Hi,

I'm troubleshooting the HT-41. The 3B28s rectifiers do not show any color when the HV switch is in the on position after several seconds the rectifiers flash with color (normal) for a second or two and then back to no color.
I have tried two sets of 3B28 tubes and the results are the same. I've replaced the HV electrolytics and bleed resistors. The unit has never caused its 15 amp circuit breaker to open. I don't have experience with the 3B28s or the HT41. I have had this
station for several decades and have the Sx111 and HT37 running good. Now trying to compete the station with the HT-41.

Thoughts and Recommendations are welcome,
dave
wa3gin
King George, VA

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OOooMoment <[email protected]> wrote:

I'm troubleshooting the HT-41. The 3B28s rectifiers do not show any color=
when the HV switch is in the on position after several seconds the rectifi=
ers flash with color (normal) for a second or two and then back to no color=

If no current is pulled by the finals, the rectifiers will not light.

Use an external meter with an HV probe to measure the voltage at one of the 7094 plates. You see anything?

I have tried two sets of 3B28 tubes and the results are the same. I've rep= >laced the HV electrolytics and bleed resistors. The unit has never caused i= >ts 15 amp circuit breaker to open. I don't have experience with the 3B28s = >or the HT41. I have had this station for several decades and have the Sx111=
and HT37 running good. Now trying to compete the station with the HT-41.

You know those rectifiers are supposed to the 866As? I don't know if a 3B28
is a good sub or not. The pinout is the same, I know.
--scott



--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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On Thursday, April 7, 2022 at 1:25:05 PM UTC-4, Scott Dorsey wrote:

OOooMoment <[email protected]> wrote:

I'm troubleshooting the HT-41. The 3B28s rectifiers do not show any color=
when the HV switch is in the on position after several seconds the rectifi=
ers flash with color (normal) for a second or two and then back to no color=

If no current is pulled by the finals, the rectifiers will not light.

Use an external meter with an HV probe to measure the voltage at one of the 7094 plates. You see anything?

I have tried two sets of 3B28 tubes and the results are the same. I've rep= >laced the HV electrolytics and bleed resistors. The unit has never caused i=
ts 15 amp circuit breaker to open. I don't have experience with the 3B28s = >or the HT41. I have had this station for several decades and have the Sx111=
and HT37 running good. Now trying to compete the station with the HT-41.

You know those rectifiers are supposed to the 866As? I don't know if a 3B28 is a good sub or not. The pinout is the same, I know.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Hi Scott,

Thanks for the fast response. The 3B28s apparently do not show much if any color. I haven't had a chance to drive RF into the input just yet. Testing today I discovered the 15 amp circuit breaker in the unit was defective. It was a very old bi-metal
breaker and I'm guessing it sat for two many years just corroding. The corrosion inserted resistance into the AC line feeding the unit. That resistance caused significant current to be drawn when the HV supply switch was turned on. That current has
pitted the contacts on the breaker increasing the likelihood of more resistance in the AC circuit. I worked the breaker several times and that has reduced the resistance across the breaker to about half an ohm. That was low enough for the unit to
operate normally. The HV was 1800 and filaments to the rectifier tubes was steady at 2.5 volts. I'm going to replace the two front panel switches, wire the HV switch to a 30amp 12vac relay that will handle the high current demanded from the HV power
supply and removing the load from the front panel HV ON/OFF switch.
Did you know the 7094 was developed to replace the 813 tube. I can't believe it as the 7094s appear to be a much less robust tube.

73,
dave
wa3gin

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OOooMoment <[email protected]> wrote:

Thanks for the fast response. The 3B28s apparently do not show much if any=
color. I haven't had a chance to drive RF into the input just yet. Testin=
g today I discovered the 15 amp circuit breaker in the unit was defective. =
It was a very old bi-metal breaker and I'm guessing it sat for two many ye=
ars just corroding. The corrosion inserted resistance into the AC line feed= >ing the unit. That resistance caused significant current to be drawn when t= >he HV supply switch was turned on. That current has pitted the contacts on=
the breaker increasing the likelihood of more resistance in the AC circuit=
. I worked the breaker several times and that has reduced the resistance a= >cross the breaker to about half an ohm. That was low enough for the unit to=
operate normally.

Circuit breaker? Originally I think that was just a big 15A fuse! Maybe you have a later model than I have seen. Replacing that (or putting a fuse in) would be a big improvement in reliability.

But I am glad to hear you're close to having it up and on the air!

The HV was 1800 and filaments to the rectifier tubes wa=

s steady at 2.5 volts. I'm going to replace the two front panel switches, = >wire the HV switch to a 30amp 12vac relay that will handle the high current=
demanded from the HV power supply and removing the load from the front pan=
el HV ON/OFF switch.

Why? Those should be beefy high-current switches, I don't see any reason to put a contactor on there when the switches are likely rated for 30A anyway.
If they aren't rated for 30A, just replace them with switches that are.

If there are none in the junkbox, consider: https://www.mouser.com/ProductDetail/Littelfuse/551840

Did you know the 7094 was developed to replace the 813 tube. I can't belie= >ve it as the 7094s appear to be a much less robust tube. =20

I didn't at all! They are totally different designs... the 7094 is a lot higher transconductance and lower capacitance... it probably doesn't have to
be quite so robust since the dissipation is going to be so much lower.

The 813 was hard to kill. The graphite-plate ones could produce crazy peak powers in low duty cycle radar keying applications because the plate had
plenty of thermal mass.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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