[Mondial]

Ron, looking at the schematic, it appears that PL8 is part of the circuit. When a stereo signal is received, pin 6 of IC 30 is pulled to ground, lighting PL8. Transistor TR307 is an inverter, so when PL8 is on PL9 is off and vice-versa. 

It looks like PL8 is the actual stereo light and PL9 is only on with a mono signal. The base drive for TR 307 is supplied through PL8 and if it is open, TR307 cannot turn on and light PL9.

Try connecting a 470 ohm resistor across the open bulb PL8 to supply base drive to TR307 and see if PL9 now lights in mono.

[Ron Ramirez]

You nailed it, Mondial...thanks. And upon further examination of the circuit, your explanation makes sense.

I didn't have a 470 ohm resistor so I used a 390 instead.

So it is PL8 that is open, which must be the red lamp. When I am tuned off signal, the dial pointer is illuminated. It stays illuminated in this color (it looks reddish in the photo but is actually a light amber) no matter which position the function switch is in - AM, Phono, Tape, Aux - and FM when it is not tuned to a stereo signal.

[Image: http://www.philcoradio.com/images/phorum/STA225_015.jpg]

But as soon as it is tuned into an FM Stereo signal...

[Image: http://www.philcoradio.com/images/phorum/STA225_016.jpg]

...the pointer goes dark.

I'll have to find a couple 3mm grain of wheat bulbs...6 volt, 60 mA. That is unless I want to convert these to LED. But given the circuitry that switches these bulbs to and from being an FM Stereo indicator...perhaps I should just keep it simple and stick with incandescent. I can, and likely will, install LEDs in place of the other lamps.

[Ron Ramirez]

Okay...

Today: Fun (?) with LEDs.

I wanted to convert the dial lamps to LEDs if possible. Trouble was, this receiver does not use fuse lamps. Instead it uses elongated bulbs with a wire protruding from each end.

[Image: http://www.philcoradio.com/images/phorum/STA225_021.jpg]

So, having several green LED fuse lamps on hand, I decided to try to convert them to wire leads.

Using a hot soldering iron, I removed the caps from four LED fuse lamps.

[Image: http://www.philcoradio.com/images/phorum/STA225_017.jpg]

I did not have to solder anything across the two pads on each end, but I decided to do so in order to have each wire centered on each end of the lamps.

Using bits of leftover copper desoldering wick, I soldered bridges across the pads on each end, and then soldered a wire in the middle of each bit of wick.

[Image: http://www.philcoradio.com/images/phorum/STA225_018.jpg]

I bent each wire lead so that there would be a bit of space between each LED lamp and the PC board they were going to be mounted on.
[Image: http://www.philcoradio.com/images/phorum/STA225_019.jpg]

Finally, having already removed the old incandescent lamps, I soldered each reworked LED lamp to the PC board.

[Image: http://www.philcoradio.com/images/phorum/STA225_020.jpg]

They worked like a charm...

[Ron Ramirez]

Fun (?) with LEDs, Part 2.

As it was easy to convert the dial lamps to LEDs, I decided to try converting the Auto Magic lamp and the two tuning meter lamps.

The Auto Magic lamp was easy. I did add a 300 ohm resistor in parallel with this lamp since it is part of a switching circuit which I will show you shortly (plus, the Auto Magic lamp did not go out without the shunt). That worked fine once it was shunted. I just picked 300 ohms at random.

I then attempted to convert the two lamps behind the tuning meter to LED.

That did not go so well.

There is a circuit in the receiver which makes the tuning meter change colors depending on whether Auto Magic is on or off when the receiver is in FM mode. It is always green in AM, and is not illuminated when the receiver is in Phono or Aux mode.

When the receiver is in FM mode and the Auto Magic button is pushed in, the tuning meter is illuminated green.

When the Auto Magic button is off in FM mode, the tuning meter is illuminated white (actually, a bit off-white due to the incandescent lamp used).

Here's what happened.

Just by using 300 ohm, 1/2 watt resistors in parallel with each lamp, the tuning meter remains green whether Auto Magic is on or off.

Now, when I add a 47 ohm resistor in parallel with the white lamp, the lamps act as designed - but the resistor gets very hot.

I tried a 100 ohm, 5 watt resistor in parallel with the white lamp and it did not get hot - but at 100 ohms, the green LED does not go completely out.

With 100 ohms across the white lamp, Auto Magic on:

[Image: http://www.philcoradio.com/images/phorum/STA225_022.jpg]

Now with Auto Magic off:

[Image: http://www.philcoradio.com/images/phorum/STA225_023.jpg]

Notice how the green LED below the white LED is still on (although at low brightness)?

Here's a partial schematic. Click to enlarge it.

[Image: http://www.philcoradio.com/images/phorum...5_024a.jpg]

PL6 is the white lamp while PL5 is the green lamp. TR706 is another inverter, yes? That conducts when TR705 does not, causing the circuit to switch between lamps?

I could use some more help here, Mondial (or anyone else). I really don't want to go back to incandescent but I will if necessary...the LEDs light up the tuning meter much better than the incandescents did...

[Mondial]

How do the voltage readings on the collectors of TR 705 and TR706 compare to those shown on the schematic  in both auto-magic positions?

As far as I can see, the circuit should work fine with 300 ohm resistors across the leds for PL5 and PL6 unless there is a problem with one of the transistors being leaky or not completely shutting off. Also check the base voltage on TR705 and TR706 in both auto-magic conditions.


There is another lamp PL7. Did you replace it with a led and bypass with another 300 ohm resistor?

[Ron Ramirez]

Yes, PL7 is the Auto Magic lamp; and yes, replaced with green LED fuse lamp converted to wired lamp, and 300 ohm resistor. It works fine.

I will check the voltages at TR705 and 706.

[Mondial]

OK Ron, I think I see what the problem is.The lamp driver circuit is very poorly designed.

TR705 which lights the green lamp is never completely turning off. With the incandescent lamp, a bit of leakage through TR705 makes no difference because it takes 250 mA to light up, but the LED is much more sensitive and still lights with a low current.

TR703 is what controls TR705. TR703's collector switches between 1.6 V and 15.5 V, and supplies base current to TR705 through R711. The problem is with 1.6 V at TR703's collector, there is still enough voltage at TR705's base to turn it on, leaving the green led lit. The solution would be to decrease the resistance of R715 to less than 1K, which shunts the base of TR705. This will form a voltage divider with R711, so that even with 1.6 V at the collector of TR703 there will be less than 0.7 V at the base of TR705, insuring it turns off completely.

Before you do anything else, connect a 1K resistor across the existing 2.7K R715 and see what happens.

[Ron Ramirez]

Well, another one of those DUH! moments for me...

After some experimentation with the receiver tonight, I noticed that I had forgotten to add a 300 ohm resistor in parallel with the green LED replacement of PL5.

So, after adding a 300 ohm resistor across PL5, I tried it out without jumping a resistor across R715. When Auto Magic was turned off, PL5 dimmed slightly and PL6 (the white LED) did not light at all.

Then I jumped a 1000 ohm resistor across R715.

The circuit now works as it should. The white lamp comes on and the green lamp goes out when I turn Auto Magic off.

I'm going to replace R715 with the 1000 ohm resistor and call it fixed.

Thanks again for the assist, Mondial.

I'm waiting for some lamps to replace the dial pointer lamps, then I can button this one up and call it finished, I think...

[Ron Ramirez]

In review:

In FM mode, Auto Magic on:

[Image: http://www.philcoradio.com/images/phorum/STA225_025.jpg]

In FM mode, Auto Magic off:

[Image: http://www.philcoradio.com/images/phorum/STA225_026.jpg]

The new LEDs really make a big difference in how the dial looks. And I did replace R715 with a 1000 ohm resistor.

Strangely enough, the red (FM Stereo) pointer lamp has now decided to start working.

I won't be putting the front panel back on until the new dial pointer lamps arrive. I've decided to go ahead and convert them to LED as well.

[Mondial]

Glad you got it all working. Dial looks great with the LED illumination. If you use LEDs for the pointer illumination, make sure to add the shunt resistors and I think there will be no problems with the circuit.

Did you remove the 100 ohm 5 Watt resistor? The 300 ohm resistor should be fine there also.

What exactly does the Auto-magic do? I saw a circuit on the schematic which senses when you touch the tuning knob. It says something about defeating AFC. Does this make the meter change color too?

Where do you get the LED fuse lamps? I have a Sansui receiver which need new lamps and it seems LEDs are definitely the way to go.

[Ron Ramirez]

Mondial

Thanks again. I won't forget shunt (and series) resistors for the dial pointer LEDs.

I did remove the 100 ohm 5W resistor, yes. I forgot to mention that in post #23.

As far as I can tell, Auto Magic is a fancy name for AFC. The meter does not change color from green to white when I touch the tuning control, but the receiver stays in tune over a large range if you turn the tuning knob to the left or right somewhat with Auto Magic turned on. Sort of like a properly restored 1937-38 Philco with Automatic Tuning does when the Automatic Tuning control is turned on - it keeps the receiver tuned to the correct frequency.

I've been buying my LED fuse lamps from eBay seller projectblk86, also known as Joe's Quality Wholesale. He has fuse lamps in warm white, blue and green.

http://stores.ebay.com/projectblk86?_trk...7675.l2563

I see he is now selling bi-pin lamps in LED and incandescent form for Marantz receivers as well as dial pointer lamps. Didn't know he had these - I just bought a kit of bi-pin LEDs and a dial pointer lamp for my Marantz 4230 from someone else.

Yes, I think LEDs look much, much better than incandescents. Especially in this Realistic - the original incandescent bulbs were very dull and lifeless. The LEDs make the STA-225's dial look in a way that Radio Shack could have only wished they could have looked in 1975-76.

[Ron Ramirez]

I've done some more reading up on the Realistic receivers and tuners that have the Auto Magic feature, and I have found that the tuning meter should definitely turn white when the tuning knob is touched, and the AFC should then go off while the tuning knob is being turned - and then the AFC should return and the meter turn green again when the tuning knob is released.

This is not happening with my STA-225. The meter stays green, and AFC remains on when I turn the tuning knob.

Hmmm...???

[Ron Ramirez]

Full schematic may be downloaded here.

[Mondial]

Check the voltages on TR701 and TR702. This is the circuit that detects touching the knob.

It basically works by detecting the hum created when you touch the tuning knob. Just like when you touch the top cap of a 75 tube in a radio, an AC signal is produced. TR701 and TR702 amplify the AC hum signal which then is rectified by D702, providing a DC control voltage. The DC voltage created when the knob is touched is applied to the base of TR703 which switches the AFC off and changes the meter illumination.

It seems that TR703 and the following circuitry is working fine because pressing the Auto-magic switch turns the AFC on and off and changes the meter illumination color. This is accomplished by applying DC to the base of TR703 through R329 from the Auto-magic switch.

So the problem is either the knob is not connected to the TR701, TR702 AC amplifier or the amp circuit itself is not working. Touch the tuning knob and see if you see a change in DC voltage at the cathode of D702 (19 TP on schematic)

[Ron Ramirez]

At the workbench.

Mondial, the first thing I noticed when I began to look this area over tonight is that it is apparent that TR701 and TR702 have been replaced previously - and whoever did it did a very sloppy job.

While I did not see any solder bridges, it is a wonder that there aren't any. There are large solder blobs on topside of the PC board, on the leads of TR701. And the "serviceman" used too much solder to solder TR701 and TR702 into place.

My readings:

TR701, schematic readings followed by my actual readings.
C - 1.6 / 0.5
B - Not given / 0.495
E - 0.02 / 8.8mV

TR702, schematic readings followed by my actual readings.
C - 9 / 15.3
B - 1.6 / 0.5
E - 0.77 / 60.2mV

Voltage at the cathode of D702 does not change when the knob (actually, the knob shaft as the knob is not currently on the shaft) is touched. The wire from the tuning knob shaft is in place and has continuity from the knob shaft to point 15 on the tuner PC board.

It looks like a rebuild of this circuit is in order.

Since I am faced with that chore, and since my LEDs for the pointer have yet to arrive, I think I am going to set this unit aside for a little while. I'm not giving up on it, though - I am hoping that I can restore this circuit to proper operation. I'm looking forward to this unit having a properly working Auto Magic circuit.