Voltage drop

[The Mad Scientist]

Excessive voltage drop under normal load is almost always an open cap or diode in the HV supply. If more than 50% of the voltage is lost under load, it's probably an open cap. I recently had the same problem in an NCL-2000 that one of the caps opened up in. I hadn't even considered this as the problem because 99% of the time this makes the cap short due to the larger voltage drop and take out the HV fuse. To my surprise this was one of the rare times the cap actually showed a virtual open connection and would not support any current. The diodes would be the next thing to check.

The Drake sounds like something may have been damaged in the output section from excessive power. See what the plate current is when it's making 400 watts. Low output and larger than normal voltage drop usually means high current is being drawn as a result of mistuning the tank. That could be the result of a part failing in this area like the plate blocking cap. It's also possible the bias has shorted and loaded the supply way down with idling current. The larger power transformer will bring out the next weak link in the chain.

I was thinking the same thing. I would suggest that if you find a bad cap that you rebuild the voltage doubler with caps of roughly double the mf value. This will help stablize the amplifiers voltage drop while under load.

Just be very carefull to make sure those caps are discharged before working on it.

 

[Crusher]

I am going to check cap bank, but I am leaning more toward bias resistors and such. When the guy used it last, he said he saw a spark in the back right corner on top. Which would be around filament transformer. Leaning toward bias resistor section opened up.

 

[Captain Kilowatt]

I am not disagreeing with you one bit. I think a 400W carrier is perfect. Not 4 x that.

Far too many people think that an amp capable of 1000 watts can be run at 1000 watts carrier, after all that is how power is measured right? Using a carrier. They cannot understand (or don't want too understand) that means 250 watts of AM carrier. They feel cheated and push the limit.

 

[Shockwave]

When I run into people from "that group" I suggest they buy a peak reading watt meter. That keeps the operator happy as well as the output stage.

 

[Captain Kilowatt]

When I run into people from "that group" I suggest they buy a peak reading watt meter. That keeps the operator happy as well as the output stage.

The ones I run into usually say "Yeah but it only puts out that power for a split second. I want it all the time." :headbang

 

[Crusher]

The bias has already been changed, Fused and about 15V of bias using 6A diodes. Uses a 50K resistor for cutoff and uses the relay to switch it in and out. So anyone that suggested the cap bank may be right. Bias circuit appears fine. I still plan on changing the 50K resistors. the cap bank was just replaced about 3-4 months ago, but it is possible there is a bad cap. Has 100k 3W resistors across each capacitor and new 6A diodes for diode stack as well. If in doubt, just replace it all.

 

[Shockwave]

Crusher, do you have the cutoff resistor wired directly to the cathodes of the tubes? I ask this to insure a current path from ground in the event the fuse opens under load. That will keep the cathode voltage at a safe level so the tube doesn't arc inside. Then to key it you just use the relay to add the bias path through the diodes, never allowing the cathode voltage to float with an open connection.

 

[Crusher]

This is correct. Everything is in series. Only thing the relay does is short across resistor. And I have a 10k 10W across fuse holder as well. I actually did this about a year ago for bis. Worked fine. Guy replaced tubes and put the parasitic chokes on backwards. Caused a direct short. That is when supply went out. I rebuilt supply and thought I did bias right, but maybe I wired up the 50k resistors and the 5k wrong. Will have to check schematic.

 

[Crusher]

Since I'm coming right off B- for bias. I don't need the 50k /50w resistors of the 5k resistor. Since I'm coming right off the B- right at the 1ohm 10W. But I think I'm going to upgrade that. I have a 5ohm @ 50W I think I'll put in its place. I have seen those resistors blow in past. Usually it cuts tube off, but in this instance, it might be sending 3kv to cathode. Still going to do what the L4B page recommended and put a 1K at B- in supply to ground in PS case as a safety measure. In case floating resistor in rf deck opens. 1 think bias doesn't have is a 10k resistor across bias supply if diode string would open. Wish there was more room in deck. Its tight. Think I'm going to redo caps and diode doubler circuit as well. These are new, but whose to say it doesn't have a bad cap. Best to rule out problem. Old cap bank had 5 caps per section for a total of 10 to give higher V rating.

 

[Crusher]

Can anyone explain to me why the grids use a capacitor to ground and not just a piece of copper strap? What is the purpose of this cap?

 

[Crusher]

Never mind. Figured out the caps are on the pins so it is at 36 ground potential and can measure grid current. Caps in it are .2uF ceramics

 

[wavrider]

Never mind. Figured out the caps are on the pins so it is at 36 ground potential and can measure grid current. Caps in it are .2uF ceramics

 

[Captain Kilowatt]

Never mind. Figured out the caps are on the pins so it is at 36 ground potential and can measure grid current. Caps in it are .2uF ceramics

Yup. RF grounded but DC isolated.

 

[Crusher]

I had to replace the 1 ohm 10W resistor. It was discolored and you could tell it was heated up. I replaced with 2ohm, 40W so it should be safe now.

 

[Shockwave]

The fastest way to check if voltage drop is related to bad filter caps is to look at the output on a scope. Bad filter caps will place a lot of AC ripple on the carrier. You'll see the low frequency hum on the scope and hear it on the air.