RX-5250 - identify circuit board component

[frontside5]

I've posted in the Panasonic subforum but suspect that this is a generic question.
I'm at my technical limit and do not know what these four burnt-out components are. Two are stamped 104 and two are stamped 502, and have a three point connection.
No sound came out, and because of the burning smell I had to turn it off.

Can anyone let me know what they are and if generic replacements can be used?

 

[Tinman]

They're variable resistors and are easy to find.
Why do you think they're burned?

 

[frontside5]

Whenever I power this up there is an immediate burning smell and there are no other components with charring on them.
My next option is to power it up again and look for smoke or sniff it out but that is obviously not the preferred course of action.

 

[Tinman]

If you're referring to the horseshoe shaped black coloring under the metal tops that turn, that's resistive material and supposed to be there.

 

[Superduper]

2 components that frequently emit bad burning smell when powered up is leaking capacitors (especially if they experience over voltage or reverse current), and the transformer. I think you should immediately check the voltage setting if this is a multi-voltage unit. Those variable resistors do seem to frequently look like that for some reason but it's not unusual. Ordinarily, those variable resistors only carry tiny current so they would typically never be exposed to the current level required to char something. If you are sure that the voltage setting is correct, then other components that can emit burning smell is resistors but they usually need to have a lot of current going through them before that will happen. Those are the common ones. Transistors can be high current devices and can burn up. Since they are already black, you may not notice until they start melting. If you have a infrared camera, you might be able to see the hot spots.

 

[frontside5]

2 components that frequently emit bad burning smell when powered up is leaking capacitors (especially if they experience over voltage or reverse current), and the transformer. I think you should immediately check the voltage setting if this is a multi-voltage unit. Those variable resistors do seem to frequently look like that for some reason but it's not unusual. Ordinarily, those variable resistors only carry tiny current so they would typically never be exposed to the current level required to char something. If you are sure that the voltage setting is correct, then other components that can emit burning smell is resistors but they usually need to have a lot of current going through them before that will happen. Those are the common ones. Transistors can be high current devices and can burn up. Since they are already black, you may not notice until they start melting. If you have a infrared camera, you might be able to see the hot spots.

Thank you for these ideas - I'll ignore the resistors.

Now that the voltage setting is correct the bad smell has subsided. I did identify the source though. The black component seems to act like a heat sink. At the right voltage it is still incredibly hot within one second, but no longer smells.

The only other observation is a capacitor that glows.

I'm honestly beyond my skill level and should really sell this piece as-is so that I can fund repairs for my grail.

 

[Tinman]

It's hard to see on my phone but that looks like a resistor.
I didn't know they could glow like that without blowing.

 

[Superduper]

You have a short. That black thing is a transistor functioning as a voltage regulator, and directly powers that power resistor (the thing that is glowing). Remember what I said about likely components that heat up (transistors, power resistors)? The circuit beyond that transistor is what's drawing excessive current. There are several components that could be the culprit but if it was me, I'd immediately check capacitors C342 and C326 to see if they are shorted. To check to see if those 2 caps are shorted, you can either take them out of circuit and see if it powers up normally without that resistor heating up, or you can first jump the 2 leads of each capacitor to bleed off any storage current, then test with an ohmmeter. If it shows 0 or near 0 ohms, they are shorted.

The other components that are powered off that circuit are the Dolby, AGC/REC and ambience IC's but those are less likely be the culprit. The 2 caps I mentioned are far more likely to cause the issue you speak of.

If you are unable to test those 2 caps or find the short, you should refrain from operating that set any further since continued operation can cause significant damage to the circuitry. If you really wish to sell it, PM me with what you are wanting, I might be interested in buying/repairing it.

-------- edit---------

When you say that........ "now that the voltage is correct....." are you saying you previously operated the unit using incorrect voltage setting?

 

[frontside5]

Thanks for your diagnosis. I understand the troubleshooting steps but am closing this one back up. If I went slowly I could definitely get there but I have a shoulder that needs my immediate attention, and a car error code, and some shelves. This will be sold as-is to get it off my list.

 

[Superduper]

Nice box, good luck with the sale.

 

[ironwolf]

Oh daaaamn.....I have never seen a resistor glow like that....that is one hot puppy! lol The transistor looks to be malfunctioning but is an easy replace (once youre fit) but I would replace that poor resistor too as its probably on borrowed time now!