Palomar Elite 400 500 600 bias circuit! what bias?

[BayouRadioAmplifier]

I have a Palomar Elite 600 to work on.
told it keys up, but no RF out.
I downloaded the attached schematic and took a quick glance at it,
and noticed a bias circuit. a "quick glance"
I thought, "cool! this amp is worth working on!"

so keyed up the amplifier with a resistor to ground at the key-up
transistor and was going to check bias first. it had no bias or real low bias.
found no .6 volts at any of the bases of the 5 RF transistors.
took a close look at the driver transistor, and the base goes
to DC ground through a choke.. then looked at the schematic closely and see
that the bias circuit on this board is not the same. the bases of the 4
output transistors also go to DC ground through a choke. so
no way there is bias to any of the 5 RF transistors. as the bias circuit is
drawn for the 4 output transistors, it can't work. but bias at the driver
can be done by just soldering the choke over to the bias circuit trace.
there is a note to cut the DC ground at T2/T5

so anybody from long ago know about this?

I guess they noticed the bias was way too high, due to the simple
circuit, so they made these class-C. the Elite 400/500 bias is messed up
also, on the diagram. maybe incorrect drawings. ?

 

[AudioShockwav]

The guys building them found that the gain was high enough to cause them to be unstable, relays latch on after unkey, squealing ect
Grounding out the bias reduced it and required less though that actually fixxing them.....
Adding some negative feedback and hooking the bias back up helps.
There's several different versions out there

73
Jeff

 

[BayouRadioAmplifier]

OK, on the history.
that schematic I uploaded says to add the negative feedback to them.
this one does not have that, but
This amp here has 5 Toshiba 2SC2879's in it.
obviously been worked on, big heavy red/black 12V power wires.
and some good soldering.

it has potential! hopefully jut the first stage driver transistor is bad.

 

[9C1Driver]

 

[AudioShockwav]

I ran across this from Bill years ago about the 600.

73
Jeff

 

[BayouRadioAmplifier]

OK on that info about these amps.
I have an update also. -

I connected an old Teaberry tube 3 watt output radio to
this amp. I had the power supply to this amp current limited to a few amps,
for this quick test. I could key-up this amp, relay comes on, but no RF out,
it did not draw much current, no bias of course as mentioned.
So I probed around with my O-scope.
I could see the first driver had some gain. then I moved on over
with my scope probe to the input section of the four MRF455's.
I was checking for RF at the compression trimmer capacitor
and when I touched it, all of a sudden, the power supply went into
current limiting. I turned it up to 10 amps, and still the same.
So it appears to be working now. The owner took it back and
we were going to test it in his truck where he has the large
wiring needed for the high input current.

So I found the dielectric on that trimmer capacitor to be flaking
in pieces and breaking away, and some of the plates were shorting out.
I opened up the capacitor for spacing of the plates.

So a lucky and quick repair. but not "official" yet until he puts
it back in his vehicle, and he tests it. I told him about the bias stuff,
that I can work on it, make it clean. But, He is an
AM-er and wants that high power, loudness, splatter coming
out from his pick-em-up truck!