Matching ultrasonic transducer

[watches01]

I also found this video and I’m going to see if I can use this method to find the actual resonant frequency. It says in the original manual that the board has to match the transducer and both should be replaced at the same time, so it may not be enough just to adjust to 100khz. I’d like to verify in case it’s off.
https://youtu.be/-E7zlQEk5MA

[watches01]

By the way on the right side of the board as pictured there is a coil with the core sticking out. That core was rubbing on the steel machine cover. I’ll have to see if I can move the board slightly, I don’t know if I’ll have room.

[Armadillo]

By the way on the right side of the board as pictured there is a coil with the core sticking out. That core was rubbing on the steel machine cover. I’ll have to see if I can move the board slightly, I don’t know if I’ll have room.

Can you post a picture of the underside of the board, so that I can trace and identify the coil.

Of course, the driver board is kind of ridiculous compared to the modern board, but it was matched to the old transducer.

[Armadillo]

, so it may not be enough just to adjust to 100khz.

Refresh memory and the tong meter; Essentially you want energy to be transferred efficiently per manufacturer designed horns/transducer fit up.

... adjust trimpot R5 on the circuit board until somewhere around 100KHz, while your tong meter placed on the line of the driver board read maximum peak current [around ~4A peak, my guesstimation].

[watches01]

Okay, that makes sense. If the current draw is correct it won’t burn up the board. I’ll post a picture of the underside tomorrow from my computer. I have to resize it.

[Armadillo]

Correction, your circuit board shows that the fuse is 2A. Ehm..... lower capacity. [What part number is the transistor used. A closed up photo of the transistor would be helpful].   
So, find the peak current at somewhere around 100KHz. Essentially is tuning for best power transfer at around the resonant frequency.

[watches01]

The large power transistor is MJ12005. The other transistor is bent over and glued to the backside of the board so I can't read it, but the schematic says it's UMT3584.

[Armadillo]

Thank you for those nice photos.

 MJ12005 is rated at 8A continuous current, much higher than the 2A fuse I would say. Maybe this is a universal board.? There is a series diode resistor just after the fuse to limit the current which is a relief. The isolation transformer should be able to supply at least 300 watts but I suppose the transducer is only less than 100W type.

The ferrite rod you can use a diamond tile cutter to cut it shorter if you want. I suppose that ferrite coil and the 1uf film capacitor form a series crude filter shaper circuit. Careful ferrite is brittle.

R5 has a definite control on the resonant frequency. I would suggest tune R5 for Peak RMS current at around 100KHz and then fine tune the ferrite coil again for improved Peak RMS current. Note that the tong meter cannot response faster than your tuning. So I suggest tune from 80 KHz to 120 Khz in step of 2 KHz and tabulate the results on a piece of paper to graph it.

Those were just my suggestion.

Edit: The isolation transformer refers to an external power isolation transformer to be use to power the driver board up for the test.

[watches01]

I won’t try to cut the ferrite rod because I would be afraid of breaking it. I’ll try to move the board slightly. I can probably drill the mounting bracket holes a little bigger to get some movement.  That coil is not one that is adjustable.  The adjustable one is towards the center of the board. It has 4 colored wires coming off of it. That’s a good idea to tabulate the results, I’ll do that. Is there any danger of burning out the board by running too far out of frequency? These boards are known to burn out and I’m wondering what happens to them. Is it possibly too much current draw from bad adjustment? Maybe it’s just a component failure. I’m trying to prevent it, but maybe I can’t.

[Armadillo]

When its out of frequency, the impedance is higher and thus the current is lower. When is at resonant, the impedance is at the lowest and the current is highest. It is actually here that I am a bit concern whether the design was originally catered for at resonant. But the designer did put a series resistor there to limit the current and protect the transistor.
IMO, I do not think it will burn out the board because as you have seen, the transistor is rated at 8A. At most the fuse will blow first, but since this is a "monitored tuning" by the tong meter, it won't be like catastrophic uncontrolled event. The important thing is to fill the tank with water at a level you normally will use to wash the parts and not empty.
Alternatively you can use a variac to power the isolation transformer and at every frequency steps, you slowly adjust the variac voltage up from 50V to 110vac while monitoring the tong meter not to overshoot 2A. For me, I am happy with the fuse protection and the board is so simple it didn't fear me at all. The first time I did micro smd, I was all shaky, but after that I realized that I put too much fear into it. It was all so simple and forgiving. I laugh at it while I crossed the learning hurdle.
That Heathkit IT-28 you mentioned "Green eyes" is interesting. You should also check the capacitance while you are at it. Interesting it can charge and check the leakage though I don't know how low is low leakage it can measure. I have no experience with the Heathkit. I think the bidding price is increasing after you mentioned it here.
So, we look forward to after your components have been replaced, hopefully is the component failure like you said.
Regards;

Edit: The components are aged and may not be able to be subjected to off-frequency potential. But don't you think is a good test to eliminate failing component also? Do cater for it nevertheless.

[watches01]

Everything has been ordered, the caps and test equipment should be here this week. When my other machine went bad I remember it smoked and burnt components but it was 10 or 15 years ago so I don't remember exactly what. I'm wondering if I should change all the resistors now just for dependability. I just repaired a timing machine that had a burnt resistor and some bad caps. Luckily it didn't damage anything.

[watches01]

I think I just found a problem. I was checking the resistors and there is one that I believe is R10 on the schematic. It's kind of hard to tell the third color, but if it is R10 it should be brown, that would be orange, black, brown, gold which would be 300 ohms +/- 5% like the schematic says. It measures 2.9 ohms in circuit. It's in series with R9 which does measure 2.7 ohms as it should. If the third band was gold it would be a 3 ohm resistor, but it's definitely not the same color metallic gold as the tolerance band. I also found a brown, green, orange, gold resistor, which should be 15kohms, that measures 11.5kohms. I don't see a 15k resistor on the schematic. maybe this is a revised board, I don't know. I'll have to try to trace these componenets. Either way there is something wrong with the resistor that I think mis R10. I'm attaching a picture of it.

[3db]

I think you should lift one end of the resistor and measure its value.

3DB

[Armadillo]

Agree.
That's because the resistor could be in parallel to other components.
In case you are measuring high ohm resistor, make sure your hands are not in contact with the probe, otherwise it will measure your body resistance as well.

[watches01]

You’re right. 278ohms.

[watches01]

I changed all the capacitors and I did change that one resistor because it was slightly out of tolerance.  I put the board back in and ran the machine to check it. It was still weak.  I used a multimeter to check the frequency and it said 41 kHz. I disconnected the transducer and using the method in the video above I determined the resonant frequency to be 84.4 kHz. I connected the oscilloscope and tried turning R5 but it didn’t seem to make any difference, so I disconnected terminal four and I connected the oscilloscope to terminal four without the transducer connected. The board made a buzzing sound like a transducer and I was afraid it might burn up but I found that turning R5 did adjust the frequency now.  I adjusted it to 84 kHz and then reconnected the transducer. It seems to work fine now and I think it’s adjusted correctly but that second adjustment in the center of the board doesn’t seem to make any difference. I thought maybe it would vary the duty cycle but it doesn’t seem to.  It looks to be running that about a 50% duty cycle and the board is drawling .82A. I’m very happy with the outcome and I very much appreciate the help!