SX-525 FM muting and stereo indicator

SA-300

New Member
I have an SX-525 that I've just finished recapping. When I went to replace the burned stereo indicator bulb I found that the there is a constant ~10v across the bulb whether I'm tuned to a station or not. I also found a constant .8v at the base of Q12, the npn transistor that drives the indicator bulb. The schematic shows .8v too; is that supposed to be when the tuner is tuned in to a station?

Looking at the schematic I see that there is only a .47uF capacitor between the base of Q12 and ground. How is this supposed to work? How does the Q12 base ever get close enough to ground to shut off Q12 and turn off the indicator light? Does the capacitor get slowly discharged by the base current? Some SX-626s (with the AWE-011 tuner) have a resistor in parallel with the capacitor which I presume would pull the base to ground when the stereo indicator light driving transistor should be off.

Looking for a reason for the indicator to be stuck on I checked Q12 for shorts (in circuit) but didn't find any. I checked the base of Q11 (which pulls up the base of Q12) and found that it's voltage swings between 2.2v and 2.9v as the tuner moves between stations. The swing seems appropriate, but should the range be lower so that Q11 can shut off completely?

Another clue might be that the FM muting switch seems to have no effect. I don't think it ever engages judging from the hiss I get between stations. I checked for continuity in the switch and it seems to be behaving. After studying the schematic some more I can't really sort out how the muting or indicator light should work. Turning muting on seems to add an extra capacitor to pin 29 of the tuner, and force the signal from Q9 through an extra transistor/transformer stage. I imagine that should raise the threshold of the signal the tuner will respond to, but I don't really see how. Can anyone elaborate on the theory or suggest what might be wrong?

Thanks,

/Mark
 
... How does the Q12 base ever get close enough to ground to shut off Q12 and turn off the indicator light? Does the capacitor get slowly discharged by the base current?

C82 should be a 0.47µF electrolytic. The circuit operates near 19KHz, and the capacitor begins to react like a resistor. The matching transformer (T14) outputs two segregated signals (channels) and those outputs drive the base of Q11. The rate of oscillation is a function of T13, T14, and T15, and those need to be adjusted to best amplitude at 19KHz.
 
C82 should be a 0.47µF electrolytic. The circuit operates near 19KHz, and the capacitor begins to react like a resistor. The matching transformer (T14) outputs two segregated signals (channels) and those outputs drive the base of Q11. The rate of oscillation is a function of T13, T14, and T15, and those need to be adjusted to best amplitude at 19KHz.

Hmm. If the transformers are out of adjustment doesn't that imply that the tuner, and not just the stereo indicator, is affected or perhaps less sensitive than it could be?

Might that explain why the FM mute doesn't seem to work?

I don't have an FM signal generator. Is that the only option for trying to adjust the transformers? I've aligned AM and shortwave radios, but never FM.

Thanks,

/Mark
 
I have an SX-525 that I've just finished recapping. When I went to replace the burned stereo indicator bulb I found that the there is a constant ~10v across the bulb whether I'm tuned to a station or not. I also found a constant .8v at the base of Q12, the npn transistor that drives the indicator bulb. The schematic shows .8v too; is that supposed to be when the tuner is tuned in to a station?

Looking at the schematic I see that there is only a .47uF capacitor between the base of Q12 and ground. How is this supposed to work? How does the Q12 base ever get close enough to ground to shut off Q12 and turn off the indicator light? Does the capacitor get slowly discharged by the base current? Some SX-626s (with the AWE-011 tuner) have a resistor in parallel with the capacitor which I presume would pull the base to ground when the stereo indicator light driving transistor should be off.

Any voltage above 0.6v on the base of Q12 causes current flow through Q12 from collector to emitter, turning ON the lamp.
Period.
Your symptoms say q12 is dead.
period.
2sc373: npn 35v 100ma 0.2w 70-400hfe 80-200mhz Ft

replace it with:
512-KSC2383YTA to-92L ecb 160v 1a .9w 50mhz 160-320hfe

and yes, the higher the gain of the transistor, the longer the lamp will take to turn off.
the transistor base current discharges the capacitor.

the 626 you reference has a time constant of 0.016 seconds, with 33k and 0.47uf
0.8v / 33000 ohms = 24uA 50ma lamp / 100 gain = 500uA, /400 gain 125uA

older caps were leakier, and they found quality improving unexpectedly. Gain went up too.
If the lamp refuses to turn off with newer caps and transistors, add the 33k resistor across that cap...
 
Last edited:
Any voltage above 0.6v on the base of Q12 causes current flow through Q12 from collector to emitter, turning ON the lamp.
Period.
Agreed.

Your symptoms say q12 is dead.
period.
This part I don't follow. Which symptoms say it's dead? Just the fact that the indicator doesn't go off? Ordinarily I'd expect a steady .8v on the base to keep the indicator on, which is what it's doing.

Either way, that's good news since it's an easy fix, and I have an extra KSC2383.

Thanks,

/Mark
 
damn....

punchy...

indicator won't turn OFF.

New cap? original cap? original transistor?

just try the 33k resistor...
 
New cap? original cap? original transistor?

just try the 33k resistor...

I replaced just one cap on the (AWE-003) tuner:
C35
.22uF/25v CSSA (Sanyo light blue) cap
647-UKL2AR22KDD Aluminum Electrolytic Capacitors 100volts 0.22uF

Everything else, including Q12 is original.

I tried adding the 33k resistor and it seemed to have no effect at all. I should point out that I have a temporary bulb with a series resistor in place of the indicator bulb which glows pretty dimly. While trying out the 33k ohm resistor between Q12's base and ground I noticed that I could get the bulb to dim by tuning back and forth (with and without the 33k resistor) but it wasn't easy. It seems like Q12 is trying to shut off but that it barely gets below its threshold voltage.

Also odd is the fact that the stereo/mono FM switch position makes no difference to the indicator light. The audio definitely changes, but the indicator stays on just about all the time.

Could this be a symptom of the FM mute not working (as described in the base note)? It seems suspicious that the replaced cap (C35) is on the signal path to Q9 which drives the signal through the FM mute switch.

/Mark
 
Last edited:
sigh... temporary bulb? what type - voltage and current and how many ohms of series resistance.

If the draw is less than 35 - 50 mA the circuit balance between discharge and charge is upset... a transistor is a current RATIO device. Less collector current, less base current.

the 33k is predicated upon that 35 - 50 mA current draw.

as for the stereo / mono question:
stereo / mon pushbutton only mixes the left and right channel audio just before the balance and volume controls in the control amp.

the FM mono and FM auto on the FUNCTION SELECTOR SWITCH might have an effect.

c35? it comes off of the IF, before the Q6 ta7060 detector. Audio shows up at "A", tp1...
Looks like the signal strength meter to me.
 
If the draw is less than 35 - 50 mA the circuit balance between discharge and charge is upset...

I didn't realize the Q12 driver behavior was dependent on the collector current. I don't have a replacement bulb (unless I borrow one from the SX-626). I only have an LED substitute I got from another AK member. It stuck on too when I tested it so I put it aside and used something more disposable. I'll try the bulb again with a series resistor that will get me the 50ma current.

the FM mono and FM auto on the FUNCTION SELECTOR SWITCH might have an effect.

It's the FM mono/auto selector switch that doesn't affect the stereo indicator light.

c35? it comes off of the IF, before the Q6 ta7060 detector.

I don't see that. I see C35 between GND and pin 21 (upper right), which is tied to pin 24 (lower left), then through R58 to the base of Q9. (AWE-003 tuner, right?)

/Mark
 
I measured the resistance of the good stereo indicator (6v 30ma) bulb I have (installed in my SX-626) as 23.8 ohms. I then put a fine adjust potentiometer in series with an arbitrary, ~4 ohm bulb I have, dialed in 23.8 ohms for the pair and installed that in the SX-525. The bulb in that hack did vary as I tuned across the dial but it rarely went out. Maybe that's just the way it's supposed to be given that this is apparently a resonant circuit rather than a dc circuit.

Once I convinced myself that the temporary bulb was working, I swapped in the LED replacement I bought and that stayed on pretty constantly as I tuned back and forth. It flashed off just for an instant a couple of times as I tuned back and forth it but wasn't really consistent or reproducible. I think I'll need to find an old school bulb.

Lesson learned: the AWE-003 stereo indicator bulb circuit is dependent on the bulb.

Thanks for the help,

/Mark
 
You can use the LED as long as the circuit passes 30 to 50mA, use a shunt resistor across the led and resistor combo, because the LED won't like that much current, as well as being too bright....

Don't go by the lamp cold resistance, it drastically changes as it heats up to glowing temp.

Figure a Q12 saturation voltage of about a volt with the transistor conducting, it could be less, but the calculations won't be far off.
 
Back
Top Bottom