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Drake L7

 

I want to get my L7 going. Yes I know the voltages will kill you. I purchased the amp at a hamfest. It looks to be all original and in almost mint condition. The amp worked somewhat but the output seemed to be low except on 20 meters. It was around 1400 watts on 20 and around 400 watts on 40 and 80 meters. Now you cannot even tune it on 40 and 80 but can still get around 400 watts on 20 meters. The plate voltage didn't change. I have 2600 volts according to the meter on the amp. Just trying to decide where to start the trouble shooting process. I have a Harbach kit to go in the power supply to replace all those old capacitors but I don't think that is going to fix my issue.?
Thanks?
Roger
KW4EMF
 

Roger,

If it were me, I'd start by using a quality dummy load, and then measure the SWR on the input to the amp. ?Then I would make a visual inspection of the input/output bandswitch contacts along with the fixed capacitors that are part of the tuned output circuits. ?A complete visual inspection of all components in the RF deck is warranted, especially the tapped RF choke in the plate circuit.

Too, it might be helpful to log grid current for each band during testing, along with plate current, to establish a baseline for correcting problems. ? I would not recommend updating the power supply until basic operation is established. ?Too, you might want to think about a soft key interface for the PTT line.

The experts on the Reflector will be able to give you better advice as well.

73,

Evan, K9SQG

On Saturday, January 4, 2025 at 10:35:42 PM EST, Roger KW4EMF <rogerlhill@...> wrote:


I want to get my L7 going. Yes I know the voltages will kill you. I purchased the amp at a hamfest. It looks to be all original and in almost mint condition. The amp worked somewhat but the output seemed to be low except on 20 meters. It was around 1400 watts on 20 and around 400 watts on 40 and 80 meters. Now you cannot even tune it on 40 and 80 but can still get around 400 watts on 20 meters. The plate voltage didn't change. I have 2600 volts according to the meter on the amp. Just trying to decide where to start the trouble shooting process. I have a Harbach kit to go in the power supply to replace all those old capacitors but I don't think that is going to fix my issue.?
Thanks?
Roger
KW4EMF
 

You pretty well covered it. I was thinking also tube pins and receptacles and overall screw tightness in the plate circuit (including anode caps). It may look minty but we don't know how well it was treated when actually used.

On Sun, 5 Jan 2025 03:44:29 +0000 (UTC)
"Evan via groups.io" <k9sqg@...> wrote:

Roger,
If it were me, I'd start by using a quality dummy load, and then measure the SWR on the input to the amp. ?Then I would make a visual inspection of the input/output bandswitch contacts along with the fixed capacitors that are part of the tuned output circuits. ?A complete visual inspection of all components in the RF deck is warranted, especially the tapped RF choke in the plate circuit.
Too, it might be helpful to log grid current for each band during testing, along with plate current, to establish a baseline for correcting problems. ? I would not recommend updating the power supply until basic operation is established. ?Too, you might want to think about a soft key interface for the PTT line.
The experts on the Reflector will be able to give you better advice as well.
73,
Evan, K9SQG
On Saturday, January 4, 2025 at 10:35:42 PM EST, Roger KW4EMF <rogerlhill@...> wrote:

I want to get my L7 going. Yes I know the voltages will kill you. I purchased the amp at a hamfest. It looks to be all original and in almost mint condition. The amp worked somewhat but the output seemed to be low except on 20 meters. It was around 1400 watts on 20 and around 400 watts on 40 and 80 meters. Now you cannot even tune it on 40 and 80 but can still get around 400 watts on 20 meters. The plate voltage didn't change. I have 2600 volts according to the meter on the amp. Just trying to decide where to start the trouble shooting process. I have a Harbach kit to go in the power supply to replace all those old capacitors but I don't think that is going to fix my issue.?Thanks?RogerKW4EMF
--

73

-Jim
NU0C
 

I've looked at several internet posts about checking the 500z's with a multimeter. If I understand this correctly. Two of the pins are the filament and three of them are grids. What should the readings look like when checking them with the ohm meter? The tubes are out of course.?
Thanks
 

On Fri, Jan 10, 2025 at 10:21 PM, Roger KW4EMF wrote:
<I've looked at several internet posts about checking the 500z's with a multimeter. If I understand this correctly. Two of the pins are the filament and three of them are grids. What should the readings look like when <checking them with the ohm meter? The tubes are out of course.?
<Thanks
?
Pins? 1+5 are the cathode...and will read zero ohms, or close to it.??
Pins 2+3+4? ?are all tied internally to the grid.? You will see zero ohms between any of the grid pins.?

And u will read a wide open? between cathode and grid.

Grid to cathode shorts are not uncommon, but that never happens till the fil is lit.? A good eimac 3-500Z will hi-pot test 5 kv between grid and cathode/fil .? ?A chinese? 3-500Z will hi-pot test 1 kv? between? grid and cathode.?
U need to go through every inch of that amp.? 1400 watts on 20m..and only 400 watts on? 40+80m is indicative? of either a bandswitch problem / coil tapping issue, on the kw output side, or a tuned unput bandswitch problem, or tuned inputs.? ?and now u are down to 400 watts on 20m.?

You can also pull one tube out...and test with remaining tube, and vice versa.? ?It could also be an issue with one or both sockets.? ? It's a simple amp, in its basic form it's a pi tuned input and pi tuned output.? Metering,? B+, fil V, and a tr relay.?
 

check the TR relay thoroughly.....esp the center pole.? Mine had the center pole contacts burnt right off after years of use.... tnx to drakes fubar idea of cut off bias, etc.??
 

¿ªÔÆÌåÓý

I agree with Jim on the relay, I eliminated the cut off bias scheme on my L7.?

Tom W9TAB?

On Jan 11, 2025, at 1:35?AM, Jim VE7RF via groups.io <jim.thom@...> wrote:

?
check the TR relay thoroughly.....esp the center pole.? Mine had the center pole contacts burnt right off after years of use.... tnx to drakes fubar idea of cut off bias, etc.??
 

I have checked the relay. There was a little bit of carbon on one side of the relay contacts but hardly no pitting. Also while checking the tubes with a multimeter I found that there was no connection on one of the grid pins. It appeared that it had lost its connection inside the adapter (the round phenolic piece with the large hollow metal pins). I could slide the multimeter probe between the glass and phenolic and contact the small portion of the pin where it exits the glass. Here I had continuity between it and the other two grid pins but not when connecting to the lower soldered portion. It was that way on both tubes same pin. I desoldered all the pins and pulled the adapter off. In that pin there was very little solder. I'm now in the process of resolderind both tubes. I've got one done.

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