[Collins] FW: AN/VRC-24 UHF radio

[Marco]

I've pushed reply... 73' Marco

From: marcocasagrande2407 at hotmail.com
To: antqradio at sbcglobal.net
Subject: RE: [Collins] AN/VRC-24 UHF radio
Date: Tue, 24 Jun 2014 19:34:18 +0200




Dear jim and Steve,Many thank's for Yr prompt replies:I've checked the K-101 and carefully cleaned the long ball end contact of K-102 relays, the 20 ohm resistor so all the rf connectors and checked all low power tubes, including the three 6j4WA. All tubes exibit more of 140-150% of efficiency!! From mine old box 6AK5,2C51 ecc only 90-95.... ;-) As from tech serv manual, I've checked all tube sockets dc voltages so unworking status resistance (pulling connectors) and retouched the various oscillators and relative tracking filters obtaining two valuable RF voltages at V-101 tx mixer (about 6-8 V rf up to dc, of plate voltage and 0.8-0.9 RF Volt to the catode)During signal tracing (pulling the three 6J4) have fused the 7554 (aghh!!) the filament is in series/parallel to 6442 and its filament adapter caps was leaking.. but now waiting the spare part, i've mounted a 5876 with similar gain and output voltage at 7554 output (about 5 RF Volt to 6442 catode at 399.9 Mhz) but only 1 RF Volt is measured to the 4x150 control grid with tube pulled and max retouching of various trimmers (C127-C141). Could be out of minimum C class drive RF voltage limits or... the 6442 is Out?I've tried to test the 6442 on the table at 160 vdc and -15 to -2V grid biasing but only 5 mA at -2V is been measured. UmmmhCiaoMarco

> Date: Tue, 24 Jun 2014 08:59:28 -0700
> From: antqradio at sbcglobal.net
> Subject: Re: [Collins] AN/VRC-24 UHF radio
> To: marcocasagrande2407 at hotmail.com
> 
> Marco
> I am familiar with the other variations of this UHF radio and there are many.  The VRC-24 was the first but I worked on later versions like the GRA-53, GRA-54 and the TRC-68.  There is also the Navy version, the URC-9.  If RX seems to be working and TX isn't, then there are only two tubes and associated circuits that  could be the problem.  V-102, V-103 and V-104 are used in both RX and TX.  V-105 is a 6442 and V-105 is the 4X150D, the D indicates a 24 volt filament.  These two tubes are used in TX only.
> 
> First, I would check K-101 and K-102 relay contacts.  These two relays do the reconfiguration between RX and TX.  You should be able to clean the relay contacts with alcohol and some paste board like a match book cover.  I would not use a file or any such rough material.  If you have a burnishing tool, so much the better.  If you still don't have TX at this point, I would suspect either the 6442 or the 4X150D or the B+ to these two tubes also check the fuses on the front panel.
> 
> Once you get an indication of RF out in TX, then let me know.  There is a trick to limit the need for tab bending when tuning the RF and Power Amplifier for max output.
> Jim
> --------------------------------------------
> On Tue, 6/24/14, Marco <marcocasagrande2407 at hotmail.com> wrote:
> 
>  Subject: [Collins] AN/VRC-24 UHF radio
>  To: "collins at mailman.qth.net" <collins at mailman.qth.net>
>  Date: Tuesday, June 24, 2014, 8:51 AM
>  
>  Hi guys, I'm a couple weeks that I'm
>  going crazy to run this UHF radio in tx mode: don't push
>  enought driving signal in the rf power amp. module (J104)
>  but seems" receiving ok (no calibrated sign gen.).I bought
>  this "anvil to very little money but since it is very well
>  built, have motor, step actuators relays and many gears so I
>  would like her to sing her song. Is there here any Collins
>  wizard who has some additional tricks respect to the service
>  infos contained in its liberated" manual
>  (TM-11-5820-222-35). Thanks and 73 'Marco IZ0MFI
>     
>          
>            
>    
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