The AM Forum

Hello everyone,
I am working on an Eico 720 Transmitter. It's in pretty good shape and replaced the electrolytic caps as they were gone.
I am having trouble settling down the 6cl6 oscillator. When I key down and hold it for about 5 seconds it stars to drift downwards and then the tone drops real low all of a sudden. I have not taken any voltage readings as of yet but something is way off. All tubes check ok. All rotary switch connections were cleaned too.
Any ideas of where to start would be great, other than the above, would be greatly appreciated.

Cheers

Bill w3kbs Havertwon Pa.

Bill,

Check actual resistor values on that tube socket. Bet you find something way off.

John, W1TAG

Thanks John.

Cheers

Bill

Bill,

   I presume you are using a crystal. The VFO for that rig, as I recall is an optional external box.

Since you are using a crystal, perhaps the "rock" is heating up?

Is the crystal an original FT-243, or one with a tiny crystal implanted within?

Either way, try another crystal, 80m, and 40m rock...do the symptoms change?

Jim
Wd5JKO

Hi Jim,

I tried a 7040 Crystal in the 243 holder. Also a 7070 Crystal that i have used in the past, and it does the same thing.
The 7040 crystal was recently purchased and appears to be old stock and not made for QRP operation.

Cheers

Bill

Check Voltage at the junction of L1 and R2 on Keydown; is it steady or changing to a lower voltage? If so, you have a bad tube, changing resistance in L1, changing resistance in L2, or R2 is changing value at Keydown.

Was the tube checked in a conductance checker to see if the conductance changed say for 15 seconds or more under test? Was it checked for shorts?

Check resistance of R1.

If frequency is still changing, replace C1 and C2.


Phil - AC0OB

  

TY Phil,

I have a Hickory Tester and it works well.

Bill

Frequency stability in AT crystals varies a great deal due to the angle of the cut and in a FT-243 crystal, you have further difficulties due to the mechanical spring method of mounting it.

I wonder to begin with- what you are using to measure frequency, by the way?
A good frequency counter or modern synthesized receiver should assure your measurements are real compare to an old boat anchor receiver.

Back to the oscillator- have you checked the C1 and C2 caps for stability?
They should be NP0 caps, but if one is opening up when it heats up, it could cause the jump and if it is not NP0, it could shift the crystal load.
That oscillator puts the crystal in the parallel mode, which is a narrow curve, but increased loading can shift it down the curve closer to the series point and stability will be more dependent on load.
These old oscillators drove crystals much harder than modern oscillators, also and modern AT crystals might not stand it.
How much shift are you seeing? Depending in the motional capacitance, the motional arm may be long or short with lower or greater pullability and therefore differing stability.
As a novice, a few khz did not matter, so maybe these old ft243 rocks have always been funky…😉

Notice the new components in the attached file.

I always place a carbon resistor in series with the crystal to reduce crystal current since crystal currents can reach a level of over 100 mA in these vintage transmitters.

Crystal currents can cause the crystal wafer to heat up changing the relative crystal-to-holder dimensions, thus the frequency drift.

But first check the voltages on Keydown as was suggested earlier. 

Phil - AC0OB

TY.

Bill

Good Morning Everyone,

Transmitter working after small Mica caps replaced a few resistors.

Cheers

Bill

Ps. Sweet little transmitter

Yay!

The VFO is Sweet also next the modulator !!!

Cheers

Bil

i guess after all those years, even the mica caps can go south. Always a problem with our older equipment as it gets to the half century mark
Fred

Nice work by Phil as usual.

The parts list in the manual lists 22 and 220 pF ceramic discs.  Any benefit to using 500 VDC silver mica, or just stay with the discs?

I have a couple of questions for Phil:

1) You suggest a current limiting resistor in your modified circuit: The 720 uses a Colpitts oscillator.  This circuit is known for having rather small crystal currents.  I would definitely do it for a Pierce.

2) The 47K 2 watt: Was that added to improve keying characteristics?


Thanks!

PS: New tubes installed.  Test soon.

If the ceramic caps are working, stay with them.

Phil

That series resistor is not going to reduce the oscillating voltage to any extent but it will reduce parasitics.

The 47k is primarily there to limit the KeyUp voltage so it doesn't exceed any of the filament-to-cathode specs on the cathode buss, and it will reduce chirp.

I usually replace J1 with a pure mono plug so it doesn't key the transmitter before I am ready to operate and plug in my dummy load or tuner.

Phil