Sony receiver model STR-V5. MV12N diode - what is it?!

[stevehertz]

I'm working on a Sony receiver model STR-V5 and there's no stereo on FM. I'm gradually tracking down some incorrect voltages around the tuner board and in doing so I'm trying to check out a diode type 'MV12N'. Resistance wise it reads 'infinity' one way and about 5k the other way, on an Avo 8? I mean, if it's just a simple signal diode I'll replace it. But I'm not sure if it is..

[HamishBoxer]

Possibly a "Varistor" diode but the rest is in Chinese on a google search.

[G6Tanuki]

"MV" was a designation often found on varicap diodes [I remember using a lot of MVAM115 a few years back].

[ms660]

Shown as a single diode in the tuner schematic (D204) and a double diode (two diodes in series) in the power amplifier schematic (D506 in one channel..D556 in t'other channel)....?

Book of words here:

http://sportsbil.com/sony/STR/STR-V/STR-V5.pdf

Lawrence.

[Goldie99]

Looks more like a VD1221 type on the schematic (e.g., D506).

Googling a couple of Audio forums seems to agree (Audiokarma & Analog-Forum.de) - so a couple of 1N4148 in series should work.

Alan

[stevehertz]

But what about my reading on an Avo 8 - would that suggest that it is faulty given what people think what type of diode this is?

[Mooly]

The most common cause of 'no stereo' across many makes and models is simply that the 38kHz oscillator preset needs a tweak (i.e. whizz it from end to end a couple of times).

Adjust on a weak signal and set midway between the points the decoder locks to the stereo broadcast.

[stevehertz]

Oh? There's a labelled '19KHz adjust' pot that comes off the MPX decoder chip's pin 16, the VCO block? is that it?

[Radio Wrangler]

Most probably.

If you cant get a circuit for a tuner, then the datasheets for the IF and decoder chips will have typical application circuits which won't be far from what you'll find in real equipment.

The PLL will have a fairly basic phase detector with not much pull-in range. Add in the large tolerances in RC oscillators on chips, and you get the need for a twiddler.

Phase frequency detectors are much better at pulling in offer very wide ranges, but they have siily moments after momentary glitches and drop outs in signals.

David

[turretslug]

The AVO 8 readings for forward and back are the sort of thing I'd expect under the circumstances for a good one- but it's difficult to say if the "5k" forward reading is reasonable without a known good one to compare against. I'd be teaching to suck eggs by saying that the resistance reading itself is meaningless and utterly dependent on battery voltage, series meter circuit resistance, temperature, phase of the moon.... but if you're working on a circuit with lots of 1N4148 or 1N5402 or whatever, then a habitual meter deflection can become apparent and a suspect one can sometimes be indicated by a differing deflection but it's all a bit vague without more sophisticated testing.

Do you have a DMM to hand?- these often have a diode test function that provides sufficient voltage to turn on a diode and give a forward voltage reading- the forward current is usually very low, though, so an Si diode will often come up as something like "0.55V" or thereabouts. With practice, you can grade LEDs by colour vs. Vf without even looking at them!

[stevehertz]

I've tried adjusting the 19KHz pot with no joy. I've also tried the stereo separation and FM signal level pots, again, no joy. Oh well, I'll plod on looking for faults. Cheers.

[Mooly]

Try checking to see if the oscillator is running, both with a scope and frequency counter if possible.

Make sure the supply voltages to the decoder chip are correct, any variation from normal could be enough to stop it working.

[stevehertz]

I applied freezer spray to the HA1196 stereo demodulator chip. After a while the set started making noises (along with the broadcast) like the groaning noises that a huge ship makes in very rough seas; kinda metallic straining noises. But, guess what? the stereo lamp lit! Also, as the effect of the freezer spray wore off, the tone of the sound changed, as it did when I pressed on it. Then as the temperature of the chip normalised, the stereo lamp went out again. So I'm pretty sure all's not well with that chip. Another one is being ordered and will be fitted in due course.

[Mooly]

Did you try altering the oscillator ?

Freezing a good chip should not affect its operation but it might cause any oscillator to shift frequency (to be expected) enough to thus appear to fix the fault.

[stevehertz]

I tried adjusting the '19KHz pot' if that's what you mean? Anyway, it did not cure the fault.

[Mooly]

Worth trying. Lets hope the new chip is a fix

[stevehertz]

The new chip did not cure the fault. After much effort (many hours..) I found that D205, a signal diode connected to the 19KHz pin of the stereo demodulator ship was short circuit - or near enough. I popped a new one in and hey presto, the stereo lamp lit.

A touch of distortion was cured by re-setting the bias currents that had drifted high.

[Mooly]

Excellent news, well done.