Vintage SONY Digital Clock Radio ICF-C12W no audio

[UserXP]

And here are the readings on R108. The manual says it should drop 2.5VAC to 2VAC. Yet on this unit, instead of incoming 2.5VAC I measured 5.92VAC, which this resistor still drops to 2VAC for the display. The supposed 2.5VAC should come from the first PT tap here (pin 1 or A), but it reads as 5.92VAC. What the hell? 🙃
I get the same reading on the main board when I connect the probes the way R_J suggested: black on C36 negative and red on PT pin 1 in AC mode.

[CapLeaker]

And here are the readings on R108. The manual says it should drop 2.5VAC to 2VAC. Yet on this unit, instead of incoming 2.5VAC I measured 5.92VAC, which this resistor still drops to 2VAC for the display. The supposed 2.5VAC should come from the first PT tap here (pin 1 or A), but it reads as 5.92VAC. What the hell? 🙃
I get the same reading on the main board when I connect the probes the way R_J suggested: black on C36 negative and red on PT pin 1 in AC mode.

R108 is a current limit resistor. As long there is 2.5AC as per spec, on the display board or after R108, or 5VAC before R108 is o.k.
Q7 is not in the rectifying circuit, it switches the the ground in and out fot the radio to turn on / off. That's all.

So far nothing explains the half voltage problem on D1 and D2 at the same time.

What about that 5.1V zener diode D101? Is it shorted?

[UserXP]

R108 is a current limit resistor. As long there is 2.5AC as per spec, on the display board or after R108, or 5VAC before R108 is o.k.
Q7 is not in the rectifying circuit, it switches the the ground in and out fot the radio to turn on / off. That's all.

So far nothing explains the half voltage problem on D1 and D2 at the same time.

What about that 5.1V zener diode D101? Is it shorted?

No, the D101 is functional. 0.5 volts in diode mode, O.L. in reverse. This diode, according to the manual, limits the voltage to 6 volts (there is a black arrow with the 6V symbol pointing at the trace leading to R108).

I have a question for you. It may sound silly and please go easy on me as I am not technical, but how does this AC voltage get converted to DC on the main board? Because, when I detached the function board completely but left the AC inputs on the main board, I only got around 0.8V instead of those 7.5 or ideeally 15.
It's like it is dependent on something from the function board to complete the rectification. Unless that "something" is the signal which makes Q7 complete the rectification circuit regardless of the radio being turned on or off.

[stj]

https://www.youtube.com/watch?v=l2y-w9aS98k
https://www.youtube.com/watch?v=_TRUsM--IRw

you should watch his video's

[UserXP]

https://www.youtube.com/watch?v=l2y-w9aS98k
https://www.youtube.com/watch?v=_TRUsM--IRw

you should watch his video's

Thank you, watched. I have a general understanding of how AC is converted to DC with diodes and a capacitor. It just didn't tell me why this Sony board cuts it in half. 😃

[UserXP]

So, to summarize so far:

1) the radio section didn't work and several caps found to be bad. Properly replaced all the capacitors with brand new ones and identical values, triple-checked while installing and comparing the photos of the old and new ones for any reversed cap(s). The radio woke up, but is low in volume as voltages are cut in half.
2) removed and tested D1 and D2, they both passed the test and even replaced with 1N4004, still low voltage.
3) removed and tested VD1220 D4, read OK (min voltage 1.12, max 1.2 as per specs, it read 1.16 in only one direction - normal).
3) tried removing C34, R30, R33, Q5, still no change.
4) tested Q4-Q7 voltages.
5) checked for bridged solder joints or cut traces.
6) the transformer outputs and maintains 6 and 12 VAC normally all the time, yet still only 7.5V DC.
7) everything on the function board seems to work as intended even under lower voltage.
8) tested everything in between the good and more knowledgeable people here could have come up with...

...and still no difference. 🤔

[R_J]

I just did that and I got the same 7.5V reading: black probe on C36 negative and red probe on D1. 😐
On the working unit, I got those 15V by putting the probes as the manual suggested.

When black probe is on GND and red on D1, I get 7.5V reading.
When red probe is on GND and black on D1, I get -7.5V reading.

based on that voltage I suspect your transformer secondary may be connected wrong, If you supply 5.2vAC to D1, the DC voltage will be 7.4VDC I suspect the secondary is not wired the same as the working units transformer
Check the AC voltage with the meter negative on C36 negative and check the AC voltage on D1 or D2 anode, My guses it is around 5.2VAC not 11VAC
Post a picture of the main board and how the transformer leads are connected, It may be that the wires from the transformer are reversed Check the AC voltage coming from transformer on the board of the working unit , then on the non working unit, I think this will show that they are different

[UserXP]

based on that voltage I suspect your transformer secondary may be connected wrong, If you supply 5.2vAC to D1, the DC voltage will be 7.4VDC I suspect the secondary is not wired the same as the working units transformer
Check the AC voltage with the meter negative on C36 negative and check the AC voltage on D1 or D2 anode, My gusess it is around 5.2VAC not 11VAC

Yes, I have already posted about the AC output, but here it is again.

The transformer has 3 taps: 1, 2 and 3:
The voltage between 1 and 2 is 6VAC
The voltage between 2 and 3 is 6VAC
The voltage between 1 and 3 is 12VAC
This is the same in the working unit.

OK, I did what you suggested. When negative probe is on negative of C36 and positive probe on transformer output tap 1, it reads 6VAC. On tap 2 it reads 0VAC. On tap 3 it reads 6VAC.

Now here is for the diodes:
black probe on C36 negative side, D1 anode reads 6VAC.
black probe on C36 negative side, D2 anode reads also 6VAC.

[R_J]

Yes, I have already posted about the AC output, but here it is again.

The transformer has 3 taps: 1, 2 and 3:
The voltage between 1 and 2 is 6VAC
The voltage between 2 and 3 is 6VAC
The voltage between 1 and 3 is 12VAC
This is the same in the working unit.

OK, I did what you suggested. When negative probe is on negative of C36 and positive probe on transformer output tap 1, it reads 6VAC. On tap 2 it reads 0VAC. On tap 3 it reads 6VAC.

Now here is for the diodes:
black probe on C36 negative side, D1 anode reads 6VAC.
black probe on C36 negative side, D2 anode reads also 6VAC.

The transformer is wired to the board wrong. It looks like the center tap is connected to C36 ground and one of the red wires is connected to the d1,D2 anodes. The center tap should be connected to R108 and the two red wires are connected between C36 ground and D1.D2 anodes supplying them with the 11VAC they need to produce the 12~13vDC

Those transformer numbers don,t tell me much but 1 and 3 need to be connected between C36 negative and D1 anode
Which wire is the black lead, 1,2 or 3 ?

You must have the center tap connected to C36 negative to get those voltages and that is not right

[UserXP]

The transformer is wired wrong. It looks like the center tap is connected to C36 ground and one of the red wires is connected to the d1,D2 anodes. The center tap should be connected to R108 and the two red wires are connected between C36 ground and D1.D2 anodes supplying them with the 11VAC they need to produce the 12~13vDC

Those transformer numbers don,t tell me much but 1 and 3 need to be connected between C36 negative and D1 anode
Which wire is the black lead, 1,2 or 3 ?

But, in the manual, look at where the transformer is connected: pads A, C andD (B is not connected to the PT).
The red line-arrow between A and C shows 5.2AC. And a "continued" red line-arrow between A and D say 11AC. The outputs on my board are soldered exactly like that, only the TP obviously has two 6VAC, not two 5.2VAC output. Plus when I measure those contacts I get the same reading order as in the manual.
(as I described in my previous post).

Or do you suggest I should switch the wires soldered on points C and D?
If we are wrong, we could seriously overvolt a 6VAC circuit on the function board, hm?

[R_J]

The only way to get what you measured it to have the transformer connected like this, which is wrong, someone must have changed that transformer and connected it wrong
I do not see any pads numbered 1,2,3,4, I do see A,B,C,D on the main board layout
Post a picture of the transformer wires and how they are soldered to the main board

[UserXP]

The only way to get what you measured it to have the transformer connected like this, which is wrong, someone must have changed that transformer and connected it wrong
I do not see any pads numbered 1,2,3,4, I do see A,B,C,D on the main board layout
Post a picture of the transformer wires and how they are soldered to the main board

Yes, it depends on the diagram you are looking at, the UK model has numbers and the AEP model has letters.
So, you think the first output wire (let's call it zero) goes on pad A, the center tap wire (6VAC) should go on pad D, and the third wire (the second 6VAC) should go on pad 3 and deliver combined 12VAC there?

[UserXP]

The only way to get what you measured it to have the transformer connected like this, which is wrong, someone must have changed that transformer and connected it wrong
I do not see any pads numbered 1,2,3,4, I do see A,B,C,D on the main board layout
Post a picture of the transformer wires and how they are soldered to the main board

R_J, you may be on to sommething here! I just noticed the board layout of my radio matches that of the AEP model, not the UK model.
The power input really seems to be 0-12-6VAC between pads 1, 3 and 4!
This radio shows signs that it has been repaired before, so maybe the PT really IS connected wrongly!

[CapLeaker]

Finally something that does make sense to me. Somebody used the center tap as ground. That definitely would cut all power in half.

[stj]

dont you hate stuff other people have worked on!

[UserXP]

dont you hate stuff other people have worked on!

Yeah... maybe the other person had the same manual and looked at the wrong PCB. There are two traces that have been mended with a piece of soldered wire.
Gosh, I really hope this it. Reattaching the wires now, I will post back the results soon.

[UserXP]

R_J, I put the wires like you suggested, but I haven't attached the ribbon cable. I plugged the AC in and I, be prepared, still have only 7.64V.
The two anodes of D1 and D2 are connected by a trace on the board. I followed your advice with the C36 cap, but I get 6.1VAC on both anodes.
In DC mode, the C36 area reads 7.64V on the black probe.
This is without the function board attached.
Does this sound normal?

[UserXP]

Guys, it didn't work. It is actually worse. I reattached the function board, everything is the same. Now the maximum voltage I get is 7.2V in DC. 😕

There is one big BAD difference. I can see three glowing lines inside the VFD tube and the display becomes burning hot to the touch. This wasn't the case before. 😕

[UserXP]

Can you help me with how to wire the PT?
It has three orange wires coming out.
The first wire comes from where 0 is printed.
The second wire comes from the tap where 6 is printed.
The third wire comes from the tap where 12 is printed.
There is a sticker on the coils housing and this where the numbers are printed, right above each of the taps. The TP says it is 230VAC, 50Hz, 6VA.

[stj]

meter the resistance between the wires to work out which is which.