[Mondial]

Ron, from looking at the voltage readings my guess is that TR701 is bad. 

This transistor is subject to a lot of abuse considering its base is directly connected to the tuning knob. Think of what happens to that transistor when you walk across the carpet and touch the knob on a cold winter day! 

After you get the circuit working, it might make sense to add some protection diodes to the input to protect TR701 from being zapped by static charges.

[Ron Ramirez]

...it might make sense to add some protection diodes to the input to protect TR701 from being zapped by static charges.

Can you explain, please? Since I am going to have to put together a parts order (TR701 and 702, as well as the electrolytics in the circuit as well as parts for another upcoming receiver), I might as well go ahead and buy these protection diodes at the same time.

Re: TR701 - I would not be surprised at all that it is bad. You should see the poor, poor soldering job on it and TR702. TR701 is the one that even has solder blobs on top of the PC board (opposite side of the traces and solder pads). What was this so-called "repairman" thinking by doing that?

P.S. This really is a sickness. I'm going to look at another Realistic receiver tomorrow and, most likely, buy it and bring it home. An STA-64. Silver face and no Auto Magic, one channel out, but for $15 I can't go wrong...

[Mondial]

Can you explain, please? Since I am going to have to put together a parts order (TR701 and 702, as well as the electrolytics in the circuit as well as parts for another upcoming receiver), I might as well go ahead and buy these protection diodes at the same time.

All you need is some 1N4007 rectifier diodes placed in reverse parallel and connected across the input wire from the knob to ground. Something like this from another posting here where the diodes were recommended to protect a VTVM meter movement.


[Image: http://philcoradio.com/phorum/attachment.php?aid=11799]


Wait on installing the diodes until you get the circuit working properly first, as they may reduce the sensitivity somewhat and it would be good to compare with the diodes in and out of the circuit.

[Ron Ramirez]

10-4, thanks again.

I wanted you folks to see the sloppy job that had been done previously by someone who replaced TR701 and 702:

[Image: http://www.philcoradio.com/images/phorum/STA225_027.jpg]

[Image: http://www.philcoradio.com/images/phorum/STA225_028.jpg]

Wire leads not trimmed, and those solder blobs on the leads of TR701...strange.

[Ron Ramirez]

So...I looked around in my stash of stuff...and discovered that I had the capacitors to replace electrolytics C701, C703 and C704.

I replaced C701 (0.47 uF) with a stacked ceramic unit. The other two are Nichicon electrolytics. I prefer to not use electrolytics when the value is 1 uF or smaller, but I only had 1 uF (C704) in electrolytic form.

I also discovered that I had a bag of fifty 2N2925 transistors. I further discovered that 2N2925 would substitute for the original 2SC828 transistors.

Here's my replacement work under the PC board - I used less solder and tried to be neater in my work, including trimming excess lead lengths.

[Image: http://www.philcoradio.com/images/phorum/STA225_029.jpg]

If you look closely in the picture, you will notice one lead that is not soldered or trimmed - the emitter of TR701.

There's a story that goes along with that...

I did this work, which did not take very long.

I then tried the receiver out.

The Auto Magic still wasn't working correctly - the meter still remained green when I touched the tuning knob!

So...

Having returned upstairs to my home office to post the last three pictures I have posted in this thread, I noticed that unsoldered lead in this last photo just above.

I returned to the basement workbench, plugged the soldering station back in, waited a few minutes, and set out to solder that unsoldered transistor lead.

The pad where the emitter lead should have been soldered promptly came off. Not the entire trace, just that pad. Probably too much heat was used on it by the sloppy repairman previously, damaging it. Fortunately the emitter lead was more than long enough to go to the other component in that short trace, so I merely cut the lead long enough to extend to the next point in the short trace and soldered it at that point.

I then flipped the receiver right side up, plugged my set of cheap test headphones into the receiver, plugged the receiver into my test bench power strip and...

Auto Magic now works as it should! When I touch the tuning shaft, the meter turns white and AFC goes off, allowing me to tune in a station. When I let go of the tuning shaft, the meter turns green again and Auto Magic locks the station into place.

A big thanks to Mondial for all of his help with this receiver.

I just wish I had those 3mm LEDs so I could replace the pointer lamps and put it all back together. They will arrive, eventually.

...I wonder if 5mm LEDs would fit, or if they would be too big...I have some 5mm LEDs in red and in white...

Edit: No, 5mm LEDs will not fit. I took the pointer apart to check. They will have to be 3mm. I will have to be patient and wait.

[Ron Ramirez]

I've finally received some 3mm LEDs, so hopefully this weekend I will wrap up the work on this STA-225.

[Mondial]

Ron, did you ever try installing the protection diodes on the knob sensing circuit?

[Ron Ramirez]

Not yet. The receiver has been sitting on the shelf, waiting for the LEDs for the pointer.

I will construct and try out the protection diodes in the knob sensing circuit when I install the LEDs for the pointer.

[Ron Ramirez]

Good news and bad news...darn the luck...

Good news: The conversion to LED bulbs in the pointer worked extremely well. The pointer is now white at all times except when an FM Stereo station is tuned in, at which time it turns red.

Bad news: I did rig up a set of diodes to form a protection circuit, and installed them in the receiver. The neat feature of touching the tuning knob to make AFC and Auto Magic go out still worked when the unit was on the workbench, not in its wood case. However, after I buttoned it all up and brought it upstairs, it had stopped working...

I should have left well enough alone.

I'm not going to revisit it right away. I've spent enough time on this unit for now, I think.

[Ron Ramirez]

Well, this is the craziest thing!

Having to know if the fix would be simple or not, I took the receiver back to the bench and removed it from the case. I proceeded to undo the modification I had made to add the protective diodes.

After removing the mod, I hooked it up (out of the case), touched the terminal on the board where the tuning shaft wire connected and...the meter turned white as it should.

I then hooked the wires back to the terminals on the board (still out of the case) and...it worked perfectly.

I put the chassis back in the case and...it still worked perfectly.

Took it back upstairs, hooked it back up and...it worked once but when I touched the tuning knob again, the meter stayed green. It stopped working again!

I seem to recall reading about an instance where the tuning shaft itself had become dirty and was impeding the Auto Magic action. I wonder if that is the case here? Don't know what else to check...

But in the meantime, it sounds great hooked up to the Bozak E-300s.

Edit: Just now, I grabbed the tuning knob again and the meter turned white for about half a second, then turned green again. It sure seems like something is not making good contact.

[Ron Ramirez]

Well...I noticed while letting it play this morning, that the treble is slowly falling off. So it's time to replace electrolytics on the tone amp board. I'll also have to try and find out why the Auto Magic has mostly stopped working.

So this one isn't quite ready for prime time. In short...back to the drawing board, er, workbench.

[Mondial]

I wonder if there is anything different between the AC power or grounding at your bench location and upstairs?

You mentioned that even with the diodes installed the Auto-magic worked fine at the bench, but stopped working when you brought the receiver upstairs. Since the circuit depends on picking up the stray hum voltage present on your body, is there any difference in hum pickup at the two locations.

 Another thing you might try is reversing the power plug in the wall socket, as one side of the line is connected to chassis through the 2.2 meg resistor. Depending on whether this resistor is connected to the line or neutral side may have some effect on the hum voltage activating the Auto-magic circuit.

[Ron Ramirez]

...is there any difference in hum pickup at the two locations.

No idea.

Another thing you might try is reversing the power plug in the wall socket...

Tried that. I had it plugged into an APC P11VNT3 surge protection power strip. Tried both ways. In one direction the tuning meter turned white once for about half a second, the stayed green.

I then tried plugging it direct into the wall outlet. In one direction, Auto Magic would not work at all. In the other direction, it worked for about three seconds before quitting again.

Well, it will be taken back to the basement. I need to replace all of the electrolytics in the tone control board anyway. When I get ready to do that, I will also do some more research into the Auto Magic circuitry. Perhaps I should attempt to remove and clean the tuning control shaft and its associated collar. I wonder if it originally used some sort of conductive grease, and perhaps that is failing, causing it to block rather than transmit the stray hum?

[morzh]

Just my two cents: BJT does not need any static protection. If however you want one so bad, buy an esd surge suppressor diode, that's what they are made for.

[Ron Ramirez]

Mike

Come on, man! Read post #40 again. Actually I didn't really want it at all. I was merely following Mondial's suggestion while I had the thing apart. I am actually quite happy with leaving the circuit original....and I came >thisclose< to not even bothering to add the diodes (which are now gone).

...hmmm...wonder what would happen if I increased the size of the 0.47 uF coupling capacitor C701 between the tuning knob and the base of transistor TR701?