North Hills CS-152 Current calibrator

[pigrew]

Here is the RS1 Oven controller for my unit.

Here are the load resistors.

I can get more pics of the oven controller If you want. I will be putting it back together later today /maybe Monday.

Thanks for the photos, that is the same board as my resistor oven uses, which a few minor component changes.

Does your voltage reference have a trim-pot for adjusting it and setting the current?

I think that since the LM301 uses less current than the old op-amp, they use higher resistance power resistors.

Looking the new board's diodes, the bottom one is a 1N485B, a switching diode. The two zeners for regulation on the new board are 1N5245. I can't read the glass diode's number without desoldering it. Also, this board uses a 2N3904 for triggering the SCR instead of a unijunction transistor.

[pigrew]

Here is the RS1 Oven controller for my unit.

I just checked my resistor oven, and its power supply has a few volts of ripple (up to 9V ripple when the oven is triggered). I assume its electrolytic cap is failing. My old cap looks JUST like the one on your unit.

You may want to check your board (and replace its capacitor), too, before assembling it.

Adding an extra 12 uF capacitor to the board made it behave MUCH better. I'll receive my order of capacitors tomorrow. See attached.

[Vgkid]

That 10uf cap was leaking on my co-10 board. It tested not to bad, replaced it with 2 22uFv 25v caps in series.

[Vgkid]

I went to use my CS-152 yesterday. Flick the power switch on with a loud chunk. Dial the knobs to all 9's and an X, Set the current multiplier to 100mA. And...
nothing. Check the fuse, it worked last month. Check the 10v output(mighty convenient), nothing.
Lets open up the reference.

Well, there's your problem. The ground lead became disconnected(broke off)
Spurned on by Pigrews tear down, lets go further.

Looking at the back of the reference we can see a few wires. The 2 heats hrunk white wires lead to a 1Meg thermistor.  This thermistor bead is sitting in a tunnel that is filled with sillicone. The 2 red wires go to the heater. The Blue/purple wires connect to the black thermal cutoff. This is used to shut of power to the heater, in the event of thermal runaway.

From this angle we can see the voltage reference in a metal can. The whole board is wrapped in a layer of tape. Helps keep the wires from getting pulled out, and makes it easier to slide in. You can see how there was a cutout machined inside the case to allow fitment of the wires. Very nice. I will give NH some credit. They can do a really good job on machining.
 Drum roll please.
Presenting the voltage reference.
…
..
My phone decided to delete 4 pics of the board when I uploaded them to my computer
 Well here is the bottom of the board.

The voltage reference is a 0.005% accurate, 2ppm%/C Burr Brown REF101, made in the USA!!!
I will not tear down that voltage reference board again.
REF101JM specs
http://pdf.datasheetcatalog.com/datasheet/BurrBrown/mXqxxqw.pdf

[pigrew]

I went to use my CS-152 yesterday. Flick the power switch on with a loud chunk. Dial the knobs to all 9's and an X, Set the current multiplier to 100mA. And...
nothing. Check the fuse, it worked last month. Check the 10v output(mighty convenient), nothing.
Lets open up the reference

Nice photos. My oven controller board is mostly depopulated now, it's looking quite sad. I'm planning to verify my resistor oven controller works tomorrow, and then do a parts order for the two controllers. I just took some photos of the new controller, and I'll build up a schematic soon for it.

It seems our two voltage references are completely different. As was mentioned before, I likely have two Zeners in series, as my reference. Once I get the oven working, I'll try to check its stability versus a 34401A over a day or two.

[pigrew]

My resistor oven controller works now. I replaced its cap with a 22uF capacitor. I made a mistake and ordered (2000hour) 85C caps for the oven. The capacitor should last a few years, I guess? Since I doubled the capacitance value, the board will be able to tolerate its failure for a while.

Now, the oven warms up and its exterior holds temperature at about 50C. The heater will trigger the SCR at the zero of the power line cycle, so there's less EMI generated.

When the heater is on, the oven uses about 45W. Perhaps I'll add some insulation around the oven next time I open it up, in order to reduce its average power consumption (and help it warm more quickly).

I'm surprised that there are no decoupling capacitors on the 30V rail... only on the intermediate rail. Is this just a cost-saving measure? Since the circuit was designed like that, I'll leave it.

[pigrew]

I'm surprised that there are no decoupling capacitors on the 30V rail... only on the intermediate rail. Is this just a cost-saving measure? Since the circuit was designed like that, I'll leave it.

I have a couple 470n 100V 85C poly capacitors. Would it be a good idea to add one on the 30V rail to ground? Or maybe parallel to each of the 15V zeners? I'm worried that the sagging of the power supply is adding some sort of intentional hysteresis to the circuit. I just can't think of a good reason that they didn't add any decoupling to the 30V rail....

[tautech]

I'm surprised that there are no decoupling capacitors on the 30V rail... only on the intermediate rail. Is this just a cost-saving measure? Since the circuit was designed like that, I'll leave it.

I have a couple 470n 100V 85C poly capacitors. Would it be a good idea to add one on the 30V rail to ground? Or maybe parallel to each of the 15V zeners? I'm worried that the sagging of the power supply is adding some sort of intentional hysteresis to the circuit. I just can't think of a good reason that they didn't add any decoupling to the 30V rail....

I'd only be concerned with the IC's supply being clean and if you were to do a bodge add some decoupling to the VDD of the IC. You could always connect a scope to the IC's V+ and check the ripple was low and any switching spikes from the oven cycling don't upset the IC. Ceramics or poly's like you have should be fine.

[pigrew]

That 10uf cap was leaking on my co-10 board. It tested not to bad, replaced it with 2 22uFv 25v caps in series.

Be careful, you may need a higher voltage rated capacitor. That power node is more like 40 or 50 VDC, as it is not regulated by the zeners directly. Also, unless you add resistor in parallel to each capacitor, the voltage will not be balanced (if the capacitance of the capacitors are not equal), causing one to fail before the other.

[Vgkid]

I had the balancing resistors, but good point on upping the voltage. I assumed it was running on 30 ish volts, I will order some 63v caps.
i will most likely go with a 100v rated cap. I only need 2

[pigrew]

I had the balancing resistors, but good point on upping the voltage. I assumed it was running on 30 ish volts, I will order some 63v caps.
i will most likely go with a 100v rated cap. I only need 2

The best solution would be to mount a radial 100V 105C capacitor. It'll be ugly, but it's hard to find high temperature axial capacitors. That's what I just ordered for my voltage reference oven.

Do you know what temperature your resistor oven runs at?

Mine is about 50C measured on the oven's outside using an infrared thermometer (module shield removed, 19C ambient). This seems a bit on the hot side to me, but not too unreasonable.



Sent from my LG-D850 using Tapatalk

[Vgkid]

Which oven do you want me to measure?
Hopefully I can do both over the weekend.

[pigrew]

My resistor oven is repaired. The root cause seems to have been that the SCR was only holding off about 167V, so it was triggering when it shouldn't have been. The switching that I saw the other day was actually the thermal protection cycling! Replacing the SCR with a NTE5418 fixed the oven.

Of course, it took many component changes (and some mistakes) in order to finally discover the issue. Most of the components in the burned up regions have been replaced.

Alterations:

• Replaced 100V 14uF capacitor with 200V 22uF capacitor
• Replaced 1.5k 5W resistor with 3.9k 5W resistor. I initially replaced it with 1.5k 10W, but it still got way too hot (70C). With the 3.9k resistor, the unit draws about 10 mA in steady state (20 during power-up). With the new resistor the power rail is about 22V during warm-up, but rises to about 28V once warm. The droop is due to the larger Zeners, which need higher currents to regulate properly.
• Unknown PNP transistor replaced. For a moment I thought it was took leaky, but it was probably ok. Substituted a 2N2905A.
• Added a small signal silicon diode in series with the PNP's emitter. This helps compensate for the fact that the op-amp can't output close to the positive rail (about 1.8V away), so the PNP transistor can be more fully turned off. Kapton tape added to the top, in order to prevent the capacitor shorting the chassis (close clearance). A slightly higher capacitance would be preferable after the resistor change (perhaps 33uF).
• Added 100nF 100V poly film capacitor to decouple the op-amp (added to the back of the board)
• Replaced GE C611B SCR with NTE5418.
• Shorted 120V to ground with my oscilloscope probe ground. Vaporized a trace, cracked a diode, and tripped the building's AFCI breaker. Diode replaced from one on another board (which I'll replace once I get to that board)
• Broke off a ground wire (similar to vgkid's photo). Reconnected.
• Replaced 15V zeners with a 1W model (1N4744A) The leads of the 5W zener were too thick to go through the board

Next is to tweak the knobs on the reference to output about 10V (referenced against a 34401A). The current sense resistor is so close to 1V, that I think that I should just adjust it to be 1V. I can't find any marks suggesting a proper value. After that, I'll tweak the output resistance network to make 10V.

Is there a good way to choose the proper current through the Zener?

Vgkid, I was wondering about the outside of the resistor oven (the big metal block), not the shield of the module. Mine heats to 50C, and I'm wondering if that's too hot (or not). Regardless, I'll probably just leave my module alone.

Attached oscilloscope plots show the UJT E1 (SCR trigger) and the common terminal of the heater. The front panel light now stays lit constantly. I'm sure that it generates lots of EMI due to triggering the SCR at power line peak.

[tautech]

This is typical unjunction switching: a train of pulses to the SCR gate.
Only the first pulse is the one that matters, the SCR will continue to conduct until the voltage falls to the zero crossing point.



There might be some EMI but it's relative to the type of load and the V and I values.
Low W resistive load, it shouldn't be much.

[pigrew]

Attached is the start-up transient of the voltage reference. It go's WAY past the point of zero-temperature coefficient. Coincidentally, the Zener maximum is at almost exactly 10.000V.

I think I want to run the reference at the peak where the temperature coefficient is zero?

I'll experiment with varying the thermister pull-up resistor value, until Vref reaches that peak value. Imonitor=0.961V, Vzener =10.38V.

[pigrew]

One of the op amp modules was starting to open up, so I took a look inside. It's filled with a white powder. I hope it isn't too toxic.....

Asbestos wouldn't make sense (and it was very very fine power, didn't look fibrous under a weak microscope).

What else could it be? Talc powder? Zinc oxide? I hope not cadmium oxide. What would have been used in the 60s?

[Vgkid]

Zinc Oxide sounds reasonable(It is a heatsink compuound). I know it get a crumbly feeling when it starts to dry out.
I didn't forget about your request. I undid the wrong set of nuts on the resistor oven module(It works for the V-ref), and now I have to pull off the whole side of the unit, those screws are really tight.

[pigrew]

Zinc Oxide sounds reasonable(It is a heatsink compuound). I know it get a crumbly feeling when it starts to dry out.
I didn't forget about your request. I undid the wrong set of nuts on the resistor oven module(It works for the V-ref), and now I have to pull off the whole side of the unit, those screws are really tight.

I'm somewhat stuck with my unit until some more parts arrive. I'm working on the TR-1 module. One board (send like a voltage limiting circuit) has some bad Zener diodes (overheated). With the bad board installed, the transformers in that module buzz at 60Hz. Replacement 1N5240B diodes should arrive early next week. I'll probably replace a few BJT, too, in a related part of that circuit.

 The old diodes are beautiful.

Here's a photo of a (good) 1N714A in my unit. In the bad diodes, the white coating had turned orange.

[Vgkid]

Found out why one of those screws was really tight. It was either cross threaded, or the nut is slightly undersized.
Started cutting open the chopper amp module. I can see a few caps, a few carbon comp, some carbon film, some mlcc caps, and a mysterious copper cylindrical object.

[Vgkid]

Guessing on the amplifier topology, opamp based howland current source with vos trim + psrr/ ratio trim.
there are 7 pins on the can.
+ power
- power
Non inverting input(plus of diff amp)
inverting input(minus of diff amp)
output
ground
shield???
Lets find out.

[Vgkid]

So I tore apart the the amplifier module(literally), started using side cutters, then broke out the tin snips + dremmel. Much faster, and neater.

In like Flynn.  After trying to be gentle, but messy, the dremel comes to the rescue.

This doesn't look like the setup I was expecting, even when taking trim resistors into account. We do have a single sided board.

A precision op-07 opamp is ar the heart of the precision circuitry. After all of the effort they went through with the ovens, precision resistors, and over the top voltage reference. This was a let down.
At least this makes tracing out the circuitry easier, that will be another update. Need to clean out the inside of the unit first.

[pigrew]

So I tore apart the the amplifier module(literally), started using side cutters, then broke out the tin snips + dremmel. Much faster, and neater.

I didn't clean out my module add much as you did! I'm hoping it just works.The lid of my module gently pulled off.

Here's what mine contains. It uses the same BJTs as the rest of the board, and no trim pots.

[Vgkid]

My module was not filled with anything, just empty.
Here is the schematic overview, my scanner does not like scanning this manual. I'm having to take pics with my phone.
I cut off the resistor section.
EDIT : This module would be similiar to the chopper amp that would be on your TR-1 board. Which one, I'm not sure.
I will also get some pics of my power supply.

[pigrew]

Here's what I put together about the boards I'm currently working with. The module has a pair of transformers. It seems like one is used for the negative supply and the other is used for the positive supply. I can't think of a reason that they'd need multiple transformers.

There's a small interconnect board which rectifies the AC, and also has a pair of voltage followers, seemingly to regulate the supply voltage.

That module powers three boards in the module.

[pigrew]

And here is board D, which seems either be a feedback amplifier for regulating the voltage going to the op-amps, or else could provide a voltage limit for the output of the current source.

It gets some feedback from the E board, which it seems to be using produce an output going back to the interconnect board's common base transistors.

The board has nearly the same circuit twice. But, on the top there is a resistor in place of a huge 1N2163 zener diode. I don't understand, and am wondering if someone changed a component over the years.