Wavetek Model 130 Function Generator - Power supply problem

[Dgtech]

I have this old function generator that I got recently and it has a power supply issue that I cant quit lock down.

Please view the schematic.

Incoming voltages 120VAC = OK
Fuse = OK
Using TP 1 as GND in this section;
TP2 = -2.7 to -4.8 varying with frequency knob\potentiometer. (Bad should be +15VDC
TP3 = -15VDC (Good)
Top of R116 = 26VDC
Bottom of R117 = -26VDC

I dont really know how to disect further to pinpoint the bad component. At this point I'd guess and just change out IC6 maybe?

[budm]

Re: Wavetek Model 130 Function Generator - Power supply problem

Did you already test the Q17,and Q18 (they are the pass transistors setup as Darlington), Q19 (is for Over current protection)?
Check the DCV between TP1 and Cathode of Zener CR19.
Right now Q17/18 are not being turned on.

[redwire]

Re: Wavetek Model 130 Function Generator - Power supply problem

I thought either the vreg is bad or there is an overload between the +15 to -15V rails.

Q17 and Q18 are normally on full-tilt (from R88) and then IC6 turns them off to set +15V. Backwards from most designs. I think the lowest it could then go is CR19, so this means the two transistors are probably not working or overloaded.

I would solder in two reverse diodes like 1N4001 across the +15V to GND, and -15V to GND to stop the +15V rail going to -V can damage IC's and capacitors. Try not to run it like this.
Maybe you can open both TP2 and TP3 (jumper) and see if the +15V comes up, for troubleshooting.

If IC6 is dead I worry the +15V goes to 22V.
You can use LM741's to sub for the uA709's; Still available in the cans wow.

Check for shorted transistors Q1, Q2. They could overload those rails.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

Sorry guys that I took so long to get back to this.

So... on the cathode of CR19 with respect to TP1 there is -23Vdc using the correct polarity. Black on TP1

[redwire]

Re: Wavetek Model 130 Function Generator - Power supply problem

That reading doesn't make sense to me;
-ve voltage at CR19-K and TP1(GND) would make filter cap C48 blow up.
Unless the xfmer center tap/rectifier is wonky.

Do you have the +/-22VDC rails at least?

Measure the voltages on Q17, Q19 and across R96 and post.
That will show what the vreg is doing I think.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

I measured wrong. I measured from the positive side of C48 instead of C49. Sorry, I was rushing. OK I have 6.1 volts on the cathode of CR19 to TP1.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

Just for the hell of it, I double checked the voltage on TP 2 and it shows .85 volts to TP1. Should be +15.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

Here are some other voltages to work with as I do some comparative probing around. All of these voltages will be from TP1 to each location.

Cathode of CR 11 = 29VDC
Anode of CR 14 = -27 VDC

Cathode of CR19 = 6.4V Anode of CR19 = .05V
Anode of CR 20 = -17.9V Cathode of CR20 = - 11.3V

R93 side of base to Q22 = -15V
R92 side of base to Q19 = .79v

Where R91 and R90 come together = .8V
Where R94 and R95 come together = -16.6V

IC6 pin 6 = -3V
IC7 pin 6 = -1.8V

[Longbow]

Re: Wavetek Model 130 Function Generator - Power supply problem

DG, we are going about this the wrong way. Instead of stabbing the circuit in random places, take a moment to predict what you SHOULD be seeing in the circuit rather than making sense of a bunch of unrelated voltage measurements.

First, you know the output should be +15v (obviously it isn't at the moment)
Then, you must have 15.6 volts at Q17 base and 16.2 volts at Q18 base.
Q19 is there to protect Q17 in the event of over-current. If it is shorted, it would then shut down the 15 volts. Thus, check for shorted Q19 first.
CR19 zener is probably about 15 volts (check in the parts list). That should put the d.c. level of IC6 pin 6 at around 0 volts.

Other than that, CR21 zener is the reference for the whole supply, along with R104 adjustment. As you say, if you have -15 volts, then the reference zener is working along with the adjustment R104. That means you DO have +15 volts up until some point around TP2 or J you have a cracked circuit trace. There is no way that you could have -15 volts without the +15 because IC7 pin 2 depends upon the +15 volts being correct. At this point your tools are a magnifying glass and an ohmmeter.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

DG, we are going about this the wrong way. Instead of stabbing the circuit in random places, .

Thanks Longbow for your help. My component level troubleshooting skills sometimes are not where I'd like them to be. I saw that there was not that much difference between the +15 side and the -15 side component wise, thats why i resorted to comparison of one base voltage to the other base voltage, both to ground and so on.

I will check for Q19 and report back.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

Q19 is not shorted. I checked. How about uA096? Is there a way to check this for correct operation? I ohmed out point to point of the components in question.

[budm]

Re: Wavetek Model 130 Function Generator - Power supply problem

How about q17, q18?
How about +vcc on pin 7 of IC6, IC7, what do you have RIGHT AT THE IC PIN7 of BOTH ICs?

[redwire]

Re: Wavetek Model 130 Function Generator - Power supply problem

DG, the comparison technique is ok but in the end it won't tell you why the voltages are different. It's easy to get lost in that. You have to roll back to circuit theory. Or test the parts in the section that is not working.

Q17/Q18 make a Darlington pass transistor with R88 supplying base drive to turn them on. IC6 turns them off when the output goes over +15V.

cathode CR19 = 6.4V so Q18 or Q17 E-B is OPEN
IC6 is powered from the output of this vreg, so it will not be doing anything until the pass transistors start working and give IC6 sufficient voltage to run.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

How about q17, q18?
How about +vcc on pin 7 of IC6, IC7, what do you have RIGHT AT THE IC PIN7 of BOTH ICs?

.8v directly on the solder side of IC6 pin 7
.8v on solder side of IC7 pin 7 too, which is weird if this side is working.
They are both can type of IC's so I can only get to the bottom/solder side of each - but directly on the pins themselves. Solder joints checked as well.

Q19 Out of circuit

620 ohms from C to E. Is this ok? Shouldnt it be open?
e to b 2 meg one direction and 330 K the other.
b to c 2 meg one direction and 343 K the other.

Q18 out of circuit
CE both directions - open
EB both directions - open
BC both directions - open

Are both bad or just Q18?

[budm]

Re: Wavetek Model 130 Function Generator - Power supply problem

Q19 is bad if that is what you are seeing between C and E which I mention for you to test in post #2.

"Q18 out of circuit
CE both directions - open
EB both directions - open
BC both directions - open'

So how does it test when using Diode mode for Q18?
How about Q17?

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

I had to take a business trip to DC yesterday. Im back home and my parts are here now. Q18 showed no voltage when trying the diode test out of circuit. Q17 tests ok. I'm going to replace the two that I have out (Q18,19).

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

ok. I replaced Q18 and Q19 Still no voltage on the positive 15V rail.
I pulled Q17. It tested good out of circuit but I replaced it anyways cause I had the new one and its kinda hard to get back in. Same problem.

So.... I pulled IC6 to see if it was pulling down the voltage somehow. Still no voltage for VCC with it removed.

Next I disconnected the jumper at TP2 separating R102 and R103. Still no voltage at TP2

[budm]

Re: Wavetek Model 130 Function Generator - Power supply problem

ok. I replaced Q18 and Q19 Still no voltage on the positive 15V rail.
I pulled Q17. It tested good out of circuit but I replaced it anyways cause I had the new one and its kinda hard to get back in. Same problem.

So.... I pulled IC6 to see if it was pulling down the voltage somehow. Still no voltage for VCC with it removed.

Next I disconnected the jumper at TP2 separating R102 and R103. Still no voltage at TP2

It will be impoosible not to have full output of 22V (instead of regulated 15V) when IC6 is removed because Q17, 18 will be bias fully on by resistor R88 (4K7). So unless R88 is open circuit or the two new transistors are bad or not properly installed or bad trace in that section, you do have +22V feeding the Transistor so it has to have output Voltage.

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

I did check that R88, all is well there (resistively).

Ok, do we agree that if I lifted the jumper at TP2 and TP3, thus, in my mind, disconnecting the load from the power supply, I should have the rail voltages for sure. Please correct me if I'm wrong.

Just to refresh - what has been changed so far:
Q17
Q18
Q19
IC6

29VDC at Q17 and Q18 Collector (same point)
0.8VDC at Q17 emitter

[Dgtech]

Re: Wavetek Model 130 Function Generator - Power supply problem

Well, I removed both jumpers. I had to test a theory of that a friend had suggested that something downstream may have been loading down the circuit somehow.

With jumpers removed

TP3 shows -19VDC
TP2 shows -3.6VDC
The only clue that I gain now is not only that the problem is definitely in this area for sure but... why negative 3.6 at TP2?