Well, never thought I would end up doing this.
I ended up taking almost everything apart. Finally I had enough and I thought that if the potentiometer won't go farther, I will make it go farther. R470 is a resistor that is right before the regulating potentiometer. It's value is only 4320 , I grabbed a similar resistor that was 5.9K and swapped the two. I could now regulate it to where I wanted it and I got all the other lines to where they needed to be. No problem anywhere. I plugged in the units and the fuses have not been blowing anymore (crossing fingers). I forgot to mention that they where blowing. I did not find anything odd after some testing. Is this a band-aid, or a real solution? I think it's fine now.
Also the fan was not turning very fast at all. (maybe 2 RPM). Taking it out was a pain! Really! I oiled the shaft after a disassembly and now it's blowing enough air that I can feel it on the side vents.
Anything else I should look for. Right now it looks really nice with everything setup
I'm still looking for a solution to the crack in the frame, but the essential is working. In total it cost me 140$, I think it's a good deal for a unit like this.
I'm still looking for a solution to the crack in the frame, but the essential is working. In total it cost me 140$, I think it's a good deal for a unit like this.
Have a look at this thread for a possible solution to the cracked frame;
https://www.eevblog.com/forum/testgear/hewlett-packard-740b-dc-standard-digital-voltmeter-(and-740a)/msg1419955/#msg1419955
It's something I will be looking at doing since I have a lot of HP delivery victims here too.
David
It's something I will be looking at doing since I have a lot of HP delivery victims here too.
David
It's something I will be looking at doing since I have a lot of HP delivery victims here too.
David
Well hopefully your still around. I thought that what I had done was good enough. I haven't use it a lot this past few months because of other projects that were in the way and work. I wanted to see if a certain frequency I was emitting was going to show up on the analyzer, I was just going to do this for practice. The minute I saw the screen light up I knew something was wrong. The display had shrunk and there was a brighter than usual spot. For some reason the beam was overlapping! (see video)
Back on the bench, opened it up again. I knew it wasn't the problem, but I thought I would check the voltage rails. 81.3V Something is deteriorating I don't know what! I haven't checked any of the caps for leakage or correct value. I have to check the schematic again.
Anyway not the main problem, I'm guessing something is wrong inside the IF section of the analyzer. I need to look into it further. I found the manual and a block diagram. This one is going to be interesting.
On the bright side, I got the crack repaired! The welding shop asked 50$ to get it repaired, I asked if I could just pay for the supplies and time on the welder, but they didn't want to. I ended up buying some aluminum brazing rods. I had some trouble at first, heating a large piece of aluminum takes longer than you would expect. I forgot to clean more than just the crack on one side and it bulge a little. It holds well, I got some 220 grit sand paper and finished up the frame. I have some before and after pictures.
For a first time I don't think it looks too bad. I've only done copper pipping before.
Things to consider next time:
#1: hold the torch farther away from the work. It's easy to think that more flame will heat the work faster.
#2: Clean the work area thoroughly before applying the heat. I did one side, but the other did not get as much cleaning.
#3: put down the brazing rod before taking of the safety goggles. I have a nasty red spot on the side of my cheek now.
I'll study the schematic, there's a lot more components in the IF section than in the power supply. I think that some component is dying slowly in the power supply.
This is taking longer than I had anticipated, but there's a lot to learn in the process.
Temporary link to video, I had to put it on my onedrive. It also plays it that way you don't have to download it.
Oops, the link timed out early. The problem is not a shrinking display, but the signal is not being drawn correctly. I took individual pictures. I believe it is the IF section because it is the one who draws on the X axis. As you can see in the picture one spot is brighter than the rest of the display. As I move the wave form to the left it "wraps around" and goes to that spot. I've never seen a problem like this one. This one won't be easy to identify. I'm also going through every component and seeing if the values match the ones written in the schematic.
I used these rods https://www.amazon.com/gp/product/B0000CBJGI/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1
They worked well after I learned to use them.
I did a thorough check of all components testing every single one of them. That included de-soldering one leg and checking for correct values. I found 4 resistors that where completely off value (28k instead of 40k!). I tested the diodes for forward voltage, but I don't know how to test the zener diodes for reverse voltages. I'm going to order a new set of capacitors just in case if changing the resistors does nothing. I'm getting tired of ordering a few parts and then finding out I need more. Where I live everything has to be bought online. Anyway, I'll be testing the bigger can capacitors.
I did forget to mention.
Some of the light bulbs had burnt out in the IF section. I replaced them all and the ones that where too expensive to buy where replaced with LEDs with resistors. It looks nice now with all the lights turning on.
I still can't believe that you can buy a replacement bulb on amazon for a 55 year old machine!
I'm curious as to where the drifted resistors are in the circuits, are they related to the problems you've had?
It's probably best to check to Zeners powered up in circuit for the correct voltage given on the diagrams (most will have a 5% tolerance, voltage reference ones may be better tolerance), they could also be tested out of circuit with an appropriate power supply & current limiting resistor (to limit to something like 5mA), but they can get easily damaged by removal (if glass) or from overcurrent. Equivalent types of Zener diodes don't always behave the same in circuit as the originals either.
I'm curious as to where the drifted resistors are in the circuits, are they related to the problems you've had?
It's probably best to check to Zeners powered up in circuit for the correct voltage given on the diagrams (most will have a 5% tolerance, voltage reference ones may be better tolerance), they could also be tested out of circuit with an appropriate power supply & current limiting resistor (to limit to something like 5mA), but they can get easily damaged by removal (if glass) or from overcurrent. Equivalent types of Zener diodes don't always behave the same in circuit as the originals either.I'll just grab my 100V bench power supply and do it! (I wish) or I could measure the voltage drop while it's running. I'll have to see
CM2107 (for CHICAGO MINIATURE / CML) I couldn't find them cheaper then 2.99$ for one. I needed two and that doesn't include the shipping.
The other ones where 367 bulbs. I got those for .75$ a piece and free shipping.
Question: do film capacitors work just as well as electrolytic. They seem to be the only kind you can buy that have a 200VDC rating. I probably won't replace them, but I would just like to know.
I'll get a picture soon.
The components with the scribble and the lifted leg are off value. Mostly burnt ones. I will replace the resistor I had swapped thinking it would fix it.
Make sure that any replacements are similar physical size or they may overheat too, you may need to up the wattage when looking as 1/2W (from the manual) are usually far too small these days.
Make sure that any replacements are similar physical size or they may overheat too, you may need to up the wattage when looking as 1/2W (from the manual) are usually far too small these days.Well darn, I hadn't thought about that when I order the parts on Thursday. They are all about half the size of the old ones. I'm still learning how to order the right components. Next time I'll place an order I'll make sure to buy different kinds. I'll use the ones I ordered just for testing purposes. Then I'll remove them.
What's the difference between metal film and thin film resistors?