The AM Forum

I have a Clegg 66er I have been working on. Power output is roughly 13 watts. I have replaced numerous resistors that wandered out of tolerance as well as replaced some electrolytics but not all of them. The Clegg 66er has what I would call anemic TX audio chain gain and sounds less than clean. I can get 60-80% mod if I close talk my D104 with the speech gain cranked to max. I know I have an issue somewhere. All the tubes have been tested. The 6AQ5s are good. One of the odd things I have come across is the 6AN8 audio stage tubes are borderline good. I have numerous others that fail in one part of the dual Triode/Pentode. I use an EICO 667 for checking everything. As a matter of fact, I have two and they test close together in results. What is up with the 6AN8s ??? Pricey too if you try to find them.

Without signal levels in the manual I am troubleshooting by assumptions. I would imagine with a pair of 6AQ5s contributing 5 watts of AU-DEY-OH power in parallel to modulate a 2E26 should should good, right ? I have heard nothing but good comments about Clegg gear. My Zeus is a perfect example. It sounds excellent.

Does someone have a 66er to run some test point levels in the audio stage ?. Like at the cathode of the 6AN8 PIN 3 and the speech gain wiper feeding the 6AQ5 inputs.

I am not a hundred percent positive but the receiver seems "quiet". Sensitivity is fairly good but the overall gain of the audio output on receive sounds low. I have the scope on and just trying to setup an audio generator to test the 6AQ5 stage.

Of course this evening I found that if I tilt the entire rig on its side power output drops but returns in an upright position. LOL. Seems like the 2E26 I have in there might by the culprit on that.

Anyway, its miserable outside and Im inside enjoying the troubleshooting adventure.

Oh and I am monitoring 50.40 on the interceptor. Crank up that vintage stuff !

Hi Stephanie ... some interesting clues ... can you get a 0-scope .... may need one to be able to determine the culprit (s) in this problem

the combination of low tx and rx audios could be Ponting to bad mod xfmr ... hope not as that would be a hard critter to find ... I would concentrate on getting the rx audio up to snuff first .... again the scope may be required to find this

good luck

Hi Stephanie

A few of things to try:

1) Feed a 1KHZ sine wave across the gain pot and see if you can get 100% modulation without distortion. Monitor the output waveform and the modulated B+ going to the RF final. This might give you some clues.

2) Feed a 1 KHZ sine wave into the mic connector and see if you can get 100% modulation without distortion.

3) Repeat 1 & 2 at reduced RF loading, say 5 watts carrier out.

I had a Gonset Communicator IV that required an amplified mic to get reasonable modulation.
I have also seen low grid drive to the RF final reduce modulation peaks. Check tuning or XTAL.

Hopefully this may help. Good luck.

73 Jerry W1ZB

Stephanie,

All good input from the others thus far. The one thing I will add that scoping things with tubes can exceed the scope input rating, and also the probe rating. I like to use a compensated 100X probe such as this one:   

Goupchn High Voltage Oscilloscope Probe P4100 with Accessory Kit 100MHz 2000V 100:1

Amazon sells these for under $20 and with Prime are just a few days away.

These 100X probes can tolerate 2KV DC with a de-rating for AC as the frequency goes up. These are good with a safety factor for most boat anchors

at the 100 watt class, such as 6146's being plate modulated to 1500v peak on the modulated B+.

The 100X is also just a few pf ac load, and essentially almost infinite DC load resistance. They come in handy in many places.

Good Luck,
Jim
Wd5JKO

Excellent advice ! I worried about this very issue. Thank you for the suggestion Jim !

Hi Jerry !

Interesting about the drive ! I noted the lower than usual output with the xtal I had. I will try another xtal. Right now I am measuring all the resistors value around the PA and driver stages. I found a number of resistors that wandered beyond their 10 percent tolerance. Each time I revisit the 66er I find another item out of tolerance. Spent some time checking components around the 6AN8. I am also repairing my RCA audio generator too. One thing leads to another. LOL Thank you for your advice

73

Stephanie

I wonder about the xfmr too. I will check that as best I can

73
Stephanie

I just ordered two of those probe ! ;D  I really needed new ones.

Hi Stephanie,

Looking at the fuzzy schematics I noticed a number of diodes that could possiblly be shorted.

I would first lift the diode from the voltage divider to the cathode of the first 6AQ5, and then working back check the other two diodes in the audio and IF chain.

Phil - AC0OB

I have never even looked at the schematic, but assuming all DC voltages are correct, you may want to cobble up a push pull to 8 ohm output xmfr backwards and use that to drive the grids of the modulators.  Run a small audio amp into the 8 ohm side and see what percentage of modulation you get when viewing the output RF chain on a scope.   Also, don't overlook a problem with the PA.  If its low on emission, when you apply the extra modulated voltage, it will just not perform.  Another good test is to float the scope and look at the secondary AC waveform of the modulation xmfr.  See what the waveform looks like when it runs out of stuff!  I always do an off resonant tuning to see if you can draw plenty of plate current.  Just a few things to try.....

   

Floating a scope can be a very dangerous thing to do. It might ruin the scope, and at worst it kills somebody. Be careful when recommending a dangerous task.

I just looked up the 66 schematic, and I attach a piece of it here.

Looks like the two 6AQ5's operate single ended in parallel during transmit, and the one on the right is biased off during receive operation.

This is similar to what I did to my Gonset G50.

The audio output transformer is likely just a 10 watt P-P output to a voice coil transformer.

On transmit, the 2e26 plate current should match the combined plate current of the two 6AQ5's. This way, there is no net DC field in the modulation transformer because that field is canceled out. Quite clever. On receive, with just the left 6AQ5 working, this acts as a low power amplifier to voice coil. A little DC here is of little worry.

Jim
Wd5JKO