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Here is a sketch of the waveforms I am seeing at the power supply. The rectified waveforms are slightly exagerated, but I do see minimal ripple coming through.
I have a new scope now and it is helping me to see that the old one was insufficient, because my ground lead had to be connected to earth ground, I could only get limited measurements....
As it has been pointed out, CR5 zener diode is regulating 10v, and it is working properly. Now, when I measure with my scope from the Base of Q5 to the Emitter, I am only seeing 400 mV. I see that CR5 is connected to Q5 base through JP8, R8, and the secondary of T2. It seems to me that A1 is supposed to modulate an AC waveform onto the 10v that the zener creates through the T2 secondary. I know that there are several more components inbetween. Does 400mV seem right for the E-B junction? I know the switch is not on, so it can't be saturation voltage...I desoldered the base leg and read the same voltages. I am beginning to think that a component connected between T2 and Q5 is disrupting the turn on voltage...PLEASE correct me if I am wrong!
I am now getting a sawtooth waveform on the DC outputs of my PSU at about 1.3 Hz! The voltage levels are incorrect, but they are proportional. I am getting 2.5 on the 5volt output, 6 on the 12 volt output, and 12 on the 24....I replaced the transistor Q4, which was obviously part of the problem. I think I'm going to replace Q1 and Q2 for good measure...but There seems to be a capacitor on the secondary side that might also be bad...perhaps?
I just want to close this string by reporting to all interested that I repaired this sucker, finally. The bad components were as follows:R2,Q4, and R17 through R22 were all high values, but not completely open. None of these components showed external signs of damage, which is weird.
The Dc outputs are now all correct, with only .06 VAC of ripple.
the only slight concern i have now is that there is a hum coming from the supply, it seems to be around 60 Hz, and seems to be coming from R15, which is a % watt power resistor, or a transformer. Is this normal for a switching power supply? Should I be concerned? i have heard hum from power supplies before, but I want to be 100% certain everything is OK.
Thanks again for the good advice in getting this thing fixed!
Hello Heathtech,
Your post is hard to read because it is so wide, so I hope I do not repeat anyone elses reply here.
Any work done on equipment with a switching power unit should be connected through an isolation transformer, for the SAFETY of YOU and your TEST EQUIPMENT!
Refering to your first diagram, T2 has high power connected to it and the high power transistor switches this to ground. This produces the required output from T2
T1 is an isolation transformer, it keeps the high power circuit isolated from the low power circuit. The AC in for T1 would come from a low power IC or something, it's ground may be at a very different potential to the powercircuit ground. Also, if the high power switching transistor were to fail, T1 would help to block the high power entering the low power IC circuit causing terminal damage.
The output from T2 is usually monitored through another isolation device so that the IC has a reference of the output voltage and knows whether to increase or decrease it's switching.
If you are getting line frequency into the output, it points towards the main filter capacitor near the bridge. If your P/S is loaded and you are getting less than say 3V it would seem O.K. Try soldering a capacitor of equal or greater voltage across the old one and see if the ripple improves. Does your instrument have some type of CRT or deflection coil running at 60Hz?
To save time, replace every capacitor in the P/S with high temperature (105 degrees celcius) low ESR type. This will improve reliability as all electrolytics increase their ESR with age. (I would not use a Low ESR for the bridge capacitor-inrush and it only filters 60Hz)
Remove one leg of all the power resistors (the big ones) and measure with an accurate ohmmeter. Replace any that are out of or nearing the edge of spec. If they are the old compound resistors (usually 20%) replace them all.
Also if you replace other components, replace with quality industrial parts as the cheap items from a hobbiest store may not meet meet the relavent standard that your instrument has to meet.
You can replace the power supply with one of equal voltage, equal or greater current, that conforms with the same or better standard that the origional one conformed to. If your instrument is no longer supported, then you have no worries about avoiding warranty.
As I had mentioned in the post right before yours, the supply is fixed. I'm not getting any 60hz on the secondary side, just good ole DC like it's supposed to be. I replaced Q4, R17-R22, And R2, which all were faulty.
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