[TMC] TMC PAL-1K Low Output Power Issues

[John Vendely]

On 1/26/2023 11:52 AM, triodes at optonline.net wrote:
> Hi John,
>
> Thanks again for your guidance!
>
> I did uncover one thing today. Someone had changed L222 in the cathode 
> circuit of the 6146 (see image. It's the large adjustable pi-wound 
> choke with the green winding). This choke plus the other components in 
> the 6146 cathode appear to be a part of the RF negative feedback 
> circuit, and this is where the sampled output from the PL-172A is 
> injected into the driver stage. L222 is 750 uH @ 100 Ma, and I have no 
> idea as to what value choke replaced it, but it is probably a similar 
> value, judging from it's size, etc. My guess is if the replacement is 
> greater than 750 uH, it should not present a problem.

<<Yep, that's the summing point for negative feedback from the IPA 
output, and also some cathode feedback from the 6146 itself.  Ages ago, 
there was probably a fault in the driver stage resulting in very high 
cathode current which burned up L222.  This was originally a typical 
pi-wound axial RF choke, and there are a few other chokes of the same 
type elsewhere in the amp. Apparently not having the correct part 
on-hand, someone replaced it with a slug-tuned variable inductor, 
presumably of similar value.  Hopefully they adjusted it to 750 uH 
before connecting it.  Unloaded Q of this part is not critical, since 
it's heavily de-Q'd by the 220 ohm cathode resistor, so it may be OK 
as-is if the value is correct.  The best thing would be to detach the 
inductor and measure it to be sure.  Adjust it to 750 uH, if needed, and 
go with it.  But if it can't be set to 750, replace it with a suitable 
inductor of the correct value.

I can't see it very clearly, but it appears that R210, originally 220 
ohms, was replaced with a 390 ohm.  There's probably some frequency 
compensation going on with this RLC combination, so best to stick with 
the original component values; you have enough problems to deal with 
already.  Doubtless you wouldn't hesitate to replace any resistor which 
was found to be 77% high, as this one is.  BTW, R222 is another one of 
the resistors that tends to go bad from overheating, for some reason.

Is C225 still in place? I can't see it in the pic, but it should be a 
button-type mica bypass capacitor bolted to the chassis like the ones in 
the top of the pic.  Gain will be very low if it's missing.>>

>
> I'm thinking of replacing it. I have 1000 uH and 600 uH available. I 
> doubt if the value is that critical....what do you think?
>
> With regard to the plate tuning, loading, and coarse loading settings, 
> here are a few of the settings I had recorded when I last fired her up:
>
> 3-4 Mhz Band: F=3850 Khz, Plate Tune: 076. Loading: 032. Coarse Load: 1
>
> 6-8 Mhz Band: F= 5000 Khz. Plate Tuning: 063. Loading: 034. Coarse 
> Load: 2
>
> 6-8 Mhz Band: F= 4500 Khz. Plate Tuning: 052. Loading: 034. Coarse 
> Load: 2
<<Is this a typo?  Did you really mean the output bandswitch was set to 
the 6-8 MHz band while attempting to operate at 4500 kHz and 5000 kHz?.>>
>
> Note that I can tune the PL-172A to resonance, as evidenced by a dip 
> in Ip, and as I tune off resonance, the output power goes up and 
> eventually the overload breaker trips. This is true on all bands, and 
> at resonance, the output power is around 50 watts on all bands, and 
> going substantially off resonance will increase the output power to 
> around 150 watts. It is when going beyond that when the breaker trips.
>
> I noted your earlier comments about tuning this amplifier only to 
> resonance, and balancing the the Ip and Isg.
<<Yes, the amplifier should never be run off-resonance, this indicates 
something is seriously wrong, either with adjustment or a component 
failure.>>

73,

John K9WT