70's Pioneer tuner, some repair questions

[fozzyvis]

Hi all,

Long time reader, first time poster.
I inherited (if that's the correct term) a '70s TX-7500 Pioneer tuner from my late grandfather.
After a lot of cleaning and degreasing it looks impeccable.
A few question about the electronics remain. As this is an inherited piece of gear I really would like to get it "completely" working.

I have three questions. The first one needs quite a bit of explanation, the other two are brief.
I better ask them in one thread instead of making separates I suppose?

• First of all, all of the lights (panel, meter and power) were broken. They were old 9 volt lamps that I couldn't find for a reasonable price, so I adapted the mount a bit and replaced them with what I could get. Works fine, looks great. The stereo indicator is my first problem. I couldn't get any markings from the bulb that was connected, it was all well...  old and used up.
  So I dug out my scope, put it in A-B mode to measure floating rails (after connecting the ground clip the first time, the fuses went out, took some time to figure out what was happening).
  Without any load one rail is +15V, the other about +14V. These are quite noisy as well. When the stereo light should be on, the second rail drops to +12V, so a 3 volt differential. With a small load on it, the difference got to 1.2V when lit. Ordered a 1.25V (260 mA) bulb, connected it hoping this would work.
  But it didn't.
  With the bulb connected and the stereo indicator off it gives about 0V, when the light should be on I have 200mV left.
  Connecting a multimeter in series adds a bit of resistance (at least, that's what I'm guessing), and makes the voltage difference go up to about the double of it, and indicates 53mA's current draw.
  What I think is that the circuit that drives the bulb can't supply enough current to light the bulb, is this correct?
  
  I have tried to figure out a way to get more current with a tranistor to ground or something like that, but can't get it to work. The voltages are positive when referenced to the ground point of the other bulbs, but I need to get a higher voltage/more current when one of the two floating lines drops a bit.
  Tried using an opamp ("dropping" line to the inverting, steady +15V to the non-inverting), but can't find out how to connect it and to which rails I should connect Vss and Vdd of the opamp. Got measurements that didn't fit what I thought I needed. But then again, I could be very wrong in my thinking or measuring. This isn't something I have done before.
  
  
• Second: simpler question. The scale is a bit off. When I tune into a station and compare the dial with the know frequency it's about 1 MHz off. I suppose I can just try and slide the needle a bit on the piece of cord that guides around the dial, the "tuning part" of the tuner and the display/needle?
  
  
• There's a "tuning" and "signal" meter on the front panel. When I hear the signal gets best (or I get stereo decoding), the tuning needle is always a bit off centre. When I go to the middle of the scale, the signal quality drops. Is this something I can easily understand and fix?
  
  

Any help (or additional questions if needed) is greatly appreciated!

regards,

Dieter

[tekfan]

This link might be helpful (google translate helps out a lot):
http://amplifier.cd/Verstaerker/Reparatur/Reparatur_Receiver.html


First of all connect the ground lead of the oscilloscope only to the chassis. All of the supplies are chassis referenced and that is their common ground. It shouldn't blow fuses or anything if you just connect the ground clip to the metal chassis.

As for the stereo bulb, are you sure it wasn't an LED? That would explain the small voltages.
If all of the parts are still original and nobody has tampered with the tuner before, there is no reason why the original circuit shouldn't work.

Moving the pointer is a nice and simple solution but it isn't the best idea since it will mess up the calibration for the AM band. There should be some smaller variable capacitors near the main tuning capacitor. Those are for calibrating the tuner. Some adjustments may interact with both AM and FM bands. Be sure to mark the positions before tweaking them so you can always put them back to where they were in case it gets worse.

A few pictures of the insides might be useful since there are none on the web.

[PA0PBZ]

A quick google turned up this:

Pioneer TX-7500
Dial illumination: 8 volt 300 milliamp X ?? "wedge type" lamps
Pilot ("power") and stereo indicators: 6 volt 30 milliamp X 2 "grain type" lamps with leads

[fozzyvis]

@Tekfan:
I thought I had tried to replace the bulb with a led, but seems I didn't or didn't do it correctly since it worked!
Didn't think LED's where being used in consumer electronics in the 70's...

About the tuning of the tuner (no pun intended): I have searched for (and found) webpages and service manuals about the tuning process for some other models of tuners, but I lack enough knowledge to adapt it to mine. I have made some pics as you asked, maybe that sheds a light...

[tekfan]

I've repaired many Japanese amplifiers from the 70's and most of them seem to have faulty small signal transistors. Looks like this was a common problem for Japanese consumer gear. You might try checking the transistors with an ohmmeter or diode test function to see if they are still OK. One transistor might be used to ground one side of the lamp. The other side of the bulb is probably connected to the positive supply.

For the tuning you might try to turn the trimmer caps labeled TCA, TCR, TCR and TCO. If you touch them with an ordinary screwdriver you will add capacitance to them and the tuning will be meaningless. Try to find an all plastic screwdriver or just use a piece of plastic to adjust them. Remember the position of the screws so if it gets worse you can always bring them back to their original position.

Is there any circuitry below the tuning capacitor? It seems to be a little too far away from the mainboard.

[fozzyvis]

There isn't really any pcb underneath the tuning capacitor, but of course there is a pcb inside the block on which the potmeters are mounted. Not sure if you mean that one.
I've made two pictures of the bottom side. One large with the whole area, one close up from the pcb in the tuning capacitor

[fozzyvis]

Quick question. How far can I reasonably turn the pots?
One of the TCR makes the signal needle move when I turn it away from its sweet spot, but only slightly.

So is it ok to make multiple turns?
Any recommended practices? I've found a guide in a service manual for another tuner, but that needs specific equipment that I don't have, and I wouldn't know how to translate it onto this tuner.

Thanks!

EDIT:

I've gone and tried all of them back and forth (the ones on the row with TCA like you said). Most didn't seem to change anything with half turns, I set those back like they where. One of the TCR made the reception better on the low and high frequency end (FM), so tuned that to be best.

You know, those little pots that are sometimes glued stuck to a certain position, they are just staring at you like: "tweak me! Tweak me!" ?
The last TCO was glued and also smaller then the others. It just kept asking to be turned , and as it turns out it was the magic one. It shifted the whole frequency range.
I've verified radio stations around the high and low end of the range, and it's all good now!

The only thing that's a bit off is that the "tuning" needle seems to be off all the time by a little bit. Best reception when it's not in the middle.
When the tuner is off, it's dead center, so I don't think it's the meter itself.

But, unless there's a straightforward way of trying to get it right, I can live with that .

So thanks a lot!

Dieter

[jh15]

Yes, the stereo pilot lamp was a low current grain o wheat.

The trimmer capacitors tune one side of the dial, and you spread or compress the coils for the other end. Very interactive.

The ratio detector is off, that is why your tuning meter is off.

I'm used to my Sound Technology generator that used a dynamic o'scope sweep to tune the detector, but manual ways are available also.

You also have to watch while you mess around that the AFC control isn't masking your tweaks.

In the 70's I would have cringed at someone poking around, but now its tinker time.

You got the frequency ok on the dial, but you messed up the delicate rf tuning caps, which must be done with a weak signal.

The good news is after you get the detector straightened out (which is probably one of the two slugs in a transformer can at the end of the IF chain),

you can use a weak station at either end of the dial to bring your rf frontend into shape.

Something to note is back in the early stereo fm days, there was an equivalent of the Great Capacitor Plaque. EVERYONE and their brother used some IC IF amps, that ALL went bad, especially the black plastic fairchild ones. The rig would work, but be poor in sensitivity, and usually the stereo decoder would not work, even though a strong station was coming in.
I probably just threw out a box full of manuals from that era, go figure...

[fozzyvis]

Well...
As said in the beginning, I don't have any experience in this field.
I'll try to make sense out of what you say by searching around for knowledge, but I'm not sure if that is going to get me much further.

Do you mean that by doing what I did, I did wrong to some of the components or the tuning?
If I did, what kind of noticeable negative effect would this have?

About the IF amps, am I correct if I think that those get a stereo signal out of the A+B and A-B signals?
And that there's a good chance that the one inside this 70's tuner isn't working properly?
Stereo demodulation isn't the best indeed, even for fairly strong signals I only get stereo decoding if I take quite some signal degrading with it...

[jh15]

Hang on,

I'll be back with some simple stuff you can try. I used to know all this by heart, but I'll look for some info in my old hifi service manuals.

You also need a small plastic coil alignment hex tool for the detector, once I find what I think you have for a receiver.

[fozzyvis]

Ok, great!

For the screws I used a wooden "spoon" (the kind that's used by doctors to examine your mouth), that I sliced off at the sides until it fit, and then ground flat, point end on it. I'll think of something for the hex screws.

[SeanB]

Use a bamboo skewer, and file it to a hex shape. Works well, just make sure they are dry before using.

[jh15]

I was about to head out for awhile and saw the update. Just be warned, if you try to turn the hex slugs without a perfect fit, you will crack the slug, making it detune, and impossible to get out.

While you are waiting:

Turn off the receiver, and let it sit for a few minutes. Look at your center tuning meter (I assume you were talking about having both a center tune and strength meter.)

This is your mechanical zero. Usually, these meters stay pretty much ok from factory unless damaged or fiddled with. Later, use this reference when you are aligning the set.

Since you've already fiddled the dickins out of it, what I'll guide you with will get it into reasonably good shape.

Otherwise I'd say the usual: Don't touch anything until you have a low distortion stereo generator and analyzer (I have Sound Technology equipment used to design your unit most likely).

I'll be back later.

[fozzyvis]

Well uhm... Shall we say I sort of skipped over the "have a low distortion stereo generator and analyzer"?

In a way I don't feel completely right about treating a piece of equipment with history like that, but on the other hand...
The tuner was really dirty and neglected for a long while, and would have gone with the garbage when my grandfather died.
It's only because my mum knew that I really liked it that it was saved. Now it has been cleaned, the lights were repaired, and the unit is working again.

So, in short, while I can't fix it the way it should be done, I am willing to search and learn and give it my best

[jh15]

Well, I said "otherwise"...

That meant if this was in the days it was new, AND if it hadn't been touched.

Since you played with it, no harm will be done in trying to get it work its best without special equipment. Likely, you can get it to work very well, following precautions.

I was an authorized full service station for several "foreign" brands when this stuff was invented. I kept up with the best equipment and factory training, when available.

Since this is not an amplifier, there is a lot less worry about blowing it up by exploring it.

I cringed with the stereo light discussion, as I remember owners patching in high current lamps and the like, and blowing the stereo decoder IC. Looks like an led will work, even they were new back then.

If you can get some clear shots of your PCB, enough to read the IC numbers, I will be able to figure out the alignment procedure.

Since you are in this forum, do you have any equipment? A counter would be ideal for the pilot osc. Your o'scope might have freq. with enough resolution, plus you can do a "poor man's" separation alignment with it in x-y.

DO NOT touch any coils without a perfect tool. In the tuner at least, the trimmer capacitors drifted more than the coils. Those are the screws you were turning. They compress, not turn like a pot.

I'll be online later watching the transit, but not near any of my stuff. I might check in.

[fozzyvis]

Sounds great, especially coming from someone with first hand experience.

To answer your questions:
Yes I can provide you with decent pictures of the pcb. Those that I posted have been seriously downsized before I posted them and are quite highly compressed.
I'll make pictures tomorrow and make sure that they are as good as possible while keeping a reasonable filesize.
If necessary I will also post a second one with part numbers noted on them so you don't have to try to decipher anything, but good pictures shouldn't be a problem. But that will have to wait till tomorrow, don't have decent lighting (or time) available right now.

I have a digital scope (Rigol DS1052E with "firmware upgrade" just like every other owner of that one), decent bench supply, decent multimeters. Also an analog scope (20MHz  Elix HC-L202), but I don't think that one has been checked in the last 20 years.
That's what I have that seems applicable. Maybe I could borrow a frequency counter somewhere if I ask around but I don't know a lot of people that might have it so that might take a while.

About the stereo light:
Just to check if I'm right about this:
With no load, the voltages across the bulb output were quite high (15 Volts with regards to chassis GND), while when I put a bulb on it (sorry about that, I had no idea that I shouldn't have done that...), the voltage plummeted.
Is that because the load (the light bulb) was much too heavy/high and "demanded" much more current from the source than it could deliver?
Rather basic question, I know, but that's why I'm in the beginners section .

Not completely sure about your last remark. The screws that I touched, those are the trimmer capacitors, the coils are the ones that need a hex key, correct?
I'll open it up again tomorrow to have a better look myself as well.

Is plastic as good as a bamboo tool? I have some plastic (http://www.rapidonline.com/Education/Polymorph-Morefoam-granular-polymer-34444) that I can make in whatever shape I want to.

Thanks!

[jh15]

No rush,

Let's leave the coils to last, whatever you do do not use a metal hex wrench, and it might even be best to leave the tuner ones alone till a last step.

an led with a series resistor for the 15 volts or so on the pilot lamp should be ok.
A plain led without current limiting resistor might be a problem, so leave the pilot light till last for now.

Once You get me the pictures, I'll know the details. I see the ratio detector from your pictures, and you should also have a phase lock loop multiplex decoder that is very common. Also very easy to adjust.
Back later...