Johnson 500 zorch.....

[kc2ifr]

My poor old 500......
Was on the air in a fine qso when I keyed up the 500 the overload kicked out. I reset it (it does that a lot) and it kicked again. I tried this a couple of times with the same results. I turned off the HV......plate and screen voltage...and keyed her again with the same results......overload kicked. I noticed that the cathode current for the final still pegged even with no plate or screen voltage. I also noticed that the driver plate current also pegged.
Anyway.....I think I either have a bad final (4-400) OR the coupling cap between the driver and the final control grid is shorted applying a high positive voltage to the grid of the final.
Before I work on this.....was wondering if u folks have any ideas. My first task will be to remove the final and see what happens. If it holds I will then measure the control grid voltage with the toob removed and see if is goes WAY positive on transmit.
What do ya think.......

Bill 

[WQ9E]

Bill,

Definitely pull your final and driver before you do anything else although either may have already been damaged.  Then check for a shorted coupling capacitor which is probably the issue.  As I recall from the 500 the bias supply is used to power the relays so if it failed the PTT should not have engaged but check it anyway.

While you have things apart, have you noticed any arcing problems previously when switching from transmit to receive?  One control relay is supposed to provide "mechanical delay" so that the antenna isn't switched to receive until the plate voltage drops however over time this delay seems to go away and causes the protective gap to flashover.  The easiest fix is a small electrolytic cap across the relay coil with an isolating diode below.

Rodger WQ9E

[kc2ifr]

Hi all,
Thanks for the advice Roger. I found that the coupling cap is indeed shorted. It a 1000 puff at 1500 volts......hope I have one. I hope that nothing else was taken out because of the short.
Bill

[AB2EZ]

Bill

Fortunately the mode switch was set in the AM (or CW) position when this happened. The schematic I downloaded from BAMA shows a 12k ohm resistor that is in series between the grid of the 4-400 and the -150 volt bias supply when in the AM or CW position... plus a about 800 ohms of additional resistance from the AM/CW bias adjustment pot.

This limited the current through the grid current meter shunt, the RFC on the grid of the 4-400, and a lot of other things (including the bias supply itself) to around 30-40 mA.

If you had the mode switch in SSB position [ ], the path from the grid of the 4-400 to the negative bias supply (and everything connected to the bias supply) is limited by only the RFC, the 6 ohm meter shunt, and a couple of hundred ohms of resistance from the SSB bias adjustment pot.

[WQ9E]

Bill,

Glad you found the problem and getting a replacement cap shouldn't be difficult.  It sounds like you caught the problem quickly so you probably don't have any "collateral damage".

What type of T/R relay are you using with your 500?  Unless it is the old electronic type where the transmitter is always connected to the antenna load or is a Dow type that has its own slow break circuit (capacitor across the relay coil) then I would seriously consider adding a small (10-12 uf) electrolytic capacitor across RY-302 and an isolating diode so that your antenna relay doesn't hot switch on break.   Since I ran into this issue with my 500 I have talked with 3 others who were also experiencing this and I believe that this is the root cause of the flashovers that many have experienced with their connectors bringing HV between the power supply and RF deck.  The original connectors used are capable of withstanding well over the rated voltage in normal operation but will flashover given the very high transient voltages created when the antenna load is removed before the plate voltage has dropped.  If the gap inside the RF cage is properly adjusted it will flashover there but it appears it was common practice to open this gap wider than recommended and so the next weakest point will be chosen.  If the gap is too wide and the other flashover points are "improved" then it is likely that the mod tranny will become the next place where arcing occurs which would be quite expensive.   My 500 is used regularly with its original power cables and connectors with no arcing problems and since I took care of the problem with the "mechanical delay relay" RY-302 the gap in the RF deck has never fired.

[kc2ifr]

HI again all.
Well where to begin......first I use a dowkey tr relay. I have never had a flashover that I know of. This 500 is modified HEAVILY! I will explain.
The 811's have been replaced with 572B's and I use a cathode follower driver for the modulators using a pair of EL34's. The audio is all external.......ART Prochannel driving an Inovonics 222 into the cathode followers. The mod iron is a Peter Dahl replacement for the original. The plate transformer is a Dahl replacement also BUT is is the 500 X as Dahl called it.Its huge and increases the normal HV by about 200 volts and its rated at 800 mills compared to the 500 mills the original was rated for. All power supply's have been solid stated and the VR toobs have been removed since I dont use the rig on SSB. Other stuff has been changed also such as the "chernoble" resistor is the VFO.
When I use the rig I only load it up to between 225 and 250 mills of cathode current. This gives me around 400 watts outpoot. Audio is clean and I get about 125% positive peaks. The 4-400 shows only very slight color if any. The poor 572B's show more color! 
I did find  a replacement cap........1000 pf at 2500 volts and will have it installed tomorrow.

Stay tooned and thanks much guys.
Bill 

[kc2ifr]

Replaced the cap with a 1000 pf at 2500 volts and the old 500 is working just fine.

[WQ9E]

Glad everything worked out OK!  The 500 is a neat rig.  Picking mine up was part of a 2,600 mile round trip but it prevented any shipping damage.   It is currently paired with a Pierson KP-81 and it and my Ranger/Desk KW are my two most often used AM rigs.

The only complaint I have about the 500 is that top heavy power supply/modulator but as long as it is sitting on the floor it causes no problems.  I do need to find a spare 6B4 before the audiophiles grab all of them.