Ok, I replaced Q1221, Q1222 and Q1223. I also verified R1212 while I had it out. Still no go - I tested all 3 transistors with my DMM and they all look like they're good. Where do I go from here? Q1220? The op-amp? Other components? What a bugger!
It seems like every time I turn the scope on to take a measurement my regulated volts keep getting lower on the 87, 42, and 15 volt lines. The 10v ref is stable. It's almost like whatever component it is, it's still continuing to slowly die.
Thanks guys!
Chris
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--- In TekScopes@..., "Tom Miller" <tmiller11147@...> wrote:
Change Q1222. It should be full off and the collector should be up around 90 volts or so. However, Q1223 could also be pulling that line down so change it also. You could lift the collector of Q1223 and see if the supply comes up just to rule that out. Also (2), is R1212 ok? If it were open, the same issue would be there.
Tom
----- Original Message -----
From: Chris
To: TekScopes@...
Sent: Tuesday, February 19, 2013 4:52 AM
Subject: [TekScopes] Re: Tektronix 2465 capacitors etc.
I had sent a response earlier but it didn't appear to have made it.
I checked some of the voltages on the board, but can't say I entirely know what to make of all of them (newb alert again hah)
Ok, I've got 94v on the unregulated rail
On the op-amp, pin 1 vref is 10.00v Pin 2 from the voltage divider is reading as ~1v or just under. Pin 1 is reading 15.19v.
Q1222: Emitter 8v, base is grounded of course, and 11.6v on the collector.
Q1220: (sorry, not sure which is emitter/base/collector, I'd have to look up a datasheet - more newb showing through). So looking down on the power supply as it's mounted in the chassis: 11.3v on the left pin, 94.7v on the middle pin (must be the collector) and 11.8v on the right pin.
Q1221: 94v on the collector, 11.1v on the base and 10.6v on the emitter.
I didn't measure Q1223 - hard to reach that one.
R1212: 94v on the unregulated rail side, 10.5v on the other side of it.
R1220: 94v on the unregulated rail side, 9v on the other side.
Curious how I got 15.19v on pin 1 of U1281A with a 10v ref and a 1v comparison from the voltage divider off the regulated line - looking at the datasheet it looks like it should be operating as two separate op-amps with one half regulating the 87v line and the other the 42v line, so I fired it up again and checked the voltage on all of it's pins:
pin 1: 15.57v
pin 2: 1.05v
pin 3: 10.00v
pin 4: -6.86v
pin 5: 10.00v
pin 6: 1.58v
pin 7: 15.58v
pin 8: 16.42
So thinking about it for a bit, the schematic shows a voltage of -1.53v for pin 1 (a far cry from the 15v I'm getting)which I assume is the voltage I should see when the line is regulated to 82v. And I assume then that the 16.42v is the positive power for U1281, and the -6.86v is the supply that lets it pull Q1222 low when the voltage from the voltage divider feeding pin 2 = the 10v ref from pin 3.
So I guess I learned something, but as far as using the voltages to give me an indication as to what's gone wrong I'm not sure. That will take a better man (or woman) that's more educated than I! I would have to resort to the shotgun repair method at this point - replace all the components :) (certainly not to the point of wanting to shoot the scope yet lol)
As a note, when reading these voltages they can vary a bit while checking them. Sometimes it's not uncommon to see some of the voltages vary ~0.5v it seems, so that accounts for some of the variance in the first vs 2nd measurements in areas. Earlier I was reading ~9.5v on the 87v regulated line, but tonight it was just under 9v.
Thanks!
Chris
