The AM Forum

I got a Central 20A on the bench. All caps check leaky.. LOL.  Like that was a suprise.  Can cap is shorted.  Ordered all new caps about $20.

I find that I need a 5mhz VFO.  They suggested you use a Command set 458. I dont have one. Nobody I know has one and most are ripped apart or junk by now.

What other VFOs out there have a 5MHZ output?  I think that a SWAN model did but I cant remember and find little on the net. 

Trying to find 5MHZ Xtals is also hard.. So I thought maybe I would get a VFO that worked and use the unit for an exiter.

C

Probably easy to build:
5 to 5.4 MHz VFO's
QST Dec. 1985 , page 39
73 Mag Jan. 1977, page 116
HRM Jan. 1972, page 27

Or modify something like a VF-1 or equivalent

Hi Clark,

Fine on the 20A.  Think that CE offered a kit of parts to modify 'your'  458 Command Set as a VFO.  The 458 etc do pop up.  There are many Command sets out there,  which are only slightly mnodified,  and would be fine as the VFO for the 20 A.

You could place an ad in the Wanted section on this site.  or ..

An ad on QTH :  
http://swap.qth.com/

AF4K does sell crystals :
http://www.af4k.com/567MHz_crystals.htm
But at $ 12 ea,  they sure do add up.

Steve WA1QIX designed a FET VFO for the Class E rigs,  and is around here somewhere ...  altho,  its output level may be  better suited driving TTL Levels rather than some grid ...   http://www.classeradio.com/vfo.htm
For your perusal.

We've all heard of Vortex Joe.  Think that there is a sub-Vortex forming at UR QTH in AZ   hi.  73  GL  Have Fun     Vic

Clark,

5.0-5.5 is a very popular VFO frequency and most of the external VFO's for the 60's and 70's transceivers either cover that range or can easily be shifted slightly to hit it.  That is also the range used by Heath for the SB (and most of the HW) line so if one is being parted out you could use it.

I have a couple of the Command sets with the CE front panel for my CE-10 and CE-20 but neither has the additional circuitry for 10 meter coverage.  They do show up and are often under the table at hamfests.

Rodger WQ9E

While not politicallly correct, I've used my drake tr7, with general coverage transmit mod,
into a dummy load and fed that into the vfo input on my 20a.  This just is just a temporary set up until I build/modify
a vfo... 

There are plenty of scrapped HT-37/HT-32's out there with a pretty decent VFO. I paid about $10 for a SSB generator, VFO and xtals from a HT-37.

Several companies made standalone VFO's: Hallicrafters, Lakeshore, CE (based on a BC-458) Globe and Gonset I think, and several smaller names that advertised in the 60's.

Carl
KM1H

Clark,

   I do 20a's here, and my main station revolves around one. These when stock can be a lot of fun, and I've went through three of these so far. In stock form these dudes are good for about about 15 watts PEP out on SSB, and maybe 3 watts carrier on AM. Modified for QRO, they can be even more fun.

   You can use 160m, 75m or 40m crystals with these rigs. The novices did just that since this bypasses the balanced modulator. You can also offset the crystal +/- 9Mhz to run phone.

   The big tubular caps and all electrolytic's all need to go. The RF bypass caps can usually stay. Some were mica's and on newer 20a's they are ceramic disks. One caution is the B+ bypass cap for the plate of the 6BA7 mixer. This cap carries RF current and is matched to a resistor across the tank coil. The combination effects the 'Q' and stage gain. When CE switched to a ceramic cap, they also changed the resistor to a higher value.

    With a stock 20a on todays high line voltage the box gets as hot as a stock Ranger.

   The VFO's are out there Clark. With just a 5.0 to 5.5 Mhz VFO you get 20m and 75m. To get the other bands the VFO is doubled, tripled, and beat against a crystal. Therefore the CE VFO's are what you want. Also the Lakeshore Band Hopper VFO's are even better, although somewhat rare. Keep an eye on the adds and Ebay.
Here is my Band Hopper with a PIC Stabilizer to lock the VFO to within +/- 10 HZ:

http://pages.prodigy.net/jcandela/Band%20Hopper/

  I could talk volumes on these rigs, but thus far on AM FONE I've mentioned this stuff many times in passing without any interest. Let me know if there is anything technical I can do.

BTW at the link above, go up 1 level and there are several write-ups on the three 20A QRO projects I've undertaken.

Jim
WD5JKO

I would like 75, 40 and 20.  I got the thing cheap.  I figured, $20 in caps and a heck of alot of cleaning and I would have an SSB rig to run into the Kings SSB port.  What sparked all of this is the SSB adapter that is in the back of ER. Its solid state and should get me decent SSB recieve on my SP600s and or R390s.  I thought it might be fun to set it up and Rag chew to the guys on something old without those pesky transistors.

I spent about an hour with my recently aquired Eico 930B cap checker.. My cap checker needed caps.. LOL. 

So far, All of the molded type or wax coated caps are just perfect. Only the Electrolytics are leaky.  I believe this thing will fire up once I pick those up at the will call window at Antique electronics supply tomorrow. 

I have a bunch of old VFOs in the store room. I have no idea the output. I am going to Grab them one by one and see what they output to the Freq counter. I know there are some FT101, 301 yeasu units and a few PAL and a couple silitronix units in there. Maybe I will get lucky.

Is it a big deal to modify a command set?

C

I have a Lake Shore Bandhopper VFO that was designed for your rig. Its very rare. Contact me off the board and we can talk.
wgr@roadruner.com

Bill

Thanks alot.. I will post pics of the thing on the bench tomorrow.  I have to take my Dog Toby in for surgery in the morning. I might find some time after I get him home and comfortable to put the caps in the Rig. 

I would also love to find an HC10 Hammarlund SSB converter if someone has one. I will post in the wanted section.

Clark

   Clark,

   Moving over to vintage SSB moves you over to a different batch of folks. With a 20a you can get very clean audio, but getting the unwanted sideband and carrier eliminated is more of a challenge. I have tweaks for the phasing circuitry to beef up the sideband rejection. The big issue however is VFO drift. On SSB, especially the hi-fi SSB folks, a drift of 30 hz is noticeable. I once engaged a bunch of hi-fi SSB guys on 40m where the 20A BC458 VFO is tripled. I drifted 150 hz, and was driven out with comments like, "hay OM your drifting out of the band".

   Even the Band Hopper VFO will drift. Figure about 800 hz over the first hour. That is why I added the PIC stabilizer to mine. Even so, the 9Mhz crystal in the 20a drifts too, maybe 200hz over the first hour. I am wanting to put that crystal in a oven, or more likely to epoxy a PTC thermistor to it where the transition temp is about 40 degrees C.

Good luck with your 20a..

Jim
WD5JKO

I am not leaving AM. I just thought this might be a fun project.  I talk Little SSB now.  One of my main interest is in using this as an exciter to Drive the 4-400s. They have a Tuned input and need little Drive to get going for AM.

Going to get the caps later today.  Should be a simple job replacing them. All other caps and parts check out good so far.

Clark

Jim, do you have any info on the Lakeshore HFVO labeled Hetrodyning VFO on the sloping front panel?  Need a schematic.

Carl
KM1H

email me.. I got the Diag and schematic carl.

Clark  AT Clark Turner.com

Clark, that was the Band Hopper that you sent, thanks. I have the "Hetrodyne VFO" which is what they call it. The rear panel decal lists the model as HVFO.

The RF circuit has the same tube lineup, maybe the crystals are underneath.
There is no PS but it has a 6U8 oscillator, a 1mc xtal and an eye tube for the calibrate function. Power comes in via an octal plug and the RF out via an Amphenol mike connector! 1950's high tech all the way!  The front is 13" wide with a slope front and a fancy National ICN calibrated dial. Serial is #24 which may or may not mean it wasnt a big seller.

Anybody have more info on this item?

Carl
KM1H

If you don't mind solid state this DDS has good specs.
http://www.pongrance.com/super-dds.html (http://www.pongrance.com/super-dds.html)

I will need a buffer amp to drive a boat anchor transmitter though.

   Carl, I never heard of this model VFO by Lakeshore. Can you post a picture?
I did find a site by k7jrl that has a heap of manuals & schematics for most any rig. Below are two links starting with Lakeshore:

http://www.k7jrl.com/pub/manuals/lakeshor/

everything else
http://www.k7jrl.com/pub/manuals/


 yes this is a good idea so long as you can make 8-10 volts peak at the 20a VFO input. With the VFO output at 250mv rms, you will need a buffer amp for sure.


Jim
WD5JKO

Drake T4/R4 line used 5Mhz PTO
You are correct, Swan did also.
So did Heathkit.

 yes but remember that gets you 20m and 80m only. For 160 the VFO doubles then mixes with 9 Mhz. For 40m the VFO triples then mixes with 9 Mhz.

For example:
160m, VFO at 5.45 Mhz doubled to 10.9. Then subtract 9 from that and we get 1.9 Mhz
40m, VFO at 5.387 Mhz, triple to 16.16. Then subtract 9 from that and we get 7.16

Jim
WD5JKO

BILL.  I emailed you but never got a response.  Can you contact me?  The 20A should be done today!

Clark at Clarkturner dot com

Jim, that link is the Band Hopper. As I mentioned yesterday I can probably use it as the RF section appears the same.

Here are some pix, they aint purty. It sat out in a shed for years.

Carl
KM1H

Finally got my computer to work again.....
Pics of my vfo......

Damn.........the front........

when I saw that mess that was posted as a Lakeshore VFO I was sick.....
Mine is a LOT better.........

Oh man.. I am going to finish the 20A tomorrow!  This is going to be fun!


Clark

Clark,

   I wish you well on this project. I have heard of folks getting an Ebay 20a, and having it on the air the same day...working perfect.

My first 20a was the exact opposite. It had the bad caps, and after that the alignment procedure of all the coils was a moving target. At times I had it tuned as good as it could be, and then I'd turn it on the next day and everything was all screwed up again. This went on for about a year and a half. I found no less than 5 intermittent issues, where the last required me to rewind the balanced modulator coils. Another 20a I went through had the two RF coils in the balanced modulator mounted further apart than stock. The mutual coupling was insufficient between them, but I could still get a carrier null. Then as the signal level went up beyond 50% the sideband rejection went to hell. These kind of issues can certainly be challenges!

Some of these rigs were kits, and miss-wired such that they NEVER worked. Also even CE made mistakes. For example for the PM mode there are two 15 ohm resistors near the Mode switch. I have found more than one 20a where these resistors were 1 meg-ohm. The band colors were the same but the order was different!

You may say, "who cares since I only want to run AM". Well, to run AM on a 20a you still need to have a carrier null at least 30db down from full output when the Carrier level control is at minimum. If you cannot get a good and consistent carrier null, then the AM downward modulation will be clipped severely.  

If the ride gets bumpy on your 20a, please persevere and hang on. These rigs are worth the ride.

Jim
WD5JKO

We will find out next week Jim.  What I reall need is at least one Xtal so I can fire this up on the bench!   I dont have any 5mhz xtals. 

I know a guy that can help me fix it if its having major problems!   YOU. hahaha




Clark

Clark,

  Just plug in a 3880 or 7160 crystal into the 20a for 75m or 40m respectively. This will be good for CW only, but in doing so you can check out about 1/2 of the rig. This is what novices did back in the day running CW on the 20a. No VFO or FONE for novices back then.

  Also be on the lookout for loose tube socket pins (replaceable, or fixable), oxidized tube pins, and cold solder joints. Run on 115v with variac, and make sure fuse is no more than 3A. The 6AG7's run very hot, and if gassy they will self destruct. The 6AL5 bias rectifier tube is prone to bad socket connections, and if the filament fails to light (pin 3 or 4 issue), the 6AG7's get no cutoff bias and fry. Fortunately with the 20a each circuit area has a series 560 ohm 1/2 w CC resistor that will act as a HV fuse in the event of a short. These resistors are all suspect  if they get overloaded just once.

  The weakest tube on this rig is the 6U8. They run hot normally and with 125 volt AC line I've had issues with them. Others debate that point saying they run high line for years without problems. For me I run high line volts and replace the 6U8 with 6MU8. These 6MU8's have the same pinout with similar spec's except for more Pd on the pentode, and a taller glass envelope. The 6MU8 is also very cheap from many tube places whereas the 6U8, 6U8A, 6EA8 types are usually more expensive.

Good Luck!
Jim
WD5JKO

I dont own any Xtals :)  My Brother has a complete Collins set. Maybe there are some in that Kit that will work for testing.

C