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REALISTIC STA-78 No Stereo Light

JB8

JB8

Well-Known Member
Squad,

I've been on a little run of finding and replacing bad transistors this last week and brought three units back to life thanks to your help - a Concept 2.5, HK 330c, and most recently I've been working on an AA-6600 that's close to being done, so I thought I'd pull out a STA-78 I'd been working on last year but shelved the project because I couldn't track down a bad transistor. I pulled it out today and found/replaced the bad dudes in record time, again, thanks to this forum.

Backstory: I've replaced the main filter caps, power supply caps and the caps on the tone phono board. Today after replacing the three transistors in the picture (TR601, TR603 and TR605) the receiver is singing like a bird and is dead silent with volume at zero, sweet! (I also hard wired the meter light into the main array along with installing a 47 ohm resistor to help accommodate the LED's)

Issue: No stereo light even with stations coming in nice in stereo - in other words, I'm hearing it, just not seeing it.

I pulled the old LED and couldn't get it to light up when removed from the unit, but also attached another new LED to the leads and it wouldn't light up either even though I know it's a good lamp.

Here are a few voltages from the power supply:

8) .03 v
9) .50 v (when tuned into known, strong stereo signal)
13) Pre 35.25 v
14) Tone 36.50 v
15) +B 29 v
16) -B 29 v

With the voltage higher than ideal on 14 (should be 29 v) is is safe to think the issue may be with TR701 or I'm I way off?

Any advice is much appreciated!

STA-78 Headshot.jpg STA-78 Transistors.jpg STA-78 PS.jpg STA-78 PS Schem.png

WARNING: I'm still pretty new to the scene and I ask some pretty dumb questions, so thanks in advance for your patience.
 
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Do you have a freq counter ? Probably need to tweek in the 19khz vco . VR205
Step 1 of the Multiplex/Separation alignment
 
JB8 said:
9) .50 v (when tuned into known, strong stereo signal)
Click to expand...
If no freq counter. Mark where VR205 is presently set. Then hook you multimeter to pin 9. When tuned to a strong FM stereo station adjust VR205 for maximum voltage at pin 9. Probably around 13 vdc.
 
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Yessir, I believe my UNI-T will measure frequency, would just need to know where to position the test leads. I've used this vco on other units to get the stereo light to turn on but have never done it the correct way, would love to better understand this and eventually be able to do a full tune up.

I digress, how do I measure the frequency with the meter?

UNI-T HZ.jpg
 
Connect the red probe to pin 12 0f IC202 and the black probe to chassis ground.
 
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Thank you, as always, @avionic.

I attempted both methods, here are my results:

1) Multimeter on pin 9 on strong station - adjusting VR205 in either direction showed diminishing voltage. Highest recorded is still .50 v in the middle of the variable resistor, lots of room on either side for adjustment
2) MM grounded to chassis, in Hz mode, touching pin 12 of IC202 with red probe yielded zero response, meter held steady at 00.00

I checked IC202 for voltage as well:

Pin 12 = .2V
Pin 6 = .57 V (supposed to be 13.4 V?)
 
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In considering this puzzle I started thinking my workaround for the meter light could be contributing to the issue, here's my premise:

  • When I first tested the unit, a year ago, I couldn't get the stereo light or meter light to come on, so I think I gave up on the stereo light and figured I'd just tap into the main array and have the meter light on all the time, not just in FM/AM mode
  • As pointed out in the picture, I disconnected the yellow lead from the meter light and just bridged up to the main array, it worked great, all green LED's glowing nicely
  • Today pulled that bridge wire and informally wired it as it should be and got the meter light to come on based on function selector position, and when it was on, the voltage at Pin 9 on the PS board read - 2 V, and of course, still no stereo light
Not trying to complicate things, instead I'm trying to untangle them. Question here is, are these two lighting systems so intertwined that one would effect the other?

STA-78 Meter Lamp Wiring.png STA-78 Lighting.png
 
I´ll bet R701 or R702 is " open" .. Or Ss-3 is dirty or has a broken wire.
 
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Thank you both. I officially reconnected the jumper so I could make sure it was back to the original configuration - still no stereo light or meter light, so I'm going to replace R701 and R702 since I'm getting good continuity from the other leads and see how it goes.

I'm not sure what was different when I had it hooked up yesterday when changing the function to FM and AM actually got the meter light to come on, but now it's not, and I know the connections are good at the lamp and that the lamp is good... Thank goodness it sounds fantastic, keeps me excited to close the loop and complete the project!

Voltage at pin 8 remained at .03 v when tuned into a strong signal. Pin 9 stayed at .50 v as well.
 
I like where your heads at, @petehall347, you may have seen in the pic of the power supply that I'd replaced F702. When I just barely touched F701 one of the holders started to butterfly and fall to the side... Great call.

I'll be working to swap that out and report back if anything has changed - resistor order is ready to go if I still have to keep digging.

Thanks again!
 
Whelp, no luck, no change. But, now I know that's not the issue.

Placing the resistor order now, and if I replace them (R701, R702) and it's still not fixed this will become a more valuable thread if I/we can eventually get this sorted!

Ironically I have another STA-78 being delivered today, it's supposed to be in great working order, so I'll take some measurements so I can have something tangible to reference and something to shoot for - failure is not an option!
 
avionic said:
Did you even bother checking them ?
Click to expand...

Of course not! Damn, I really do have a lot to learn...

Both tested fine. No resistor order placed.

Main array of lights are still working great, just no meter light when in FM/AM, and no stereo indicator when on strong station I know to be coming through in stereo - I can hear the separation, just no light (light has been tested and working).
 
Measure the resistance from pin 7 on the power supply board to chassis ground with function selector on FM. See if your getting continuity through the wafer switch.

No power applied....!!!
 
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With power off, probe on pin 7, other probe to chassis ground, function selector on FM, when first connected I get a quick beep, then the light on the meter turns red and the display eventually shows ~495 ohms.
 
Do you have the meter LED installed with the correct polarity ? The most positive voltage goes to the anode ?

led-series-diagram.png
 
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