problem in an op amp type power supply in a Wavetek 142 - Page 1
Forum section: electronics / repair

author: WPE
author_rank: Contributor
timestamp: April 01, 2025, 12:19:32 am
content: I was given a Wavetek 142 with a power supply problem. It uses a 4 section power supply with a uA709 in each, giving +/- 15 VDC and +/- 28 VDC. The +15V has a Zener and provides the reference for the other 3 op amps. The +15 and +28 are very good (both 0.05V low), but the -15 has -19 VDC on it, and the -28 has about -36 V on it. (I hope it didn’t hurt anything on the main board.)IC2 for the -15V has 0.00V on its ‘- input, but -4.63 VDC on its ‘+. All 4 diodes across the 709 inputs are good, and correctly installed. I found no out of spec resistors. The transistors in the -15V supply are all good (had to remove 2 for proper testing). R17 4.7k ohm resistor to ground for the ‘- input is correct. (IC1 in the +15V circuit has only 0.001V differential between ‘- and ‘+ inputs, at 6.43 VDC.)I substituted a TL081 for IC2. Now it has 22.28 V on the '-input, and -21.98V on the ‘+. The -15V output worsened to -24VDC.I then “splurged” for a bag of good original Fairchild uA709AHM’s (1981 to 1985 date codes) and installed one for IC2. Now I have the same as before – with 0.00V on the ’- input and -4.7V on the ‘+ input, and the -15V output has -24V on it (which puts nearly 40V across IC2 V- to V+).I hope someone is more familiar with these op amp regulators than I am – what’s it doing wrong?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 01, 2025, 05:26:41 am
content: HiAre you certain that Q8 is good?lift one end of R9, what happens?g edmonds

author: MathWizard
author_rank: Super Contributor
timestamp: April 02, 2025, 12:45:21 am
content: Do you use any circuit simulators ? Any old op-amp in place of the uA709, with the right transistors, should get pretty close -15V. And with random BJT's and op-amp, it's still not hard to get +15V.I made a model of the positive rail in LTSpice, (but with a uA723, so not exactly the same) and then I just had to adjust the feedback divider and reference levels to get the +15V.At 1st I thought this WaveTek was a DC 4-quadrant sink/source PSU, but it's a signal generator. Someday I want to copy one of these older designs and make a Sig.Gen, and a 4-quadrant supply.

author: WPE
author_rank: Contributor
timestamp: April 02, 2025, 01:13:10 am
content: Thanks for the replies. I had removed Q7 and another one so I could test all 4 for E-B and B-C diode - but today I can't convince myself that I had also checked C-E both directions before putting them back in. Anyway - Q5 - Q8 all SEEMED OK at the time. (I also had figured ANY old op amp should work in this supply - but after my weird TL081 attempt I just bit the bullet and replaced the uA709.)Tonight I lifted R9 and found that with 0V on Q5 base - I still get nearly full (-) supply voltage out due to R2. SO I applied voltage to Q5b through R9 and found that at ANYTHING 0V to -20V, the output remains around -24V, with only a SMALL amount of control - - - so I finally convinced myself I HAVE TO find something wrong with the Q5 through Q8 circuit - Q5 just won't draw enough current to take control. It should have settled on -15V with about -2.9V or so on the free end of R9 - according to what I saw in the +15V supply with its identical but complementary circuit. I'll be less hasty next time around.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 02, 2025, 10:06:39 pm
content: HiYou could remove Q7 to see what happens or lift one end of.R2Q6 is the current overload protection so should not be the problem, crudely it senses the voltage drop across R11, when this voltage exceeds say 0.6V Q6 conducts and puts a short across the base emitter junction of the Darlington pair Q7/Q8 shutting the power supply down.G Edmonds

author: WPE
author_rank: Contributor
timestamp: April 05, 2025, 01:52:45 pm
content: Thanks for everyone's help. I finally had time to get back to it, and removed Q8 for proper testing. Only then was it obvious that it measures a little flakey E-C, so I replaced it. Now I can drive Q5 properly through the lifted R9 by a pot, and get some regulation results. I soldered R9 back in and now all 4 power supply voltages are very good.