Discussion in 'Amateur Radio Amplifiers' started by AE1P, Jul 3, 2020.
Here You go:
Thanks Neil. Nice and clear closeups.
So the Parasitic Circuit PCB schematically has to look like this (see 1st .PDF file) in order for the DC to get to the plate.
If the mica capacitors are of equal value then the equivalent circuit is one where the parasitic resistor is in series with a cap whose value is 1/2 Cx and that circuit is shunted across the Parasitic Choke coil, PDF file 2.
That lower "leg" appears to be a circuit where the impedance of the lower leg drops with an increase in frequency.
I would desolder those mica caps and check for resistance across them and check for proper values.
IMO, this circuit seems like a weird and unneeded modification of a circuit that needed no changes, unless that 100 ohm resistor is not a carbon film, carbon composition, or an Ohmite (non-inducticve) ceramic resistor that had an inductance that needed to be de-tuned.
But as yet, we have not established that there is a parasitic oscillation occurring at the input ports nor at the output.
Aaah...The plot thickens...
So after pulling the 3-500Z tube, to check for shorts& opens... I reinstalled,
made sure everything was tight...and powered back up amp...
PUFF... there goes 10 ohm resistor #127 and fuse F101 (2ASloBlo)... WTH??
Nothing changed, other than pulled Tube and reseated...
Ok, found a temp replacement resistor,soldered in, new fuse...
power up, this time with tube pulled... no problem, fires up, good
HV voltage, no blown resistors, no blown fuses...
So, I power down, re-install tube...Stand back...fingers crossed, sunglasses on...
throw switch...all good!... again, no blown fuses or resistor... Interesting thing
though, the 3-500Z is glowing red... and stays glowing red for a couple of minutes...
keying amp with dummy loads changes nothing, nothing showing on meters...
stops glowing and hasn't glowed since after several more power ups and
key downs (no rf drive)
Still stumped..as this has done nothing to explain the initial problem,
and ready to delegate this amp to the dust bin for a bit... Although...
I do have a TS-520 I could sacrifice...
Oh, and I can tell you with 100% certainty...
the HV caps are most certainly good...ask me how I know?
I wouldn't sacrifice another transceiver until we find out what's going on within this amp.
R127 is part of the Inrush circuit so something down the line was loading or shorting.
It sounds as if the (EBS?) bias supply may have become faulty.
Short the RF Input connector and Keydown in short bursts and see if the tube plates redden again on various bands.
That's the same year as mine with the same initials, R. H. Mine's still working but I bought a new tube, but haven't gotten it yet.
Nothing...ran the all bands on band switch turn plate and load every which way...
in short and long bursts...
no red tube...nothing on meter...nothing out of the ordinary...
3-500Z shouldn't glow red with just "idling" current, even at zero bias and +3kV. With zero bias and +3kV it should draw <200mA and of course that should show on the Ip meter.
We bias the 3-500Z mostly just to save power, but it doesn't absolutely need operating bias. We use cutoff bias to save power and reduce the possibility of the tube generating noise in standby.
Neil, you're only a couple hours from Lou W1QJ, it might pay to just drive the amp down to him when you have some time and see WTF is going on with that amp.
I also don't know why they changed the parasitic suppressor to add the capacitors; mine is a 2000 model and doesn't have that board, just a conventional few turns of wire as a coil with a carbon film resistor. Adding the PCB with the mica caps surely didn't save any money, so they must have been trying to solve a problem.
I wonder if this change coincided with changing the single 3PDT T/R relay to two separate, smaller relays? I don't know why they did that, either, although the two small relays might actually cost less.
My original 20 year-old 3PDT relay still works fine after what's probably millions of operations.
And my AL-80A is still operating just fine as well.
As Steve said, if you have someone close who wants to investigate further then by all means seek him out.
And when and if you guys find the problem please let us know.
Has anyone noticed that according to the color code on this resistor it is either a 2.0k or a 2.7k, (color of middle band not well defined - looks purple to me) 5% resistor and not a 100 ohm resistor?
This is impossible unless those mica caps are extremely leaky and with an approximate leakage resistance of about about 50 ohms each.
I'm 99% certain that center band is Black...not purple...
I'll try and magnify & verify later today..