Pioneer: dial lamps and weirdness

I have a Pioneer TX-8800II which in the USA is a TX-8500II.

THE DOUBTING FACTOR:

Some years ago I had another TX-8800II. I replaced all the caps on it and cleaned up the board. It was great — the sound improved and all. Then a couple of years later it died — just went dead. I inspected it as much as time would permit. We were moving house, and I had a ton of audio gear piled up — gear that like the TX was bought as junk. Some worked some didn't. Well, I thought it was either a chip death or dodgy transistor. I had no time, no space — so it went to the place where it might have gone anyway before I bought it. Bye-bye!

Onto my spare TX ... same model. Now I am being very cautious before I commit all the time it takes to re-cap and fiddle. I have caps to do the power supply. But before that I wanted to check out why I was not getting lamps for "WIDE" and "STEREO". I pulled the lamps. NARROW looked OK and it lit up when toggled. Under a loupe the WIDE build was obviously open. And I thought I could hear a difference between the two settings. Well ... OK ... who really needs the lamp?

QUESTION: Why does the NARROW lamp only work when its plugged into it's rubber retainer? There is no circuit in there AFAIK ... and there is no exposed part for say, DC to ground.

Now the STEREO lamp. I pulled it, and it looked good. But why no light? With the circuit on I put it back in and then pulled it again. Then under the loupe it appeared to have gone open. Apparently these lamps are fragile.

The question nagged me — why did a former OK looking lamp not light?

So ... onto to STEREO. The lamp looked good but was not lighting up. So, next ... I put the meter on the leads and got voltage on BOTH mono and stereo settings. I assumed that on mono it would fall to minimum or zero! But no ....!

Then — out to the garage and my parts bins. I found a bulb from some old tuner. I tried soldering it on the STEREO leads but one of them broke away from the bulb.

Well ... if I really have WIDE and STEREO, I can live without the lamps. But the questions.

* How can these lamps light up only when plugged in — or is there a quirk?

* Why is there seemingly comparable voltage both in mono and stereo toggle positions

* What is the usual way we can replace these — diodes? And do we need resistors in series with the diode? What diode?

* Have I got a problem?

I feel kinda stupid. I should have some answers myself. I used the search engine here — but I am stuck.

PS: This unit probably cost me only 5 or 10 bucks here in Japan about 10 years ago. I could junk it if it is suspect, but it sounds pretty good as is.

I sure could use some guidance here ... thanks in advance

Well, I got somewhere myself. The one working indicator lamp that sometimes lights up is going open from just physical movement. I have got it to light up in my hand, and right now it lights up on NAROW toggle. So, it's off or on. Tiny, old fragile leads I guess.

I am convinced I am getting stereo. The mono setting flattens out the sound stage.

So ... what is the fix to get the lamps refurbished?

The good thing — the four "face lamps" all light — and I have spares in the bins.

I agree that wire or filament fragility probably explains most of your puzzling experiences with the lamps.

If the stereo lamp was open, a typical digital voltmeter would read about 12V regardless of whether the tuner is in mono or stereo mode. This is because even a very small amount of "leakage" current is enough to make the meter read voltage, while it would not be enough to light a lamp.

You could replace the stereo, wide, and narrow lamps with 6V 30mA incandescent lamps, or you could fit LEDs.

All three of these indicators already have series resistors located on the main ciruit board, but the resistors are sized for incandescents rather than LEDs. To use conventional LEDs, you would need to either replace the existing resistors, or add a series resistor in the wiring. You would probably want to experiment with different resistance values to obtain the brightness that you prefer - I would suggest starting with 470 ohms.

It would probably be simpler to use "12V LEDs", which have a self-contained series resistor. These should work without any other modification, but they may be brighter than you would like.

Either type of LED needs to be connected with the correct polarity. For round LED packages, there will be a flattened area of the package indicating the negative lead. According to the schematic I'm viewing, the negative leads of the wide and narrow lamps are connected to pins 47 and 48 of the main circuit board, and the negative lead of the stereo lamp is connected to pin 26.

I hope this helps. Good luck,

chazix

Thanks for this. I think that a local computer parts shop (a national chain) should have the 12 Volt LEDs you mention. That seems to be the easiest solution — especially since I have found out that the schematics of the 8800II and the 8500II are not describing the same unit. (And I trust your schematics literacy better than mine

I am not worried about them being brighter. At least they will run cool.

As for polarity, I should be able to read this on the meter, yes? The leads will bring up a (+) or (-) on the screen.

I am going to start another thread concerning my work on the TX-8800II and related issues. It may or may not be of interest since the 8800II was only sold in Japan. I have made some comments about this on the newer thread.

Thanks again ... and I hope to report success in the lighting solution.

Ah - I should clarify that I was looking at a TX-8500II schematic. I think it is unlikely that the indicator circuitry would be different, but the board pin numbers might be.

Yes, you should be able to use your meter to see which is the + lead, but remember to select either wide or narrow in order to see the + voltage on the leads for those indicators. The stereo indicator should always have about +13V DC on one of its leads. This assumes that your tuner's indicators are wired like the TX-8500II's but again, that seems likely.

chazix: I want to thank you for the help with the lamps. I replaced the 'Stereo' and 'Wide' lamps with the 12 volt LEDs you suggested. I had to innovate a way to mount them.

For 'Wide' I used a red LED. I had to gently scape the translucent green paint off the lens to get a discernible light. I left 'Narrow' alone; the original lamp still works. Fortunately the bulk of the new LED does not interfere with one of the 4 dial lamps. But, I had to sacrifice a dial lamp to get the new 'Stereo' light in. I don't miss it, and besides that whole side of the FM band is empty anyway.

I've replaced all the e-caps and the unit sounds great. Oh yes — I don't have a TX-8800 II. I was confusing it with another unit I had years ago. Its, a TX-8900 — Japanese domestic. I do not believe that there is a X-over for an export model. Later I may put something up on the 'Pioneer' forum.

Cheers — lorne