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 Post subject: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 5:18 pm 
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Location: Columbia, SC USA
I'm continuing work on my Zenith J733, which has SMD. I have a "donor chassis" from a previous 733 that wasn't packed well, and arrived smashed. I pulled one of the cans of the donor chassis to practice the "pencil eraser" technique. The can is marked 95-1150 10.7MC RF

I believe was successful, and my DMM tells me I have continuity where I should have continuity. The "capacitance" setting doesn't give a reading, and doesn't on the device under test either. I've seen posts that give elaborate methods of checking capacitance for these cans, requiring lots of test equipment. I think I'll just plug the "repaired" can in, to see if it works.

I was hoping someone could shed some light on why the DMM can't measure the capacitance on these guys. Just trying to learn and absorb. :)

David


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 5:55 pm 
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Location: Long Island NY
It depends on which DMM you have, but most of them are really not capable of measuring small capacitances. 2 nF is the lower limit for many of them. The capacitors in the IF transformers of your set's vintage are typically around 110 pF to 150 pF, so the capacitors are around 15-20 times smaller than many meters can handle. Indeed, if you intend to make fixing antique radios a hobby, you might consider getting a purpose-built digital capacitance meter that can read smaller caps. Some of them only cost a few bucks.

Seeing if the radio works properly is a good way to test your repaired or replacement cap. If the cap is good and has the right 'Q,' you'll find that the IF transformer aligns sharply at the specified IF frequency. If the cap is off value, the peak will come at the wrong frequency. If the cap is lossy or has leakage you will find that the manner in which the transformer peaks is low and broad rather than sharp and steep.

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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 6:00 pm 
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Can the DMM read a 100pf cap? Most can only read much higher values. Mine read in steps of 0.001uf in the lowest range (1000pf) and is really only usable above 0.01uf. Most IF can caps are near 100pf (0.0001uf)

Also, you need to isolate the caps from the coils to measure them.

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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 6:23 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
Agree with above comments. Most "meters" are intended for use by HVAC repair people and electricians... they will read microfarads pretty accurately, but pF probably won't register.

My little digital meter ($25) will read down to about 20 pF. The short leads added another 2 or 3 pF.

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Rich


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 6:28 pm 
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Ah, thanks! So, two mistakes I made! The meter isn't capable, and I was hooked to the coils. Thanks to you both! I like learning new stuff.

:)


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 7:05 pm 
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Location: Santa Rosa, CA
It would be difficult to actually measure the internal capacitor. It would have to be disconnected from the coil.

Image

A meter reading might tell you if the capacitor was a dead short, but not much else.

My little MK meter will actually tell me the resonant frequency of the coil + capacitor. Won't tell me anything about the internal capacitor in this case.

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Rich


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Tue 10, 2022 7:27 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
When it comes to RF and IF components, it actually is better to measure them in circuit, at the frequencies that they will be used at, as much as you can. Things like capacitors and coils can be reliably counted on to behave differently at radio frequencies than they do at lower ones.

If you did not want to disassemble an IF transformer to test the caps (might be very difficult with subminiature ones out of transistor radios), you could still test the cap by measuring the resonant frequency of the transformer. This would be done with a signal generator, a load resistor, and an oscilloscope. With the secondary loaded similar to what it would see in circuit, say with a 10-k ohm resistor, and a 50-ohm resistor in series with the primary, you should see a sharp peak in voltage across the 10-k resistor when you tune the signal generator to the approximate frequency the transformer is tuned to. The cap has to be the correct value to give a sharp rise in voltage at the desired resonant frequency.

Note, however, that this does not test the cap for breakdown under voltage, which is the problem silver migration causes. In a fully assembled IF transformer you'd need to apply some voltage, perhaps 100-200 volts, between primary and secondary through a current limiting resistor to see if the capacitor breaks down.

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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Thu 12, 2022 5:44 pm 
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The 95-1150 needs two 24 pF silver mica capacitors. There is only a few volts across each new capacitor as they are across coils, unlike the original mica design with plate voltage next to grid voltage on same wafer.
Here is a schematic for a Zenith K731 which shows the IF transformer capacitor values.

The other diagram shows values for many Zenith transformers


Attachments:
Zenith K731-7M07schematic IF caps shown.gif
Zenith K731-7M07schematic IF caps shown.gif [ 612.9 KiB | Viewed 353 times ]
zenithifxfmrs.gif
zenithifxfmrs.gif [ 517.46 KiB | Viewed 353 times ]
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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Fri 13, 2022 11:06 pm 
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When I tried to peak the discriminator on this Zenith 8H032 with the 8.3 MHZ FM IF it wants to peak at 7.66 MHZ. Can silver mica capacitors increase in value and reduce the resonant frequency? The discriminator will dip but not zero. Any thoughts would be greatly appreciated!


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Sat 14, 2022 11:07 am 
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Location: Victoria, Australia
As a general statement electrolytic caps either loose polarity ("form") and present as a short & on the odd occasion when the set, after sitting for eons is powered as a first step, explodes. Alternatively some, in transistor radios in particular, "dry out" and they decrease in capacity. Electrolytic caps are meant to leak. following HV method is not suitable.

Waxed Paper capacitors tend to gradually absorb moisture and become resistors. I, when I do bother, will test them with an insulation tester first as 99.99% will leak. In the unlikely event that you will get a good one, if the resistance is below 200Meg that's a dud if its coupling. Relative to an IF The Mica cap may have say 250V on it, but as it is across a coil it may have next to nothing across it. It has to have a minimum of one lead disconnected to test it. I do not care

The locally made Mica Caps toward the late 30's here, were rarely an issue & the Silver Mica similar: Failures were rare. It is more likely for a Silver Mica to short, especially if it has HV DC on it. Many of yours tend to succumb to the nature of silver to grow metallic whiskers, which results in shorting. The insulation tester @500V DC may burn the whiskers.

You need to understand the circuit when testing. You cannot test a cathode bypass cap in circuit, nor accurately the resistor, when they are parallel. Understanding measurement is critically important.

Marc


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Sat 14, 2022 11:19 pm 
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Probably the best method is to use 6 clip leads and start the radio up and listen to it. 5 clip leads for the 5 lugs plus on for the can cover to chassis. No snap crackle and pop after a few hours or run time and there is a good chance that IF can is OK. Doesn't of course mean that next month or year that can may well get SMD.
John k9uwa

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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Sun 15, 2022 12:00 am 
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After 50 years+ of fixing radios (and other things) you do develop sixth sense as to what in it is likely duds. Firing it up to see if it works as the first step, can be a very expensive one. Once assessed & repaired as required, then you power it. If a set runs for 3hours, or more, the probability of fail moves to the higher percentile, of not failing.

I always monitor "B" on start-up (analogue meter).

Marcc


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 Post subject: Re: SMD Checking Capacitance with DMM
PostPosted: May Mon 16, 2022 5:34 pm 
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