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Old 20th January 2013, 03:05 PM   #21
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K strange stuff seems to be happening.

Earlier on it was mid 4v now its 2.4 and goes up and down. it is not constant. Pin 3 of the IL393, D206, and pin 2-1 on output card have this fluctuation. Could a cap be the problem?
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Old 20th January 2013, 03:38 PM   #22
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I did a quick recheck of all the PS IC voltages... here they are:

KA7500BD
Pin 1: 6.01 (high)
Pin 2: 2.519
Pin 3: 4.25
Pin 4: 3.574 (high?)
Pin 5: 1.546
Pin 6: 3.658
Pins 7, 9, 10 & 16: 0.000v
Pins 8, 11 & 12: 12.64v
Pins 13,14 & 15: 4.95v

IL393
Pin 1: 6.60 (high)
Pin 2: 2.473
Pin 3: Started out @ 4+v and as I held the lead on it, the voltage slowly went down to 2.43v (lower than pin 2) then will jump up above to around 2.6or 2.7v then slowly go back down over and over. climb and drop, climb and drop. Whats up with this?
Pin 4: 0.00v
Pin 5: 1.025v
Pin 6: 3.096v
Pin 7: 0.118v (low?)
Pin 8: 12.64v

Pin 3 of the IL393 (IC2) appears to go to Pin 11 of the PS card and out to pin 2-1 on IC 201 (output card) via D206. They all share this voltage fluctuation problem.

Last edited by Deadly Sones; 20th January 2013 at 03:50 PM.
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Old 20th January 2013, 04:19 PM   #23
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TH1 says 20K on the schematic but I only get 8.37Kohms. Is it fine?
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Old 20th January 2013, 04:57 PM   #24
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If the fault was related to the thermistor, pin 7 would be going high.
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Old 20th January 2013, 05:11 PM   #25
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Should I lift D206? What should I test or be looking for after doing so? I know to watch current, heat, etc...
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Old 20th January 2013, 06:25 PM   #26
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D206 is for the DC offset protection. If the anode of D206 isn't being driven towards 12v, it's not the problem. You can lift it if you want to confirm that it's not the problem.
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Old 20th January 2013, 06:44 PM   #27
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The anode has 0.000v, the cathode is doing the fluctuation thing from 2.81 (sometimes much higher) to 2.4??? then back up again. So if the anode isn't being driven high, then I can rule out the Q225/226 circuit?

R263 & 264 should be 22K but I'm only getting 18.5K...

Last edited by Deadly Sones; 20th January 2013 at 06:50 PM.
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Old 20th January 2013, 06:47 PM   #28
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I would think so and since the rest of the circuit was isolated, it's unlikely that the battery had anything to do with the failure.

Have you checked the legs of all of the outputs to see if any are broken? Sometimes they break just inside the body of the transistor so it's difficult to see.
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Old 20th January 2013, 07:34 PM   #29
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Yes I did, however I just went over them all again.

They all have high resistance through all pin configurations.

The transistor readings set to diode check were as follows:

All IRF640N: 1-2: 0.961v, 1-3: 0.540v, 2-3: OL
leads reversed - 1-2: OL, 1-3: 0.586v, 2-3: 0.454v

All IRF9640: 1-2: OL, 1-3: 0.67v, 2-3: 0.440v
leads reversed - 1-2: 0.942v, 1-3: 0.539v, 2-3: OL

The amp looks brand new. Clean, no smells or obvious signs of damage. What should I look at next? Thanks

Last edited by Deadly Sones; 20th January 2013 at 07:42 PM.
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Old 20th January 2013, 08:30 PM   #30
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Set the timebase to 10us

Set the horiz display to A instead of A inten.

Set the coupling to hf reject

Synchronize the timebase knobs

Set the timebase 1 more click clockwise

Adjust the trigger level
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