Wavetek Model 25 repair - Page 1
Forum section: electronics / repair

author: byoungblood
author_rank: Contributor
timestamp: February 17, 2018, 07:07:39 am
content: I just bought a cheap Wavetek model 25 off of eBay and powering it up it had no output (frequency counter reading 0, doesn’t respond to the frequency adjust knob).Opened it up, found 2 blown fuses (500mA slow blow PICO fuses) immediately ahead of the rectifier for the 25v supply. Further investigation I found 4 burned out resistors between the output transistors (2n2219) and the output attenuators. They hadn’t opened up, one ohms, other 3 between 100-110 ohms. They were marked R128-131, I assume that they are the ones setting the output impedance?This was a quick 20 minute check out, so I haven’t seen any other apparent damage. I am guessing someone tried turning it up to its maximum output and connected a large load on the output and the resistors and fuses went before there was more carnage. My thought is to replace the burned up resistors and open fuses, check the output transistors, and see if it comes back to life.I can’t find a schematic, so I’m hoping that perhaps someone has one or can at least tell me if I’m barking up the right tree or can suggest anything else I should investigate before powering it up again.

author: tautech
author_rank: Super Contributor
timestamp: February 17, 2018, 07:25:17 am
content: Here's one for the 22, it might be close to the same:http://usedpribor.ru/data/Wavetek_22.pdfBut its' AKA TG550https://www-users.cs.york.ac.uk/~pcc/Service_manuals/TG550/TG550%20Service%20Manual.pdf

author: byoungblood
author_rank: Contributor
timestamp: February 17, 2018, 04:28:37 pm
content: Thank you! Looks like if I can get the 15v rail up and going (hopefully) I’ll be back in business. It looks like a couple of the fusible resistors around the output amp show some signs of heating, so I’m thinking that the overload was contained to that part of the generator.

author: floobydust
author_rank: Super Contributor
timestamp: February 17, 2018, 08:22:10 pm
content: I'd get it to power up with the output transistor's (collectors) disconnected, and see if anything upstream got damaged.Driver op-amp IC15 is "CLC430" in the TG550 service manual, bit of a rare part from National Semiconductor.

author: byoungblood
author_rank: Contributor
timestamp: February 18, 2018, 12:25:55 am
content: Ok, here is what I’ve got.Both output transistors are open from the base to emitter. PNP is fine from base to collector but the NPN only shows about .2v drop.With my scope hooked up to R175 I get normal sine and sawtooth signals, the square wave output is distorted (first three images) They respond to the frequency control, and the frequency display agrees with the frequency measurement on my scope.Connected to the anode of D17 (output of the CLC430) I get a square wave that is distorted. (last image)My understanding of op amps is rudimentary, is the CLC430 being used as a differential amplifier? If you feed a sine wave into the non-inverting and inverting inputs, what you should get out is a square wave output equal to the frequency of the inputs, and with an amplitude of the gain of the op amp, correct?I’m asking this because as mentioned, that op amp is more or less not available. I’m just trying to make sure that it is functioning as intended.

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2018, 03:11:05 am
content: Not to worry- if you have no output transistors in-circuit, then the output op-amp will just make square waves because it is receiving no feedback signal via R121.The CLC430 is in the inverting configuration acting as a summation amplifier. It sums four things - DC offset trimpot VR16, front panel DC offset output buffer 1 through R175, and buffer 2 through R120, and output stage feedback through R121. I think the AC signal comes in on CLC430 (+) input so it is a voltage-follower as well. Not sure where PJ5_15 goes.I would put in new transistors and make sure the four 10R fusible resistors are OK, D15, D16, C53, C54 have survived.Most signal generators have output protection diodes, but none here so this model is vulnerable.

author: byoungblood
author_rank: Contributor
timestamp: February 18, 2018, 05:11:43 am
content: PJ5_15 appears to go to VR30, the front panel attenuator control. Per the block diagram the signal path is through the attenuator VR, so I do believe you are correct about the + input on the CLC430.Thanks for clearing up the operation of the circuit. Most of my experience has been with fairly basic electronics and I've been trying to broaden my knowledge, hence buying the signal generator to begin with. Guess I got a good troubleshooting lesson out of it as well.Any tips on adding/modifying the output so this doesn't happen again?I also assume outside of using the (marginal) usefulness of the markings on the front panel, and space constraints, there's nothing preventing me from changing out the frequency adjustment pots with 10 turns? The coarse adjustment knob in particular was VERY twitchy. It may just need some Deoxit, but it was jumping around 5-6 Hz at a time on the 50Hz range, and the fine adjustment has maybe that much adjustment range.

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2018, 08:45:51 pm
content: Example Wavetek 22 signal generator includes a 1/2A fuses, and1N5349 (12V 5W)/1N4002 clamp, as well as output (to rail) clamp diodes, for protection. I didn't see any protection on this TG550 modelThe coarse freq. control pot might be noisy. I would clean the pot before adding a fine freq. control pot or going multi-turn.Some sig gens are very non-linear with the freq. control. Stay in mid-setting and change range if you are way at one end.

author: byoungblood
author_rank: Contributor
timestamp: February 20, 2018, 04:17:36 am
content: Awesome! I've been out the past couple of days, but I think there should be enough room to squeeze in a small proto board to add the clamping diodes to the output after the attenuator.I've got the next few days off and my parts order from Digikey should be here Thursday, hopefully I'll have it up and running then.

author: tautech
author_rank: Super Contributor
timestamp: February 20, 2018, 04:23:20 am
content: Good simple mod floobydustArguably only a single fuse could be needed but one for the GND too gives some protection to the DUT from GND loops.

author: byoungblood
author_rank: Contributor
timestamp: February 22, 2018, 05:33:15 am
content: Partial success. Replaced the output transistors, resistors to output, and fuses between the transformer and rectifier and it came back to life. Let it run for 20 minutes, started going through the functions, hit the triangle wave function button and the output dies. Check the power and the same fuses blew again. Transistors are OK in circuit.Checked the voltages again, and the +/-12V rail was at about 10v. Took the zeners out and they were breaking down at roughly 5v instead of 3. Interesting....

author: floobydust
author_rank: Super Contributor
timestamp: February 22, 2018, 11:55:53 pm
content: That's strange. Eight rails to check: +/-15V, +/-12V, +/-5V, -6V, +5VAThe Capacitance Multiplier also has a push-pull output, so it could load down + or -15V rail if the output went into a shorted triangle amplifier?

author: byoungblood
author_rank: Contributor
timestamp: February 23, 2018, 03:55:05 pm
content: That's what I was checking out. It must be intermittent, because I connected a ammeter across one open fuse, put a jumper across the other, saw a peak of 300mA (max amplitude, 5MHz output, all function types) and usually between about 220-260mA depending on the output and frequency. This was before the rectifier that supplies the 15 and 12 v supplies.As mentioned, the 12v +/- was reading about 10.4v, per the specs should be no lower than 11.5. Removed the load from the 12v rails and no change. Checked the zeners as mentioned, and found them breaking down at 4.5-5v instead of 3v. Replacements are on the way.All other supply voltages were fine.Considering replacing the leaded fuses with some fuse holders accessible from the rear of the generator, the traces where the leaded ones are in pretty bad shape from the damage before I bought it. At least this way I'm not creating more problems constantly soldering those traces.

author: byoungblood
author_rank: Contributor
timestamp: March 01, 2018, 07:43:15 pm
content: Keeping my fingers crossed, but it appears to be fixed now. Replaced the two 3v zeners and now I have a proper 12v instead of 10.4v. Ran it through every range and function and outside of the square wave undershooting at the high end of the frequency range, everything else appears to be working fine. I'll let it sit on the bench running for a while just to ensure that my problem is not related to a component that only fails after its temperature rising.Thanks to everyone for their help.