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Old 8th Jan 2019, 7:11 pm   #1
Martin G7MRV
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Default Can a capacitor's value increase with time?

Im currently hunting an annoying fault in one of my PRC320 HF manpack radios - the radio is slow to switch back to receive after transmitting, and the time it takes seems to be proportional to the length of time in transmit, ie, the longer i transmit (even just in Tx mode with no modulation in SSB) the longer the switch back takes,

To complicate matters, there are three relays involved in the tx/rx switching (ive ruled one out!) plus some audio 'gates', plus a transistor bypass for the contacts of one relay! (to absorb current surges).

Listening to the relays, I can only hear two of them click but seem to 'drop out' immediately the PTT is released.

But my suspicion is falling on the possibility that something somewhere is either breaking down, or charging up, more than it should, resulting in the slow release.

I have noted that one of the relays has a 68nF cap across its coil, and is fed with 24v on transmit from the master relay, via a 100 ohm resistor. Im seeing an RC timing circuit here!!!

I understand that the 100ohm resistor may have gone high, but that to me would cause the relay to engage slowly, not release slowly, as its not in the discharge path, which is via the coil. But could the capacitor have gone high?
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Old 8th Jan 2019, 7:25 pm   #2
Skywave
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Question Re: Can a capacitors value increase with time?

"Could the capacitor have gone high?" you ask.
Obviously that's a possibility.
But the leading questions are:
1. How difficult is it to remove it?
2. Do you have the test kit to measure its capacitance?
3. Failing 2, do you have a suitable replacement cap. to fit? (Ideally a new, unused one, preferably with a stated small tolerance of its stated capacity).

Al.
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Old 8th Jan 2019, 7:28 pm   #3
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Default Re: Can a capacitors value increase with time?

Leakage in a capacitor can make it 'appear' to have a higher capacitance when used as part of a R:C time-constant.

The leakage-resistance drains-away charge that is being supplied via the proper charge-resistance.

The PRC320 uses a number of 'wet' Tantalum capacitors: the most-problematic of these are the high-voltage ones used in the +120V supply [module 5] and the frequency-synthesizer, but to be honest they're all suspect. Modern electrolytic capacitors are vastly better than 1970s wet tantalums.
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Old 8th Jan 2019, 8:21 pm   #4
Martin G7MRV
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Default Re: Can a capacitors value increase with time?

Cheers,

I have the test gear, and the spares, its very much the difficulty in actually accessing the components! The 320 is modular, but they are soldered in modules. I've already managed to rule out one of the three relays, the 'master' relay, and the power supply module, by exchanging them with my spare radios parts. But these are about the only 'pluggable' modules!

The tantalums in the synth were replaced several years ago with modern electrolytics, as were those in the spare radio's PSU. The PSU in the radio under test is a complete modern rebuild!

So that leaves me with the task of stripping the damn thing down to get at the two slave relays and their associated 68nF caps and 100 ohm resistors! I do note that neither relay has a protective diode across the coil! One of the relays is easier to access than the other, as its on the 'motherboard' and not in a screened module, so i'll start with that one!
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Old 8th Jan 2019, 10:04 pm   #5
G8HQP Dave
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Default Re: Can a capacitor's value increase with time?

A paper dielectric capacitor could go high if moisture gets in. This is because water has a high permittivity - around 80. However, enough water to change the value would also be enough to make it leaky.
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Old 8th Jan 2019, 10:35 pm   #6
Martin G7MRV
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Default Re: Can a capacitor's value increase with time?

There should be almost no moisture gets into these sets Dave, they are well sealed, admittedly though mine are no longer hermetically sealed as I dont have the means to pressurize the case!

I've got the motherboard up into its 'service' position and tracked down the first of these caps (nowhere even close to the actual relay of course!), it is some tiny little blue plastic encapsulated thing, probably a polyester at a guess. Haven't found the resistor yet! There is a 180uF tantalum as part of the relay circuit but it's across one of the contacts that routes 6v to another module, so I'm pretty sure that isn't involved.

Trouble is, with the board up in the 'service' position to access these parts, it's impossible to actually operate the set! (as the rear unit cannot be attached) so I can only do DC tests using a bench supply and meter/scope in this state, not actually test to see if the fault clears!

Tomorrow, I'll 'dry' test the relay/cap/resistor circuit with the bench supply to see exactly what it is doing, but since the set is open I will probably change the cap and resistor anyway
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Old 8th Jan 2019, 11:25 pm   #7
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Default Re: Can a capacitor's value increase with time?

I have had one capacitor in the past go high by a factor of 10. It behaved as though it was the electrical equivalent circuit of its original value, plus a value 10 times bigger with a resistor in series, in parallel with the terminals, and in parallel with that, some overall leakage resistance. I called it the capacitor with two time constants. It only showed up though because it was in the grid circuit of a horizontal oscillator with a high impedance there and caused the oscillator to run slow. In another circuit, say as bypass application, or a lower Z circuit, no problem would have been noticed. This capacitor was a black bumble-bee type and I think had paper insulation that had absorbed water and probably residual salts in the paper caused the effect.

In any case in a low Z circuit, this effect would be fairly minimal. Maybe there is a mechanical issue with the relays being slow to release, or some other issue.
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Old 9th Jan 2019, 5:06 am   #8
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Default Re: Can a capacitor's value increase with time?

Have you degaussed the relay cores and armatures? You may need to.
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Old 9th Jan 2019, 8:01 pm   #9
Martin G7MRV
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Default Re: Can a capacitor's value increase with time?

Quote:
Originally Posted by Boater Sam View Post
Have you degaussed the relay cores and armatures? You may need to.
I'm not sure that's even possible with these little hermetically sealed relays is it?

This is proving a nightmare to fault find! All I have to go on is the short delay from releasing the pressel (PTT) and the audio returning. Its not an extreme delay, perhaps 1.5s at most (enough to make amateur operation awkward), but it's shorter than the response time of any of my meters, so metering the relay voltages isnt helping. Comparisons with the spare only prove that there's an issue, but don't help in locating it!

It looks like im going to have to start module swapping, no easy task on these!
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Old 9th Jan 2019, 9:35 pm   #10
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Default Re: Can a capacitor's value increase with time?

Quote:
Originally Posted by Boater Sam View Post
Have you degaussed the relay cores and armatures? You may need to.
I have often wondered about this. In theory at least the cores in the relays should have low magnetic rententivity. But you never know the quality. One way to check this would be to remove the relay and apply reverse polarity voltage to the coil a few times and then see if that made any difference to to unlatching speed.

In small relays, where a little too much current has been asked of the contacts, the contacts can go sticky, and sometimes even weld together.
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Old 9th Jan 2019, 11:33 pm   #11
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Default Re: Can a capacitor's value increase with time?

Just thinking, if you could get a connection to the relay coils somewhere nearby (without too much disassembly), you could connect a scope or meter or LED etc and see if there appears to be an obvious delay between the coil switching off electrically and the relay mechanically opening. Also that would help confirm that the relay coil didn't have some small residual voltage supplied to it (or some residual coil current) when it should be off. If there is no appreciable mechanical delay, it might be that the circuitry driving it is defective and the relay is fine.
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Old 10th Jan 2019, 10:31 am   #12
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Default Re: Can a capacitor's value increase with time?

I have seen this kind of thing before. It is usually either a relay being slow due to friction, or that a relay is not being completely switched off due to leakage.

I have not worked on PRC320 but I have seen this sort of thing on some of the many Syncal-30 sets that I have fixed. They were plagued by failures of the transistors used to perform the simple logic functions relating to tx/rx switching and related duties. Sometimes they just went very low gain, or had high leakage.

The clue is in your description - "and the time it takes seems to be proportional to the length of time in transmit".
So that is something heating up most likely. Heat makes things leak better.

Check the circuit and try to imagine what would prevent a relay current going right to zero. So transistors leaking, or a capacitor leaking causing a transistor not to switch off completely.

You might want to try to measure the voltage across the relays - but I know what you mean about the Clansman modules - total nightmare! That's why I like Syncal-30 so much!
---
In answer to the topic question - YES, but that isn't your problem.
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Old 10th Jan 2019, 11:01 am   #13
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Default Re: Can a capacitor's value increase with time?

Certainly yes. Two examples I know are:

1) The capacitors which set the time constant of the frame oscillator in the later type of Rigonda / Vega portable TV set. They are film types, and increase in value so much that the timebase will only lock at half speed. Note that this can only occur as the effect of a value change, leakage would cause the circuit to oscillate at a higher frequency. Measurement on a capacitance bridge confirms the diagnosis, they are marked 1uF (+/- 10%) but usually read over 2uF.

2) Certain types of electrolytic capacitor used in the Sony TV9-306 also show value increases, sometimes to many multiples of the marked value. There was two types used, both have grey sleeves but one has blue printing and the other green. It is the green ones which mess about, the blue ones normally measure perfectly. This shows that the effect is a function of materials, design and quality, rather than just age. Again this is a genuine value increase, not leakage, confirmed that its effect is to make the relevant oscillators run slower.

What the cause is I don't know. Thinning of the dielectric layer perhaps?
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Old 10th Jan 2019, 7:41 pm   #14
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Default Re: Can a capacitor's value increase with time?

I'd guess thinning of the dielectric (aluminium oxide) layer as well. Normally an electrolytic stays 'formed for life' if used above a certain small percentage of its working voltage, but maybe the electrolyte decomposes in a way that prohibits this.
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Old 10th Jan 2019, 11:03 pm   #15
Martin G7MRV
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Default Re: Can a capacitor's value increase with time?

Argus - ahead of you there! Rigged an LED last night to check the DC side of the first slave relay, and the LED lights (ie rx 6v returns) before the audio returns. Ive 'dry' tested the relay with a 24v supply and continuity meter on its other pair of contacts, which carry the IF. It looks like its ok. I cant absolutely rule it out, but I dont think it is this relay.

Since that slave relay looks ok, then I think I can rule out the master relay - which switches the slaves.

My thought is still with either a capacitor failing, or as GMB suggests, a semiconductor breaking down. My instinct at this stage is something withing the later receive stages or AF stages. The demodulators, AGC generators and AF stages are all within module 6b, and a review of the circuit diagram shows another of these tantalum jobs in its supply rail. I think I will swap that module with the one from the spare radio tomorrow. Its perhaps the easiest of the suspect modules to swap - a mere 18 connections per module! So just 72 desolder/solder tasks... (assuming I bother refitting the one into the spare radio!)

The other slave relay only switches LC filters, the fault doesnt 'feel' like that one is involved, but if it is thats a real pig to get to!
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Old 11th Jan 2019, 10:01 am   #16
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Default Re: Can a capacitor's value increase with time?

This is a strange one. The popular and reliable Dubiliar 'MDC' [mixed dielectric capacitors] used by all the top manufacturers including those that were supplied by RS are often discovered to be 100% higher in value than originally intended. This goes for NOS as well as originally fitted components.
These capacitors have enjoyed a very long and reliable life but many are now 50 years over their design life. The odd thing is, they do not show any leaks with high voltage testers but their values appear to be exactly double! Most odd and can be confusing when rebuilding television timebases. Regards, John.
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Old 11th Jan 2019, 10:25 am   #17
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Default Re: Can a capacitor's value increase with time?

Quote:
Originally Posted by Heatercathodeshort View Post
This is a strange one. The popular and reliable Dubiliar 'MDC' [mixed dielectric capacitors] used by all the top manufacturers including those that were supplied by RS are often discovered to be 100% higher in value than originally intended.
I would say this is perhaps to be expected. From that result I would suggest that one of the dielectric materials has failed completely, leaving a now thinner layer to do all the work.

This could be verified by measuring their exact characteristics in detail. This should indicate which dielectric has failed.
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Old 11th Jan 2019, 2:32 pm   #18
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Default Re: Can a capacitor's value increase with time?

Quote:
Originally Posted by Heatercathodeshort View Post
This is a strange one. The popular and reliable Dubiliar 'MDC' [mixed dielectric capacitors] used by all the top manufacturers including those that were supplied by RS are often discovered to be 100% higher in value than originally intended. This goes for NOS as well as originally fitted components.
These capacitors have enjoyed a very long and reliable life but many are now 50 years over their design life. The odd thing is, they do not show any leaks with high voltage testers but their values appear to be exactly double! Most odd and can be confusing when rebuilding television timebases. Regards, John.
My 2 cents: they don't show leaks because the dielectrics are in series and only the paper is leaky. The increase in value is because of both the higher dielectric constant and the conductance of the broken down paper.
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Old 11th Jan 2019, 2:36 pm   #19
Martin G7MRV
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Default Re: Can a capacitor's value increase with time?

Well, fingers crossed its fixed!

Ive swapped the receive audio module 6b, and it seems that the faults gone, although ive further testing to do to be sure.

My suspicion is still with the tantalum capacitors, and one in particular, but fault finding the module itself will have to wait
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Old 11th Jan 2019, 5:07 pm   #20
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Default Re: Can a capacitor's value increase with time?

Yes, a mixed dielectric capacitor could go high in value without going leaky. However, what was the point of using two different dielectrics? Maybe nobody trusted this new-fangled plastic film stuff?
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