Electrolytic capacitor questions (reversed polarity)

[Richard9025]

Hi! I recently replaced all the electrolytic capacitors in an old 70s radio. I had a problem with the huge 4700uF/50V filter caps (2 of them), I could't determime which are the pozitive and negative leads. (I'll attach some photos with them and let you determine , spoiler: one cap has 5 leads.) So I put the new ones randomly.
When I pressed the power button a big spark came from the primary fuse (the fuse that is between one terminal of the primary winding of the transformer and the 220V line. The fuse is rated 500mA/250V. What I did? I replaced the fuse and powered it again. Same thing. Note that the fuse blew exactly the same time i pressed the power button. I realised that the filter caps were exactly reversed connected after I checked the service manual.

Now the question:
Are the two 4700/50V capacitors good after they were reversely connected for a fraction of a fraction of a second at ~30Vdc ? They look brand new, no buldges or any sign of damage. Shall I replace 'em ? They're nice nichicons...

Note: When I powered it the 2nd time, I connected a multimeter to the leads of one of the filter caps, when(and after) it went on, no voltage was on the terminals. I'll attach the schematic later. Maybe the nature of them connected reversed instantly led to the blowing of the primary fuse, without the capacitors being exposed to reverse voltage. Also I should say that the radio worked (poorly) before I changed the electrolytics. Thanks!

 

[kellys_eye]

The only reliable way to test them would be with an ESR meter (equivalent series resistance) to check for internal damage.

Experience proves that the capacitors DON'T survive the reverse connection though.

Remove (or disconnect) the suspect capacitors and drop in any reasonable value you have available (that has the minimum working voltage) and re-power it to see if the fuse stays intact - just to prove there isn't a fault elsewhere in the circuitry.

 

[davenn]

I could't determime which are the pozitive and negative leads. (I'll attach some photos with them and let you determine

you haven't attached photos

spoiler: one cap has 5 leads.) So I put the new ones randomly.

some of those connections are probably not electrical, but rather just mechanical connections to hold it to the chassis

but to be sure, please post some clear photos

 

[CDRIVE]

You used exceedingly poor logic. Before the cap replacement it worked poorly (undefined statement) but it worked. You're unsure of the cap's polarity but solder it up anyway. It then blows the fuse when powered up. Instead of a logical response to a blown fuse that didn't blow before you try frying it further by replacing the fuse. I guess it could have been worse. At least you didn't replace the fuse with a higher amperage one... I think!

Chris

 

[Richard9025]

you haven't attached photos

Side (this is the only writing on the side of the can, the rest is blank, no minus sign or anything)


Top


And bottom (notice faintly 1, 2,3 next to the leads) (I used the flash because it captures more details, at least on this camera)



You tell me which are the negative and positive leads

some of those connections are probably not electrical, but rather just mechanical connections to hold it to the chassis

Yea, 3 leads are for mechanical and only 2 for electrical (it should be one more because it says 2minuspoles and there is one cutout for another lead that isn't there)

I guess it could have been worse. At least you didn't replace the fuse with a higher amperage one... I think!

It could've gone much worse What if the two 4700uf caps puked all over

Remove (or disconnect) the suspect capacitors and drop in any reasonable value you have available (that has the minimum working voltage) and re-power it to see if the fuse stays intact - just to prove there isn't a fault elsewhere in the circuitry.

As I only replaced the electrolytic caps, it can't be any other problem except the polarity because there aren't any solder cracks or solder bridges. After some searches I found this drawing that helps me check if I soldered them properly:
(censored with red to avoid cheating at the above questions ) ) (this is only a part of the full image)
I got loads of capacitors rated at or over 35V what value should I put just to check? 1000, 470? I think that I can put some new 4700uf s in the right position and check 10 times if the rest are not reversely connected using this drawing that I discovered after I messed up.

Also: schematic of the caps and the fuse that likes to blow.



Many thanks for all your support!!

 

[Alec_t]

it says 2minuspoles

It doesn't. It says "2 Minuspol". Perhaps that signifies pin 2 is the minus pole?

 

[Richard9025]

Perhaps that signifies pin 2 is the minus pole?

Yea, I thought that 2 minuspol means 2 negative leads, that's why it had a third cutout but why would you have 2 negative terminals for 1 capacitor? (maybe that cutout is for a double capacitor with one shared negative lead). Anyways I'll fix the problems and see if the radio works. Thanks!!

 

[kellys_eye]

Are those the 'right' capacitors for the job? I mean - the ones you fitted are 'polarized' in the sense that they can only be inserted in one position due to the tab positions.

If they were 1-for-1 replacements how can you get the job wrong?

Another thing - how OLD are those capacitors? Even unused they can 'dry out' and not work as they should. There is a technique for 'reforming' old capacitors before using them - putting old capacitors straight into circuit and expecting them to work is often problematic hence the 'reforming' method.

Another thing - putting them in backwards WOULD cause them to explode but WOULD'NT cause the fuse to blow instantaneously - it would take second or two for that to happen so perhaps there is a fault elsewhere?

The transformer primary is wired via an input voltage selector - has this been set correctly?

Another thing - check C813 as they are often faulty. Use an X2 class replacement.

Another thing - if C813 IS short (and I bet it is) then your capacitors may have been saved by the fact.....

 

[Richard9025]

Ok so what I did is I checked the polarity of each capacitor and replaced all the electrolytics execpt some 0.15ufd's. Only the big 4700ufds were reversely connected. I grabbed another 4700 pair and I connected them as they should be, after I powered it on: Sound in the speakers! It receives very good, stations on MW, LW and FM ! On SW it does nothing, no static no nothing.

Another thing - putting them in backwards WOULD cause them to explode but WOULD'NT cause the fuse to blow instantaneously - it would take second or two for that to happen so perhaps there is a fault elsewhere?

The capacitors were causing the fuse to blow, how? Idk magic but with the new caps in the correct position it works flawlessly.

The transformer primary is wired via an input voltage selector - has this been set correctly?

Of course, set it to 220V and the primary fuse 500mA (it says 0.5A-220V 1A-110V on the schematic)

Another thing - check C813 as they are often faulty. Use an X2 class replacement.

Looks like it isn't.

The next thing to do is to replace the bulbs with leds. They're 7V 0.3A. I got a lot of 3mm standard LEDs, 3V ones. How could I wire the DC LEDs to an 7V AC source ? I saw people breaking the glasses of the bulbs and soldering 2 leds in there. Thanks!

 

[kellys_eye]

The next thing to do is to replace the bulbs with leds.

Simple diode wired across the LED and a series current limiting resistor will work. Fitting them into the old bulb holder is a good idea - possibly fiddly, but a great looking result.

 

[CDRIVE]

No, don't do that. With a 7VDC supply you will need a 200Ω resistor in series with your LED. This is based on the voltage drop of 3V for a White LED.

Chris

Edit: Just saw Kelly's reply. Yeah a diode across the LED (Diode Cathode to LED Anode) if the supply is AC.

 

[duke37]

Or use two LEDs in inverse parallel if they will fit.

 

[Richard9025]

Or use two LEDs in inverse parallel if they will fit.

I connected 2 leds in inverse parallel, and when I powered the radio, they lit up for half a second then they blew.

With a 7VDC supply you will need a 200Ω resistor

Then I connected 2 leds in inverse parallel in series with a 200Ω resistor and they work! problem solved. Many thanks! BTW: how do you determine this value? If the source was 6.5Vac what value should the resistor have?

 

[duke37]

The series resistor can be set by getting the same brightness as a led fed with 20mA.
Some time back, I calculated the average current from the pulse of current, this involved calculus, last used 65 years ago. I am not doing it again, it would be quicker and easier to use LTSPICE.