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Old 4th Jun 2018, 8:52 pm   #1
stevehertz
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Default Technics tuner is dead, well almost

A Technics ST-9600 stereo tuner lights up, that's all, when switched on. No movement of the signal meter, no audio output. All fuses are ok, checked. It sounds like something fundamental is wrong, major power supply failure or something. I've only just located the service manual so I've not had chance to dive in yet, but I wondered what others thought? As an aside, the build quality of this tuner isn't the best, with lots of extra, 'rat's nest style' wires connecting points on PCBs. It reminds me of a NAD amplifier I worked on.
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Old 4th Jun 2018, 9:42 pm   #2
Ted Kendall
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Default Re: Technics tuner is dead, well almost

First guess would be a duff rectifier or something thereabouts - the lamps could run happily off AC, after all. A quick look at the circuit confirms - the dial lights run off their own transformer winding.

Funny how some of even the most "pro" Japanese kit has the rat's nest syndrome - if Revox could make a tidy interior, why not Japan. Inc?

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Old 5th Jun 2018, 12:06 am   #3
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Default Re: Technics tuner is dead, well almost

Start by taking some basic voltage readings. You should be able to check for broadly plausible values without service info.
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Old 5th Jun 2018, 7:26 am   #4
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Default Re: Technics tuner is dead, well almost

i think this is one of those tuner that uses a voltage doubler to generate the supply. suspected the diodes or capacitors a quick run round with a meter should confirm if so a blanket replacement of it parts should give back long term reliability
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Old 18th Aug 2018, 4:10 pm   #5
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Default Re: Technics tuner is dead, well almost

Finally got round to having another look at this set. The voltages coming out of the power supply appear to be ok, although I haven't as yet checked voltages all around the circuitry. I did check the rails on one IC and it was fine, +/- 15V. So anyway, the symptoms are the same as before, absolutely no movement of the signal and centre tuning meters when tuning on FM or AM, and no sound output or variance of the same in any way when tuning. Out of interest, with the power off, the meters are rock solid even if you move the set side to side quite violently. That would suggest they have somehow jammed, but I'm not sure how much an in circuit dynamic braking effect will have on them until I remove a wire off the back of them. If they've jammed, then that possibly suggests corrosion, but the rest of the set doesn't tally in that respect. Oh, I should say they both appear stuck in their 'proper' positions; bottom end of scale for signal strength and in the centre for the centre tuning meter.

It could be that I have separate jammed meters and audio output problems, maybe the RF side itself is ok
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Old 18th Aug 2018, 10:00 pm   #6
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Default Re: Technics tuner is dead, well almost

Not strictly a Technics fault, but it may be worth a close inspection for physical damage if it doesn't add up. I had a Panasonic tuner which was working perfectly until something was dropped onto it by mistake. Not a large item but one which gave it a sharp shock. The tuner went off immediately. I thought it would be a simple fix like a connector out or something, but it had actually caused the fracture of a glass diode on the PCB. I didn't have a diagram and never discovered what the diode did, but I suspect it was in some form of selector circuit. It stopped AM and FM working. It was probably the most obscure fault I have ever had on a tuner. Thermal stress could cause the same thing to happen I suppose.
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Old 18th Aug 2018, 11:02 pm   #7
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Default Re: Technics tuner is dead, well almost

It's not unusual for glass RF diodes to physically break in half, I remember we worked through a fault on a forum member's old CB radio where that was precisely the cause of their problem.

I think it often happens where the diode has been inserted into two holes which are a little too close together so the leads have been bent at sharp angles close to the glass body in order to get them to fit.
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Old 19th Aug 2018, 8:36 am   #8
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Default Re: Technics tuner is dead, well almost

Most often the meters in Japanese gear have 200uA movements. A 1.5v battery with a 10k series resistor will check them for life without risking things. If you suspect a short outside the meter, disconnect the wires from one end.

For repairing a tuner without a schematic available, identify the IF and decoder chips. The datasheets will give a lot of voltages and waveforms to check as a first stab.

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Old 20th Aug 2018, 4:12 pm   #9
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Default Re: Technics tuner is dead, well almost

Just checked the meters with an Avo. They swing fine. So clearly they're not being fed any signal at all when tuning.

I'll take a look around for glass diodes. Otherwise I'm thinking it's a 'down the line' power supply/rail issue as the tuner has a pink noise generator that you can switch in, and that doesn't give any sound output either.

I do have a cct diagram but it prints out in minuscule detail that is frustratingly illegible most of the time. However, when I zoom in the detail is not too bad, so I'm going to do that and take screen grabs of the enlarged sections.
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Old 20th Aug 2018, 6:21 pm   #10
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Default Re: Technics tuner is dead, well almost

Might be worth checking switches in case you have a bad contact somewhere. I suppose the best thing is to check power supply voltages right at the point where they feed the circuit. The fact that the fault is affecting AM and FM will narrow it down a bit.
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Old 20th Aug 2018, 11:36 pm   #11
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Default Re: Technics tuner is dead, well almost

Also check for any of those 3 legged ceramic filters. The middle leg is ground and i have known them go short circuit and stopping everything.
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Old 21st Aug 2018, 7:19 am   #12
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Default Re: Technics tuner is dead, well almost

But surely, the pink noise generator not working points to a broader fault than a purely RF one?
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Old 21st Aug 2018, 5:42 pm   #13
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Default Re: Technics tuner is dead, well almost

It does seem to point towards a common supply problem. Check that the supplies are accurate. I once had a Technics tuner that had slightly low supplies and it stopped everything. Turned out to be a faulty chip in the audio output/mute circuit which was taking a lot more current than it should have been and dragging the supply down a volt or so. Removing the chip temporarily from the board brought everything back to life, but not before I had wasted a couple of hours chasing my backside changing other things.
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Old 21st Aug 2018, 7:26 pm   #14
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Default Re: Technics tuner is dead, well almost

Thanks for the 'inputs ongoing' fellas. At great cost of time and trouble ive managed to make up a large, largely legible cct diagram. I took four screen shots blown up in size from the original cct that itself was in two halves. There the fun began as when printed out and offered together, all the components and connection lines etc did not line up - a right muddle. So I had to re scale each one, print it off, try it and then keep trying again and again until a size match was obtained, then tape them together. Anyway, sorry to digress on a seemingly trivial printing issue, and as soon as I get time again I'll have another go, hopefully with more ease now that I have a readable cct diagram.
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