Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: HF-2500 Plate resistor failure  (Read 2838 times)
WD0FAA
Member

Posts: 2




Ignore
« on: September 23, 2012, 03:23:37 PM »

I just bought a used HF-2500 export model.  Three 3CPX800A7 tubes.  It tested okay on 10 and 15M last week.  I finally got it installed in the shack and while testing it on 80M very low drive (5w) I burned out the carbon comp plate resistors on one of the tubes.  Full grid current on the meter with no drive. 

Is it possible to high pot the tube, grid to plate to determine if the tube is damaged? 

I can replace the resistors but I am also concerned that something else is wrong with the amp.  Is there anything else I should be looking for before bringing it back up and risking another tube?  In other words what is the best way to bring the amp back on line.

-Aaron
WD0FAA
Logged
W1QJ
Member

Posts: 1444




Ignore
« Reply #1 on: September 23, 2012, 04:37:54 PM »

What I usually do first is to remove the tubes and bring the amp up slowly on a variac to make sure the power supply OK.  Once I see the amp comes up to normal plate voltage with no noises and smoke I can rule out a problem with power supply.  Then I would install the tubes and again slowly bring up the voltage again on a variac watching for any plate current or grid current either negative or positive.  If you have a bad tube you will usually quickly see one of the two which indicates a bad tube.  Then you would pull the tubes one by one to see which one is problematic.  This system works every time.  Of course if you have a high pot you can do that too.
Logged
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #2 on: September 23, 2012, 05:13:07 PM »

Lou,

Doesn't that amplfier have an excessive grid current shutoff circuit?

Tom
Logged
WD0FAA
Member

Posts: 2




Ignore
« Reply #3 on: September 23, 2012, 05:43:50 PM »

I don't have the correct manual for the amp so I don't know about an excessive grid current shutoff circuit.  I could not find the manual for the export model with 3 tubes. 

The amp did shut down first.  All I heard was a click and the power LED went off.  I cycled power and brought it back on.  The tubes held off plate voltage.  When I keyed the amp with no RF drive is when the resistors opened and I saw the grid current meter pegged.  Does this sound like some oscillation?

I read the tube datasheet and they recommend 50ohm 25W.  All the amp had was 33 ohm with the paracitic choke wrapped around three 1W carbon comps.  I am not sure if a higher power rated resistor would have helped. 

Tomorrow I will highpot the tubes grid to plate and see if they conduct.  Most likely I will have to buy a new tube.  If they hold off the high potter I will install them again and beef up the resistors.

Logged
W1QJ
Member

Posts: 1444




Ignore
« Reply #4 on: September 24, 2012, 06:12:27 AM »

Lou,

Doesn't that amplfier have an excessive grid current shutoff circuit?

Tom
Tom, depending on the vintage it either does or does not have grid shut down.  Since these amps are the worst for documentation one needs to actually verify these things by visual inspection.  The very early units might have been lack the grid overload, however apparently one was put in shortly after.  So some where out there without it.  The schematic documentation on these amps is useless in some cases,  Many revisions to the actual amps where never shown on the schematics.  Often times repairs are seat of the pants, although a schematic gives you a general idea of his intentions. 
Logged
AH6RR
Member

Posts: 803




Ignore
« Reply #5 on: September 24, 2012, 11:25:38 PM »

If its in the power supply board take lots of pictures to get it back together because Lou, you and I know what a pain that is to get to. The only one I have done took 4-5 hours of disassemble time just to get to it. I guess they were not thinking on it ever having a problem and all that to replace the HV diodes because one opened up and change out the meter resistors because they had changed values and the meter was not reading HV correctly. If that is the case I would cut a hole in the bottom and cover it with a plate so the next time you will not spend all your time just getting to the board that's under the tubes.

Roland AH6RR
Logged
W1QJ
Member

Posts: 1444




Ignore
« Reply #6 on: September 25, 2012, 06:14:23 AM »

If its in the power supply board take lots of pictures to get it back together because Lou, you and I know what a pain that is to get to. The only one I have done took 4-5 hours of disassemble time just to get to it. I guess they were not thinking on it ever having a problem and all that to replace the HV diodes because one opened up and change out the meter resistors because they had changed values and the meter was not reading HV correctly. If that is the case I would cut a hole in the bottom and cover it with a plate so the next time you will not spend all your time just getting to the board that's under the tubes.

Roland AH6RR
You are so right Roland, but all the units made have the low voltage control board accessible on the right hand side of the units.  But you are right, all the power supply stuff is underneath where it is a PITA to get to.  I suppose the thought was that the PS will outlast the warranty.  Wink
Logged
WA2GO
Member

Posts: 6




Ignore
« Reply #7 on: October 03, 2012, 09:17:00 PM »

Hate to say it, but the best thing to do with that amp is probably to export it per the manufacturer's original intentions...   :-)
Coincidentally, there are a couple of mint-looking HF-2500s of the US-legal variety on ebay at the moment.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!