Vintage SONY Digital Clock Radio ICF-C12W no audio

[stj]

need a 455KHz source,
i wonder how cheap one can be made!

[UserXP]

Checking levels isn't much good at this point, since you are lacking an RF generator and other test equipment. Unless you actually find a faulty component, I think this is as far as it goes without firing the parts cannon at it. The only thing what you may want to do is to compare a working one with this faulty one and do some voltage measurements and compare them.

My next step would be doing a full FM Rx alignment and see what that would do. Then again... you don't have the necessary equipment.

Oh... shoot. In that case, I'm stuck. Before I started this topic, I actually called a few local repair shops in my town to ask if I can bring the radio to them to have a look at the problem. All of them said "We don't do that, we don't fix eletronics that old". I guess they got used to to microprocessors, chips, "all-in-one" circuits and what not. Old-school electronics repairman is impossible to find.
So, I tried my luck here and all of you people who stepped in and guided the noob me through all we have done, ant that is really amazing. From a non-working radio and a 40% lit display we got to an almost fully working radio, sorted voltages and a VFD display that illuminates like a brand new one. For all this I am eternally grateful. I am still clinging to some hope that there could be something else to give a go to, still hoping.
OK, tomorrow I'll have some time to open up the working unit again and compare the voltages. I guess I can use it as a reference point, since it uses the same power supply, maybe the reading will really tell something. Hopefully.

[CapLeaker]

Makes me feel even older now.

[UserXP]

Makes me feel even older now.

Naaah... You have been awesome. 🙂

[UserXP]

I couldn't find the reating in the manual, but CF seems to be an E10.7A filter and has a red dot on one corner. It also has some symbols below its value symbol, I can't see them well, but they might be manufacturer logos or so.
I found that this is likely a 10.7MHz ceramic filter. What do you think, should I try and find a replacement and see if it will have any impact?

[Agent24]

I couldn't find the reating in the manual, but CF seems to be an E10.7A filter and has a red dot on one corner. It also has some symbols below its value symbol, I can't see them well, but they might be manufacturer logos or so.
I found that this is likely a 10.7MHz ceramic filter. What do you think, should I try and find a replacement and see if it will have any impact?

No point really looking into that unless you have reason to suspect it, probably it's okay, and even if it's not, good luck finding another one anyway.
I speak too soon - you can get them all over AliExpress... https://www.aliexpress.com/item/1005006274906905.html

Did you check the D5/D6 diodes in the FM detector circuit? We already know the VD1220 diodes have a habit of going bad. I don't know for the others.
They are a 1T26 germanium type and should show about 0.3v drop when tested with a multimeter in diode mode.

Testing C24 in circuit probably isn't going to tell you much. Desolder it and check it out of circuit, or replace it.

[UserXP]

No point really looking into that unless you have reason to suspect it, probably it's okay, and even if it's not, good luck finding another one anyway.
I speak too soon - you can get them all over AliExpress... https://www.aliexpress.com/item/1005006274906905.html

Did you check the D5/D6 diodes in the FM detector circuit? We already know the VD1220 diodes have a habit of going bad. I don't know for the others.
They are a 1T26 germanium type and should show about 0.3v drop when tested with a multimeter in diode mode.

Testing C24 in circuit probably isn't going to tell you much. Desolder it and check it out of circuit, or replace it.

I tested VD1220 and I would say it is OK. It has a 1.16V drop, but that is within its specs (1.12-1.20V). Plus, while it was removed from the circuit, there was no sound in either mode. So it is safe to assume that if it provides normal volume for AM, then it should do the same for FM.
Now, D5 has a 0.38 voltage drop, and D6 0.37. So, that's another dead end, I guess.
R12 measures 0.97K and it really is a 1k resistor, even though the manual suggests a 2.1K one here.

Now, I attached a probe with a clip to the ANT pad to serve as an antenna. The volume increases, BUT, even though FM is still quiter than AM, it is now loud enough to hear a constant background buzzing noise in both AM and FM modes. You know the one which sounds like an AC unfiltered power, or when a rectfying filter cap goes bad. Only this one is affected by the volume change.

[UserXP]

Ignore the previous test, the original antenna wire provides the most stable reception, so I using ut now (the volume is still low).

also, I removed C24. It measures 10.0nF, but it took like almost two seconds to reach the value in gradual increments, and then it stabilized. Without this cap, no sound is produced.
Can I temporarily introduce a different cap of this type and see how it affects the volume (if it would), or does the test reveal that C24 is actually fine?

[UserXP]

It's not C24. I put a new 103 cap and there was no difference. 🙁
So far, if we don't count the bad electrolytic capacitors, every other original Sony component I desoldered and tested turned out fine.

[UserXP]

I checked the IF voltages on the IC01 of the working unit. Everything is almost the same, except I got 4.3V readings on pins 6 and 7, whereas on the faulty unit it was 4.5. Other voltages differ in 0.0X volts. Another dead end. 🙁

[CapLeaker]

Couple more things you can do with what you got. You can check L1 isn’t open and T3. Maybe touch up their solder. But for the rest you need different gear.

[UserXP]

Couple more things you can do with what you got. You can check L1 isn’t open and T3. Maybe touch up their solder. But for the rest you need different gear.

Will do. In the meantime, I did some additional checking on the working unit. Visually, it is identical, except it has original capacitors. But, I removed the tunning drum wheel. There are two small capacitors soldered onto the board. The faulty unit doesn't have those. I took a picture.
Then, I unsoldered its antenna wire. The working unit was still receiving radio signal at almost the same strength as with the antenna wire soldered. The faulty unit can't do that.
The main voltage is 13.3-14.8 volts DC, slightly lower than on the faulty unit. But the volume is crisp and prominent even on lower settings.
Are the two extra capacitors with ratings 101 and 24 somehow involved in the volume or reception?

[UserXP]

CapLeaker, OK, I checked L1, it is OK. I also reflowed the solder points on T3. It didn't make a difference. I also tried touching different solder point pads. Touching Q2's pins caused to switch to a near station frequency.
This is getting tedious... 😔

Hmmm... here is a thought; provided nobody messed with the trimmers and pots on the board... is it possible that replacing the capacitors alone has somehow influenced the alignment, maybe due to different resistance of the new caps or current flow or whatever?
I haven't discarded the old caps. Most of the small ones measured fine, but the bigger ones were bad.

[CapLeaker]

This radio doesn’t have an RF amp stage. There is the oscillator and the mixer. The mixer heterodynes the frequency you tuned in to a single frequency 10.7 MHz called the IF. The IF is steady regardless of what you have on the dial. So if the mixer would be dead, no heterodyne, nothing would be on the IF. So Q1 gotta be good and working, unless it has no hfe (gain),

[CapLeaker]

Nah, I don’t think the cap swap did it. But it could be that an adjustable inductor lost a wire and thus is open. Old ceramic trimmer caps are an issue too, but I don’t want you to mess with that, because other wise you might end up with nothing since you don’t have any RF repair gear.

[stj]

swap Q1 and Q2
see if it changes at all
i still think Q1 is vulnerable to damage from static or spikes on the antenna and it could have poor HFE (gain)

[CapLeaker]

swap Q1 and Q2
see if it changes at all
i still think Q1 is vulnerable to damage from static or spikes on the antenna and it could have poor HFE (gain)

Q2 is the oscillator… the RF goes though the base of Q1 (mixer). On the older radios there would have been actually an RF amp stage first, but this thing doesn’t have that. It just 2 IF amps, one before and after the 10.7 MHz CF. From there it’s just a couple of transformers, the detector and everything is audio from there on. Either it is misaligned, Q1 doesn’t have any gain, or IC1 has a bad IF amp on one of the two, or the two transformers T3 & T4.

Q1 and Q2 have the same part number… so yes, might work or it’s going to be dead.