Radio stopped working - Hitachi KH-1190E

[martfoxy]

My radio (Hitachi KH-1190E) stopped producing any sound when I was using it.I have tested the supply voltage from the batteries (4 x 1.5) to the pcb and is about 5.8v. The mains fuse inside the radio is also ok. The radio makes a slight noise when I turn it on.
I have tested with a DIN lead connected to my stereo and get very quiet sound from the radio if turn the volumes up to maximum. Also if a plug in earphone I get soft white noise.
Can anyone help with this? I have a multimeter and a capacitance meter. The capacitors don't look swollen.
I have attached a schematic - its not brilliant.

[Renate]

Hi.

Is the audio low for both AM & FM?
The DIN output is the takeoff point, the speaker and the headphone go from there.
If both AM & FM are low check the voltages on Q10. The voltages are on the schematic.
It could be something as simple as a wonky switch on the DIN socket. Try exercising it.

[martfoxy]

Thanks for replying apologies it took me so long to answer.

I measured Q10 and only got 0.02v on the base and 0v on collector and the emitter.

[martfoxy]

I got a paper copy of the service manual and took a photo of the schematic which I have attached. Hopefully cleared than previous pdf.

[martfoxy]

There is no sound on both AM and FM.Only time I got a very quiet sound was when I connected my hifi to the DIN socket - that was on maximum volume on both - plus it was distorted (that was input from hifi).

I tried exercising the DIN socket but that hasn't fixed it.

Thanks

[Renate]

Well, we may have hit a good debug spot there.
There should be ~2.5V on Q10 collector
That gets fed from C45 is the extra filtering for the audio.
That gets the 4.7V zener D5 that gets fed from the 6V.
Something is wrong around there. Shorted cap? Shorted zener? Open resistor?
(C45 & D5 are right on the crease of the schematic.)

[Audiorepair]

I was looking at that.

If D5 or the capacitors across it were short, the power supply would be shorted, probably a fuse would blow, or the batteries would be flat.

So more likely a short the other side of R18.
Or maybe R18 just failed open as you suggest.
C45 could short without taking R18 out, Q10 could be shorted without taking R18 out.

Well, we may have hit a good debug spot there.
There should be ~2.5V on Q10 collector
That gets fed from C45 is the extra filtering for the audio.
That gets the 4.7V zener D5 that gets fed from the 6V.
Something is wrong around there. Shorted cap? Shorted zener? Open resistor?
(C45 & D5 are right on the crease of the schematic.)

[Renate]

If D5 or the capacitors across it were short, the power supply would be shorted, probably a fuse would blow, or the batteries would be flat.

D5 is fed by 150Ω
A dead short is only 1/4 W on that resistor.

[Audiorepair]

Ah, didn't see that, the schematic is rather hard to follow.
Silly me, there HAS to be a resistor before the zener.

So maybe best to meter for a short across D5 and C45 and see what that turns up.



Edit:  And measure R18 and R37 in circuit at the same time.

That should give enough clues.

If D5 or the capacitors across it were short, the power supply would be shorted, probably a fuse would blow, or the batteries would be flat.

D5 is fed by 150Ω
A dead short is only 1/4 W on that resistor.

[martfoxy]

Thanks for your replies.

R18 measures 218 Ohms. The zenor diode D5 measures 21 Ohms when the power switch is switched on and 1.2 Ohms when turned off. I took the zenor diode out to test and is not shorted out of circuit.

R37 measures  156 Ohms.

C45 measures 219 Ohms in circuit.

[Renate]

The zenor diode D5 measures 21 Ohms when the power switch is switched on and 1.2 Ohms when turned off.

Well, you're confusing me with this talk of power switch on.
You shouldn't be measuring resistance with the power on, or did you just mean the switch without batteries in?

Measuring resistance might go to a low reading for a short time but then rise to a steady value if there are capacitors around.
It might even show negative resistance if one of the capacitors are charged.

C45 should not measure 198 ohms if it is out of circuit and the probes are on the right direction.

It sounds like R18 is burned out, double check it out of circuit.

Check voltage on R37, R18, D5 with the radio on.

[martfoxy]

Sorry for confusion I was getting confused myself and modified my message a few times. Please ignore the messages about switch on/off.

I tested R18 out of circuit and it measures 219.

Also measured C45 out of circuit and it measures open. Also tested on meter measures 258uF.

The voltage on one end of R37 measures 5.2v and the other end 0.02v (where D5 is connected).

On R18 0.02v at the D5 end and 0v at the other end.

D5 measures 0.02v and 0v.

[Renate]

Ok, we're getting somewhere.
Something is shorted to ground in the whole area of D5.
That's probably C29, directly to the right of D5 on the schematic.
I'm going blind looking at that thing.

You can pull C29 and check the D5 voltage without it in circuit.
It won't harm anything, just noisier operation.

[martfoxy]

I have pulled C29 and still getting 0.02v on D5. Have you got the second image of the schematic I sent - it is clearer.

[Audiorepair]

C31 and C85 are also in parallel with C29, might be one of them.

C22 and C26, if shorted, would also give a low resistance path to ground at this point, but probably not zero volts.

[Audiorepair]

C27 would short this to ground also, but only if FM is selected.

C4 would short via 100 ohms, but again only if FM selected.


Maybe try unselecting FM to see if the situation changes.




Ok, just saw you already did that.

[Renate]

C31 and C85 are also in parallel with C29, might be one of them.

True, but they're little ceramics, I wouldn't suspect them first.

Most of that line is connected through higher resistors and caps.

You might try removing D5, the voltage is low enough it won't harm anything.
I'm not sure we can trust the previous checking of it.

[Audiorepair]

I agree, it really isn't possible to check a zener is good with a multimeter.

You might try removing D5, the voltage is low enough it won't harm anything.
I'm not sure we can trust the previous checking of it.

[martfoxy]

I tested C31,C85,C22 and C26 but not shorted.

C27 and C4 are also not shorted.

I tried unselecting FM but makes no difference to the situation.

Thanks for your ideas though.

I will try removing D5 tomorrow to see if that makes any difference.

[perieanuo]

R37 then R18 gets the static polarisation point for Q10 if I see well your schematic, have you checked them?
(also R65 ?  near Q10)
you get those 6V on IC1
maybe DIN socket is not OUTPUT but INPUT

[martfoxy]

I disconnected one leg of D5 and I have a component tester and it recognised it as a zenor diode 4.3v. The voltage on the rail was 0.02v still.

[martfoxy]

perieanuo

Yes tested R37 and R18 they seem to be ok.

Just tested R65 and is 3.2k

I get 6v on pin 1 on both ics.

I am not sure if the DIN socket is output or input to be honest or both.

Thanks for your reply.

[Renate]

Well, you do have a short to ground on that D5 bus.
We've already suggested the most probable nearby components.
The other components on the bus seem to be isolated by large enough resistors to make them seem unlikely.

Hook up your ohmmeter using clips on the D5 bus (without batteries or power plug).
You said that it measured 1Ω.
Is there anything that changes this value?
Check all the switches and even the positions in between the clicks.

[jdragoset]

Might as well check C 16, as it is also across 4.5 volt bus to ground bus

[Renate]

Might as well check C 16, as it is also across 4.5 volt bus to ground bus

C16? Are we playing "Where's Waldo?"

C16 under Q3? That's kind of isolated.