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- Опубликовано: 15 сен 2024
- In this episode Shahriar takes a close look at an Agilent pulse generator which does not produce any output signal. The generator has an output ON/OFF function which can disconnect the unit using a microwave relay. The output spectrum of the faulty unit shows the presence of the desired spectrum and residue of the square wave signal.
The teardown shows that the output relay is not being engaged with the output is enabled. The fault is traced to the relay switch transistor (NPN) which does not respond to the control signal. The replacement NPN corrects the problem and the unit produces the correct output. The unit's maximum output swing as well as frequency accuracy are verified using a R&S MXO-4 Series & a Rubidium frequency counter.
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*Several people have suggested to check the protection diode across the relay, which is a good idea. I went back and checked. Although it does appear to be OK, the forward voltage did read a little too high. So I changed the diode to be on the safe side. Thank you all.*
Nice repair. I am surprised it was the driver transistor and not the relay (I've replaced a lot of relays). I wonder if the flyback diode around the relay coil is flaky, and that might have induced a failure in the bipolar switch transistor - probably worth checking.
I was happy that it was the transistor and not the relay!
I love how you call it luck when you're a walking super computer of knowledge. Can you add the thank you button please.
Is the other SOT-23-3 device that is connected to the same net as the negative terminal of the relay coil and the collector of the NPN BJT a flyback diode? Probably worth checking that too. If it has gone the way of the dodo, the new BJT might not live long.
Repair isn't done until Pooch signs it off
Your troubleshooting skills are always impressive.
Good stuff sir.
I forgot how good these channel was. I Have to go back and look at some of the old content.
A quick, clean, obvious fix. I love those. Intermittent problems, like ghosts in the machine drive me nuts.
That was a satisfying repair...I swear this is the type of thing a big company will e-waste right away, especially if the unit is a bit older. Just took a $0.14 transistor and your ~$6k pulse generator is ready to go
That worked out well! The team in Germany made pulse generators for many years, maybe still at it today. My 8112A, although much earlier than this one, shows some similarities in UI philosophy. Maybe they were trying to have continuity for replacement models or maybe it was just the same people working on it.
Awesome content and explanation as usual. Thanks shahrihar
Glad you liked it!
Nice!!! I love fast pulse generators. The pre-pre-predecessor hp8082, 250Mhz, < 1ns tr, was also designed and built in Germany, Böblingen, and looks as well quite empty. Edit: Yep, my HP8082A (obsolete) was succeeded by the HP8130A (obsolete) and successor of the latter was the HP81110A (obsolete as well) 😁😁
Thanks for sharing!
Always excited to see a new video - thanks Shahriar!
If only medical diagnoses were this precise!
Great find! But is there no snubber across that relay coil then? I suppose it's easy enough to replace it again if it blows (not like 2n3904s are that uncommon), but... 🤷♂👍
>100k subs well done,,, I remember when you couldn't post videos longer than 10mins :-)
I wonder if the back EMF from the relay coil killed the transistor :(
Another multi thousand dollar instrument rendered inoperative by something worth a few dollars. Good repair. I could actually follow this one 🙂
That transistor is pennies! It could be a contest between the cost of the component and cost of electricity to heat the soldering iron used to do the replacement.
i just wondering the oscilloscope you using this video. What is normally noise level that scope?
We have to sometimes investigate THD in audio freq. very low noise level and tried to found out proper measurement equiment for that. Originally product tested with Audio Precision equipment but we need something for toubleshooting
Exactly the same fault on my Philips Pm5771 pulse generator. What a coincidence!
Nice repair
Thanks for another great video. Any ideas why the crazy big power supply?
Could be a standard power supply they use for a wide range of products in this form factor. For this particular model, it certainly is an overkill.
Plugs a signal to the scope: "and we see nothing". I personally genrally conclude that I have messed up the trigger, the scale, the mode, or the wiring when I don't see a signal, not that there is no signal.
I think 2N3904 transistor just opened. That's all. It happened many times to me. Sometimes base opened, sometimes collector or emitter. It is not diode fail.
Nice Video...!! Interesting..
Awesome !...Cheers.
Nice , can you take x-ray of that faulty part to see what is behind semiconductor failure?
It is nearly impossible to see semiconductor failures with X-Ray - at least not with any reasonable x-ray machine.
Did you check the presence of a flyback diode on the relay coil, either internal or external?
Yup.
I will also say that i am quite curious what the output transistors are
Any speculation as to why that trani failed? Maybe the flyback from the relay coil has no clamp diode? Nice repair. How about a video showing how you use it in the real world?
I did change the clamp diode. The clamp diode did read a little high. So perhaps over time, the transistor died.
I wonder if the other unit has an H-Bridge?
Really enjoy the vids dude, nice work.
I also think that a pooch appearance should be mandatory :-)
Woot
There seems to be no clamping diode on the relay so when the relay closes it makes high voltage go to that transistor(collapsing magnetic field) Or I am mistaking it here... Still good repair
There is one next to the replaced transistor.
The base current of about 4.5mA is rather high. Maybe the 1k base resistor is too low in value and should have been higher. Best to check the max base current or your device also so.
Relay seems to be 15mA (5V / 334ohms), so its good to have at least 1.5mA base current, but 5mA isn't going to be a problem, after all you'd want to put 20mA base current to drive it to the 200mA max output current in switching circuit, so we know the base can handle that sort of current.
@@MarkTillotson Many of the small bipolar transistors have a maximum allowable base current of 2mA. If the HFE is 100 and Ic = 15mA you really only need 150uA into the base to switch it on. Not sure why they chose such a low value for the base resistor.
So I thought Germans were renowned for 'record keeping' but it seems now they learned 'their lesson' 🥴 and don't give documentation now? 🤦♂️
I could imagine this being infinitely more useful with a differential output
but thats twice the drive circuitry
First !
?
Lol
Love your videos.
Also, was there no snubber or inverted diode across the coil? Usually there is something to absorb the energy of the collapsing magnetic field of the coil. Maybe that's what killed the transistor if someone was flipping the enable on and off quickly.
@@HarblesTheSkeptial Pretty sure it was the SOT-23 part just above the transistor that he replaced (if you watch his close-up of the transistor and resistor, the trace from the coil to the bad x-istor has another trace branching off going to a second device which I expect is the freewheeling diode).
@@icesoft1 Seems that very few noticed the T branch in the trace.
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