Wavetek 182A repair

[dirty_glam]

Hello to you all,

This is my first post, so thanks to everybody for having this forum. I read some random topics and I'm glad I ended up clicking the link to this site on Google because I can see that I can benefit from your experience and I hope I can help others with the little bit I have.

Being said that, let's get to the point...

I live in Spain and I few months back I bought this wavetek 182A function generator from a seller in Indonesia. I had a LOT of problems in the Spain customs department because the seller didn't especify the charges correctly and I ended up paying big bucks for an instrument that I later found out it's not working (returning it would cost me more than keeping it and accept I was ripped off  )

The thing is, it came with the fuse blown, I purchased a new one put it in, selected the 230V (Spain) switch and when pressed the on/off button it turned on for 0,00000something milliseconds and then went off again, checked the fuse, blown, put a new one, same thing all over again.
Opened the generator, couldn't find anything blown, burned or suspicious to me, but since I'm thinking I have a short somewhere I did some research and I came up with the idea that it might be the transformer, so I took it apart, put a 100W light bulb in series with the main transformer and connected it to the wall and it seems to work fine. I have 25,6 or so Vdc in two of the three secondary transformer wires and 2,something in the other.

Then I thought it could be the electrolytic capacitors since it is an old unit, so I de-soldered one of the capacitor in the rectifier area and it seems to work fine, but I only could test it with a digital multimeter that its capacitance range goes only to 20microF and the one tested is 1000microF. So I could only check if it charges and discharges "correctly" using the resistance meter.

So...HELP!!!! Please
Any suggestion, usual suspect, any advice. I would like to repair it myself instead of taking it to a tech, so, let's see if I can do it with your help

Thanks very much

[Vgkid]

Have you checked the resistance of the primary transformer

[dirty_glam]

Ok thanks, I'll check that as soon as I find some time and let you know what I find

[Khashoggi]

Schematics-
http://bitsavers.informatik.uni-stuttgart.de/pdf/wavetek/182A_Mar86.pdf

Power supply section looks straightforward. As suggested, check primary winding and then filter caps and bridge rectifier and regulator.

[dirty_glam]

Hello guys.

Today I received and ESR meter I bought on eBay and measure the two 1000microF 35V filter capacitors and they give a resistance of 0,161 and according to the "worst case" table for this cap it should be 0,07, so, it's obviously bad, right?

The resistance in the primary of the transformer gives: blue cable to chasi or brown cable to chasi more than 2Mohm and blue cable to brown cable 185 ohm. That's ok, right?

And finally the 4 diodes of the rectifier give an impedance of 0,580 ohm in one way and infinite in the other, so, they are fine.

Since I don't have a dc source I don't know how to test the voltage regulator, but with these results I give you is it possible that the short that's blowing up my fuses is caused by the bad filter caps???

Thanks again

[edavid]

Hello guys.

Today I received and ESR meter I bought on eBay and measure the two 1000microF 35V filter capacitors and they give a resistance of 0,161 and according to the "worst case" table for this cap it should be 0,07, so, it's obviously bad, right?

No, probably OK.  This is just a line frequency supply, ESR is not critical.

The resistance in the primary of the transformer gives: blue cable to chasi or brown cable to chasi more than 2Mohm and blue cable to brown cable 185 ohm. That's ok, right?

Sounds good.  Disconnect the transformer secondary and see if the fuse still blows.

[dirty_glam]

Well, when I first tested the unit I did that with a 100watts light bulb in series with the primary just in case the transformer was shorted and It worked fine I measured around 50V on the secondary cables

[Khashoggi]

It's unfortunate to not have a bench supply. But, pull VR1 out of circuit. Does it still blow? If it does pull q2 which isolates everything after the bridge.

If it doesn't blow after pulling vr1, measure its resistance. It may be shorted, which wouldn't be surprising.

[dirty_glam]

Thanks a lot, I'll try that and let you know how it goes.

[dirty_glam]

Hi, I've got news on the subject

This thing is driving me crazy, I pulled out VR1 from the circuit and bingo, the fuse didn't blow and the power led went on, so I thought, that's it, replace the voltage regulator and you are done, but then I measure the resistance in it and it wasn't shorted, I had 0.56 between two pins and out of scale with the other. And then, I started measuring things again and found out that the 3 terminals from the secondary winding have continuity among the 3 of them. I desoldered the transformer from the circuit plug it to ac, nothing blown but there's no voltage in them and if I measure continuity the multimeter beeps.

How could this be??? I tested it before, it was the first thing I tested and I had voltage across them.

What do you suggest?

Thanks

[alm]

Depending on the current the resistance of the transformer windings may very well below the threshold for continuity of your meter. Stupid question, but you are measuring AC voltage when you plug in the transformer, right? If you measure AC voltage on the primary but none on the secondary, then my guess would be an open in the primary (for example a blown thermal fuse) or a short in the secondary. What's the resistance of the primary winding and the resistance between the three secondary taps?

[dirty_glam]

Ohhhh man!!!! Thanks Alm!!!

Not a stupid question at all, the stupid was me!!! Ha ha ha, I'm pretty sure that today my multimeter wasn't on AC voltage.

I'll check again later

THANKS VERY MUCH

[dirty_glam]

Ok, I checked again, my bad. The transformer is ok

So now I ran two jumper cables from the PCB holes where the "IN" and the "GND" pins of VR1 would connect to a breadboard and connected the voltage regulator there to be able to test it and it works, the output voltage is 14,8 Vdc

So, what are your guesses now? Q2? C4? I'm totally lost :-(

Thanks

[alm]

It would surprise me if a short after the regulator would blow the 1/8 A (?) fuse, since the regulator should limit the current to 1.5 A. Is the transformer connected by all the wires and screws? I wonder if you maybe (temporarily) cleared the short while testing the transformer, for example by removing it from the case and putting it on a non-conducting surface.

I would also connect the output of VR1 to the PCB to see if it still blows fuses (or lights the light bulb). If so, measure the resistance between the VR1 output and ground pin on the PCB without VR1 connected. If this is very low resistance (less than about 20 ohms), something might be shorted. Are there any tantalum caps across that power rail? Those would be my first suspect. After that I'd check for any component that's getting hot, and maybe pull R10 to isolate the +5V section and see if that resolves the fuse blowing issue.

[dirty_glam]

Ok thanks Alm I'll check all that as soon as I can and let you know what I find

[drtaylor]

I've contributed designs to a few Wavetek generators, and although the 182A was already in production before I joined Wavetek, I'm familiar with it. I also know and have the email address of the chief designer of the 182A who might be willing to help you. That being said, I would isolate the regulators from the rest of the circuit and see if the fuse holds. This is best accomplished by removing VR1 and Q2. If the fuse still blows, then its been isolated to the earliest components in the power supply. I'd be surprised if it is the transformer, but I would check the bridge rectifier and the main filter caps C1 and C2 carefully.

After the regulators, I would suspect the output amplifier over all other circuits. Specifically Q16 and Q17. Check these transistors and the resistors around them. Also the electrolytic caps are probably all dried up and could use replacement. Old aluminum caps rarely cause a short, more likely just noisy or bad performance.

[dirty_glam]

It would surprise me if a short after the regulator would blow the 1/8 A (?) fuse, since the regulator should limit the current to 1.5 A. Is the transformer connected by all the wires and screws? I wonder if you maybe (temporarily) cleared the short while testing the transformer, for example by removing it from the case and putting it on a non-conducting surface.

I would also connect the output of VR1 to the PCB to see if it still blows fuses (or lights the light bulb). If so, measure the resistance between the VR1 output and ground pin on the PCB without VR1 connected. If this is very low resistance (less than about 20 ohms), something might be shorted. Are there any tantalum caps across that power rail? Those would be my first suspect. After that I'd check for any component that's getting hot, and maybe pull R10 to isolate the +5V section and see if that resolves the fuse blowing issue.

Yes, the transformer is connected by all the wires and screws. I'm doing all the tests with the transformer attached to the PCB and to the back (aluminum?) panel of the case as it would be when the unit is closed inside the case.

As I mentioned earlier I ran 2 cables (now 3) from the PCB holes where VR1 is mounted, to a breadboard and when I push the "ON" button the power light goes on (through VR2) and I get 14,56 Vdc at the "out" pin of VR1 but as soon as I connect this pin to its corresponding hole in the PCB, the fuse blows.

I measured the resistance across the output and common pins (holes) of VR1 (when VR1 is removed from the PCB) and in fact it gives a very low value, 5,2 ohms, and when test for continuity I get the beep, so I guess that yes, something is shorted there but I can't figure out what is.

I pulled out from the PCB or at least desoldered one of the legs of R3, R5, R6, R9, R10, C4 and U2 and I still get the 5,2 ohms across the pins mentioned before. There are in fact two electrolytic tantalum caps (C6 and C7) but when R9 and R10 are removed from the PCB they are out of the path.

[dirty_glam]

I've contributed designs to a few Wavetek generators, and although the 182A was already in production before I joined Wavetek, I'm familiar with it. I also know and have the email address of the chief designer of the 182A who might be willing to help you. That being said, I would isolate the regulators from the rest of the circuit and see if the fuse holds. This is best accomplished by removing VR1 and Q2. If the fuse still blows, then its been isolated to the earliest components in the power supply. I'd be surprised if it is the transformer, but I would check the bridge rectifier and the main filter caps C1 and C2 carefully.

After the regulators, I would suspect the output amplifier over all other circuits. Specifically Q16 and Q17. Check these transistors and the resistors around them. Also the electrolytic caps are probably all dried up and could use replacement. Old aluminum caps rarely cause a short, more likely just noisy or bad performance.

Thanks very much for your help.  I'll try to check more things tomorrow and let you know what I find.

[Khashoggi]

Now that you have tested the positive rail regulator, separate the positive from negative rails to confirm what could be a negative problem.

Pull q2 and r1 and check your resistance. It may be OK now and you can reconnect vr1 and you may have your positive rail working without blowing the fuse.

At this point you can concentrate on the negative rail transistors. Check q2, q1 and q3 with your meter,

Good luck! Your getting closer...

[SArepairman]

maybe you used a fuse that cannot handle the inrush current?

[dirty_glam]

Good news guys!!! I did it!!

Today I started checking along the +15V rail from VR1 till I could find the short, started lifting +15V jumpers so I could isolated different parts of the circuit and finally, when I got to the capacitance multiplier circuit..... Boom!!! C41 (a tantalum capacitor) connected to the COM rail was shorted. I removed it, tested out of the circuit and yes, guilty as charged!! And the short across the COM and OUT leads of VR1 was gone, in the end Alm was right, a tantalum cap.

I soldered again all the jumpers, transistors, resistors, etc, etc I had removed in my search, plugged the unit, crossed my fingers and pushed the ON button.... The fuse didn't blow!! Woo hoo!!!

Next week I'll go buy the capacitor and as soon as I solder it and fully test the unit I'll let you know.

But meanwhile I want to thank you all for your useful help and good advices, THANKS!!!

[dirty_glam]

Well, hello again.

I couldn't wait till I buy the cap (that I bought today and is already soldered) so I tried the unit during the weekend and again today, i connected it to my oscilloscope and it works fine, the only things that's bothering me is the DC offset control. 

What does it do? Because if I'm not mistaken I think that if I have, for example, a sine wave and I turn the DC offset control to the (+) position it should raise the sine wave above the X axis and if I move it to the (-) position it should lower it. Am I right???

Because if I am, I'm gonna need your help fixing this control because what it does it to "cut" the sine from the top if I move the control to (+) and from the bottom when I move it to (-).  What do you think??

Thanks

[casinada]

Make sure that the Signal is small enough so it won't clip.
As an example, start with a 1vpp sinusoidal signal at 10khz and add a positive or negative offset of 1 Volt. At those levels the signal shouldn't be clipped, if it is then there is a problem; if not keep increasing the signal amplitude until it starts clipping. Look at the manual for the maximum specs, check to see if the instrument meets those specifications.
 

[Vgkid]

What is the output voltage of the main transformer secondary?

[dirty_glam]

Ok guys, once again I made a mistake. The generator works perfect.

As casinada says the signal is clipped at certain levels but my main concern was that the signal wasn't moving up or down the X axis, it stayed put.

Then last night before falling asleep I was thinking and thinking what could I have done wrong and I came up with an idea that I checked this morning and yes, that was it.  I had the oscilloscope channel in AC instead of DC and that was the reason the signal didn't have a DC component in the scope.

Thanks very much.