bias of the first stage dramatically, but the voltage
> never dropped enough that it was really noticeable. Bottom line is that
> both the -150 and -155V supplies have a profound effect on the bias of the
> 6Cl6, and the -150V supply goes through a relay.
> 
> I took the relay apart and used Deoxit and emery paper on the contacts and
> now it works great. My other power supply was doing the same thing and
> cleaning the relay fixed it too. This really changed the way the amp
> behaves. It is completely consistent now. This was a funky problem since
> every fraction of a volt drop through the relay of the -150V rail greatly
> affects the gain of the 6CL6!
> 
> 
> 
> *Both of my power supplies are later models for the PAL-500. Early PAL-350
> power supplied are a bit different circuit. I have not traced them out to
> see if the relay in them can produce this same problem but it is certainly
> worth a look if your PAL 350 is misbehaving.
> 
> 
> I hope these notes help someone else working on PAL 350-500's.
> 
> 
> Best regards,
> 
> Chris
> 
> KI7NXY
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