The AM Forum

Hello:
My Valiant has developed a problem with High Plate current.

With 8ma grid drive the Plate Current meter is pegged to over 450 M
with only 30 watts output and I also see no plate dip.
With the grid drive turn down to the zero level I see 400 M plate current with a plate dip with 15 watts output. These test wre into a dummy load.

This Transmitter is a K1TLI rebuild. I switched ina new set of finals and it acts the same.

L8-L9-L10 parasitic Supressor resistor look burnt, but I dont think that is the problem?

The 300-600-900-1200mf loading caps had been previously replaced

Any ideas??
Thanks, Frank
WØKH

Every band; one band; crystal or VFO controlled ??

Thanks for the reply:

The transmitter works correct on 40 and 160 Meters with the correct plate curent with a good dip at the correct grid drive and 150 watts output.

 The 10-15-20-80 meter bands is having the problem

Thanks

I forgotto mention the problem is there in both VFO and Crystal positions.
I only have 3.880 and 3.885 rocks. The tests on the other bands were with the VFO
only

The second sentence had me thinking parasitic suppressors.

Carl

You might check the VFO bandswitch coupler block.
If it has come loose the VFO could be on the wrong frequency.
The adjustment procedure is in the assembly manual for sure...I dont remember if the operators manual mentions it.

Agreed- Check the synchronization on the VFO's bandswitch vs the chassis/front panel/RF bandswitch. Depending how they get out of they get out of sync, the rig will transmit OK on some bands but not on others.

Bill

I checked the VFO Freg, with a Sony 2010 receiver tuned to 3.885 and the Valiant  on ZERO and verified the VFO is on 3.885.

Assuming the plate and load caps are ok by virtue of 160 and 40 working, ya might wanna check the bandswitch section that chooses the plate inductor tap. It might be dirty/burnt/evaporated.

Has to be in the crapstal oscillator/ VFO amp stage. That stage doubles for 80, 20,15, 10M. Are you sure you have the osc. and buffer tuning right?

Phil

I suspect that you might have a PA bias problem, along the clamp being inoperative. There could be problem with the P10 jumper plug.  I have not fully traced out the path of the bias supply, but the clamp should kill the PA screen supply if the grid bias is gone. Check the plug. The PA grid bias should be on P10 jumper lug between pins 1 and 2. Check the procedure for clamper adjustment. Do you have a Valiant 2 with the separate bias pots? Without grid bias, it could still work correctly on 160 and 40. How?  Well each of those frequencies are fundamentals of the VFO for the selected group of bands. They will have the highest amplitude and perhaps provide enough grid drive to "bias by drive" and appear to work correctly. The grid current is the grid acting like a diode and building up negative charge on the coupling cap. MAny lower power  rigs will depend on this to provide proper  bias for class C operation. The danger is that with loss of drive, the final will run away and burn up. The clamper tube is suppopsed to protect the finals by killing the screen supply if the bias supply fails.  

As to the VFO cam slippage, consider this:

Well, you can verify if the VFO band switch cam is working correctly. Just place the unit in spot mode and set 160 Meter band. Set dial to 1.8MHz. and verify you can find it on receiver. The vfo is used harmonically. The VFO part of the band switch has three positions: First, 160/80; Second, 40/20/15/10, and Third, 11. You should be able to "hear" the VFO at 1.80 MHz for both 160 and 80 meter positions. It should jump away when you switch to 40. Tune it to 7.00 MHz and find it on receiver. It should be heard there on 40, 20, 15, and 10 meter positions. and, it should jump away from 7.00 MHz when you switch to 11. This will verify that the cam action has not slipped. However, slipping of the cam doesnot seem to explain the combination of bands that work and those that do not. AT least not for a single point failure. Suppose that the cam slipped in some way and that the first switch occurs between 160 and 80 and not between 80 and 40. 160 would work correctly, 80 would not, but if 40 worked, so should 20, and 15. For 10M the cam switch might move to 11M, but it would probably be close enough to give some output.

Correct CAM alignment

Band   VFO Frequency
160    1.8 MHz
80      1.8 MHz

40      7.0
20      7.0
15      7.0
10      7.0

Possible CAM slippage

Band   VFO Frequency
160    1.8 MHz  will work
80      7.0 MHz  will not work

40      7.0    will work
20      7.0    should work
15      7.0    should work
10      6.xxx  might work poorly