Vintage SONY Digital Clock Radio ICF-C12W no audio

[CapLeaker]

Now figure out:
A: Is the faulty unit drawing so much current that the voltage drops that much on the transformer
B: Is the transformer delivering proper voltage?

[UserXP]

Yes, the power transformer in the faulty unit outputs dual ~6 volt. It is a center tap PT and measures 5.9 and 6.1 volts, giving 11.9-12 volts total. This is almost identical in the working unit, differences are maybe 0.1 volts up or down.
Which part would you want me to check about the current, what to probe in series with the multimeter?

[stj]

do you get the 15v and 30v from the power rectifier circuit?

[UserXP]

do you get the 15v and 30v from the power rectifier circuit?

No, I only get 2.56v and 7.56v after recification.

[stj]

double-check none of the new caps are in backwards

[UserXP]

double-check none of the new caps are in backwards

Already done that. I visually checked each cap. Also, I took pictures of the top of the board with the old and with the new caps and compared the two. All negative stripes are oriented the same way in both images.

[stj]

try isolating the power circuit from each section and see if the voltages return to normal

[UserXP]

try isolating the power circuit from each section and see if the voltages return to normal

I will attempt that. I am awaiting the original Sony ICF-C12W service manual. I have the C12L version. Although the function board is the same, I noticed there is a slight difference in the components layout where the power supply connects to the main board. Maybe C12W has slighly different voltages to check against. The manual should tell. I don't know what else to try, all the caps are properly soldered and in their right place, all the components I removed (diodes and a few ceramic capacitors) check out OK, but the power is still low. I only haven't checked the transistors, though the power and heat in tose sections are so low I couldn't imagine how they may have become blown or something.

[stj]

it sounds like something is pulling too much power - probably a diode or ceramic cap

[UserXP]

I did some additional measurements. I measured voltages on the transistors Q4, Q5, Q6 and Q7 (in volts). The C12L manual I currently have has two models covered: UK (with FM, MW and LW bands), and AEP (only FM and AM bands, like mine).

Q4 voltages:
E = 0.00
B = 0.62
C = 3.07

Q5 voltages:
E = 3.67
B = 4.27
C = 7.44
Voltages here are also less than in the manual.

Q6 voltages:
E = 3.67 (manual suggests 6.8)
B = 3.06 (manual suggests 6.2)
C = 0.00
Something appears to be drpping the voltage here by 3.2V.

Q7 voltages:
E = -0.18
B = 0.64
C = 0.00

[UserXP]

Update: I removed both 1N4001 diodes from the board and they both passed the test, 0.590V in bias, 0.L. reading in reverse, which indicates they are OK. I then tested D4, which is a VD1220, it measured 1.2V in both ways with reversed probes, which I also think is normal for this old type of diode. The two 5.1V (1S1555) zener diodes also tested OK. I then removed a small 1nF capacitor which was in paralel with D2. It also passed the testing. I powered on the board without it and the power was still the same.

Then I disconnected the flat ribbon cable - and only got around 0.6V on outputs where 2.56 and 7.56 used to be with the ribbon cable connected.

i also noticed that, when increasing the volume quickly, it hits a random place on its pot where the sound completely mutes for a second or so with a slight "pop" sound from the speaker. It stays like that for a second and then the sound reappears with the same slight pop sound. It is as if when increasing the volume fast the circuit experienced a lack of already low voltage, insufficient to momentarilly increase the volume but needs that little time to "recharge" to continue.

I am at a loss what else to try. Something is draining the voltage, nothing gets hot. In some areas the voltage should be around 13.5 to 14V, yet only 7.5 is there. If the radio is off from the AC power for a while, that voltage turns on at around 6.9-7.1V upon plugging AC back in, and as it stays on, it increases to 7.5 and just stays there.

Any ideas what else to try? I can't really understand the rectification circuit on this board if it is dependent on the small function board and which section. Please guide me with detailed steps so I can follow. Thanks in advance, guys.

[CapLeaker]

A VD1221 (D4) is another component you wanna replace on sight, these destroy things downstream when they fail. Measuring 1.2V forward and reverse? I don’t think that’s good. Should only go one way, not the other. Replace it with two x 2N4148 soldered in series to get the 1.2V drop.

If I look at Q6 (AM RX) there are different voltages on the schematic than you say there should be. Are we taking the same component?

[UserXP]

A VD1221 (D4) is another component you wanna replace on sight, these destroy things downstream when they fail. Measuring 1.2V forward and reverse? I don’t think that’s good. Should only go one way, not the other. Replace it with two x 2N4148 soldered in series to get the 1.2V drop.

If I look at Q6 (AM RX) there are different voltages on the schematic than you say there should be. Are we taking the same component?

Thank you for writing. OK, I can remove and replace D4, it is VD1220. Will two 1n4148 diodes in series work? Also, after connecting in series, I still obey the "-" (stripe) end on the diode and solder it to the "-" end on the PCB?
Is there a "one component replacement" for this VD1220 type of diode?
End-to-end or back-to-back connection?

Regarding the voltages, I measured those and labeled them according to the B, E an C markings on the PCB. The schematics I have is for model ICF-C12L, so the layout of the transistors is silghtly different than on my PCB, but almost everything of the AEP model section in that service manual matches what I have here.

[UserXP]

Update: I desoldered VD1220 and measured it then. It gives 1.16V one way, and 0.L. reading the other way. According to the specs, the voltage for these old diodes is 1.2 volts. So this old diode appears to be working within specs?

But when soldered onto the board, it measures 1.2V in diode mode on both ends (depending on the probes orientation, it just displays + or - 1.2v on the multimeter).