[ICOM] IC-751A errata in Service manual & more

[Jim W7RY]

Icom uses the same style caps in the VCO as well as the carrier oscillator 
adjustments. Remember how un-reliable the VCO caps are. Replace the caps 
that control the RTTY shifts. They are easy to replace because you 
don't  have to remove any shields. As I remember, there is also one of the 
defective caps in the LSB carrier adjustment as well.


Good Luck

73
Jim W7RY

At 03:22 AM 4/25/2005, you wrote:
>Hello Jim
>Thanks much.
>If  I understand correctly now, L201 should only be tweaked for 3v on 
>8.0000 Mhz, LSB at R202.
>I have also found that the calibrator voltage specified to be meassured at 
>J1 pin5  should be done
>at pin 6 instead.
>
>My TTY problem is different; the shift is 170Hz at first but becomes 
>narrow (100Hz or so ) after some
>time of operation (not always though...). It than comes back to 170 on a 
>random time basis (If at all).
>Probably some trouble with the FSK generator (Xtal or some caps I guess).
>
>Again thanks & 73
>Isaac 4Z4TL
>---- Original Message ----- From: "Jim W7RY" <w7ry at centurytel.net>
>To: "ICOM Reflector" <icom at mailman.qth.net>
>Sent: Mon, Apr 25, 2005 7:41 AM
>Subject: Re: [ICOM] IC-751A errata in Service manual & more
>
>
>>Yes.
>>The service manual is wrong when it comes to the VCO alignments.  Turn 
>>the dial until you hear the filter relays switch. then stop.  When the 
>>relays do switch, that is where you measure the VCO voltage. I believe 
>>the resistor is the correct place.
>>
>>There are four VCO ranges. Tune the VFO dial with the TS switch engaged 
>>(or in the AM mode) to speed up the dial. You can watch the VCO voltage 
>>change as you turn the dial. When the voltage jumps up (also where the 
>>filter relays switch) you will be in the next VCO range. As I remember, 
>>the voltage is highest at the highest frequency of the VCO range. Adjust 
>>to the specified voltage when the VCO is on the highest (shown on the 
>>VFO) frequency.
>>
>>For the RTTY shift, there is a procedure for adjusting the RTTY shifts. 
>>The procedure is in the service manual.  I simply use the monitor 
>>function and an external audio generator and adjust for zero beat. I just 
>>went through the alignment on my 751A and the service manual was accurate 
>>for the RTTY alignment.
>>
>>I hope this helps.
>>
>>73
>>Jim W7RY
>>
>>
>>At 07:55 PM 4/24/2005, you wrote:
>>>Hi Everyone,
>>>I'm looking for the correct PLL adjustments procedure as I suspect
>>>the service manual has some errors in this section.
>>>Specifically - what adjusts the 3 volts at R46 on 8,15, & 22 Mhz? It 
>>>sure isn't
>>>L201 as one might think from the manual I believe.
>>>Also - has anyone dealt with FSK being too narrow shifted? (100 Hz in 
>>>place of 170).
>>>Tks in advance, Isaac, 4Z4TL
>>>4z4tl at iarc.org
>>>http://www.iarc.org/~4z4tl
>
>
>----
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