Test Equipment.....

[trem]

.....do you guys use anything like a Sencore Z.?
Or is there a modern day instrument that is smaller...cheaper...faster, or more useful/practical.?
I am just wondering about abilities similar to a .....Sencore LC53 'Z Meter' Capacitor Inductor Analyzer | eBay
I have always thought I needed a way to "test" a capacitor...for ESR and/or for capacitance at (or close to) its rated voltage.
I guess what I am asking is...what pieces of equipment would you suggest a Hobby/Amateur amp repair person have handy to test capacitors in a way that are relevant to tube guitar amp repairs.?
What do you guys use.?
Thank You
besides my DMM that is

[R.G.]

I really like my kit-built ESR meter. Doesn't work at high voltage, but that hasn't impaired its usefulness much. It's deadly on picking out old caps that are going bad. The kit was cheap, and offered a high value per dollar.

ESR is somewhat affected by the temperature of the insides, but not terribly affected by the polarizing voltage during testing, so I've always thought that measuring at low voltages is OK.

But then I don't make my living doing repairs, so the quantity of tests I do is low.

[g1]

The little searching I've done has mentioned the Der EE DE-5000 as a fairly good unit. I have yet to order one so I can't say first hand.
Der EE de 5000 Handheld LCR Meter | eBay

[R.G.]

Dang! That's nice! I'm saving my pennies.

[Jazz P Bass]

Nice unit.

Still.
Most of the capacitor issues that I run into (power supply) can be resolved by looking at the Vac ripple.
10 second test.
Go. No go.

[R.G.]

Yeah, and I'd probably stay with that if I wasn't having to look at more transformer/inductor issues these days. I can measure a cap pretty quickly with a precision resistor and a voltage. Inductors, not so much.

The classical inductor test for big inductors with high currents is the pulse inductance test. You set up a way to switch a voltage onto an inductor, and sense the current ramp. Often this was the voltage across a small sense resistor in the ground leg. For inductors, V = L* di/dt, so the inductance was equal to V*dt/di, or the inverse of the slope of the rise of current. That sounds pretty convoluted, but it's easy to set up with a scope, a square wave generator with a variable pulse width, and some high current, switches, like a power transistor or MOSFET. The hidden beauty of this technique is that you start with a low pulse width and get the low signal inductance, increasing the pulse width gives you a visual representation of the incremental inductance at each current, right up into saturation. When the slope of the current versus time starts to turn up towards the sky, you're seeing the onset of saturation.

But boy would it be nice just to hook a meter on.

[trem]

Nice unit.

Still.
Most of the capacitor issues that I run into (power supply) can be resolved by looking at the Vac ripple.
10 second test.
Go. No go.

Don't laugh at me.....is that just putting a DMM across the cap, and set it for VAC.?
Or are you saying to put a scope across the cap.?
Is it a percentage of the VDC that you use to judge whether or not the cap is good.?
Thank You

[R.G.]

It depends on the capacitor and DC loading, and it's pretty much only good for power supply caps.

Power supply caps charge at a repeatable time, twice per AC half cycle; in the USA this is 120 Hz (I'm presuming no one is using half wave rectified power these days). The cap charges up to the peak of the incoming voltage and would stay there forever if charge was not pulled out by a load. Each half cycle, the cap runs down by a voltage equal to the resistor-capacitor discharge. For big voltages and low-current loads, this can be simplified to dV = I*dt/C, where I is the load current, dv is the change in voltage, and dt is the time between recharges, about 8.3mS in the USA.

With your meter set for AC volts across the cap, there is a BIG dc voltage that the meter ought to ignore; you have to check your meter, as not all meters do ignore the DC, and some would be damaged by looking at a few volts of AC on top of a few hundred DC. But if your meter can take it, you read the AC volts.

This tells you what I*dt/C is; you know the value of dt very accurately, you can arrange things so you know the load current I accurately, and you ought to know C; it's the capacitance value printed on the cap, with tolerances taken into account. So you can directly calculate what dV ought to be, as a "no more than X volts AC."

If the voltage you read is bigger, then either the capacitance is smaller than you thought or the load is bigger. Another possibility is that the cap has a too-big ESR, which is not accounted for in the simplistic equation for cap voltage.

For a 22uF cap that's supplying 400Vdc at 100ma, the ripple is dv = I*dt/C = 0.1A*8.3mS/22uF = 37.9V peak to peak. That's only true if the cap is **perfect** and exactly 22uF. But that's a guide. If the cap had a -20% tolerance, the reasonable voltage might be 37.9Vac*1.2 = 45.4. And your meter probably does average, not peak to peak, so it might call that "16Vac" unless it's a fancy true RMS meter.

But for a good 22uF cap, it will be in that range. If the ripple voltage is much bigger, then either the capacitance is getting smaller, the ESR is getting bigger, or both.

Given the need for knowing the load current and the meter characteristics, this is a good test only if you have measured a lot of stuff. But with mild knowledge, like having measured a lot of amps like this one, or having measured this amp when it was new(ish) and having a good comparison, it's a good indicator.

[trem]

OK...I see what you are saying.
I have a scope...a cheapish one... BK Precision 2120B.
My meters are:
35XP-A Compact Digital Multimeter | Amprobe
and a
http://extech.com/instruments/produc...=48&prodid=625

Does a scope "ignore" the VDC as you were discussing.?
And the DMM...what would I look for in the specs that would tell me.? Is there a name or phrase that would tell me.?
Thank You

[R.G.]

Does a scope "ignore" the VDC as you were discussing.?

No. It does what the switches say - for DC, it's DC coupled from input to screen. For AC, it swaps in a cap in series. The imponderables are at what frequency the input cap rolls off low frequencies. That's something the scope specifications should specify. You want this frequency to be less than 1/10 of power line frequency, or about 6 (5?) Hz or lower. Well, I guess the cap switched in does make it ignore DC after the cap charges up.

Even low frequency, cheapish scopes are good at low frequencies, so that's not an issue.

A scope is by far the best way to look at ripple for these purposes. Peak to peak is what you want to know.

And the DMM...what would I look for in the specs that would tell me.? Is there a name or phrase that would tell me.?

Look for the low frequency response of the meter on AC ranges for the frequency rolloff - same issue as the scope - and also for the way it creates an AC reading. Most analog meters used to rectify the incoming AC to DC, then measure the average DC, and "correct" the average DC to "rms" by scaling the average DC to read a bit higher. The math behind that presumes that what you're reading for volts AC is actually a sine wave, which is not necessarily true. It's something like "average reading, rms indicating" in the specs on the AC voltage ranges.

More modern uC instruments may actually read many instantaneous values and compute RMS. I'm not familiar with the insides of more modern DMMs.

[trem]

10-4
Thank You

[J M Fahey]

Agree and add: a common (wouldn't call it typical though) value is .047uF x 630V cap or thyereabouts, and they tell you NOT to measure above, say, 400V DC.
Since a common Scope input impedance is around 1M ... do the Math

Out of the top of my head around 3Hz, which is fine to measure 50/60/100/120Hz with acceptable "Engineering/Tech" precision (say within 10%).

"Physics/Lab" precision must be much higher, of course, but we don't need that much in our everyday life.

And if you have an absolute need for that, you may compensate all or most DC and leave ripple quite visible, while setting the Scope to DC .

[nevetslab]

I spoiled myself earlier in my career, working as a product development engineer, had money to burn, had access to great used prime lab gear at pennies on the dollar at the Radio Amateur's Technical Swap Meets (TRW Swap Met Redondo Beach, General Dynamics Pomona)...before the days of ebay. I invested in GenRad, HP, Tektronix, Wavetek, Bruel & Kjaer, etc. So, regarding measuring capacitors, if I was looking to measure the cap value at or near rated voltage, I have a GenRad 1617A Capacitance Bridge...manually tuned (capacitance & DF dials), where you can apply DC voltage up to 600VDC to see what that effect has, as well as measuring leakage current.

If I was looking to see what the ESR was, of say, an electrolytic cap, I'd normally set up a 'network analyzer' to measure impedance, using a log swept frequency generator, with it's signal turned into a psuedo-constant current source, a good Log Converter (like an HP 3575 Gain/Phase Meter), either a storage scope or XY plotter, and a few resistance standards to calibrate the graph. The Y-Axis would be calibrated in ohms, X axis in Log Freq, usually 4 decades wide, and then I'd sweep the components...looking at several different manufacturer's parts of the same value, and you'd see vast differences between them out in the ESR & self-resonance domain. Of course you only see where the component's impedance finally converges close to the part's ESR. It won't show you the ESR at the lower frequencies. But, that can be computed from the measured C & DF values ESR = Df/(2pi*F*Cs), one of the many formulas and definitions from the GR 1617 Cap Bridge manual. Film caps are generally far more linear, then hit their resonance. All of them turn inductive once you've past the ESR plateau and the impedance curve begins rising again with increasing frequency.

But, for every-day work, I've an inexpensive GenRad 1658 RLC Digi-Bridge here in the shop (bought for $50-$60, I think, and agrees with my other analyzers). All the other instruments rarely get set up for the daily-dirt work Back home, I've an ESI 296 with fixtures, and an HP 3570A Network Analyzer. Haven't used it for years......

I mostly stick with my Tek 7633 or 7834 Storage Scopes, a number of plugins, good selection of Tek scope probes, a Fluke 8060A DMM, a good selection of generators...though I favor B & K 1027 for Pink Noise & Sine....and an old GenRad 1396B Tone Burst Generator...pass filtered pink noise thru it when I'm burning gear in. And, a 30A 0-140V Superior Electric Powerstat with a 5A-25A-100A Magtrol Digital Power Analyzer. I start with my ears, and either my bass or pink noise, after I see the gear is going to power up. If I need the heavy artillery to make more exotic measurements, I have it available.

[trem]

http://www.amazon.com/dp/B000VBMLIW/...productDetails
I always thought one of these would be cool to have.
The price has not really changed (much) in the last 5-6 years.

[Merlinb]

You can make a remarkably good capacitance/ESR meter with a handful of parts:
http://conradhoffman.com/cap_bridge.pdf