Test Equipment Anonymous (TEA) group therapy thread

[tautech]

I was going to start painting today but that's not going to happen. It's raining, it's cool, and humidity levels are sky high, even inside. Not a good combination for painting. I did get some outside activities completed yesterday like an oil change on one of the cars. So.....guess I'll do some TEA today. 

I still haven't resolved the hum issue on the OL-1. And it is indeed 60Hz hum, not 120Hz ripple from the B+ supply. I've implemented some wiring lead dress changes which have helped. Going to do some more today. I'll show what I've done so far later. I also have a thought as to where the hum may be coming from and an idea to fix it. We'll see. 

Were the heater supplies twisted ?
Maybe they just need rerouting. 
And/or tucking in hard against the chassis.

[Specmaster]

I was going to start painting today but that's not going to happen. It's raining, it's cool, and humidity levels are sky high, even inside. Not a good combination for painting. I did get some outside activities completed yesterday like an oil change on one of the cars. So.....guess I'll do some TEA today. 

I still haven't resolved the hum issue on the OL-1. And it is indeed 60Hz hum, not 120Hz ripple from the B+ supply. I've implemented some wiring lead dress changes which have helped. Going to do some more today. I'll show what I've done so far later. I also have a thought as to where the hum may be coming from and an idea to fix it. We'll see. 

Were the heater supplies twisted ?
Maybe they just need rerouting. 
And/or tucking in hard against the chassis.

Yep, I was just going to say the same. It's the most likley reason as most of the other supplies are DC and ripple would be 120 hz.

Sent from my POT-LX1 using Tapatalk

[med6753]

I was going to start painting today but that's not going to happen. It's raining, it's cool, and humidity levels are sky high, even inside. Not a good combination for painting. I did get some outside activities completed yesterday like an oil change on one of the cars. So.....guess I'll do some TEA today. 

I still haven't resolved the hum issue on the OL-1. And it is indeed 60Hz hum, not 120Hz ripple from the B+ supply. I've implemented some wiring lead dress changes which have helped. Going to do some more today. I'll show what I've done so far later. I also have a thought as to where the hum may be coming from and an idea to fix it. We'll see. 

Were the heater supplies twisted ?
Maybe they just need rerouting. 
And/or tucking in hard against the chassis.

You're on the same train of thought as I am. But here's the rub. Heath went cheap here. They tied one side of the heater to ground. So the 2 heater leads come out of the power transformer to the 6X4 rectifier. Then one side grounded and then one lead up through the chassis to heater distribution on the circuit board. Not sure that was the best way to go but it's what I got to work with. But I have an idea but I want to complete the other lead dress issues first and see where I'm at.

[syau]

Anybody in UK will purchase the following item:

www.ebay.com/ulk/itm/303186482094

Lots of active probe up to 2Ghz

[med6753]

OK, the OL-1. If I could eliminate the 60Hz hum this guy would be done. But first....as I mentioned the other day the replacement CRT is about 2 inches longer and the socket was extremely close to the back of the metal case. There's 1KV back there so not a good thing. I fashioned this plastic cap which fits perfectly over the socket. Problem solved.



And I hate to admit this but I got bit for the first time in almost 40 years. I was moving the scope with power up and my arm accidentally brushed against the B+ terminal strip. It reached out and touched me....OUCH. 

Here's what I've done so far to try to get rid of the hum. All the "flying leads" on the bottom side of the circuit board have been eliminated and all that black cabling is shielded and grounded to the chassis. On the top side the 2 vertical outputs to the CRT are now shielded. It has helped but when you've got one side of the AC heater circuit tied to the chassis it explains why no matter where I scope in this instrument I can see 60Hz floating around.




So what's the next step? I have to do something about the heater circuit. Going to try this. The 6X4 will remain on the 6.3VAC heater directly from the transformer. Going to cut the heater ground at the 6X4. Going to cut the heater wire up to the circuit board which feeds the 4 - 12AU7's and 1 - 12AX7. Will power those tubes with an external supply of 6.3VDC. That should eliminate all 60Hz floating around on the chassis. I'll get this set up and report the results.

   

[bd139]

Ugh that's horrid. Just looked at the schematic. Wonder if it's possible to stuff a bridge rectifier and cap in there and run the heaters off DC as the CRT has its own heater tap so no nasty flashover problems? I've done that before in the dark ages to fix a tube guitar amp for someone.

Also are the heaters grounded directly next to the tube sockets or some distance away?

[med6753]

Ugh that's horrid. Just looked at the schematic. Wonder if it's possible to stuff a bridge rectifier and cap in there and run the heaters off DC as the CRT has its own heater tap so no nasty flashover problems? I've done that before in the dark ages to fix a tube guitar amp for someone.

Also are the heaters grounded directly next to the tube sockets or some distance away?

It sure is a shitty design. If running the tubes off of DC works I'll probably will try stuffing a bridge in there and running it. If I can't get enough voltage due to loading I may have to build up a separate supply. There's plenty of room in there for that.

Nope.....there are long runs to the edge of the circuit board to the ground side of the filament. All in all......shit design.

[beanflying]

Ugh that's horrid. Just looked at the schematic. Wonder if it's possible to stuff a bridge rectifier and cap in there and run the heaters off DC as the CRT has its own heater tap so no nasty flashover problems? I've done that before in the dark ages to fix a tube guitar amp for someone.

Also are the heaters grounded directly next to the tube sockets or some distance away?

It sure is a shitty design. If running the tubes off of DC works I'll probably will try stuffing a bridge in there and running it. If I can't get enough voltage due to loading I may have to build up a separate supply. There's plenty of room in there for that.

Nope.....there are long runs to the edge of the circuit board to the ground side of the filament. All in all......shit design.

Quick and dirty idea that might help is re twist the pairs heading to the tube. Love yet another kwality @Med bodge too 

[med6753]

Pictures to follow. Running the tubes off of 6.3VDC results in a perfect trace. 

[med6753]

Sharp trace, no noise. 5KHz/2V p-p. The retrace is normal. This scope does not have a retrace blanking circuit.




The test set up. Linear supply. The tubes are drawing about 1.3 amps.





So.....going to order a hefty bridge in the order of 5 to 10 amps with some capacitance and modify the filament to 6.3VDC.

[med6753]

Ugh that's horrid. Just looked at the schematic. Wonder if it's possible to stuff a bridge rectifier and cap in there and run the heaters off DC as the CRT has its own heater tap so no nasty flashover problems? I've done that before in the dark ages to fix a tube guitar amp for someone.

Also are the heaters grounded directly next to the tube sockets or some distance away?

It sure is a shitty design. If running the tubes off of DC works I'll probably will try stuffing a bridge in there and running it. If I can't get enough voltage due to loading I may have to build up a separate supply. There's plenty of room in there for that.

Nope.....there are long runs to the edge of the circuit board to the ground side of the filament. All in all......shit design.

Quick and dirty idea that might help is re twist the pairs heading to the tube. Love yet another kwality @Med bodge too 

That's a bodge that I'm proud of. 

[BU508A]

So.....going to order a hefty bridge in the order of 5 to 10 amps with some capacitance and modify the filament to 6.3VDC.

Btw, I recommend to do a soft power on with a smooth ramp for the filament. This will increase lifetime dramatically.
I've done this with the filament of a russian GU-81M for my diploma thesis.

Edit: example picture:



BU508A

[worsthorse]

Perhaps there's a time nut inside me somewhere

perhaps?

 

[med6753]

So the experiment was a success.  I don't see how this scope ever worked properly with that type of filament circuit. At the time this was the low end Heath scope and I know the 5 inch scopes worked fine because my father had an O-10 and I used it. Never any issues with hum on the trace.

I had to shut it down for a while. My home built power supply was kicking off some serious heat. I originally built it to power mobile CB's and it's rated for at least 3 amps but not continuous. That 1.3 amps continuous is giving it a good workout.  I do have a supply which can provide 10 amps continuous but it's in the closet. That's one thing my bench is lacking. Some good quality high power supplies. I may have to "rectify" that. Hello HP.   

[bd139]

Cool stuff 

Can thoroughly recommend a 6284A. Rather attached to mine

https://www.ebay.com/itm//372366684281 

Add porno:

[Specmaster]

Sharp trace, no noise. 5KHz/2V p-p. The retrace is normal. This scope does not have a retrace blanking circuit.




The test set up. Linear supply. The tubes are drawing about 1.3 amps.





So.....going to order a hefty bridge in the order of 5 to 10 amps with some capacitance and modify the filament to 6.3VDC.

Don't forget bridge rectifier and cap will increase the heater voltage.

Sent from my POT-LX1 using Tapatalk

[mnementh]

Make sure it is a low-noise voltage regulator! And surely a trimcap would deal with the 47kHz.

Standard 78L08 with LC pi network after it. Not overly worried about noise from this bit (yet). Mostly concerned with getting it working cost effectively, fixing all the large broken bits and poor bits of design and then optimisation. Call it an "agile" analyser

Trimcap won't do it unfortunately. The trimcap is actually not used for pulling the crystal here but acting as a tank for the larger air wound inductor. The oscillator is actually better described as a crystal disciplined LC oscillator. When you set it up you actually don't install the crystal to start with and replace it with a resistor of approximately the same ESR as the crystal (about 33 ohms here) and a 10nF ceramic cap. It will oscillate without the crystal in then. Trim it to 100MHz then pop the crystal in and it locks to the 5th overtone of the crystal. Unfortunately being a cheap fundamental cut 20MHz crystal, it wasn't cut for the the 5th overtone so it's 47KHz off. Not really a problem as the first IF filter bandwidth is actually nearly 2MHz at 112MHz but I'd rather it was closer because I like seeing more zeroes in the right place. Perhaps there's a time nut inside me somewhere

Ah, OK. The Tek time mark generators have a standard crystal oscillator, then separate active and passive components for the overtone tank. As for timenut... clearly if you get an 8 digit anything, then the *nut mentality is being primed ready for explosion.

Yes I prefer that approach myself, particularly after dreaming about crystals all night. This oscillator was more of an experiment to see "can I get 100MHz out of 20MHz crystals forced into an overtone" as it was an interesting design. The objective of the person who did this original design was more to cost cut the original QST article's oscillator which was a far more reputable piece of engineering.
   I may build this one yet. Main problem is finding MAV-11's in quantities and quality that isn't a royal buggering. They are a compatible replacement of an old HP MMIC part so I may just look for that once I've worked out what part it is. On time nuttery I am really after an HP 53131A with OCXO and channel 3 option but being able to get hold of one of them at a reasonable price has proved impossible so far.

That right there is some first-rate technical gobbledygook... reading it warms the cockles of my old hacker heart, and it was just the thing over my morning cuppa. Thank you my friends, from the heart of my bottom.

OK, the OL-1. If I could eliminate the 60Hz hum this guy would be done. But first....as I mentioned the other day the replacement CRT is about 2 inches longer and the socket was extremely close to the back of the metal case. There's 1KV back there so not a good thing. I fashioned this plastic cap which fits perfectly over the socket. Problem solved.

   And I hate to admit this but I got bit for the first time in almost 40 years. I was moving the scope with power up and my arm accidentally brushed against the B+ terminal strip. It reached out and touched me....OUCH. 

Here's what I've done so far to try to get rid of the hum. All the "flying leads" on the bottom side of the circuit board have been eliminated and all that black cabling is shielded and grounded to the chassis. On the top side the 2 vertical outputs to the CRT are now shielded. It has helped but when you've got one side of the AC heater circuit tied to the chassis it explains why no matter where I scope in this instrument I can see 60Hz floating around.

      So what's the next step? I have to do something about the heater circuit. Going to try this. The 6X4 will remain on the 6.3VAC heater directly from the transformer. Going to cut the heater ground at the 6X4. Going to cut the heater wire up to the circuit board which feeds the 4 - 12AU7's and 1 - 12AX7. Will power those tubes with an external supply of 6.3VDC. That should eliminate all 60Hz floating around on the chassis. I'll get this set up and report the results. 

I vaguely remember.... and I was thinking about this when you first mentioned the 60Hz hum issue... reading on one of the antique radio forums about a fellow with a similar issue with an old Dumont or Eico scope that had the heater tied (shorted?) to the anode. His solution was to rectify/filter the heater voltage to the CRT. I don't think he did that with the entire 6.3V heater chain; I could swear the circuit I remember seeing had only the CRT heater involved, so it is possible he did it by inserting a dedicated XFMR just for the CRT heater. Or maybe he used a CRT brightener XFMR.  No idea if that would cohabitate with the HV tripler circuit in your scope.

mnem
 

[mnementh]

Pictures to follow. Running the tubes off of 6.3VDC results in a perfect trace. 

Woops. Looks like sick minds think alike. 

Hunt up a 6.3V CRT Brightener. They typically are made to bring 6.3V up to 8-ish volts. Turn it around backwards, and you should be right at 6.3V after rectification/filtering.

mnem
 

[med6753]

Pictures to follow. Running the tubes off of 6.3VDC results in a perfect trace. 

Woops. Looks like sick minds think alike. 

Hunt up a 6.3V CRT Brightener. They typically are made to bring 6.3V up to 8-ish volts. Turn it around backwards, and you should be right at 6.3V after rectification/filtering.

mnem
 

The CRT filament is on it's own circuit and tap from the power transformer because it is raised to HV potential to prevent internal arcing in the CRT itself.

The possibly of one of the other tubes having a heater to cathode short did cross my mind and I swapped out all four 12AU7's with no change. I don't have a spare 12AX7 and I should probably get one just in case. But that tube is in the sync circuit and if it did have an issue I would have poor sync. Instead I have excellent sync for a recurrent sweep scope.

As soon as I get parts and build up a DC filament source I can put this bad boy to bed. 

[mnementh]

In the meantime I suppose you could try probing with the 12AX7 out of circuit; see if the 60hz noise you're "seeing everywhere" goes away. 

[EDIT]      I just tripped over this while looking for something else. [/EDIT] eBay auction: #362619939251

mnem
*tinker-ily*

[Terry01]

In the meantime I suppose you could try probing with the 12AX7 out of circuit; see if the 60hz noise you're "seeing everywhere" goes away. 

[EDIT]      I just tripped over this while looking for something else. [/EDIT] eBay auction: #362619939251

mnem
*tinker-ily*

What is that and what is it used for?

[Cerebus]

In the meantime I suppose you could try probing with the 12AX7 out of circuit; see if the 60hz noise you're "seeing everywhere" goes away. 

[EDIT]      I just tripped over this while looking for something else. [/EDIT] eBay auction: #362619939251

mnem
*tinker-ily*

What is that and what is it used for?

Voltage trim adjuster for a You-Bastard!^TM brand electric nipple clamp.

[mnementh]

[EDIT]      I just tripped over this while looking for something else. [/EDIT] eBay auction: #362619939251

mnem
*tinker-ily*

What is that and what is it used for?

   

It is a 1KΩ wirewound rheostat; a distant cousin to the VARIAC in this. Of course, the eBay listing tells you the basics. They were commonly used for small motor speed control and and to dim lighting. And on the bench for substitution to determine  best values for a fixed resistor in a circuit.

For those interested, I just posted a full teardown of this beauty in its own thread on the TE Forum.    PM me if you're interested in giving ugly sister a home; of course my TEA brethren will get first crack and a great deal. 

mnem
*Simplify*

[mrz80]

Motivation arrived in the shape of a really nice Kennedy machinist's toolbox which I cleaned up and stuffed full of all my electronic & precision type tools. That got me looking at the workbench and I moved onto some more buying! I got tired of trying to figure out what's sideways with my two fried FG501s    , and... wait for it... yeah, you guessed it, bought ANOTHER FG501 of eBay  . No, really, I can quit any time!. It's just that I need a working sig gen so I can get further into what's off with that pile of dead stereo gear in the corner.  I also picked up one of those 3' long JAMMA extender cables, so some under-power testing of the two fried units may now proceed apace, with maybe less frustrating results.

[mnementh]

That's right... I don't need less tools; I need more (and bigger!) toolboxes!!!

Welcome to the nuthouse... you clearly belong here. 

mnem