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Author Topic: how to key d&a mdx 200?  (Read 2180 times)
KC9VZB
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Posts: 51




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« on: May 06, 2013, 06:05:45 PM »

 knowing these amps are frowned on by ham community,a customer gave this thing to me.I would  like to try it on 20 meters and take a look at output  (haven't used scope in years)perhaps try to clean it up.You know, tinker.question is what triggers the relays if shorting rca jack doesn't.I haven't gotten schematic yet which would tell allot. 
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WB2WIK
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« Reply #1 on: May 07, 2013, 03:36:55 PM »

You'd have to install a hard-wired method of keying the relay.  The "key" jack is for a code key, not T-R keying.  You could re-wire that jack so it closes the coil connection to the T-R relay, and make that your "amp keying" line.

You'd also have to replace the input and output tank components, which are all set up for 10m (and 11m!) and won't cover 20m at all -- you need "bigger" parts to do that.  And you'd want to re-wire the "VFO" input jack to the cathode tank of the PA tubes so you can use a transmitter to directly drive the PA stage.  The way it's factory wired, if you transmit into the jack labeled "VFO," you're likely to blow up everything that follows that, since it's a low-level input.
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W9GB
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« Reply #2 on: May 07, 2013, 07:51:26 PM »

Quote from: KC9VZB
... knowing these amps are frowned on by ham community ...
Richard -

There is no provision (or correct parts) for 20 meter operation, as noted by Steve (WB2WIK).
Notice lack of band switch, small coils and tune/load capacitors?
http://www.cbtricks.com/Amp/d_and_a/mdx_200/tour/index.htm

IF you desire to use it for a DIY 20 meter amplifier project, get ARRL or W6SAI Radio Handbook from late 1950s or 1960s, for various plans -- when they wound coils and bent sheet metal for DIY fabrication.

As I noted a couple months ago, D&A used two AC transformers that were common late 1950s/early 1960s Stancor (Chicago mfg.) used for vacuum tube audio amplifiers and some SW receivers.
 Far too many obsolete TV sweep tubes for 21st century usage,
especially when surplus Russian 400-500 watt ceramic triodes (circa 1990) sell for $25 !!
« Last Edit: May 07, 2013, 08:04:27 PM by W9GB » Logged
KC9VZB
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Posts: 51




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« Reply #3 on: May 07, 2013, 07:58:42 PM »

      thanks,that makes sense,the guy I got it from was using a modified cb. So the label 20 meter was just on it to keep them out of court?I thought the caps looked awful small. getting bigger coils and capacitors will be real fun.getting rid of 4 tubes might make some room.  they wired this thing with all same color wire.this will give me a chance to add some color.I have 2 of Bill Orrs books to help figure henries and mmf'ds. Thanks for not bad dogging me,to me its the journey, not the destination thats important.In reality I may never use this thing.A Local ham says its good enough just to see that another CB isn't hooked up to it.  
Second thought is to gut it and rebuild .


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W9GB
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« Reply #4 on: May 07, 2013, 08:09:14 PM »

Quote
thanks,that makes sense,the guy I got it from was using a modified CB.
So the label 20 meter was just on it to keep them out of court?
D&A was out in Scottbluff, NE.  They tried various gimmicks and flaunted their product,
but FCC continued to pursue and D&A eventually closed business (bankrupt).

Personally, I would just strip for parts.  Forget using about 1950s glass TV sweep tubes,
 pick a newer tube, designed for HF or VHF radio amplification (which is what you want).

The Russian GI-7b ceramic triode has its own web page.  Look at what other radio amateurs,
around the world, are doing with this capable (HF to 1 GHz range) surplus tube.
http://gi7b.com/
« Last Edit: May 07, 2013, 08:18:00 PM by W9GB » Logged
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