Tektronix 422 low voltage PSU problems ? - Page 1
Forum section: electronics / repair

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 12, 2018, 06:40:39 pm
content: I got this beautiful 1960s scope used, saw a green dot or something few seconds months ago, but I didn't want to risk it with its 1960s electro. cap's, so I stopped until caps got here.I just recapped the LV PSU, which is modular and separates fully off the back of the scope. So it's just running its bleed resistors. So checking some voltages, the +12V reg might be bad. And the high voltage astable multi-vibrator is not working.the full schematic is on page 35http://w140.com/tek_422_ac_powersupply.pdfI'm running off 120VAC, the transformer is wired right and the outputs from it, and on the other side of the rectifier diodes, seems about right (if a little high since it's supposed to be 110V or 115VAC)In the 55V reg at the top of R655, which is 5.6k and ok, I get 79.1VDC (instead of 63V from 110VAC but I think it's ok)The bottom of R655 is 38.8V (schematic says 34.5V, IDK if thats with the scope running tho or not) But thats probably fine.Reliant on the 55V reg is the 12V regQ653Vc=26.3VVb=2.9V should be 13.2V how can that be fine, or is it just with no real load ?Ve=2.9 should be 12.5R654 6.8K is good, Q653 reads fine out of circuit(they plug in and I have continuity on the plug)The +12v output from q657 Ve is 18.6, but with just a bleed resistor, so maybe thats ok ?The HV driver is not working rightHV driverq675 (c/b/e) -14.04 -14.2 -14.5q685 -14.1 -14.2 -14.5It shows the collector waveform in the schem, but all I see is it going from -10 to -15V, at 120Hz, and a lot more like a 30degree triangle < than a Sq wave pulse generator.? I'm pretty new at this, both those BJTs read fine in-circuit, so what's going onOr what do I not know/realize ? I've measured /checked most things and they seem fine too

author: orbanp
author_rank: Regular Contributor
timestamp: April 13, 2018, 01:31:00 pm
content: First of all, I would wire the PS to the right input voltage.Pg. 9 in the documentation shows the transformer wiring for various mains voltages.Do see if you are getting unregulated voltages closer to what is listed there with e.g. the 125V input wiring.Look at the block diagram on Pg. 13 of the manual.The +12V regulated voltage relies on the correct -12V regulated output.Do check if the -12V is correct!The high voltage driver, that multivibrator, also depends on the correct -12V and +12V regulated supply!It also needs the "high voltage transformer primary" from the rest of the scope, it gets the +12V through that.You will not see correct waveforms without that transformer.So, as mentioned, do start with the -12V!Regards, Peter

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 13, 2018, 01:41:22 pm
content: The -12V is at -14V and seems fine. I should hook it up, or well move on and re-cap the scope 1/2

author: orbanp
author_rank: Regular Contributor
timestamp: April 13, 2018, 03:00:26 pm
content: -14V output for a regulated -12V is not fine!There is something amiss there!The pass transistor is a Si device, so there should not be that much leakage current that you would need a bleeding resistor!Do check the voltage across that D622 9V Zener diode.Also, do check C637, that 1uF elco, it could be leaky.Peter

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 13, 2018, 06:50:34 pm
content: Its good, 8.9V across d622, the cap I replaced is fine.-12v regK adz622 9v -5.6 -14.5c b eq633 5.7 -4.45 -5.1listed 1.27 0.64q634 -4.45 -4.35 -5.1listed 1.27 -7.5q624 0 -5.6 -5.1gnd -6.9 -7.5q637 5.8 -5.1 05.3 0.64 gndWhy is the world is the base of q653 off by so much. I don't really know what I could do a load test with on the order of 10s of watts to see how much the outputs change.I opened up the scope 1/2, it sure is a work of art. It's super clean too.

author: orbanp
author_rank: Regular Contributor
timestamp: April 13, 2018, 09:06:11 pm
content: Hi Lordvader,I guess we forgot the obvious: Can you adjust the output voltage to spec with the R639 pot?If not, what range does it adjust?Then do check R637, R638, as well as the full resistance of the R639 pot.If those are all fine, do check the rest of the components, including the transistors.If those are fine too, disconnect the -12V from the rest of the circuits, from the +12V stabilizer.You should be able to get the proper output voltage without any external additional load in the first place.Then you can worry about what happens when you load the PS. (For 12V loads you could use automotive bulbs. Just be aware that the initial load is higher as the cold resistance of a bulb is lower.)EDIT:You need +12V for the -12V stabilizer to work, so no disconnecting the +12V circuits as suggested above.The accuracy +12V is not paramount, it just needs to be at least a couple of volts. You can see it from the schematics.Peter

author: orbanp
author_rank: Regular Contributor
timestamp: April 13, 2018, 10:17:35 pm
content: Did a simulation of the -12V circuit.A couple of things.Not all the voltages on the schematics are correct!The voltages on Q624 are not correct! If you see the Zener voltage is not right. On the anode there is -12V, on the cathode -6.9V. The difference is not 9V!So either the Zener is not a 9V Zener, or the voltages are not right.Using a 9V Zener in the simulation the output voltage is correct!Left out the caps, not needed for DC voltage simulations.Used 2.9K as the sum of R638 and R639, it gave close to -12V output.The voltage on the collector of Q633 and Q637 depends on the load. I used 1.2A load.Peter