Sony ICF 7600 showing wrong AM frequency

[Oivindx]

Hi from Norway

My SW7600 has been recapped.

When listening to Radio Caroline on 648 kHz in the evening, the frequency shown is 630 kHz. How can I adjust the frequency reading to be correct on AM?

/Oivindx

[coopzone]

Thats a big difference. Normally the service sheet will give the procedure to adjust this.

I take it the re-cap was only the electrolyte caps, not small ones (ie pf or nf) caps changed?

[Graham G3ZVT]

Originally Posted by Oivindx

Hi from Norway

My SW7600 has been recapped.

When listening to Radio Caroline on 648 kHz in the evening, the frequency shown is 630 kHz. How can I adjust the frequency reading to be correct on AM?

/Oivindx

Hi there, you cite ICF-7600 and also SW7600
Is what you have actually an ICF-7600D?
If so, have you checked the switch hidden in the battery compartment? It should be set to the 9kHz position.

[Oivindx]

Hi,
Thanks for answers and help to find out about this.

I have two ICF-SW7600 receivers. (No GR or D in the model) Both are recapped, one by me. The one in question was recapped before I bought it.

I had another test this evening. It turns out that fine tuning does not work with the "630/648" radio. I checked this with the other set tuned to 648kHz. By turning finetuning-knob to full stop in one direction I received a station on ca 639 kHz. By turning the finetuning-knob in the other direction I heard 648 (midways) and ca 657 kHz..

Maybe finetuning covers +/- 9 kHz from tuned frequency? If so, the "630kHz" position with signals belonging to 648kHz could be because the fine tuning function is "stuck" in the "low frequency" position? Do you agree?

Next question is then how to make the fine tuning function work? I have opened the radio, but have not seen any faults (yet).

Would appreciate your comments..

[coopzone]

It may be stuck.

I would check around S1-1 since it seems more likely a dirty contact etc. See attached.

Derek

[Oivindx]

Thanks for info and schematics.

Two of the contacts of the switch where connected by solder. I desoldered these, but it made no difference.

I opened both my two radios to compare. The R1 fine tuning seems to work on both radios according to my Ohm-meter (radios off). When turning on the radios, one of them had a variable voltage reading when moving the fine tuning wheel. The "problem radio" had no voltage. I guess this is the problem. I need to look more into it, but I did not find any problems with the switch or connection with the fine tuning wheel....

[frsimen]

The voltage on the supply side of R42 is shown as 15V. If the 15V is missing, it's likely that there is a problem with the DC-DC converter. The DC-DC converter is shown on the bottom right hand corner of the circuit diagram in the service manual.

Paula

[Oivindx]

Thanks for help.

While looking for the R42 I found this close to the DX-Local switch, ref picture.

Should it be like this and what is it ? The radio works except for the fine tuning.

[frsimen]

That odd looking component is T1 and couples the MW/LW signal to the mixer via the diode switch D26. It is shown on area B5 of the circuit diagram. It won't have anything to do with the radio tuning too low in frequency.

If the supply from the DC-DC converter is missing, the radio will be tuning to a lower frequency than it should be.

If you can't find R42, measure the voltage on the three terminals of the fine tune control. You should find in excess of 10V on one of the pins. If that's not present, it confirms that the output of the output from the DC-DC converter isn't reaching that point.

Trace back along the PCB to find R42. Check the voltage on both sides of that. If both sides are 0V, the trace then to the DC-DC converter, which uses Q31 and Q32. Have a look around C130 and C131. If they have been changed and one of them is fitted the wrong way around, that could cause the trouble.

Paula

[Oivindx]

Thank you for a most helpful explanation.

I will investigate this further tomorrow and will let you know the outcome.

This is really "learning-by-doing" for me...

Oivindx

[Cruisin Marine]

I have the G version of this radio, and from memory you have to adjust the 2nd L.O to get correct frequency alignment.
I don't know if your version is the same.

[Oivindx]

Paula,

Checked the radio once again this morning. There is no supply of voltage to the fine tuning fuction. I also checked the R42, nothing there...

Further, I checked all condensors. Orientation should be correct.

Re DC/DC converter. There is an output of 14.14V, ref photo showing where I measured the voltage.

It seems to me that next step should be checking if condensors are well connected. The 14.14Volt supply disappears somewhere on its way to the fine tuning wheel... Agree?

BTW; on my other radio (working well) I did recapping of all aluminium condensors. I did not replace the three large blue condensors as they are different and look a bit hard to replace. Is it essential that these condensors are replaced ?

Looking foreward to you comments.

Oivindx

[Oivindx]

Originally Posted by Cruisin Marine

I have the G version of this radio, and from memory you have to adjust the 2nd L.O to get correct frequency alignment.
I don't know if your version is the same.

Thanks for your commments.

Initially, I thought frequency alignment would solve the problem. When I realized that the fine tuning function did not work, I believe that the fine tuning problem needs to be solved before doing the frequency alignment..

Oivindx

[frsimen]

It would appear that there is a break between R42 and L18. L18 is fine as you are measuring the 14V on the second PCB and that is fed from L18 too.

I don't think you need worry about capacitors at the moment. The problem is more likely a broken track or a poor connection between a track on the top side of the PCB and the bottom side. Check for continuity with your meter and have a very careful look for a cracked PCB track.

When you find the problem, bridge the crack with a short length of wire.

If you can't find the break, try connecting an insulated wire from L18 to R42.

For the working set, it's up to you as to whether to change the blue electrolytic capacitors. If access is difficult, the chance of doing damage probably outweighs any benefit to be gained by changing them. As the set is working, if it were mine and there were no other sign of trouble from them (such as leaking contents), I would leave them alone.

Paula

[Oivindx]

Thank you so much for your advice.

I checked the connections thouroughly, but could not observe anything wrong.

Soldered an insulated wire from L18 to R42.

Wow, It now works as it should. BBC is now on 810kHz as it should be and the fine tuning works!

Thank you again!

[SiriusHardware]

Bearing in mind that Oivindx said in the first post that the radio had been recapped, probably the most likely cause of this problem is that a through-hole 'via' was pulled out when one of the original capacitors was removed from the PCB, breaking a connection between a top side track and lower side track.

Good detective work by Paula, and I'm glad the OP's radio is now working, but it does illustrate the potential pitfalls involved in doing unnecessary rework to any item of electronic equipment - occasionally, it ends up causing problems which weren't there in the first place.