Tek 453 - Power supply

[dan3460]

Hey guys, looking for an explanation for the power supply for the above scope. This is my first 453 and puzzled by the design of the power supply. All my voltages are high and I understand that all voltages hang from the -12V power supply. In my case this is running about -15V, which makes all supplies high ending with the 75V that is running at about 88V.

By reading the theory of operation and by measuring voltages I suspect that Q1124 is faulty. I grabbed my cheap Chinese tester and it shows that Q1114 and Q1124 are ok but with a beta of 50 and 62. I ordered the transistors and I'll replace them on the circuit to see what happens. I do not suspect that Q1133 and Q1137 are defective but I have not been able to test them as they are mounted on a platform under the power supply board.

In the meantime I decided to investigate the design. I try to simulate the circuit and I cannot make it regulate at -12V. On my simulation I left out Q1129 as it is just a protection transistor. In the simulation the output always stays at around -14. Q1124 is always cutoff as I cannot make the base of Q1124 -9.1V.

I have been using a circuit simulator in http://www.falstad.com/circuit/, I have simulated many power supplies to understand their workings but this one just stomp me.


Can anyone explain in lame terms how this circuit operates?

[strawberry]

R1157 bleed rezistor paralell to Q1137 may prevent from good simulation resaults. Did you compare all voltages in -12V supply? Sometimes fix hide in unsuspected place.

[VK5RC]

Can you measure the voltage drop across what I think is a current limiting resistor R1157 hence determine if there is excess current draw? I can't read what is written on the circuit above q1155 is it 0.5A ?
Aren't q1155 and q1137 the main power regulators, presumably off circuit board and on the chassis for heat dissipation, I would look at those , ? has one having shorted collector to emitter .

[dan3460]

R1157 bleed rezistor paralell to Q1137 may prevent from good simulation resaults. Did you compare all voltages in -12V supply? Sometimes fix hide in unsuspected place.

Yes, on my simulation I left out R1157 and also Q1129. I did compare voltages and by memory: the base of Q1114 was at the indicated -9.1 the voltages at the emitters was close the indicated -9.6, the base of Q1124 was complete off sitting at -2V aprox. Voltage at the collector of Q1114, Q1129, base of Q1133 was sitting at -9.5 and the voltage at collectors of Q1133 and Q1137 was sitting at about 2.5V. Posting expanded view of the circuit in next post.

[dan3460]

Can you measure the voltage drop across what I think is a current limiting resistor R1157 hence determine if there is excess current draw? I can't read what is written on the circuit above q1155 is it 0.5A ?
Aren't q1155 and q1137 the main power regulators, presumably off circuit board and on the chassis for heat dissipation, I would look at those , ? has one having shorted collector to emitter .

Thanks for the answer here is a better view of just this part of the circuit.
I think you referring to R1137 (on my original post is hard to read) I have a drop of about 2.5V, so I was thinking that there is too little current draw, therefore I think that Q1124 is acting up.
On top of Q1133 shows +5.95.
Yes Q1133 and Q1137 are the main power regulators but I thought that they are slaved to the combination of Q1114 and Q1124 that set the voltage drop. It has been know that many times I complete wrong.

[dan3460]

I was thinking and I think you are right VK5RC. After I wrote "...I think is too little draw..." and I looked at the diagram, the way to increase the draw is by activating Q1133 and Q1137. Those two transistors are easily found, I think I will order some just in case.

[siggi]

Can anyone explain in lame terms how this circuit operates?

Q1114 and Q1124 are a differential pair. When in-regulation, the voltage at the bases should be near-identical. D1114 provides the reference of -9.1V, and R1121, R1122 and R1123 provide a scaled sample of the output for the diff pair.

Q1133 and Q1137 are the darlington pass element. They get base current from R1117, which is robbed by Q1114 to keep it in regulation.

R1129 is a current sense resistor, and Q1129 is the over-current "comparator". When the voltage drop across R1129 exceeds its VBE, it'll also rob base current from the pass element. Looks like the overcurrent protection should hit around 1.2-1.4A.

Note that due to R1137, you won't achieve regulation unless there's a sufficient load on the output to soak up the current going through it.  I(R1137) = (input voltage - 12)V / 50Ohm. The schematic is fuzzy, but it looks like the input to the pass element is tagged at 5.9V, which would mean you need a minimum load of ~120mA.

I sketched this in LTSpice, and it regulates just fine once the minimum load is satisfied.

By reading the theory of operation and by measuring voltages I suspect that Q1124 is faulty. I grabbed my cheap Chinese tester and it shows that Q1114 and Q1124 are ok but with a beta of 50 and 62. I ordered the transistors and I'll replace them on the circuit to see what happens. I do not suspect that Q1133 and Q1137 are defective but I have not been able to test them as they are mounted on a platform under the power supply board.

It should be fine to replace Q1114 and Q1124  with just about any jellybean small-signal NPN transistor you have at hand for testing at least.
Also note that you should be able to test Q1133 and Q1137 in circuit, as you have access to all their leads through the interconnects. You can also see whether they're grossly bad by measuring the voltages at "I" and "L" while operating. Point "L" should be one VBE above ground, and point "I" should be two.

[w2aew]

Don't forget to check simple things, like the resistor values.  I had a power supply problem in a 465B years ago, where a resistor had changed value for no apparent reason. 

[dan3460]

Thanks Siggi for the detail explanation. What you said about your simulation on LTSpice gave me an idea. I know that the scope has other faults, I started with what I always start when repairing equipment, making sure that the power supply is working ok. I would have to study the schematics a little more, but maybe the circuits that supposed to draw from the -12V are disconnected or broken and they are not drawing any current from the power supply.

W2AEW, I check some resistors and those have been within tolerance. I will check all resistors. Thanks.

[dan3460]

Yesterday I got the 2N930 to replace the ones in the board. It was no change, so I have to wait for the other transistors to come.
Another question for someone that have worked on this scopes: How do you get to the frame that holds the transistors under the regulator board? I look on the manual but could not find a reference to that.

Thanks guys.

[mzacharias]

A little off-the-wall, but I got fooled once by a crap multimeter. It's battery was low resulting in a much higher than expected reading on the display. An expensive part got replaced for no reason. Since then I stick with higher quality meters (mostly Fluke) for anything important. (The meter functioned normally with the battery replaced.)

[strawberry]

R1121 or R1122 or R1123. can not show -2V in any case if output is -14V

[dan3460]

Thanks for replays, my multimeter has a low battery indicator. I just measure another project and gave me the correct voltages. I got one of the power transistors and waiting for the other one to continue troubleshooting.
In the mean time, how do you get to the platform where the power transistors are located, I did see explanation in the manual.

Thanks

[VK5RC]

If getting to the transistors are too hard, use the diode function on your meter (with new batteries - or if you have/borrow a transistor tester) on the leads as they come out.

[dan3460]

I will test the transistors before I dig in, but I strongly suspect them. In any case, does anyone knows how to get to the power transistors without dismantling the whole scope?

[VK5RC]

If I recall correctly , I think you get to those with the rear panel off, in mine, the feet (on the rear panel) screw off taking out the rear panel (and the voltage selector mechanism) but also a rear cast metal chassis component, came off relatively easily I recall. It was pretty close to the fan mechanism I was trying to get at.

[dan3460]

ok thanks.

[dan3460]

Indeed was easy to replace the Q1137 (2N3055), the only thing that I had to unsolder was the connections to the 2 legs of the transistor. Did not take me more than 5 minutes.
After replace I could adjust all voltages to the correct value and the scope came back to life. There are some dirty pots and switches but it is working great.

Thanks to everyone.

[VK5RC]

Great to hear, Saved another classic from the scrap heap, I recall they were really made for IBM techs to take to service mainframes and could fit under an airline seat and the scope itself could be repaired on the job, hence the easy to remove covers but robust frame etc.
Don't ask me how I have 4 of them! 1 working very well, 2 mostly, 1 parts unit.

[PaulAm]

I had a 453 for years and liked it so much I got a 454 (150MHz) and restored and calibrated it.   I have other scopes, including DSOs, but I like the feel of the controls.  It tends to be my goto scope when I just need a quick look at something.

I still have the 453, it was retired out to the shop.  I was just using it the other day to look at a tach signal going to a flaky tachometer.

[dan3460]

Indeed, this are nice scopes. I like the way they were built. My only problem with them is the small screen, my DSO is so big compared.

This weekend I will go over it, I have to clean pots and selectors and I will do a full calibration.

Cheers.

[dan3460]

In case that anyone is interested, just finished with the scope. I cleaned all contacts and knobs, did a calibration and works fine. Here is a picture of a 2 Mhz square wave which has a period of 0.5ns, the scope is set on 0.1nS per division, this is the highest setting that it will go. The specs reads a 50Mhz , but you have to use the X10 magnifier to be able to see the wave.
I will clean the face a little, but I'm happy with the results.

Cheers,

[VK5RC]

The tube looks in quite good condition, Congrats re 'new' scope. 

[Martin.M]

put a drop silicone oil on the little fan motor,
when this stops, your psu get some problems.

greetings
Martin