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Greetings!

I am working on restoring an SC504. It is cosmetically perfect, but has other issues.

Channel selection is not working. Channel 2 is always selected. The vertical positioning is also not working. I have experienced different symptoms over the last few days. Channel 1 would move both traces while Channel 2 functioned properly.

At this moment Channel 2 is displayed regardless of the channel selector. Channel 1 is not displaying at all.

Any ideas?

You might want to verify that U2170 is working. It is a JK flip-flop and it's in charge of channel switching. I have an SC502 that had this issue with channel switching and replacing that IC fixed it.

Benjamin

Benjamin

Thank you Benjamin!

I had already tested the U2170 and it tested good for logic. I will replace it and see.

Barry

Benjamin,

I swapped U2170 from an SC502. The SC502 still works and the SC504 is still not working.

Barry

CLA Clean Lube adjust all controls, switches

J

Jean-Paul,

I did that a few days ago. Thanks!

Barry

Can you verify that pin 13 of U2170 is low when the channel switching is set to CH1? Pin 2 should be high.

The junction between R2176 and R2275 should be -5V in this position too.

Pin 3 of P1570 should be at +5V as well.

Check if the voltage at the anode of CR1457 changes when switching between ch1 and ch2. Same with the collector of Q1555.

Benjamin

On Sat, Feb 1, 2025 at 03:49 PM, Benjamin Badrakh wrote:
Benjamin,

Thanks for the help!

Can you verify that pin 13 of U2170 is low when the channel switching is set
to CH1? Pin 2 should be high.

Pin 13 is low and Pin 2 is high.

The junction between R2176 and R2275 should be -5V in this position too.

The junction between R2176 and R2275 -is 0V. (The switch is working, -5 on both sides.)

Pin 3 of P1570 should be at +5V as well.

Pin 3 is at +5V.

Check if the voltage at the anode of CR1457 changes when switching between ch1
and ch2. Same with the collector of Q1555.

The anode of CR1457: CH1=-0.65, CH2=-0.37
The Collector of Q1555: CH1=+1.12, Ch2=+1.01

Again, thank you!

Barry

Hey Benjamin, did you see my responses?

Thank you!

Barry

Benjamin,

I made an error in my previous response.

The junction between R2176 and R2275 is -5V.

Barry

On Sun, Feb 2, 2025 at 07:21 AM, Barry Breaux wrote:

...
The anode of CR1457: CH1=-0.65, CH2=-0.37
The Collector of Q1555: CH1=+1.12, Ch2=+1.01

Anode of CR1457 is the same node as collector of Q1455. Seeing -0.37 to -0.65V doesn't have an easy explanation when collector of Q1555 is ~ 1V. What voltages do you see at Ch2 hybrid, HY1430, pins 1, 3, 4, 5, 16, 17, 19?

Pin1 should be at ~ 1V when channel is disabled, ~ 2V when it is enabled. It is the same node as the collector of Q1455 but perhaps there is a broken trace between them so it is best to measure at the pins of the hybrid.

Ozan

The condition of t; his SC504 has changed. I am currently unable to see a trace from either Ch1 or Ch2. From the beginning I have been seeing an intermittent problem of not seeing either channel. If I short out the delay line from the F&I board the traces come to the center of the display. Therefore the problem is before the delay line. What I don't understand is why shorting out the delay line from the Main board doesn't produce a trace.

OZAN, pin 1 isn't ~1V OR ~2V.

CR1457 Anode: CH1=-0.645, CH2=-0.360
Q155 CH1 Select E=-0.676V B=-0.000V C=+0.708V
Q155 CH1 Select E=-0.679V B=-0.000V C=+0.688V

OZAN, these are the pins you questioned.

Pin, Ch1 Select, Ch2 Select
1, -0.65, -0.35
3, -0.03, -0.03
4, 1.37, 1.37
5, -0.01, -0.01
16, -0.57, -0.57
17, -0.05, -0.05
19, 4.99, 4.99

I took all these measuements today, but I suspect warmup was not sufficient.

Barry

My comments are inline.
Ozan


On Sun, Feb 9, 2025 at 03:20 PM, Barry Breaux wrote:

..... If I short out the delay
line from the F&I board the traces come to the center of the display.
Therefore the problem is before the delay line. What I don't understand is
why shorting out the delay line from the Main board doesn't produce a trace.

Most likely the delay line is made of a lossy coax (there is an explanation in Tektronix technical series) and it has some resistance from end to end. Shorting at the F&I board side or at the attenuator presents a different resistance to the next amp.

Pin, Ch1 Select, Ch2 Select
1, -0.65, -0.35

This reading shows an issue, according to schematic should be either 1V or 2V but this could be because of pin 5 below.

3, -0.03, -0.03
4, 1.37, 1.37
5, -0.01, -0.01

It looks these are also wrong. My guess is common mode (pin 5) has an issue, more about it later.

16, -0.57, -0.57
17, -0.05, -0.05

Is the Y position knob at one of the extremes? This can also be the result of pin5 so for now it is OK. We expect 16 and 17 to be similar and independent of channel switch in normal operation. They will deviate with position knob or input signal. However, no need to think about them for now.

19, 4.99, 4.99

This is good.

I took all these measuements today, but I suspect warmup was not sufficient.

The voltages are quite a bit off, warm up is not an issue.

Pin5 at 0V doesn't sound right, it may also be yanking pin3 and also setting pin4 ~0V. Most likely Q1462+Q1460 is pulling too much current. There could be several possibilities:
- Is J1350/P1350 connected properly? They dump current to emitters of Q1462 and Q1460. Without this current Q1462/Q1460 will pull extra current from pin 5
- It is unlikely extra current is because of bad Q1462/Q1460

I recommend:
- Look at voltages at base and emitters of Q1462 and Q1460
- Look at B/E/C voltages of Q3237 and Q3236. They are sourcing current to emitters of Q1462 and Q1460. Without them extra current is pulled from pin5 of the hybrid.

Ozan

Ozan,

J1350 is correctly positioned.

Q4660B and Q1462B are +0.672
Q1460E = -0.42
Q1462E = -0.42

Q3237E 4.2
Q3237B = 3.525
Q3237C = 0.184

Q3236E = 4.288
Q3236B = 3.623
Q3236C = 0.151

Thank You!

Barry

Greetings!

I made a little progress. One of the delay lines was open. Perhaps that was my interment problem. I fixed that and got CH2 displaying. The problem is (as before I lost both traces) that CH2 is displayed when CH2 is selected(correct) AND when CH1 is selected(incorrect).

Barry