Ah, the good old Vp-p vs. Vrms. I have fallen in that one too - but maybe not for days ;-) . The other inexplicable factor of two that’s easy to fall into when calibrating stuff is high impedance vs 50 Ohm terminated, or worse, 50 Ohms at one end vs 50 Ohms on each end. Spent a few confused hours exploring these holes too. But great work on saving this magnificent scope, it’s a joy to see it coming back!
I got so obsessed with the result, I totally neglected to look at the process! It seems there's a ton of those little pitfalls you can stumble into, but I feel a little less bad knowing it's gotten you before too, haha. Thanks for coming along on the journey as I bumble my way through restoring it! HP really were wizards of the analog (and knew how to make a good looking piece of kit)!
Also a nice trap is the resistance of the scope coaxial cable. Years ago used one as handy adapter from solder posts to bnc and was wondering for hours why my amplitude at the sink was off not knowing that these 1x/10x probes have resistive inner conductors to avoid ringing and reflections…
The "Phase Inverter" is just a part of a push-pull amp which inverts the drive signal to create an anti-signal for the other power tube in the pair. In this case it's a simple inverting amplifier followed by the 2nd half of V19 as a cathode follower, but there are several standard phase inverter designs. You'll find the "long tailed pair" inverter in Fender amps and there's quite a bit written about that one. Each design has its own strengths and weaknesses.
Thank you for the explanation! Though, I should have been a bit more specific in the video in that what I'm really not sure of is what role the phase inverter has in the horizontal amplifier. Having said that, I didn't know that Fender used a long tailed pair design in some of their amps, that's really awesome! I played around a bit with the long tailed pair design at low voltages building the tube 555 and the tube 741!
@@UsagiElectric V20 and V21 provide differential outputs to the horizontal deflection. One pulls left and the other pulls right. V19 just inverts the V20 drive signal for V21 to get a differential drive signal. Without V19 you'll only get sweep from one edge to the middle, or whatever range is dictated by static conduction in V21. If V19 is weak, you might get a very nonlinear sweep with reduced range. Class A amps are nice for audio, but to drive an instrument and maintain linearity, an AB amp is far better suited. The phase inverter here is running in class A, but as it doesn't need to provide gain it can operate in a very linear part of its curve. Side note: a lot of great Rock n' Roll tone comes from misbalancing the phase inverter to create asymmetric distortion in the power tubes just before the signal starts overdriving earlier preamp stages. Add enough static bias to keep the plates glowing cherry red and the output transformer dissipating enough to warm your coffee on a cold day, and you have the basis of SRV's Texas Flood tone.
Thank you so muich! I really enjoy digging through the schematics and logic flow charts to try to understand how the machine works and it's great to hear others enjoy coming along with me on the journey!
A warning! I also used to venerate HP classic 1960's/70's gear to God-like proportions. Until one afternoon I found myself lifting a 19" rack-mount VNA over a low coffee-table, from here to there and CLACK! Drat & curses. My lowest vertebrae went; - displaced and then arthritic as time passed. I'm still hoping epigenetics will step to the plate, and there'll be a miracle healing but there's the rub: always remember correct lifting posture and movement when lifting heavy kit. So now, I look upon my HP gear with less of a glossy-eye, in fact a red-flag goes up whenever it comes to any lifting of that type.
Thank you! HP were true masters of the analog design back in the day and even though this scope is giving me a proper run for my money to bring it back to life, it's such a beautiful piece of engineering, I really look forward to spending time with it trying to diagnose it!
Back in the 80's, I built a 30 Watt per channel SQ Quadraphonic system using valves. If I recall correctley, It used about 15 valves in total. (psu/pre-amp/decoder/main amp). Each chan had a magic eye VU meter. It sounded awesome with great seperation. During final callibration it gave me a 1200V shock that sent me flying accros the room. LOL. (I've no idea what I touched) Happy days.
@@frankowalker4662 when I was in graduate school, early 2000s, part of my funding came from me maintaining lab equipment for my program. My main responsibility was keeping an NMR spectrometer running, which involved tending to a bunch of 5000 W output RF amplifiers. These monsters were fed with a 24 V, 70A (yes, seventy amps) power rail. I was very fastidious about turning off the mains when I worked on these things, but one day I forgot about the capacitors, and ended up discharging a capacitor the size of a soda can through my finger. One minute I was digging in the amplifier, the next I’m on the other side of the room, next to my toolbox, wondering what I needed from the box, followed by wondering how exactly I ended up laying on the floor next to the toolbox, and why I would lay down on the kind of gross lab floor, and finally wondering why my finger hurt so much. That was fun, for certain values of “fun”.
@@frankowalker4662 I suspect if it were the live rail rather than a partially discharged capacitor, I’d probably not be around to discuss it. I still have a couple of little burn scars on my finger.
Fantastic! Thanks! … Question: How many years have you been working on electronics, and how many years specifically with vintage/vacuum tube electronics, and also, how many hours per day or week, on average, would you say you practice your skill (work on electronics)?
Thank you so much! I had always piddled around with small electronics kits and stuff as a kid, but I didn't start getting into building my own things until I was in university. Though, at that time, I was mostly just focused on building little BEAM robots (very minimalist bots that don't need any programming). At the start of 2020, like many, I found myself with an abundance of spare time, so I figured I'd finally take a stab at trying to understand vacuum tubes, and I just sort of spiraled down the rabbit hole from there, haha. As for how many hours per day or week, it's hard to judge. Even when I'm not working on stuff, I'm still thinking about it. But mostly though, I very rarely study something just to learn it. I almost always have a project in mind, and learning the electronics/logic/design work is really just a by-product of working towards accomplishing the project. Though, it is still quite fun to do some reading about how things were done once upon a time!
Thank you! It's actually an Accurate Instruments Model 257 tester I got used for cheap, but the original case it came in had some pretty serious water damage and wasn't salvageable. So, I just knocked up a quick wooden frame to mount it to out of some scrap 2x4s from a pallet I had laying around!
The primary tube used in the scope is the 6DJ8/ECC88 dual triode, which is a pretty popular audio tube. That's good in that you can pretty much always find really good condition tubes on eBay. That's not so good in that they're often quite expensive, haha.
You realize film caps have a polarity and should keep the leads as short as possible. Not polarity like a electrolytic but they have an outside foil like the old wax caps.
I am indeed keeping that in mind. Unfortunately, I don't have a good way to test them to figure out which side is the foil side, and if I'm honest, I don't think that's the cause of the near 400% increase in gain and 30% decrease in frequency. Once I get the big problems hammered out, if I'm still having trouble bringing it into spec., I'll look into getting to work on figuring out cap polarity.
@@UsagiElectric Mr Carlson's Lab has a video on how to figure it out with a scope. Another option I know of is using a Guitar Amp, what ever side has the less noise is the proper polarity. Yea it wouldn't cause the 400% gain but keeps noise out and makes things stable.
