Central Electronics 20A low output on 10-40 meters

[WB8PFZ]

 My issue is the transmitter works fine on 80 and 160. It tunes nice and puts out 18 watts or so. The other bands 40-10 puts out very little power. Less than 1 watt. The capacitor padders in the amplifier stage are peaked for max. I am just getting into it but looking for ideas from anyone who might have owned one of these. Started checking signals on the transmitter. What does not make sense is I have a nice 5.1 mhz signal out of the VFO. That mixed with the 9 mhz signal from the crystal oscillator should give me a 14.1 or a 4.9 mhz output signal from the mixer. I am seeing 9.4 mhz at about 12 watts. I tried adding subtracting and multiplying and still can't come up with 9.4 mhz unless something is going weird with the crystal oscillator but it works good on 80 and 160? Mike

[AJ1G]

FWIW, if you are using a command set transmitter with the factory conversion kit as your VFO, mine came to me with a shaft and knob attached to the large breadslicer variable padding capacitor (the one rhat is normally set with a tweaker through an access hole in the side of the chassis and then locked in place in the original unmodified transmitter). I have to adjust that cap when changing bands.  The peak output of the 20A ends up being about the same from band to band, but the setting of the ladder cap is different from band to band.

I do see somewhat lower output across some bands from the low end to then high end of the band, that can not be corrected by tweaking aforementioned bread slicer.  In assume that tracking of in band alignment is probably off.a bit.

[WB8PFZ]

I have given this some thought. Looking at the output of the VFO on my scope the amplitude changes a lot as the frequency changes. I am going to use my signal generator as a temporary VFO and see if that is the problem. Mike

[w1zb]

I run my Central Electronic 10A exciter with a DDS signal generator and it works fine. The 10A requires about 12V P-P signal for full output on 160, 80, 40 and 20.
BTW I did at on time use a CE 458 VFO but it had a bad drifting problem which I never could fix.

73 Jerry W1ZB

[WB8PFZ]

The more I play with this the more I think it is the VFO.

[WD5JKO]

There are several 20a threads on this forum. Here is one:

http://amfone.net/Amforum/index.php?topic=37143.0

The last post is on point here for this topic, and it explains some of the quirks of the Mixer plate tuning circuit.

Oh, BTW, forget about 15m with a 20a...spurious hell, and low output. For 10m, there is a crystal controlled converter at is designed to fit inside the BC-458 Command Tx.

Also, it has been my experience that 40m is a low output band because the VFO output is tripled in frequency.

The best 2 bands are 80 and 20 since the VFO output is applied directly.

Jim
Wd5JKO

[WB8PFZ]

My best bands are 160 and 80. The rest don't really work. That post you mentioned was one of the first I read. Thanks Mike

[WD5JKO]

My best bands are 160 and 80. The rest don't really work. That post you mentioned was one of the first I read. Thanks Mike

Well that is really a good clue. On those two bands, the 1/2 12BH7 RF driver cathode follower has a passive RF Choke return, and on 40-10m, the cathode follower return is an LC tank. Looking at those tank circuits, they all return to AC ground through capacitor C79, .005uf. Capacitor C79 could be the culprit!

Good Luck!

Jim
Wd5JKO

[WB8PFZ]

Checking the VFO output for 40 meters shows little output. I am going to put this in the corner for a while. Merry Christmas to all. Mike

[WD5JKO]

Realize that for 40m, the BC458 VFO is set to triple the oscillator. So for a transmit frequency of 7.2 Mhz, the VFO needs to put out 16.2Mhz.

(3*5.4)-9 = 7.2Mhz

Running the VFO direct (5-5.6Mhz) only works for 80, and 20m.

9-5 = 4Mhz
9+5 = 14Mhz


for 160m, the VFO doubles.

(5.5*2)-9 = 2Mhz

The crystal input on the front panel is for direct operation on CW. The 9Mhz balanced modulator is not used.

You can only null the carrier, and work voice (AM PM DSB SSB) when mixing with the 9 Mhz internal crystal.

Merry Christmas All!

Jim
Wd5JKO

[WB8PFZ]

With a 8 megacycle crystal (all I have) in the 40 meter band switch position I get a little more than 2 watts output at 8 megs. Mike

[WB8PFZ]

Just purchased some 40 meter rocks. With the transmitter using the crystals the output is near 20 watts. Unfortunately the AM does not work

[WD5JKO]

Just purchased some 40 meter rocks. With the transmitter using the crystals the output is near 20 watts. Unfortunately the AM does not work

Hi Mike,

I updated my last post to further clarify.

Back in 1951, Art Collins at Collins Radio calls Wes Schum at Central Electronics, and Wes sells Art five 10a kits. The Collins staff builds up the kits, and they have the exact same problem you are experiencing! You are in good company Mike!

Play the video below, and around 28 minutes into it, the story is presented by Nick Tusa, K5EF:
https://youtu.be/q0SDwieAeLw

To get a 20a on AM/PM/SSB on 7.2Mhz, you need a VFO input at 16.2Mhz (5.4x3), or use a crystal on the front panel stamped 16.2Mhz fundamental.

Using a direct frequency crystal as you have been doing, works for CW only since the 9Mhz section and the Balanced Modulator gets bypassed.

Note: You should be able to operate direct at 9 Mhz and balance the carrier, and run voice modes. I forget where the band switch gets set at, but it is either 40m, or 20m to find the 9 mhz peaks.

Better use a dummy load!

Jim
Wd5JKO