HW-12A, Power Loss

[Bob Burns]

In a message dated 30/04/2009 01:05:46 GMT Standard Time,  
krkress at BELLSOUTH.NET writes:

I have a  very nice HW-12A that rapidly lost output power, now has minimal 
output.  Receive is okay. Replacement of driver and finals did nothing to 
correct the  problem.
I am a little rusty on trouble-shooting this gear, as it has been  quite a 
few years (a couple of decades, really) since I did much service work.  
So... I would appreciate suggestions, likely causes, first steps, etc to  
diagnose and fix this problem...  Thanks in advance... I know I'll get  some great 
advice from this group!

73 de  WB4SYU,

Kent



Hi Kent
 
Ken has already provided some suggestions to you but can I add a few  more?
 
At the more basic level, check that:
 
1. All of the components visually look OK and are not obviously faulty or  
"fried".
 
2. All of the valve heaters are glowing when the unit is switched  on
 
3. With the unit switched off, gently remove each valve and replace it a  
few times to make sure that the socket contacts have not oxidised or weakened 
 over time
 
4. With the unit switched off, cold and disconnected from its power  
supply, measure the value of all resistors - carbon composition resistors will  
probably be at least 30% high after all these years and those outside of their 
 original specification should be replaced. Also check any preset 
potentiometers  for correct value and track contact. The carrier balance pot on the 
SB series  transmitters for example is known to fail.
 
5. The relay contacts are clean - a little switch cleaner and a clean  
postcard will do this job well unless the contacts are pitted (tears the  
postcard) in which case you may need something a bit more pursuasive or a new  
relay. I never use sandpaper as it will remove the contact plating although you 
 could try very fine 'wet and dry' rubbing down paper to remove any pitting 
 problems although this is only a short term fix.
 
6. I was going to suggest using a voltage chart to check the DC conditions  
of each stage but I cannot see one in my single band HW manuals. However, 
you  can check the obvious voltages like the supplies which are shown on the 
circuit.  You can measure the voltage at each cathode (to ground) where 
there is a  cathode resistors and work out the anode current using Ohms Law. I 
would  expect something in the region of 1 - 5v as a rule of thumb. Zero or 
very high  cathode voltages should initially be treated as fault conditions 
but do look at  the circuit to see how the stage is intended to work. Check 
that screen grids  and anodes have a fairly high potential depending on their 
function.
 
Does another member of this forum have a list of known good voltages or  
could measure some and provide the information as a file upload?
 
Hope that  helps but please do report back with your findings so that we 
can accumulate a  list of failures for these units.
 
73

Bob 

G3OOU
G-QRP  6907
_www.g3oou.co.uk_ (http://www.g3oou.co.uk/) 

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