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Author Topic: What VFOs used 5MHZ output? Central 20A  (Read 15119 times)
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WD5JKO
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WD5JKO


« Reply #25 on: January 16, 2010, 08:50:24 AM »

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
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ke7trp
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« Reply #26 on: January 16, 2010, 10:25:30 AM »

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
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WD5JKO
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WD5JKO


« Reply #27 on: January 16, 2010, 11:03:30 AM »


What I reall need is at least one Xtal so I can fire this up on the bench!   I dont have any 5mhz xtals. 

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
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ke7trp
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« Reply #28 on: January 16, 2010, 10:56:15 PM »

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

C
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