#1820

[@IanJohnstonblog]

This has quickly become one of my favourite channels. Thanks for making excellent content, and sharing your knowledge and experience with all of us. You have the perfect balance of in-depth explanations delivered in a concise and efficient fashion. You rock.

[@1shARyn3]

Wow -- a site without April Fool Derangement Syndrome. Thank goodness!!!

[@jj74qformerlyjailbreak3]

Did the WhiteHouse ever wish anyone a Happy Easter yesterday. No. It must have just slipped their minds, however they did celebrate a holiday nobody has even heard of so I get your sarcasm here. 😂😂😂 Great comment.

[@GeorgeGraves]

Did EEVBLOG do one this year? His were the worst - I refuse to watch him anymore. Mr. "911 was an inside job" Jones.

[@peteroneill404]

I agree with Defpom's comment about checking the bridge rectifier. I have a Keithley 196, which dad bought new in 1988, the bridge rectifier shorted a few years ago and that killed the power transformer. The line fuse didn't protect the transformer as from new the 196 had a 3A fuse not 250mA, 30+ years too late for a warranty claim. I was able to source a replacement transformer and my 196 is still great instrument.

[@featheredskeptic1301]

More than 100V rectified voltage seems a bit too excessive for a 0 - 50V power supply. Even if that voltage drops to say 75 - 80V under load that's still way too high. I suspect the transformer primary got a section of it shorted out, and then the shorted section blew open circuit due to the high current that would have been induced in it. That could have ended up both making it look as if the transformer is fine, while also increasing the secondary voltages, thus killing the electronics, especially the op amp. Either this or there's an open circuit somewhere on the power rails of the op amp. That thing takes a maximum of +/-18V on it's power rails, so there has to be a voltage drop circuit of some kind. that would power it. Maybe it's a small simple regulator with a transistor, or maybe it's just a voltage divider, but there has to be a voltage drop from the + rail going to the op amp. Ultimately you need a schematic with displayed voltages to properly diagnose this.

[@simontay4851]

17:01 Those 2 grey power transistors are probably in a transistor-zener regulator circuit. The zeners apply a regulated voltage to the bases.

[@mariomionskowski6223]

Indeed !

[@mariomionskowski6223]

Supplyvoltage for the 709

[@wa4aos]

You'd think a power supply problem would be duck soup, easy, to repair but if I had a dollar for every time a PS issue ate my lunch, I'd probably be able to buy a nice new scope. LOL Good luck Professor !!

[@KeritechElectronics]

High test voltage insulation resistance meter (be it old-school cranked megger or an electronic one) should be enough to check any leakage on the power transformer. Speaking of which, I still need to recombobulate mine.

[@davebleamwa2bxy799]

I have an old school magic eye Conar RCL tester that has variable dc voltage to test capacitor breakdown voltages.

[@bayareapianist]

I would start replacing the transformer with two simple PS. The maximum cap voltages are good clues what voltages you need. Make sure the bridge rectifieers aren't shorted and and they are good. Once someone told me long time ago fixing broken stuff needed a lot of intuitions since you would he lucky to find any schematics in 70s and 80s. However, i am sure you have fixed it by now since your videos are behind by a few weeks. Also, you don't need to disconnect the transformer if the dioides are ok because the diodes separate the path from the transformer.

[@t1d100]

Please see it through and take us along for the ride. Thanks!

[@TheDefpom]

I would be suspecting the bridge rectifier before suspecting the transformer, maybe disconnect the transformer and check the circuitry, oh never mind, you did LOL.

[@romancharak3675]

Drawing the circuit diagram (as you suggested) might help you figure out a solution.

[@Manf-ft6zk]

There is a classic way to power up suspicious equipment via control transformer or if not available via light bulb. I always like to see that, - maybe it does not really help but it avoids going over to the fuse cabinet in the dark. (The other idea is of course to have the fuse on the bench.)

[@iceberg789]

crazy heatsink mount arrangement outside.

[@Radioman.]

An OP amp in a TO case. Haven't seen that in awhile.

[@charlesdorval394]

I stumbled upon a few in an old electric organ I took apart, used a couple different part numbers on the oscillator boards (who knows why, cost maybe?), one day I might find datasheet for them, in an old GTE databook or something I hope

[@klumpy103]

Trust the design...if the voltages you're getting are exceeding the original capacitor ratings, you don't really have to go much further...

[@gorak9000]

It's very odd that it's rated for input frequency from 47-420 Hz, when the only 2 common frequencies used are 50/60Hz in normal line power, and 400Hz in aircraft systems (which is why everything in an airplane sounds like a slightly out of tune "A") - so it works on a huge range around normal power line frequency, and up to and slightly beyond aircraft power frequency as well. That could be a key to it's intended use!

[@mikesradiorepair]

Am I overstating the obvious. 100 Volts rectified to a 75 Volt bulk filter capacitor. Sounds like a recipe for rapid magic smoke evacuation.

[@xenoxaos1]

It can work on aircraft aka 400hz AC

[@MrXenon1977]

Is it possible that the 75V electrolytic is of this quite exotic wet hermetic tantalum type? They are astronomically expensive (mostly used for space and aviation...) but ultra rigid and of quite competetive energy density, even from a nowadays point of view.

[@GeorgeGraves]

So "a power supply needs a negative voltage to get down to zero" - so like a LM217 can only go down to 1.2 volts, right? So I assume you are making a new "ground" that's below that so that it can get down to zero? But I assume you can't just use a "virtual" ground - something like a diode drop or a resistor divider? Would it need to supply the full current in that region, right?

[@TheElectronicDilettante]

Can the vintage PCB be delaminating? I have a cherry transformer from a Tek 465 scope, if it’s of any use for your project, let me know and it’s yours.