Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Checking Capacitors

Status
Not open for further replies.

RetiredHAL

New Member
Just a quick question. What quick method is used to check for open or shorted capacitors. Indication only for short or open. I am not after a capacitance reading. I am restoring an old radio. Most caps after 50- 60 years have some leakage. I use an Analogue (moving coil meter, 15 v battery for X1000 Ohm Scale) to check.
An electrolytic will usually show a dip and then slowly climb to Mohms.
I tried to check some .1mF --- .oo1 mF (not electrolytic) and the meter did not dip at all. (indicate open) So I used an extra resistance 4 Mohm in series with the cap under test. Still no dip. I thought that with the extra series resistor I would have seen a small charging effect resulting in a small dip. eg 4M x .1mF = .4 CR sec. NB I have not checked this method with known good caps, I will tomorrow.
cheers RH
 
Hai R H,
Ac signal are allowed thro' these capacitors, as you already know. Many multi-meters provide for capacitor measurement.just disconnect from radio and connect one by one to DMM in capacitor range and you will read their value approximately.
a crude test is also possible passing an audio signal thro the device and connect the output to an amplifier, if you are able to listen the output in speaker, the capacitor can be certified, provided that it is tested for short circuit earlier.
 
mvs sarma said:
Hai R H,
Ac signal are allowed thro' these capacitors, as you already know. Many multi-meters provide for capacitor measurement.just disconnect from radio and connect one by one to DMM in capacitor range and you will read their value approximately.

has he said he is not after a capacitance reading... :rolleyes:


@RetiredHAL your analog meter didn't deeped at all because the capacitor you checked was too small and it was charged very fast!

if you used an analog multimeter and it stays infinite ohms or at least close to that then i guess your capacitor passed your leakage test.

if you want more accurate ressutls about your capacitor's condition then use an ESR meter.
 
Last edited:
Thank you for your replies.
Picasm, I am aware of that site and in fact I subscribe to that forum. As I have restored some other stuff.
The reason I posted here is that perhaps someone has another method of checking other than as per that site.
Unfortunately, ALL capacitors are of the suspect black type. I guess I will have to either do a mass replacement or test each one for some time .

I am testing each one using a 1K Ohm 10 watt resistor using the EHT from the set (all valves removed, except for the power tube) 320 V dc in series with the capacitor under test:

B+ 320V ---R (WireWound) 1k 10 W----- CAP under test-----Chassis 0 V.
I then check the voltage drop across R to calculate the Leakage Current.

If the leakage current is less than 2 mA , I then use a scope to see if 80K 9Volt squarewave (555 astable setup) will pass to make sure the cap is not open.

This is a time consuming effort as this set has about 35- 40 caps.

That is why I posted here to see if there was a quicker method.

I'll keep at it, so far two caps found that need replacing.

Cheers RH
 
Last edited:
whiz115 said:
if you want more accurate ressutls about your capacitor's condition then use an ESR meter.

Are you sure you want to use ESR meter on capacitors with capacitance of 0.1uF or less?
 
RetiredHAL said:
Thank you for your replies.
Picasm, I am aware of that site and in fact I subscribe to that forum. As I have restored some other stuff.
The reason I posted here is that perhaps someone has another method of checking other than as per that site.
Unfortunately, ALL capacitors are of the suspect black type. I guess I will have to either do a mass replacement or test each one for some time .

I am testing each one using a 1K Ohm 10 watt resistor using the EHT from the set (all valves removed, except for the power tube) 320 V dc in series with the capacitor under test:

B+ 320V ---R (WireWound) 1k 10 W----- CAP under test-----Chassis 0 V.
I then check the voltage drop across R to calculate the Leakage Current.

If the leakage current is less than 2 mA , I then use a scope to see if 80K 9Volt squarewave (555 astable setup) will pass to make sure the cap is not open.

This is a time consuming effort as this set has about 35- 40 caps.

That is why I posted here to see if there was a quicker method.

I'll keep at it, so far two caps found that need replacing.

Cheers RH
If some of the capacitors are not rated for such high voltage, perhaps they may fail now while you test them. My opinion is generally Disc capacitors and polyester caps don't generally fail. Due to aging only electrolytics will be dry and loose their value. Perhaps you can overcome all this hurdle by replacing 5 to 6 electrolytics without even going for test. But ensure their working voltage is maintained. your problem will be mostly sorted ot. At the end, it is every likely that the Final audio amplifier Valve ( perhaps(EL84 in many cases) might fail due to aging. this will be revealed by lower cathode voltage. also the G1 voltage might turn +ve compared to a -ve value originally expected. try to replace if available.. Otherwise, perhaps solid-state audio amplifier module can be substituted by suitably powering it .
 
I would agree, check and change the electrolytics, but for the non-electrolytics I would only check the grid coupling capacitors, these commonly go leaky and overload the valve. I wouldn't blanket check them otherwise, they only rarely fail (apart from anode/grid couplers).

To check them just measure the voltage on the grid it feeds, if it's leaky the grid will be positive.
 
Hi, Sarma, I use the B+ 320 to test 400V working, 600/1000v test rated caps and use a B+ 40V (lucky I found that) to test lower rated caps. Problem is not final output (6V6G) -- I can inject 555 variable frequency square wave into Pick Up (phono) setting and not have squeal -- but in mixer/IF/detector or auto gain cct as problem is very faint reception but squeal (not 50HZ motorboating) when I turn up volume.
A solid state radio in a beautiful 1947 cabinet without the rich sound of valves is not acceptable.
Cheers RH
 
Last edited:
eblc1388 said:
Are you sure you want to use ESR meter on capacitors with capacitance of 0.1uF or less?


an ESR meter as the one i build and you gave me your advices.. :) it will show a reading even in very small values on non electrolytic capacitors of course in such cases the indication has no importance but i guess you can check if the capacitor you coupled is open.

also a simple analog multimeter at ohms function can help to discover if there is leakage i think infinite or almost infinite indication togather with the above test can tell us something... :rolleyes:
 
RetiredHAL said:
Hi, Sarma, I use the B+ 320 to test 400V working, 600/1000v test rated caps and use a B+ 40V (lucky I found that) to test lower rated caps. Problem is not final output (6V6G) -- I can inject 555 variable frequency square wave into Pick Up (phono) setting and not have squeal -- but in mixer/IF/detector or auto gain cct as problem is very faint reception but squeal (not 50HZ motorboating) when I turn up volume.
A solid state radio in a beautiful 1947 cabinet without the rich sound of valves is not acceptable.
Cheers RH
Can you please upload the schematic of your radio in case you have, as that will help easy discussion, sir.
While i will come back on RF diagnosis, (perhaps need replace the Osc-mixer valve-- faintly recollect- whether 6K7 ?)
Can you please upload the schematic of your radio in case you have, as that will help easy discussion, sir.

you may replace the filter caps on AGC feed back path. and also the 6V6G cathode resistor decoupling capacitor. Generally it used to be 25uF/25V

i was suggesting you to use a solid state audio amplifier board like pre assembled TBA810 module to substitute final stage. No issue as you have cleared the audio stage.

Most important appear to be some 1/2 to 1 watt resistors in the screen and suppressor grid ccts, generally above 330, 470 and 680K:eek:hm: which would have eventually opened up. Please have a check.

P S: BTW, i am uploading a tentative schematic for just reference. the valve pin numbers will not tally.i have marked suspected components by arrow. you may check them at similar locations ,with values as marked in your set.
 
Last edited:
hi, mvs sarma, thank you for uploading the zip of the set.
Problem was leaky capacitor 0.1 uF 400V from screen 6U7 pin 4 (1st IF) to ground ( I think you call it a bypass capacitor--- Its been a long time since I worked on valves---) At least I think so. I did do a "blanket" cap check and replaced a few more caps which showed some leakage. Interestingly, as I checked each cap for some time their leakage reduced with time eg. most caps showed about .1 to .2 mA leakage at first, but after a few minutes and a few switch on/offs this reduced to almost nothing (barely a needle movement). The radio is now in its restored cabinet .I tried to copy the cct diagram but I could not get a good enough scan to warrant uploading.
Cheers RH
 
Last edited:
Great RetiredHAL, Congratulations to you for the effort.
We too call it decoupling capacitor and not coupling,Sir.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top