Vintage SONY Digital Clock Radio ICF-C12W no audio

[CapLeaker]

A very low audio can be a few things. The CF filter on IC1 is bad, C31 set as a DC blocker could be high in ESR, a resistor went higher in ohms that per spec, etc.

[UserXP]

A very low audio can be a few things. The CF filter on IC1 is bad

How do I go about and test that? And what does it have to do with lower voltages even when that IC is innactive when the radio is turned off? Do you think that amp IC is involved in the rectification to DC voltages?

C31 set as a DC blocker could be high in ESR

All the red electrolytic capacitors are brand new, they are of the same ratings as those old Tracon were, only newer technology. All are German Würth Elektronik, 2000 hours at 105°C general purpose AEG5 series capacatitors bought from Farnell specifically for this project. None is bad. That C31 cap coupled with the speaker ouput works normally in AM mode when increasing volume. It it were faulty, it would change the volume improperly regardless of the selected band.

a resistor went higher in ohms that per spec, etc.

Now this could be the case as it may be involved in getting lower voltages. What are the odds for a resistor going bad and increasing the resistance instead of just blowing open when overloaded? Can I test each resisor in circuit or do they have to be removed?

I think that resolving the voltage drop is the first thing to fix, to make sure components, including that amplifier IC, could be checked properly. They would at least be receiving expected voltages to start with, and then try to find the next culprit. 🙂

[stj]

resistors can go up AND DOWN in value without looking bad on the outside.

[UserXP]

resistors can go up AND DOWN in value without looking bad on the outside.

Are you suggesting I should unsolder each resistor to check it? Is there a specific section of the board with resistors to check, the ones that may be causing the voltage drop? Just as a suggestion, to narrow down the unsoldering job area. 🙂

[UserXP]

I tried unsoldering the 3.3k resistor which is between the supposed 13.5V output (in the schematics, it is R33 but labeled as 1K, not 3,3K. The colour code on the resistor, however, tells that it is a 3.3K resistor. No difference even with it out of the circuit, plus it measured 3.24K ohm on the multimeter, so that one is OK. So far, every component I unsoldered around those big caps, the resistor, the rectifying diodes (D1, D2 D4), small ceramic caps, they all measured fine. I am at a loss.

I then took apart the working unit. It supplies 15 volts everywhere where the faulty unit provides 7.5 (it turns out half of it). The voltages are stable in any function mode.

On the faulty unit, in complete Off state, I get 4.2V instead of 2.5 in Radio On mode on the L brightness switch setting. It seems that turning the radio to Off position supplies additional 1.7 voltage for the display, making a total of 4.2 volts. With that in game, even on Low brightness setting, the display turns on and is slightly brighter, and also the blinking clock state works too (before, in radio On mode, with only 2.56V at its disposal, the display couldn't even light up at all, nor blink).

Something is dropping the total voltage of ~15V by half, and then the remaining 7.5 volts are used to power the unit, which is insufficient or maybe get further divided by the components producing even less voltage.

The working unit provides 4.5-4.7 volts for the Low brigtness setting, and 14V for the High. It is very difficult to measure some pads ond its board as this would require completely disassembling the LEDZ pointer sliderthe tuning drum with its cord and volume knob.

[Agent24]

So if I understand this correctly...
You have the same AC voltage out of both transformers (working and faulty units)
BUT You have low DC voltage coming out of D1/D2 on the faulty unit?

It seems to me like there is likely no excess current draw, or you would see a drop in the AC voltage from the transformer as well, not just the DC voltage, especially when the drop is that significant.

You say D1/D2, and C34/C35 or other associated components may measure OK out of circuit, but they could be failing under load, have you tried just replacing them?

Your other option is measure current draw between the two units, have you tried this?

[UserXP]

So if I understand this correctly...
You have the same AC voltage out of both transformers (working and faulty units)
BUT You have low DC voltage coming out of D1/D2 on the faulty unit?

It seems to me like there is likely no excess current draw, or you would see a drop in the AC voltage from the transformer as well, not just the DC voltage, especially when the drop is that significant.

Yes, that is exactly right. The AC voltage between the transformer's terminals soldered to the faulty unit's board is 0-6-12V when measured. It is the same on the working unit. Yet, the faulty unit converts/utilizes only half of that.

You say D1/D2, and C34/C35 or other associated components may measure OK out of circuit, but they could be failing under load, have you tried just replacing them?

Your other option is measure current draw between the two units, have you tried this?

No, I only replaced the capacitors thus far. I also tried other suggestions, none has made a difference. Even with some components out of the circuit there was no difference in voltages. The two rectifying diodes are 1N4001.

So, measuring current draw would require to connect the multimeter in series with the parts being tested upon unsoldering, right? if so, I may do it with this faulty unit, but I am reluctant to do it with the working unit so I don't mess something up in there.

[UserXP]

A silly question: since the working unit produces 15 volts total, yet its imput is also 6 + 6 volts, is it likely that one of the Q4-Q7 transistors is bad and does not work as expected, as they may be involved in voltage amplification? So instead of amplifying it, they drop or just pass it through as it is.

[stj]

the diodes and capacitors after the transformer create a voltage booster,

[Agent24]

So, measuring current draw would require to connect the multimeter in series with the parts being tested upon unsoldering, right? if so, I may do it with this faulty unit, but I am reluctant to do it with the working unit so I don't mess something up in there.

Yes, but there is no use in just measuring the faulty one. You need to measure the good one too, to get a known good reading you can compare with.

A silly question: since the working unit produces 15 volts total, yet its imput is also 6 + 6 volts, is it likely that one of the Q4-Q7 transistors is bad and does not work as expected, as they may be involved in voltage amplification? So instead of amplifying it, they drop or just pass it through as it is.

The voltage being 15 volts DC out with a 12v AC input makes sense. When you rectify AC to DC, you multiply it by 1.414 because of the RMS value. Then you subtract the diode drop of the rectifier. That is why you get the 15 volts DC from 12v AC. One of the schematics (not sure which one yours is) shows also a voltage doubler circuit, that's how it also gets 30 volts.

If yours has the voltage doubler but the voltage is coming out too low, you may have leaky caps or diodes in the doubler circuit.

[UserXP]

Yes, but there is no use in just measuring the faulty one. You need to measure the good one too, to get a known good reading you can compare with.



The voltage being 15 volts DC out with a 12v AC input makes sense. When you rectify AC to DC, you multiply it by 1.414 because of the RMS value. Then you subtract the diode drop of the rectifier. That is why you get the 15 volts DC from 12v AC. One of the schematics (not sure which one yours is) shows also a voltage doubler circuit, that's how it also gets 30 volts.

If yours has the voltage doubler but the voltage is coming out too low, you may have leaky caps or diodes in the doubler circuit.

OK, the capacitors are all brand new and soldered correctly regarding their "-" symbol, so I doubt it's a leaky/bad capacitor.
The two rectifying diodes 1N4001 measured OK when checked out of the circuit, but I can try and put two new ones and see if it will make any difference.

[UserXP]

I strarted with one diode. The way it looks to me, 12VAC is used to provide the power for the VFD. It goes through D1 and 25V220uF capacitor and all the way down to the Brightness switch. So these two components should provide a DC voltage only for the VFD, the way the traces go. If after the rectification that voltage should rise up to 15V (like in the working unit), that is probably why a 25V capacitor is used here?
i removed D1 (1N4001) and put in a brand new 1N4004 - no difference, still producing only 7.5 volts. So it seems that the original D1 is OK, verified by the multimeter reading as well.

I then replaced the second diode, D2, without any difference. The original D2 (also 1N4001) tested out OK as well.

It is neither the diodes nor the capacitors that cause the lower voltage, guys. There is obviously something else that cuts this voltage in half.
Q4 is labeled as "drive amp", Q5 and Q6 are labeled as "power amp". Could these be involved in one of the rails being low in voltage?

[Agent24]

This is somewhat confusing when there's two schematics with different power supply configurations, one has a voltage-doubled 30 volt rail and the other doesn't., and I don't know which one yours is exactly.

Anyway, the drive and power amp sections are the audio amplifier stages for driving the speaker. They should not affect the power supply unless an output transistor was leaky or shorted and pulling everything down, but that's why comparing current draw measurements would be very helpful.

[Agent24]

Also, there is no R110 on the function board, the one labeled 56 ohms is R108 and that is the last R1xx there. It reads -12V on its outer pad, but on the inner pad I don't get any reading. It gets warm while powered on.

What do you mean that you don't get any reading? Zero volts? Where is your reference point you're measuring from?
This is the heater supply to the VFD and it is AC.

What is the voltage across the resistor?
What is the voltage across the same resistor on the working unit?

[CapLeaker]

Take Q5 and R30 out and thee if the voltage stabilizes.

[UserXP]

Take Q5 and R30 out and thee if the voltage stabilizes.

OK, I am going to try that. But here are the in-circuit tests of Q4 to Q7 in diode mode. The third measurement is X to Y, meaning black probe to red probe respecively on the transistor's collector and emitter leads. The device is unplugged from the power.

Q4 is NPN and reads:
B to C = 0.71V
B to E = 0.70V
C to E = 0.7V (reverse is O.L.)

Q5 is NPN and reads:
B to C = 0.68V
B to E = 0.68V
E to C = 0.60V (reverse is scattering around 2V)

Q6 is NPN and reads:
B to C = 0.69V
B to E = 0.69V
E to C = 0.60V (reverse is scattering around 2V)

Q7 is NPN and reads:
B to C = 0.69V
B to E = 0.69V
E to C (scattering around 2V)
C to E (scattering around 2V)

Now, regarding your suggestion to remove R30, it looks like it is used to drop the voltage from 13.5 to 11.5 volts. But in order to do that, it needs to start with having those 13.5V, which it doesn't. The voltage I can measure on that line after D2 is only 7.4V instead of those 13.5V as per manual. R30 then drops the voltage from those 7.4 to just 6.2 - which should be 11.5V according to the manual. Also, Q5 receives less voltage:
B = 4.26V instead of 7.4V
C = 7.34V instead of 13.5V
E = 3.64V instead of 6.8V

This is in Radio On mode. When it is Off, the voltages are near zero.

Do the aforementioned readings shed any light any closer, or should I proceed with unsoldering the pads you suggested to disconnect R30 and Q5?

[UserXP]

R23 should then drop the expected 11.5V to 4.2V, which are then passed through the FM setting on the switch down to IC1 and radio segments.
Instead, the provided low 6.2V is dropped to only 2.15V (instead of supposed 4.2V) and that is the voltage supplying the radio circuit. It may be one of the reasons why there's low volume in FM mode, due to insufficient power.
Sorry for the long messages, I am trying to provide as much data as I can. 🙂

[CapLeaker]

If you lift R30, it prevents the power going to the circuit further back. You may want to lift R33 as well after this test, but there shouldn’t be a lot of current draw through a 1k resistor.I wanna isolate or break up the circuitry in sections to pinpoint what’s going on. Put the clock in radio mode, plug it in and take voltage measurements AC on the secondary of the transformer and DC at D1 and D2.
In circuit tests may fool you. I chased a bad transistor years ago on a quad audio amp from mid seventies, I did find the problem, but was not there what I expected, even I checked the faulty component many times in circuit.

The idea here is to power only parts of the radio and see what makes the power go low. You may install Q5 and leave R30 out or vice versa and take more Dc measurements, Continue taking strategic placed components out to stop voltage flow in order to power up the circuitry in sections.

[UserXP]

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? 🙂

[UserXP]

If you lift R30, it prevents the power going to the circuit further back. You may want to lift R33 as well after this test, but there shouldn’t be a lot of current draw through a 1k resistor.I wanna isolate or break up the circuitry in sections to pinpoint what’s going on. Put the clock in radio mode, plug it in and take voltage measurements AC on the secondary of the transformer and DC at D1 and D2.
In circuit tests may fool you. I chased a bad transistor years ago on a quad audio amp from mid seventies, I did find the problem, but was not there what I expected, even I checked the faulty component many times in circuit.

The idea here is to power only parts of the radio and see what makes the power go low. You may install Q5 and leave R30 out or vice versa and take more Dc measurements, Continue taking strategic placed components out to stop voltage flow in order to power up the circuitry in sections.

Sorry, I missed your post while I was typing the previous one. I already tried with R33 removed and there was no difference, but I will do as you suggested now.

Also, the AC outputs from the transformer are stable all the time, be it the radio is On or Off. In the manual, power transformer contacts are labeled as 1, 2, 3 and 4. On the board, they are actually labeled as A, B, C, and D respectively, but the letters are hidden by the ribbon cable. The secondary output wiresfrom the transformer are soldered at points 1, 3 and 4 (2 is not in contact with any of the AC output). So:
between 1 and 3, I get 6VAC
between 1 and 4, I get 12VAC
between 3 and 4, I get 6VAC
All normal.