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Author Topic: SB220 Testing  (Read 1529 times)
GM6WCF
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Posts: 28




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« on: March 28, 2013, 02:34:00 PM »

I had a number of Transceivers checked by an ARC on Tuesday evening. With one exception they were all found to be working.
They were not prepared to test the SB220 I took to them. I don't think they had or needed a dummy load to handle it's potential.
The visual appraisal was that it appeared unused and was very clean and well built. I believe it was put together by someone with professional qualifications who very likely never used it to exceed the UK maximum of 400W.
It was suggested that it should be tested using a Variac to slowly increase the supply to the maximum. It is wired for UK supply voltage and has passed the PAT (Portable Appliance Test) for electrical safety and integrity.
I don't want another cross country trip to another ARC so how would I set about checking the amplifier, specifically the tubes and capacitors?
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KA5N
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Posts: 4380




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« Reply #1 on: March 28, 2013, 03:01:20 PM »

I'll probably get flamed for my reply, but what are you doing with a KW amplifier when you
obiviously don't have any idea about how to check it out and probably don't know how to
tune it (assuming that it works)Huh?  Yeah, you need a correctly sized dummy load and
while you can check for shorts etc. with a DVM or Vom  and check the diodes and capacitors
in the power supply.  The tubes will indicate whether they are good or bad once the amp
is working.  It is not a trival task to go over an amp with unknown pedigree as there are
lethal voltages present when operating. 
Your best bet would be to get a local ham with the know how and equipment (meters, variac
etc.) and have him tutor you as he goes over the amp.  Otherwise self study is something 
that most ham use to learn what they need to do to check and properly use amateur
equipment properly.
Good Luck and 73
Allen  KA5N
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GM6WCF
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Posts: 28




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« Reply #2 on: March 30, 2013, 07:28:20 AM »

I'm sorry I bothered to ask. Bye!
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K2OWK
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Posts: 1066




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« Reply #3 on: March 30, 2013, 01:30:07 PM »

Hello, Because this is a kit built amplifier. I would suggest if you do not already have one, get a copy of the construction manual for the linear even if you have to pay a few dollars for it. Because it is a kit built amplifier, the manual will contain the complete setup testing and operational procedure of the linear. The variac approach to forming and testing an old unit is a good one. I just got through restoring an old National receiver, and brought the voltage up 30 volts at a time over a few hours and the radio worked fine when I was through. A dummy load of the maximum wattage will be required. An oil filled type will be the least expensive. A tube checker would be handy to check the tubes, if you can locate one. One last thing the manual will contain troubleshooting information on the amp. and what to check if certain problems occur.

I hope this helps.

BE EXTREEMLY CAREFUL WHEN WORKING ON THIS TYPE OF EQUIPMENT, LETHEAL VOLTAGE IS PRESENT SOMETIMES EVEN AFTER THE POWER IS REMOVED.

Good luck,

73s

K2OWK
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KB4QAA
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Posts: 2407




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« Reply #4 on: March 31, 2013, 12:51:59 AM »

Joining the chorus, but not saying you can't do, be aware that overhauling  an amplifier is jumping in to the deep end of the pool, if you have no prior repair experience.

-Get the manual
-Read a lot about repairing amps
-Talk to people.
-Find somebody local who is experienced with amps to help you!!!!!
-Get a thicker hide...people WILL disagree with you on boards. Wink
-DMM with diode function
-capacitor checker (optional)
-Dummy load, appropriately rated (You need it anyway for testing and tuning up)

-Tubes can't be checked on tube testers, they draw too much power.  All you can do is a basic short check with DMM, then cross your fingers when turning on the amp.

-Caps.  One school of thought is run caps till they blow; that they can be reformed; that slow voltage bring up is best.  Other school is replace them if they appear old; turn the power on see if they hold.

-Power supply diodes and resistors should be checked.  Replacing with modern diodes is not  a bad idea.

-Resistors in the meter circuits should be checked, be precisely accurate or better replaced. 

-Check tube bases for overheating, cracked pins and springs.

-Check fuse for correct amperage, clean socket.

-Absolutely confirm that transformer is properly strapped for your voltage!!

-Check wafer/rotary switches for arcing, carbon tracks, corrosion,  clean with an alcohol swab.  DO NOT use any conductive sprays like DeOxit.

-Remove and measure parasitic suppressor resistor.  If drifted more than a few percent, replace with proper value 'non-inductive' resistor, e.g. carbon composite, etc.

-Check overall security, burn or scortched parts, bad solder joints, general cleanliness  (use a damp rag).

This is a good start and hits many of the key points.
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GM6WCF
Member

Posts: 28




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« Reply #5 on: March 31, 2013, 02:50:18 PM »

Thanks to those who provided the useful replies.

I got this amp. as a gift from the son of a deceased Amateur. I was asked to sell it on. Hence my enquiry about testing.

What I've decided is to put it on eBay untested and see what it gets. It's in wonderful physical condition and the soldering is excellent.

Is it possible to date these things? I've noticed that certain parts don't match the illustrations in the construction manual.

Thanks again for the helpful replies!
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