just finished my recap of sx-780

Getting ready to set up a try with the o-scope but first i put my working 780 on the bench and probed around the fm section near pins 36/37 with my handy signal tracer just to see what i should be getting. Armed with that info i went back to the non-working unit and found that the signal stops at the source on q28 and 29 but if if jump them then all's loud and clear-ish. So i assume that from what i think i understood merylnski to say, is that they are getting saturated and are not functioning they way they should? which would point back to the tuner board?
 
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Ok..so I've read many times on here before that the only dumb question is the one not asked so I'm gonna set an AK record.... Here goes:
Tek 465
set to channel 1...ok
volts div set to 2...ok
i assume dc?
probe connects to left top channel 1 port?
1x setting on the probe or 10x?
time delay position?
coupling set to dc?
source set to channel 1?
time/div delay set to 10 (u)s..ok
red var. knob turned all the way clock-wise puts it in un-cal?
trigger mode auto..ok
power on...lol ok
I am so embarrassed to have to ask you all of this but I have to unfortunately. Instead of saying that I'm learning how to use my scope, I should have said I have no idea how to use my scope. I hadn't really planned on using up till this point. Thank you for your understanding
 
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d.russell said:
Ok..so I've read many times on here before that the only dumb question is the one not asked so I'm gonna set an AK record.... Here goes:
Tek 465
set to channel 1...ok
volts div set to 2...ok
i assume dc?
probe connects to left top channel 1 port?
1x setting on the probe or 10x?
time delay position?
coupling set to dc?
source set to channel 1?
time/div delay set to 10 (u)s..ok
red var. knob turned all the way clock-wise puts it in un-cal?
trigger mode auto..ok
power on...lol ok
I am so embarrassed to have to ask you all of this but I have to unfortunately. Instead of saying that I'm learning how to use my scope, I should have said I have no idea how to use my scope. I hadn't really planned on using up till this point. Thank you for your understanding
Click to expand...
I spent so many years using a 465b that a different scope makes me have to stop and think. I don't much like the digital one I have now, but it was a gift, so . . . I would love to have a 465/475. Anyway here is my commonest starting take on your question about settings:
Tek 465
set to channel 1...OK
volts div set to 2...Adjust to see signal, as needed, remember which setting your probe is on, if the scope does not see selected range (depends on the probe).
i assume dc? If you are (intending on) looking at a small AC signal riding on a DC voltage, you would use AC, as the larger DC component will push the trace off the bottom or top of the screen. AC is the most usual selection.
probe connects to left top channel 1 port? Yes for channel 1 display
1x setting on the probe or 10x? I use x10 probes mostly, not switchable ones. A x10 probe loads the signal (less) with a 10MΩ resistance, a x1 probe (or direct clip lead) loads the signal with the scope's nominal 1MΩ input impedance. You can see very small signals better with x1, but may distort signal when measuring at a high impedance point in a circuit.
time delay position? Lock (time/div) knobs together on the same position. This is a special function that is almost never used, but is indispensable when actually needed, lets you scroll through a delayed sweep.
coupling set to dc? This depends on what you are looking for, you can chose + (rising edge), - (falling edge). Pick DC if you want it to trigger at a specific dc level, set it to AC is simpler, then adjust the trigger level for a clear stable repetitive display.
source set to channel 1? If you are looking at Channel 1 it must be set to Channel 1, unless you have a different trigger source selected, on purpose.
time/div delay set to 10 (u)s..Select setting that is most useful for the signal you are looking at on the display.
red var. knob turned all the way clock-wise puts it in un-cal? Normally left in Calibrated position (full CCW, click), so you can measure signal timing off the divisions on the screen. Sometimes you may use uncalibrated to look and compare, but you cannot measure time periods with the screen divisions in that mode.
trigger mode auto..In Auto the trace sweeps across the CRT face whether there is a valid trigger signal present or not. Unless you WANT to trigger on a specific event Auto is usually OK
power on...lol OK
 
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Thank You Sir...I know that was a big ask, too much to ask actually...I've been toying with it awhile and watching videos. when I hook it up to my cd player (bnc to rca adapters) I get a trace on both channels seemingly fine but when I hook it up to my fm board on pin 23 as watthour suggested I get just a straight line. I obviously assume its because I'm not doing it correctly. I've made sure to use a good ground and I have 2 new probes so I may just have to put this on the backburner awhile until I get smarter.

I think I will try cleaning the tuning fins and board with the CRC cleaner and see if that makes any difference. I'm going to tinker with the scope some more, watch a few more videos and see what I can do with it.

attaching the probe to the speaker terminals also produces a trace so i don't know why i get nothing off of pin 23...i've even tried this on my working unit and i get the same result.
 
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d.russell said:
Boy do i hope this makes sense...the first pic is the square wave from the onboard generator:
View attachment 2092194

The sine wave is from the fm board pin 23 of my working sx-780:
View attachment 2092196

to me it looks like 6.5 horizontal divisions as opposed to the 5.25 to equal 19,000 hz that was suggested and the wave is sharp at the bottom...is that to be expected?
Also the square wave is crisp left to right, the Sine is crisp on the left and gets fuzzy to the right

I haven't tried any adjustments on this machine (sx-780), i'm just using it as a reference.
Click to expand...

Ok..so using the same settings as yesterday for my working SX-780 displaying a sawtooth wave, i'm testing my non-working unit. What happens is that, connecting my probe to tp23 and then turning on the receiver, i get a quick wave and then it goes flatline. Turning off the receiver has the same effect. a quick flash of a wave and then gone.
turning on gives me a flash above the mid-line and off gives me a flash below the mid-line.
 
Does this mean that you briefly get a sawtooth wave at around 76KHz, or just a general deflection of the 'scope?
 
the wave flashes just a second and is not stable, no way to measure it.

on:IMG_0873.JPG

and off
IMG_0870.JPG

weird huh? my other 780 doesn't do that
if i leave it on i get just a flat line...i think i've detected a tiny ripple in the flat line but it's hard to tell
 
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Try setting the Tek to AC trigger coupling if it isn't already.

It appears that the relative amplitude and frequency are about right - If only for a few moments. The key is going to find out why that goes away as the power supplies come up to full voltage.
 
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Are the caps C36, C37 & C38 testing intact, or leaking like a Russian intelligence agent?

EDIT: I changed the cap numbers. It would also be good to check the voltages to the IC both at Pin 1 and Pin 16. .
 
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Lol....I will check those caps and voltages in the am. The Tek is already set on ac coupling
 
on the pa-1001:
pin 16=1.86v v 3.4v per schematic
pin 1=12.70v 13.0v per schematic

i will have to pull and test caps later ( i have a peak lcr45). i gotta go bang on the house some first.
should i just replace c37 and c38?
 
The voltage at Pin 16 is remarkably close to the 2V necessary to shut off the 76KHz oscillator in the PA1001A IC. When the Mode switch is in the AM position, that voltage should be 3-ish volts, but when in FM it should be almost ZERO.

Verify that the Mode switch is clean, and if that is intact you might want to lift one end of R34 to test resistance and see if the internal oscillator functions with that resistor lifted - No voltage at Pin 16 of the PA1001A.
 
i disassembled and rechecked the muting switch...a-ok there.
the mode switch i had previously disassembled and cleaned after all this started so, it's good to go.
i lifted the resistor and tried for sound and nothing..not sure what you meant about testing the oscillator..just sound or use the scope?
r34=62.94k schematic says 62k
c38 i just replaced with a wima, 1uf/50v
c37=7.500uf tant cap reads 6.8uf
c36= 501.pf schematic calls for 510pf

pin 16 measures 1.9v when set on fm and 2.40v when set on am.
 
I was thinking that if R34 could be taken out of the circuit PA1001A Pin 16 should then drop to almost zero volts. In that condition, connecting the scope to measure the 76KHz oscillator (TP23) could be very revealing.
 
Watthour said:
I was thinking that if R34 could be taken out of the circuit PA1001A Pin 16 should then drop to almost zero volts. In that condition, connecting the scope to measure the 76KHz oscillator (TP23) could be very revealing.
Click to expand...

I sure am glad that you are here and are putting up with me! With the resistor lifted the sawtooth trace comes thru loud and clear! Almost 1.2 divisions.
it's funny, i've gone from being terrified of this scope to loving it! I can't wait to find other uses for it. lol
 
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So we've established that the VCO works properly when the VCO "killer" input on the IC is close to zero volts. That's as it should be, and for some reason the signal at Pin 16 is still near 2V when it should be essentially off.

Keep R34 disconnected for now.

As a second test, please report the voltage measured at the inboard end of R26 (connected to Pin 2 of the PA3001-A) in both AM and FM modes.

If we proceed backward, there are diodes in the circuit (D1/D2) which should be blocking any DC from leaking around the circuit.

It could also be informative to measure voltage at Pin 11 of the AWE-099 Tuner board when in AM mode and in FM mode. If one of the switches has leakage, either Mode or Muting, that could be a problem.
 
Inboard R-26 to pin 2 Pa-3001-a:
Fm muting off:
Fm=2.57v
Am=2.88v

Pin 11:
Fm= 10.34v muting off/ muting on=10.25v
Am=13.47v

not much change on or off
.
 
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