Amp Supply LA 450 problem

[Radio Tech]

I picked up a nice LA 450 NTC at the ham fest last week. This amp is a single tube (3-500Z). After checking everything out (for the price I knew it could not be working) I really found no problems that were noticeable.

The amp came on and loaded with no problems. Was going through the bands and checking and all bands loaded fine.

Then made a mistake. I had switched the band switch to AUX. Then after I keyed the exciter there was a loud bang, no output, and the tube turned red. I spend the next 2 hours checking components and found nothing bad.

Removed the RF deck from the amp and found the 3 10 ohm 2 watt resistors had exploded. Replaced and checked the bias diode. Could not find anything again bad. Turned the amp on to take some voltage reading and BANG, Blew the resistors out again.

Sounds like a bias problem?

 

[paws264]

I picked up a nice LA 450 NTC at the ham fest last week. This amp is a single tube (3-500Z). After checking everything out (for the price I knew it could not be working) I really found no problems that were noticeable.

The amp came on and loaded with no problems. Was going through the bands and checking and all bands loaded fine.

Then made a mistake. I had switched the band switch to AUX. Then after I keyed the exciter there was a loud bang, no output, and the tube turned red. I spend the next 2 hours checking components and found nothing bad.

Removed the RF deck from the amp and found the 3 10 ohm 2 watt resistors had exploded. Replaced and checked the bias diode. Could not find anything again bad. Turned the amp on to take some voltage reading and BANG, Blew the resistors out again.

Sounds like a bias problem?

I looked at the factory roadmaps for those amps and, they did not show the 10 ohm resistors that you mentioned in the RF Deck circuit.

If, I understand the problem correctly, the amp worked on the rest of the bands until you tried to use the "Aux-10 meter" position and then there was a failure; now, after you have changed the blown resistors, it now has a high voltage short as soon as you turn it on (without putting it in transmit)?

1. Pull the tube and turn the amp on, if the symptoms change, the tube may have a short.

2. If, it still arcs, there is a problem in the high voltage section.

3. There is a Grid current meter in that amp, what are/were the indications.

4. If, the Zener in the bias circuit is open, the amp will not go into transmit

5. If, the Zener shorted, you would see high Cathode current in standby.

The schematic for the amp is here: http://bama.edebris.com/manuals/ampsuply/lk450-550

.

 

[Radio Tech]

I tried the tube in my Henry and works fine (15 - 160)
looks like a mod was done under the tube. There are 3 large .01 caps (orange) along with the 2 watt 10 ohm resistors. trhe plate voltage is showing 3400 volts and I did not check the grid reading, from the tube turning red it appears to be drawing a lot of current. I will make some more test tonight and see what I can find. I did notice a second relay has been added and the bias diode goes through this. Maybe a mod to turn cathode or grid off when idle?

 

[Radio Tech]

Update,
Problem fixed.
Insulating washer on the stud mounted zener had failed.

 

[Crusher]

Glad you got it fixed. I just picked up a SB220 today myself. Case is scratched up, but in working order for $300. Couldn't pass it up since they are going for 500-800 on ebay. Guy I bought it from worked for Dentron before MFJ/ ameritron bought them out. I think he was part owner.

 

[Toll_Free]

Not a good bet, on the Dentron part owner bs line. Dentron was owned by 2 people, all the way up until Denny screwed up.

Denny Hadd owned Dentron, all the way up until his penis found its way inside his secretary's vagina.

THEN, his WIFE owned Dentron, ALL the way up until the idiots at Mighty Fine Junk bought it. Which was VERY soon. And Vectronics came about, with similar designs as dentron, although I dunno if Dennis had anything to do with them or not, I don't BELIEVE so...

Dennis Hadd, of Dentron and other ham radio fame, is shlepping audiophool / audioidiot amplifiers now..... or he's dead, depending on what you believe... He's still sitting on the board of directors of his audio company, although his obit was published a couple years ago hahahahaha. Sounds like me. I'm legally dead, but only in the state of Texas. Divorce can be an UGLY thing!!!!!



--Toll_Free

 

[Crusher]

Ok, well this guy's name is Dan K. mentioned 3 other guys he worked with at dentron but don't remember names. He didn't say he was part owner, but the way he talked he had been with company for bit and was high up.

 

[Toll_Free]

What's kinda funny is this: Out of ALL the companies that have come and gone, it seems Dentron has more people "that worked their" or where owners, etc.

Not pointing any fingers, but I think I've heard more stories about or from people that worked at Dentron, for Dentron or something similiar, more than ANYWHERE else.

--Toll_Free

 

[Radio Tech]

Is a bit strange eh.