KD0GES
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« Reply #15 on: December 06, 2012, 06:28:41 AM » |
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I came up with a good way of testing the transformer.
I hooked my o-scope to the high voltage output and with the amplifier switched on.
Then I touched a 9 volt battery across the plug and the o-scope shows a 35 volt spike output from the transformer.
I believe this is a reasonable indication the transformer is OK without running high voltage.
I am going to put a harbach power supply into the amp.
Dale
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KD0GES
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Posts: 25
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« Reply #16 on: December 16, 2012, 02:47:47 AM » |
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Update I put in a harbach power supply.
Now I have 2500 volts on the meter but at idle it has no grid or plat current.
Would this indicate the tubes are bad?
Thanks
Dale (KD0GES)
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KA5N
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« Reply #17 on: December 16, 2012, 06:04:51 AM » |
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Do you have a manual for the SB200? Many of the questions that you have asked
of late are fully covered in the manual. With the SB200 keyed without drive (no
RF input) you should be able to read the high voltage (which you can) and the plate
current should be about 60 ma. (which you aren't reading) with no drive the grid
current should be zero (which it is). But you don't read the 60 ma plate current.
This could be because either R14 is open or disconnected from the meter switch or
from R12. Since you have installed the Harbach power supply board R12 and R14
should be OK. But have you throughly checked the wiring? to/from the new board?
There is a slight possibility that both tubes could be bad and not draw current, but that
is very slight!!!! The plate choke could be open (RFC1). Do the filaments in the tubes
light up? Have you checked the center tap connection of the filament winding?
That is the ground connection for the cathodes/filaments and if it is bad you will
have no plate current. Following your many posts, I think you need to get some
local ham to help you trouble shoot.
Allen KA5N
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KD0GES
Member
Posts: 25
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« Reply #18 on: December 16, 2012, 07:18:32 AM » |
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I have a manual.
I am going to take the amp apart again and check everything I can and make sure everything is wired correctly.
I have asked on local ham nets a few times if anyone living near me knows how to work on the amplifier and have had no responses.
I will check into your suggestions and see if one of these solves the problem.
Thanks
Dale (KD0GES)
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KA5N
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Posts: 4318
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« Reply #19 on: December 16, 2012, 10:14:27 AM » |
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That's a good idea. There might be a problem with your bias voltage. When you
key the amp with no drive the bias voltage should drop to about -2 volts to -4 volts.
If it stays high (about -125 volts) you will cut off the tubes and show zero plate
current. The components (relay, resistors etc.) in the relay circuit are the items to
check if this is the problem.
Allen KA5N
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KD0GES
Member
Posts: 25
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« Reply #20 on: December 18, 2012, 02:58:09 AM » |
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Last night I pulled the tubes out of the amplifier.
One tube had a ball of glass rolling around inside.
The heating elements would light up on both tubes.
Except for the heating elements all the other pins had showed as an open circuit on my ohm meter.
I,m assuming both tubes are bad.
I went over the wiring and found no obvious problems.
I am going to buy a new set of tubes for the amplifier.
RF parts looks like a good place to buy tubes from.
Dale (KD0GES)
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G3RZP
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Posts: 2835
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« Reply #21 on: December 18, 2012, 04:21:57 AM » |
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Except for the two thick pins, there should be noting showing on your ohm meter.
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KD0GES
Member
Posts: 25
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« Reply #22 on: December 18, 2012, 07:16:37 AM » |
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I get the impression that the only way to test a tube is in a known to good amplifier.
Thanks
Dale (KD0GES)
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G3RZP
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Posts: 2835
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« Reply #23 on: December 18, 2012, 08:37:13 AM » |
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Except for permanent filament to grid short or open filament, yes, that's about it, Dale.
One could, if so inclined, build up a test circuit, but substitution in an amp is by far the easiest.
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KA5N
Member
Posts: 4318
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« Reply #24 on: December 18, 2012, 09:59:35 AM » |
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Yeah and the only way you can see if the bias switching is working correctly is to
measure the bias voltage with the power on. With the power on and no drive the
bias voltage will be around -120 volts, when the amp is keyed (with the power on)
the bias voltage will drop to -2 to -4 volts and plate current is drawn. If rf drive
is applied (i.e. signal) then grid current will be indicated.
A small ball of glass inside a tube may just be a leftover from when the tube was sealed
and cause no harm. If one tube is good and the other is bad (but not shorted) then
the plate current will be half what is expected. If you get new tubes and they make
no difference then the problem is either a miswire or bad component (s).
Allen KA5N
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