Vintage SONY Digital Clock Radio ICF-C12W no audio

[UserXP]

OK, lifted R30. The radio now doesn't get any life. The voltage on the unsoldered R30 pad which was 6.2V before is now 0.28V.
I tested R30 and it reads 149 ohm.

I then removed Q5, no difference, the voltage is 7.64V now on its C pad on the board. The rest show almost zero voltage or some millivolts. Q5 tested fine with the same readings in diode mode that I got while it was in circuit.

Something is still draining the voltage.
The voltages I get after D1 and D2 cathode endings are now 7.64V.
Do these measurements tell anything?

[CapLeaker]

Here is anther interesting outcome. Because R23 drops the voltage to only 3.78V instead of 4.2, I traced the path and noticed the in AM mode that voltage passes through R14 and should drop from 4.2 to 3.6. Because the incoming voltage here after R23 is just 3.78, I bridged R14 and BAM!, the volume increased and worked perfectly in AM mode, but it produced static only (there no AM signals here). But this is exactly how the working unit sound in AM mode as well, at same louder volume level. So the amplifer kicked in once it got those 3.78V in AM and increased the volume. Because of the initial low voltage of just 3.78V that comes to R14, the IC gets only 1.15V after that R14 (instead of 3.6V). So my assumption that the radio unit should be OK is kind of confirmed by this. Once we remove the voltage drop, it may actually come back to normal function. It is so frustrating that finding than one faulty component causing all this is so elusive.
Any new ideas after these new findings from you, guys? 🙂

But that didn’t normalize the DC after the D1 & 2 diodes or did you check that?

[CapLeaker]

OK, lifted R30. The radio now doesn't get any life. The voltage on the unsoldered R30 pad which was 6.2V before is now 0.28V.
I tested R30 and it reads 149 ohm.

I then removed Q5, no difference, the voltage is 7.64V now on its C pad on the board. The rest show almost zero voltage or some millivolts. Q5 tested fine with the same readings in diode mode that I got while it was in circuit.

Something is still draining the voltage.
The voltages I get after D1 and D2 cathode endings are now 7.64V.
Do these measurements tell anything?

My whole point was to disable the radio. But that didn’t normalize the DC after D1 &2? Your problem should be somewhere else then probably after D1?

[UserXP]

My whole point was to disable the radio. But that didn’t normalize the DC after D1 &2? Your problem should be somewhere else then probably after D1?

Well, it didn't normalize it, but it increased the voltage from 7.56 to 7.64 volts.
So, I guess I can put back the components I removed? it seems to me that the components around those transistors are within their specs, as all of the ones I removed tested out OK.
The voltages are the same, no matter which probe I use. With the red probe they read as positive values, and with black probe as negative ones.

I am at a loss as to what to try next. I replaced the caps, diodes, tested several ceramic caps and resistors, and then even had this idea of buying new transistors for Q4-Q7, but now I feel overwhelmed by all the dead ends.

I just don't get it. The 12VAC pad is connected to the anode end of D1. Its cathode end is then coupled with the positive lead of the brand new 25V220uF C36 cap. After D1 I get 7.64V, and after D2 I get 7.5V. Frustrating!

The voltages on the negative ends of C33 and C36 are -0.18mV. Is this normal?

[CapLeaker]

Just for sanity reasons: when you take the DC voltage measurements, where do you put your negative probe?

What I don’t understand is why is it that the DC voltage is half what it supposed to after D1 and D2 yet the AC voltage (measured before these two diodes) is o.k. per spec. Doesn’t make any sense. D1 feeds the display board and D2 the radio part. There is nothing on the schematic that I can see where one board could draw excessive current from the other without affecting the AC voltage from the transformer.

[stj]

what happens if you lift r108 between the center-tap and the display heater?

[UserXP]

what happens if you lift r108 between the center-tap and the display heater?

Well, I haven't tried that. The manual says that R108 is a 56 ohm 1W metal resistor, critical for safety. The diagram appears to show it is used to drop the 2.5VAC to 2VAC for the first pin of the VFD display. When I measure it, it reads 55.9 ohm. I am kind of reluctant to remove it, what if something gets damaged?
Btw, when the device is On, that resistor is always warm to the touch. Should I proceed?

[UserXP]

Just for sanity reasons: when you take the DC voltage measurements, where do you put your negative probe?

Just as per the manual, I attach the black probe to, what I suppose is ground, where the arrow shows in the manual. I then check with the red probe the DC voltages on the desired pads. The easiest place to attach the black probe clip is at the top left corner where the volume knob metal body is soldered to the board, or the FM/AM switch housing as it is also soldered in contact to the same pathway.

What I don’t understand is why is it that the DC voltage is half what it supposed to after D1 and D2 yet the AC voltage (measured before these two diodes) is o.k. per spec. Doesn’t make any sense. D1 feeds the display board and D2 the radio part. There is nothing on the schematic that I can see where one board could draw excessive current from the other without affecting the AC voltage from the transformer.

My feeling exactly. Very frustrating. The components all pass the test, those in the rectification circuit are all new, yet when soldered they don't work as intended. I am stumped. 😓

[R_J]

If you have your negative meter lead where the vom picture (Q7) shows, try check your dc voltages with your negative lead on the negative of C33 or C36, if the voltages are correct, I suspect a fault with Q7 or it is not being turned on, it connects the power supply negative to the radio/amp section.

Place your meter negative on C36 negative and check voltage on Q7 base and collector (emitter should be same as your ground)
I suspect Q7 is either not being turned on (0.8v on base) or Q7 could be open (bad) If you have 0.8v on the base of Q7 and anv voltage on Q7 collector, try shoring Q7 emitter to collector and see if radio works.

On this clock radio, they switch the ground circuit for the radio/amp section (except for Q2 circuit which is connected to chassis ground and not switched ground)

The easiest place to attach the black probe clip is at the top left corner where the volume knob metal body is soldered to the board, or the FM/AM switch housing as it is also soldered in contact to the same pathway.

This is why you are getting weird dc readings, if the radio (Q7) is not turned on that ground is floating above the actual ground.

[stj]

if the display has an internal short between the heater wire and the grid or any segments it could draw a lot of current.
if you dont want to lift the resistor then lift the transformer center-tap.

[CapLeaker]

if the display has an internal short between the heater wire and the grid or any segments it could draw a lot of current.
if you dont want to lift the resistor then lift the transformer center-tap.

but that should be affecting the other tap on the transformer as well.

[CapLeaker]

If you have your negative meter lead where the vom picture (Q7) shows, try check your dc voltages with your negative lead on the negative of C33 or C36, if the voltages are correct, I suspect a fault with Q7 or it is not being turned on, it connects the power supply negative to the radio/amp section.

Place your meter negative on C36 negative and check voltage on Q7 base and collector (emitter should be same as your ground)
I suspect Q7 is either not being turned on (0.8v on base) or Q7 could be open (bad) If you have 0.8v on the base of Q7 and anv voltage on Q7 collector, try shoring Q7 emitter to collector and see if radio works.

On this clock radio, they switch the ground circuit for the radio/amp section (except for Q2 circuit which is connected to chassis ground and not switched ground)

As far as I recall there was almost nothing on Q7's E B and C.

[R_J]

As far as I recall there was almost nothing on Q7's E B and C.

Q7 emitter is at power supply ground, the base needs to be turned on to connect the radio/amp ground (Q7 collector) to power supply ground, If his ground probe was on the volume control ground, it would be floating and not at power supply ground
If Q7 is not conducting, the volume control ground point is NOT at ground potential, Q7 is the only path to power supply/chassis ground

[CapLeaker]

Q7 emitter is at power supply ground, the base needs to be turned on to connect the radio/amp ground (Q7 collector) to power supply ground, If his ground probe was on the volume control ground, it would be floating and not at power supply ground
If Q7 is not conducting, the volume control ground point is NOT at ground potential, Q7 is the only path to power supply/chassis ground

That explains weird voltage readings on the radio side. But does not explain the weird voltage reading on the clock or display side whatever comes from D1. I feel like

[R_J]

That explains weird voltage readings on the radio side. But does not explain the weird voltage reading on the clock or display side whatever comes from D1. I feel like

If the meter is not on power supply ground, you will not get proper voltages, to get the correct voltage on D1 you must connect the meter ACROSS C36, if the meter ground is on the volume ground, who knows what voltage it might read.

[CapLeaker]

If the meter is not on power supply ground, you will not get proper voltages, to get the correct voltage on D1 you must connect the meter ACROSS C36, if the meter ground is on the volume ground, who knows what voltage it might read.

That's what I think is going on here... the ground lead from the meter not always being at the right spot. The OP sez he puts the ground lead where the manual sez...
On the contrary all voltages are o.k. when he measures out a working one. This gotta be interesting!

[R_J]

If the radio is turned ON, on the working unit, then Q7 is conducting, connecting the volume ground to power supply ground, then the voltages would be correct using volume ground, if the unit was in the OFF state Q7 would be open, and I suspect the voltages would be weird as well.
The non-working unit may have a switch problem or Q7 may just be open, there are two ways to turn on Q7 one via R103 from the controller and one via R102 from the on/off switch

[UserXP]

Dear all,
Thank you so much for reading my lengthy posts and trying to figure out this persistent issue.

@stj
In one of my previous posts, I removed the two orange wires (E and F) which provide the L an H voltage for the display. While they were detached, the display couldn't turn on, so if there was any internal short in it, it was inactive. The main board was still producing only 7.5 volts, even with the VFD display unpowered.
But for the sake of testing, I will unsolder one end of R108 and see what happens, although I suspect the effect will be the same, i.e. the display will become unpowered.

@R_J
Thank you for going into this. I will do the measurements as you suggested. But, I can actually turn the radio unit On, it just doesn't work well, and having only half of the required voltages probably contributes to that. The 12-15V must exist regardless of whether the radio is switched On or Off. This is on the working unit. That is why I never saw Q7 as part of the rectifying circuit, but rather as a "junction" whether to activate the radio circuit or switch it off, depening on the signal it receives from the function board (either by the rotary switch or the chip itself).
Also, I measured the voltages on the working unit the same way I did in the faulty unit, yet I got different voltages. The manual shows positive (red) and negative (black) arrows from the VOM, so my logical guess was that the indicated spots are where I should put my black (negative) and red (positive) probe of my multimeter. I did it like that all the time. But I suspect that, when I reverse the probes, I will get the same numbers only with the "-" symbol (negative). However, I will try what you suggested and hope that it will reveal something.
I will post the results immediately.

[UserXP]

Place your meter negative on C36 negative and check voltage on Q7 base and collector (emitter should be same as your ground)
I suspect Q7 is either not being turned on (0.8v on base) or Q7 could be open (bad) If you have 0.8v on the base of Q7 and anv voltage on Q7 collector, try shoring Q7 emitter to collector and see if radio works.

On this clock radio, they switch the ground circuit for the radio/amp section (except for Q2 circuit which is connected to chassis ground and not switched ground)

This is why you are getting weird dc readings, if the radio (Q7) is not turned on that ground is floating above the actual ground.

@R_J
i did what you suggested:
I turned the radio on. It produces sound from, but in very low volume.
i put my black (negative) probe on the negative end of C36 and here are the readings by the red (positive) probe on Q7:
B = 670.8mV
C = 018.2mV
E = 0.000mV

I then kept the black probe on C36 negative and touched the ground with the positive (the ground indicated by the arrow in the manual) and got the same reading as on Q7's C pad (018.2mV).

is this how you wanted me to test, did I do it correctly?

By the way, using the same probing method you suggested but this time when the radio is turned Off, Q7 readings are: B = -1.1V (but it is dancing, going up and down, not stable)
C = 7.1V
E = 0.00V

[UserXP]

If the meter is not on power supply ground, you will not get proper voltages, to get the correct voltage on D1 you must connect the meter ACROSS C36, if the meter ground is on the volume ground, who knows what voltage it might read.

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.