Help! Got a sick Central Electronics 20A

[wa2tak]

Hi all:

I got this ( new to me ) 20A going with 15 watts out into a dummy load ON CW.
Then...went to AM...it DIED!
Line VAC amp meter when thru the roof!

So far, with NO tubes...just the 5U4...xmfr looks OK...got 3 amps on the 5.5 v 5U4
With NO tubes...NO 5U4..got good AC voltages from the xmfr on the 5U4 pins.
( wondering if my my HV secondary goes to heck under load? )

Here's where I'm at.
I disconnected filter choke, filter caps...leaving just the B+ 'empty'
I read ONLY 14 ohms on the B+ line.

Fortunately, made a video when working CW.
Got 1.4 Amps on the AC line meter...@ 120vac that's 168 watts
Taking AWAY the amps on the 5U4 = 16.5 A
And taking away the ( calculated ) 6.3 tube filament watts = 28 watts.
Thus 168 - 16.5 - 28 = 123.5 watts for the 300 vdc B+
Soooo  123.5 W / 300 VDC = 0.14 amps.
Thus...RESISTANCE INTO THE B+ = 300 V / 0.14 A = 731 ohms.

MY CURRENT PROBLEM:  I measure only 14 ohms to ground.
Currently disconnecting various 300 v points in the circuit...nada.

GREATLY welcome your generous help.

Kind regards,
Steve
WA2TAK

[WD8BIL]

Look for a bad electrlytic on that B+ somewhere.

[wa2tak]

Disconnected EVERY electrolytic....14 ohms.
Good idea...will check EVERY cap on the B+...had changed ALL the papers.

[WD5JKO]

Steve,

   If it was the B+ shorting to ground, that 5U4 would have visible distress inside, sparking, redness, etc...carnage. If so, that tube might be toast by now.

If the B+ line is 14 ohms to ground, you could always jump in 12V from an external source, B+ to ground....Something will get hot quick!

Once past the issue, keep in mind that the 20a has a Chernobyl problem...If the 6AL5 bias rectifier fails to light (tube pin/socket issue), there will be no bias

and therefore the 6AG7's will pull as much as they can...Depending on the fuse size, this simple issue can create lots of smoke!

The 20a B+ is closer to 400v instead of 300v with today's line voltage. During turn on, the 450v rating of the filter caps gets exceeded.

Jim
Wd5JKO

[wa2tak]

Hi Jim:

Thanks for your input!

In addition to me creating my own 'challenges'...the unit is presenting its own to me.

One concern is that the schematic is date 1954...as other info I've found.
I THINK mine is a factory 1957 ( actually, the video on CE history / Wes Schaum...by Nick Tusa, K5EF..says
he rented 5 GARAGES from his neighbors to assemble them!...great video...very worth the look.)

Most challenging is following the SCHEMATIC and MATCH it to the actual rig WIRING!
"All" the wiring is harnessed!!...and unless really necessary...don't want to open it up.
On top of this..the wiring pictorials are 'good'...it shows the most wire routing AND wire COLOR CODING...BUT NOT HARNESSED.
Alas, my wires no longer have colors!

Most disconcerting is that the harness wiring goes to terminals...and 'impossible' to trace out a schematic line!!
That wire line branching makes it tough.
And...speaker of having "my wires crossed"...I may be NOT on the 'proper' B+ out to all those branches.

And..speaking of creating my own "challenges"...I've been unsoldering certain B+ lines in my attempt to find where that 14 ohms originates!!!  It may even be "OK"??

I think I may have found something significant...the OPERATION switch..SWB, sec 2, pin 18 goes to B+...I find
no connection...reading 40 megohms!!
I jostled the harness at the switch...those megohms changed...burnished the contacts..resoldered the terminals...still that 40..
Just for the heck of it...I put an LCR meter to it...read CAPACITANCE..changing value down over seconds...settling down on 0.03uF!!!!!!

Please excuse my long note...I've am a rag chewer...

THIS 20A WILL GET ON THE AIR.

Lastly...having 'fun'...just LUV ham radio!!!!!

[wa2tak]

To all 20A"ers

Hope this helps ya.

This web site is a "must" for those fixing this rig:
https://frank.pocnet.net/instruments/CentralElectronics/20A/20A.html

and...this specific item on the page is VERY, VERY HELPFUL tracking the wiring as per schematic!!
https://frank.pocnet.net/instruments/CentralElectronics/20A/20A_WD.pdf

Steve
WA2TAK

[wa2tak]

Got it going...CW

THANK YOU ALL FOR YOUR GENEROUS HELP!!

Found a shorted 0.005 MICA - C13 and split 560 ohm resistor - R20...in the Master Oscillator 6U8 B+ line.

I now know this rigs circuitry pretty darn well know.

AND...learned a LOT about CE gear on THE GREAT FORUM!!!

Some info:
B+ to ground reads 20K ohms
B+ in STANDY = 440 vdc
B+ in Man + CW = 369 vdc
Filaments = 6.8 vac

JUST LUV HAM RADIO.

[WD5JKO]

OK Steve,

    Good on that find, those 560 ohm resistors are supposed to pop in the case of a fault like that.

If you want to keep the rig mostly stock, which many do, then you will have run it as if the year was 1958! That's ok too.

The power supply stuff, addressing the line voltage today might be worth doing. A 12.6v 1.5 to 2 amp transformer could be used to "buck" the incoming AC line voltage such that if you had 125v from the wall, then 125-12.6=112.4vac. This could be done inside, or outside the rig. Another method sometimes used, although a fairly major modification, is as follows:

* Solid state the 5U4
* Use the 5v winding for the 5U4 to buck the incoming line voltage (it may be closer to 6v)
*Switch to choke input filter on the B+
*add an additional R-C filter on the B+ to provide better filtering for the low level stages (maybe 150 ohms 10w, and 47 uf 450v). Run the 6AG7 plates off the higher B+.

Solid stating the 5U4 will increase the B+, but going to choke input filtering will drop the B+. After the switch, expect something like 275v to 300V B+. The extra AC ripple from this change will be picked up in the low level stages. The depth of the carrier null is affected by ripple on the B+. You may hear a buzz in your receiver when nulling the carrier. This is why I suggest adding an additional R-C decoupling filter.


Now that you are moving forward, all the alignments, will follow.

Keep in mind, the two bands that are easiest to get full output on (using a VFO) are 80m and 20m since a 5.0-5.5Mhz will beat +/- with 9Mhz to get you on those two bands.

For 160m the VFO gets doubled, and 40m the VFO gets tripled. Forget about 15m (spur city), and only contemplate 10m if the VFO has the 10m optional module installed.

If you plug in a ham band xtal, you bypass the balanced modulator, so no carrier null, and no AM/PM, SSB...only CW.

Jim
Wd5JKO

[WD5JKO]

Now that you are moving forward, all the alignments, will follow.

Keep in mind, the two bands that are easiest to get full output on (using a VFO) are 80m and 20m since a 5.0-5.5Mhz will beat +/- with 9Mhz to get you on those two bands.

For 160m the VFO gets doubled, and 40m the VFO gets tripled. Forget about 15m (spur city), and only contemplate 10m if the VFO has the 10m optional module installed.

If you plug in a ham band xtal, you bypass the balanced modulator, so no carrier null, and no AM/PM, SSB...only CW.

Jim
Wd5JKO

    While waiting for a VFO, the 20a can be tuned to transmit on 9Mhz. I forget whether you can tune UP from the 40m band, or down from the 20m band, but for sure, you can peak the tuning on 9Mhz, while transmiting into a dummy load. This way you can null the carrier, and setup both the RF and audio phase adjustments.

Jim
Wd5JKO

[wa2tak]

Hi Jim:

As you see...I'm very wet on getting this 20A working...other than my 14 watts on 40 CW...which any 'dummy' can do.

It's circuitry...with that 9 Mhz...is different from other "standard" rigs I've worked...my first "phase" radio.

I was befuddled why I had no AM with that 40 xtal!!

Now I know..and looking for 5000+kc xtals....from 5417 to 5434 = SSB on 40...or...16250 to 16300...

Here's more of my ignorance...how would I operate ON 9 Mhz..with no xtal or VFO?

What would I go when it does?

Tnx a bunch for your help and PATIENCE,

Steve

[WD5JKO]

Here's more of my ignorance...how would I operate ON 9 Mhz..with no xtal or VFO?

Steve,

  The 9.000 Mhz oscillator and crystal are already there in the 20a. See image attached With this, it goes through the balanced modulator, and the  phasing circuitry which generates

AM/PM, and SSB Phone signals.

At the 6BA7 mixer, the intent is to mix with 9Mhz with the VFO, or VFO 2nd, or 3rd harmonic to cover 160-20m.

Now here is the kick, if you don't have a VFO, or an appropriate crystal, just tune the mixer plate to 9 mhz, and the RF PA to the same 9.00 Mhz.

AS I said earlier, use the 40m band setting, and tune UP from 7 to 9Mhz, or use the 20m Band setting, and tune the variable capacitors ("Mixer" and "Amplifier") down from 14 to 9 Mhz.

As I recall, the 40m band setting works better. You should be able to get 10 watts or more out on 9 Mhz, and then play with carrier null, and the phasing circuitry.

At 9 Mhz output, stay with a dummy load...

My first 20a, had issues with the RF phase shifter that took me a long time to figure out. Many times, I aligned it perfectly, and then let it sit for a while. Upon powering it up days, or weeks

later, it was all out of whack again. These rigs can be a lot of fun, and they can be a bear at times. These rigs are over 60 years old now, and many have not aged well.

Jim
Wd5JKO

[wa2tak]

Well Jim:

Seems like I'm actually making progress.
After replacing that 2nd 560 ohm resistor...darn thing split in 2 just like the other...now getting 20 watts CW.

And, it appears that your 9Mhz 'works'.
Had to use 160...AM...mixer & amp tuning right about 9Mhz...got output...VERY HIGH SWR into the dummy.
And..can't seem to adjust it - whatsover - with my MFJ antenna tuner.

No settings on the tuner work...only get NOTHING at some inductor settings?

Got a beautiful Palstar AT1500DT tuner to try?

After "fixing" that...I'd guess I can use my HI-Z shure 404C mike.
Should hear myself on 9 with my 706Mk2G...took out the diode...can xmit all freq's

Now...thinking ahead...I'd expect to get some SSB on 9 too???

TRULY APPRECIATE YOU HELP...COULD NOT 'DO IT' WITHOUT YA!!!

[WD5JKO]

Steve,

  James Hagerty, WA1FFL has the solution for interfacing a SS VFO to a tube circuit:

https://wa1ffl.com/VFO_driver_amp.pdf

He might still sell a kit.

Remember though, the 5-5.5Mhz VFO needs to double for 160m, and triple for 40m. As is, a 5-5.5 gives you 80 and 20m. Do the math.

As for your 20A operating at 9 Mhz RF output, you have to find 9Mhz resonance on both the Mixer plate and Amplifier plate.

Having a receiver nearby should help...but you will not find it on 160m Band!  You may be forcing the Final Amplifier into a parasitic oscillation.

40m should do it, tune (less cap Mesh starting from 7Mhz on the silkscreen)

The RF load needs to be a dummy load, not an antenna tuner hooked to an antenna....unless you want a pink slip from the FeCe.

A 25w incandescent light bulb might work if you lack a dummy load.

Once you get past the above issues, do you have an audio sine wave generator? (IFIRC, you want 1.2Khz or thereabouts)

Jim
Wd5JKO