[ICOM] Problem with IC-751A

[Gary Fiber]

Jamie,

Remember the IC-751A will not remember that last frequency you had in 
the VFO as it does not have band stacking registers like the newer 
Icom's have. So its quite normal with you change band for the VFO to 
return to the default setting for that band. Its been a long time since 
I've been in front of a 751A so I can't say changing from VFO A to B 
will exhibit what you are seeing. I will say its very doubtful the 
lithium battery has anything to do with this issue. In my 11 years at 
Icom, I think I only saw less than a dozen actual battery issues. The 
majority were from operators trying to prevent failure by changing out 
the battery and caused their own troubles. We did see cracked solder 
joints around the sockets the ram card used to connect to radio so if 
you pull out the ram unit have a close look at the solder joints on that 
card. BUT don't remove or short out the battery supply on the card.

NOW what I suspect the trouble might be is the 8 volt DC supply. Under 
the bottom cover mounted on the side of the radio where the PS-35 power 
supply slips in is a small PC card with a couple of transistors and a 
few resistors mounted on it. This card supplies 8 VDC for most all of 
the radio. One of the resistors, R1 as I remember liked to change value 
over the years and doing a voltage measurement you will find something 
other than 8 volts. I seem to remember that resistor value being 4.7 
ohms and renewing it would fix many strange troubles on this radio. Have 
a good look at that circuit board and do check the output voltage.

I suspect it also could be some sort of logic issue but I would not go 
looking for that until you know for sure the 8 volts is correct.

Hopefully some of this helps.

Gary K8IZ

On 03/23/2013 08:47 AM, Jaime Gomez wrote:
> Hi fellow friends,
> I have a friend's 751A wich present the following problem:
> Anytime I change from A to B VFO or reverse, the VFO changes band, for
> example if I am in 24.890 in both VFOs and I change from A to B or B to A
> the frequency in the VFO in use will jump to 20.000, if I am in 1830 and
> change VFO the frequency will jump to 0.809 or something like that, if I
> operate on just one VFO A or B the radio works fine, any hint?
> The RAM card has a second battery added and the voltage is 3.02 volts, the
> radio was working fine, this happened suddenly.
>
> Apprecaite any help,
>