Sony Icf pro 80 freq fault

[Goldieoldie]

Hi
I am repairing one of these and now after replacing the caps etc it is working almost correctly .
I have aligned the rf section as per the manual ( tunning voltages ,Osc frequency’s etc ) .
This is the fault .
When using FM it has to be tuned 200 kcs higher than if should ,ie 100 mcs has to display 100.2 to tune correctly.
If using narrow fm it tunes correctly .
I have repaired a fair few of these and know them quite well ,but have never had this fault .
Anyone out there who knows of this fault please ?
Cheers
Pete

[agardiner]

Hello Pete,

This fault is usually caused by the reference oscillator not being exactly on frequency. I presume this is one of the adjustments you have checked? (CT201) Do you use a frequency counter for your alignment? The adjustment needs to be within a 100Hz tolerance, although I have found it really needs to be within 50Hz. It is difficult to be this precise with a scope so a frequency counter is really required.

If the fault only really shows up during a scan for station, then it could also be the VCO2 adjustment.

Regards,

Adrian.

[Goldieoldie]

Originally Posted by agardiner

Hello Pete,

This fault is usually caused by the reference oscillator not being exactly on frequency. I presume this is one of the adjustments you have checked? (CT201) Do you use a frequency counter for your alignment? The adjustment needs to be within a 100Hz tolerance, although I have found it really needs to be within 50Hz. It is difficult to be this precise with a scope so a frequency counter is really required.

If the fault only really shows up during a scan for station, then it could also be the VCO2 adjustment.

Regards,


Hi Adrian,


Adrian.

Hi Adrian,
I have checked the ref osc with a frequency counter and it’s correct.
I have tried adjusting it slightly ‘each way ‘ but makes no difference
.I have checked all vco voltages and they are correct.

Looking at the pd tuning voltage when you switch between fm narrow and fm wide on the same frequency the voltage stays the same .

As I say it’s fine on FM narrow it’s only wrong on FM wide.

Looking at the circuit the radio uses different IF dectectors so I don’t know if it’s to do with that circuitry but would it knock it out by 200kcs or if it’s to do with the microprocessor control ?

Cheers
Pete

[Goldieoldie]

Just to add .
Could be a case that because the tunning voltage is the same on narrow fm and wide fm the problem is to do with the microprocessor giving the wrong info to the lcd display rather than an issue with the actual tuning ?

[agardiner]

Hmmm, very interesting. It is very common to be out by 100Khz on the display, but not had them worse than that with the oscillator on frequency. Will pull the manual and take a look.

[marceljack]

Hi Goldieoldie,
How do you evaluate the exact tuning in FM wide mode ?
If it is by the best audio quality, it might be a mistuning of the FM discri.

[agardiner]

Good point. Can you also confirm what happens when you get the radio to scan for the next station? Does that result in it stopping 200Khz off?

[frsimen]

The service manual gives the alignment instructions for the FM IF stages on page 7.

The setting of T3 and/or L15 may have been altered. A ceramic filter well off 10.7MHz could also cause the same symptoms as could a faulty C60 (47pF).

Checking the alignment is the best next step.

Paula

[Goldieoldie]

Originally Posted by frsimen

The service manual gives the alignment instructions for the FM IF stages on page 7.

The setting of T3 and/or L15 may have been altered. A ceramic filter well off 10.7MHz could also cause the same symptoms as could a faulty C60 (47pF).

Checking the alignment is the best next step.

Paula

Hi Paula
The if alignment is fine.
What’s odd is if for instance the fm station is 88.5 mhz if it’s tuned 200 kcs higher or lower it works ok.
I think the thinking about the filters might be correct ?
Regards
Pete

[agardiner]

What happens when you get the set to scan? Does it stop 200Khz out?

[frsimen]

Hello Pete,

I'm not convinced now that there is a problem with the filters, if the IF alignment has been found to be correct. It would be a very unusual filter fault that gives correct results at +/-200kHz off tune, while not working when on the correct frequency.

It would be worth checking some of the voltages on IC2, they are marked on the circuit diagram. In particular, check what happens at pin 12 as you tune through a signal. Expect it to change from a lower voltage to a higher one as you tune. The centre frequency should correspond to the mid voltage that you measure.

I notice that the audio output from pin 12 feeds through an electrolytic capacitor to a diode switch. I would be rather suspicious of that capacitor, C66, as leakage there will upset the operation of the switch.

Other surface mount electrolytic capacitors in the area could also cause problems by upsetting the bias and decoupling for IC2. I'm not advocating changing them all on sight, but if nothing else seems to be wrong......

Paula

[Goldieoldie]

Originally Posted by agardiner

What happens when you get the set to scan? Does it stop 200Khz out?

If I scan it stops at the right frequency but very week audio till you manually go up by 200 kcs

[Goldieoldie]

Originally Posted by frsimen

Hello Pete,

I'm not convinced now that there is a problem with the filters, if the IF alignment has been found to be correct. It would be a very unusual filter fault that gives correct results at +/-200kHz off tune, while not working when on the correct frequency.

It would be worth checking some of the voltages on IC2, they are marked on the circuit diagram. In particular, check what happens at pin 12 as you tune through a signal. Expect it to change from a lower voltage to a higher one as you tune. The centre frequency should correspond to the mid voltage that you measure.

I notice that the audio output from pin 12 feeds through an electrolytic capacitor to a diode switch. I would be rather suspicious of that capacitor, C66, as leakage there will upset the operation of the switch.

Other surface mount electrolytic capacitors in the area could also cause problems by upsetting the bias and decoupling for IC2. I'm not advocating changing them all on sight, but if nothing else seems to be wrong......

Paula

Pin 12 of ic2 remains at 2.9v not dependant on tuning .
All electrolytes have been changed .

[agardiner]

Originally Posted by Goldieoldie

If I scan it stops at the right frequency but very week audio till you manually go up by 200 kcs

Curious. That means that the micro is detecting the signal correctly, and suggests correct alignment.

[Goldieoldie]

Bit more info
If the freq is set to 70mcs narrow the tuning voltage ( junction c of q229 and r 235 )
Is 11.9 v and tuned correctly .
If I then switch to fm same freq the tuning voltage is 10.8.
Does this suggest the fault is with this tuning voltage and how it is derived rather than being an issue with If side of things ?

[Goldieoldie]

Even more info
Checking on a known good radio same model
The tuning voltage does change between narrow and wide fm
70 mcs narrow tuning voltage is 11.5v
70 mcs Fm tuning voltage 10.4 v

So it seems tunning voltage does change between narrow and fm which I find confusing .
So am I back to tuning is ok because on scan it stops at the right display but it’s to do with the if circuitry ?

[frsimen]

Hello Pete,
When switched to FM wide, the receiver uses VCO 1. When switched to FM narrow, it uses VCO2. That will account for the different tuning voltages you are seeing, as they won't have the same alignment setting.

From your latest posts it would appear that one of the muting circuits is operating. IC2 has a mute and there is a second mute circuit at the input of the audio power amplifier IC. If the RX light is lit, the mute circuit at the input of the audio power amplifier IC should be inhibited, allowing normal audio output. If the light isn't lit, that circuit is muting the audio.

The signal detection indication is fed to the microcontroller which in turn governs the squelch and RX light. This indication is generated in a side chain, taken from pin 4 of IC2, before CF4, the ceramic filter. The scan stop etc. is governed by this signal which uses its own ceramic filter CF5, along with Q31-Q33 and IC3.

It's not clear what triggers the mute circuit in IC2. The datasheet shows that it if pin 10 is connected to the positive supply, mute is OFF. If connected to about half the supply voltage, mute is ON In this Sony, pin 10 is close to the supply voltage, so the mute is OFF, I think.

The voltage you have measured at pin 12 seems rather high. The voltages for the pins associated with FM are stated as follows:
Pin 2 1.6V
Pin 4 1.4V
Pin 5 1.2V
Pin 6 1.6V
Pin 7 2.9V
Pin 8 2.6V
Pin 9 3V
Pin 10 2.4V
Pin 12 1.45V

I suggest you check that the RX light is lit when the receiver is correctly tuned. If that is the case, it's probable that whatever is wrong lies around IC2. Checking the voltages listed above will be useful in that case. If you have an oscilloscope, or an audio amplifier, you could check for audio at the junction of R61/C62/C66, which may help to identify the trouble area.

Paula

[Goldieoldie]

Bit of an update .
I swopped the signal board for a good known working one ( had to cut and make 20 wires !)
Madness I know .
The fault went away so it’s definitely on the signal board .
I’ve checked the ceramic 10.7 filters and even changed the if chip .
I don’t fully understand the q31 and onwards circuit ending as a SD output .I think is is to do with the squelch or does SD go to the micro to change anything to do with tuning ?
As I’ve said before 200 kcs below or above the set frequency is where it outputs best .
It’s almost like an upper and lower sideband but nothing in the middle !
Cheers
Pete

[Goldieoldie]

It’s fixed !
It was a faulty FM detector L15.
Although it had continuity it was not peaking at 10.7 megs.
I had checked the c60 cap and tried tuning the coil.
I replaced it with a FM coil from a scrap radio and it all came good.
I have spent far to long on this radio but was determine to find the fault.
It all goes to show just because a IF transformer shows continuity it does not necessary mean it’s working as it should !
I hope this thread might help others with a similar fault .
Now off to repair something with a simple fault ( I hope !)
Cheers
Pete

[agardiner]

Fantastic. Well done Pete. What perseverance! I hope it feels very satisfying. It's faults like this that keep our brains young!