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

[T. L.]

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
>>
>>