Another thing not to be overlooked which is prevalent in many Ameritron amps is a shorted bypass cap on the bias line anywhere along it’s length?.??In rare cases, but possible,?a bypass cap that’s shorted on the filament choke can cause such issues. ?If ?shorted?however in the second test he did it seemed like the tubes behaved which would rule out a short and point to something intermittent like a moving contact perhaps.
On Thursday, November 30, 2023, 9:21 PM, Louis Parascondola via groups.io <Gudguyham@...> wrote:
Not all LK500 amps had QSK and vacuum relay output, it was an option. ?Some had a standard 3PDT relay like an SB220 et Al. ?Even if your amp has a QSK option it’s not working and the previous owner did a CF modification as is clearly shown by those outboard coaxial relays. ?That?said whoever did that probably has a third relay inside the amp for bias switching. ?REGARDLESS OF HAVING THE QSK OPTION OR NOT, THE BIAS SWITCHING IS NOT DONE DIRECTLY WITH ANT TRANSISTOR, THE BIAS IS STILL RELAY SWITCHED WITH CONTACTS IN EITHER?CASE. ?Don’t get bogged down with a bad transistor. ?Now that we know this amp has had the golden screwdriver done to it, it’s time to pop the top and show a photo of the rest area so we can see how the “ tinker” handled the bias switching.
Sent from the all new AOL app for iOS
On Thursday, November 30, 2023, 7:44 PM, Adrian Fewster <vk4tux@...> wrote:
The comments about a bad bias contact do not make sense to me ;
If K1C contact was welded (closed #) , which is highly unlikely
in such a low current circuit unless a fault current came through,
then that may be a possibility.
The issue was in standby , with relay open. If K1C is open and
not #; R1 (I requested check in earlier post) would need to be
shorted as that provides resistor bias cutoff.
D3 would probably need to be defective as well. The meters seems
to work so plate current should be checked on TX with and without
drive to check bias current? ?
73
vk4tux
On 1/12/23 09:46, Dave w6de wrote:
There
are burnishing tools for exactly this purpose.?
Using finger nail files and files for this purpose is
risky, as the potential damage of removing too much contact
material is high.
?
I
have one of these:
Here
are two more:
?
?
Burnishing
tools have very fine almost invisible teeth.?
Using a burnishing tool is a safe method for cleaning
relay contacts.?
Every radio amateur that works on equipment with open
frame relays should have one in his tool kit.
?
73,
Dave,
w6de
?
?
John,
you just might have a worn out contact that switches bias on
the antenna relay. ?If the contacts are severely pitted they
might not make up too good sometimes and other times ok.
?You need to inspect the that set of contacts in particular.
?If you carefully remove the rear spring on the relay you
can lift the armature off and inspect all the contacts. ?The
contacts that switch the RF can be cleaned gently, the
contacts on the bias may be in bad shape, there may be a
tiny point on the armature contact and a tiny depression on
the body contact. A gentle filing to remove the “pit” and a
cleaning should restore reliable contact. ?It’s not unusual
at all to see the bias contact hood bad with the age of thst
amp especially being used on CW a lot. ?Seen it quit often
and by doctoring up the bias?contact that problem went away.
?Lou
Sent from
the all new AOL app for iOS
?
On Thursday, November
30, 2023, 3:58 PM, John Roland <johnroland51@...> wrote:
