Ok, Tek did a lot of "referenced" designs.? In the scopes, typically a -50 volt supply is fully adjustable, and the other supplies depend on resistor tolerances to get the right voltages. If the -22 volts is off, as you say, then everything will be off. Don't remember the design in this, but check that first.? if the -22 is off, then *it* might be loaded too much.
Looking at the schematic:
1) R680 is the current sensing resistor, so measure the drop across this.? Typically, the capacitor manufacturers told Tek that that their parts could be run quite close to the voltage limit. Say 6.3 volt capacitors on a 5.0 volt line.? They were wrong.? I'd look on the -22 volt supply for overloads pulling that down.
The voltage drop on R680 has to be less than 0.6 volts, otherwise Q690 turns on and robs base drive to the pass transistor
If 6 and 7 are high on U490, then it presets the counter to 9, and I suspect that the detect circuits cause the display to flash, indicating over range.? So those being low makes more sense in a non over range situation.
What it looks like is that the counter chain is not getting any pulses, so nothing to count.? I'd be happier fixing that 5.2 volts, since Tek ran the TTL a bit hot at 5.2 volts.? You could track the signals through from say, test point #3, but without proper voltages?? I'd not get upset on that on.
Harvey
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On 2/16/2024 6:20 AM, Frank Mashockie wrote:
@Harvey
Measured voltage across 0.33ohm R624 = ~0.35V. So ~1A. Seems normal.
One thing I learned reading through the circuit description is that 5V supply is referenced from the -22V rail. Would it be a good idea to inject -22V to see if this brings the 5V rail into sepc?
I was looking at the display/counter circuit more closely - everything was looking good until I got to the state generator timing relationship waveforms. They all match except pin 12 438D - there is no reset pulses. Also, pins 6 and 7 to counter U490 are low. According to the ciruit description, these should be high.
Still need to do more digging/inspecting. Thanks for everyone's help!
-Frank
