Author Topic: Tektronix CFG253 function generator repair  (Read 4044 times)

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Offline Rgreen50

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Tektronix CFG253 function generator repair
« on: February 19, 2015, 06:43:27 am »
Hello All!
I recently picked up an old DOA Tektronix CFG253 3 Mhz function generator for $25 at my local hamfest. Knowing it was dead on arrival in advance I thought lets try and fix it :) However I have run into a critical snag... I cant find a service manual anywhere even Tektronix claims not to have one. I did find the CFG250 service manual but the boards are slightly different and I could only go so far. I believe I have found a bad regulator  but I cant be sure because I dont have the correct manual. Can anyone help me?
Thanks
Rick
« Last Edit: February 22, 2015, 01:55:49 am by Rgreen50 »
 
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Offline Bryan

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Re: Tektronix CFG253 function generator repair
« Reply #2 on: February 20, 2015, 08:21:18 pm »
I believe I have found a bad regulator KA7815 but I cant be sure because the manual may not be right and just putting in a generic 7815 did not work either.
I'd suggest you study the pinout from a 7815 datasheet and compare it with the KA7815 AS installed on the PCB.
There is only IN, OUT & GND, I'm sure you can sort it.  ;)
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Offline Rgreen50

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Re: Tektronix CFG253 function generator repair
« Reply #3 on: February 22, 2015, 01:56:52 am »
i have updated my post
 

Offline Old Don

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Re: Tektronix CFG253 function generator repair
« Reply #4 on: September 18, 2017, 10:14:07 pm »
I've revived this old thread because the link provided in it no longer has a service manual for the CFG250, CFG253 or CFG280. I'm hoping somebody has a stored copy that they could post to me. I did found the operator's manual, but that doesn't have any schematics.

I picked up three (3) CFG253's in As-Is condition and would like to know what voltages should be on the connector from the power transformer output where it plugs into circuit board input connector. I'm guessing +/- 30V and maybe some other voltages.

The first unit that is 100% there seems to have a blown transformer and I would like to use an external supply to see if the main board powers up and works if supplied from my dual output pwr supply. Or if I need to use more than two (2) voltages.

Haven't had time to jumper 120VAC to the other two units, both are missing input pwr cord jacks, to see if one or both has a good transformer. Will try that today since I hope to have free time. But in any case, having a schematic would be nice if possible to figure out any other circuit problems after I can get power to each one.

I followed up with trying to acquire the CD from this link, but email address is not working:  https://groups.io/g/TekScopes/message/14713?p=Created,,,20,2,0,0
« Last Edit: September 19, 2017, 12:55:40 am by Old Don »
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Offline lowimpedance

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Re: Tektronix CFG253 function generator repair
« Reply #5 on: September 19, 2017, 01:37:14 pm »
Try here, same site (searched the manuals section for cfg253)

http://www.ko4bb.com/getsimple/index.php?id=manuals

How ever that service manual is only for performance verification so no schematics.
If my memory serves , there was only schematics for the cfg250 online.
« Last Edit: September 20, 2017, 09:20:04 am by lowimpedance »
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Offline Old Don

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Re: Tektronix CFG253 function generator repair
« Reply #6 on: September 19, 2017, 11:42:09 pm »
I D/L'ed the service manual, but was hoping for a schematic and getting away with being lazy. So back to basic troubleshooting going in blind and working it out the old fashion way. THX!
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Offline Assafl

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Re: Tektronix CFG253 function generator repair
« Reply #7 on: September 20, 2017, 07:42:25 am »
I measured 2x 26V CT as follows:

Pin Number: Wire Color: Voltage
1                 Red            13V AC
2                 Blk             CT (0) for 1&3
3                 Grn            13V AC
4                 Wht            13V AC
5                 Blk             CT (0V) for 4&6
6                 Yel             13V AC

The two groups are isolated. On the board, the Red (1 terminal) is where the silkscreen says J6
 

Offline Old Don

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Re: Tektronix CFG253 function generator repair
« Reply #8 on: September 22, 2017, 05:07:32 am »
I measured 2x 26V CT as follows:

Pin Number: Wire Color: Voltage
1                 Red            13V AC
2                 Blk             CT (0) for 1&3
3                 Grn            13V AC
4                 Wht            13V AC
5                 Blk             CT (0V) for 4&6
6                 Yel             13V AC

The two groups are isolated. On the board, the Red (1 terminal) is where the silkscreen says J6

Thanks, that should be a big help in decoding the problem I'm having with one of 3 253's. Of note, the two secondary center taps, while on the connector, are not connected to traces on the PCB. So twin 24/28VAC connections are what the transformer is supplying to the PCB, strange unless that was a standard Xformer design and there for cheaper than a custom one. This also explains why only two fuses on the PCB - one for each AC supply.
« Last Edit: September 22, 2017, 07:21:00 am by Old Don »
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Offline Old Don

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Re: Tektronix CFG253 function generator repair
« Reply #9 on: September 30, 2017, 06:50:52 am »
Update on my repairs:

I found one defective open fuse holder that didn't pass power to the power switch and which solved the open circuit across the power plug pins. Not the transformer as I first guessed.

The other two units had their AC power sockets removed and with the internal wires lopped off requiring new runs of wire to the fuse holders and power switches. Other than that the units powered up. But had wrong fuses in a couple of fuse holders and/or on the PCB fuse locations.

All three seem to output Sine, square and triangle waveforms that vary with scale switch settings and the  rotary knob. I'm waiting on parts to replace the AC in sockets and should have these things buttoned up PDQ. Then look at frequency vs. calibration and noise levels.

A fun project. I will put a couple, if not all, up for sale.
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Offline Old Don

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Re: Tektronix CFG253 function generator repair
« Reply #10 on: October 03, 2017, 06:36:08 am »
Mouser came through with replacement parts and all 3 are powered up. I figured out my fuse holder issue, new fuses are 30mm long and fuse holder expects 32mm (1-1/4") fuses and that's what I get for ordering on eBay vs. using Littlefuse fuses, I guess. Tek scope is in the middle of a capacitor replacement repair and so the only scope I have at present is a POS DSO-94 and it doesn't trigger worth a bleep. My frequency counter sees the output from all 3 and follows the change a frequency from the waveform generators. When the DSO is working I can see square, sine and triangle waveforms.
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