Identify Capacitor Info?

[SudoSpencer]

I'm working on a vintage Grommes 503 Receiver and came across these capacitors. I assume they are 250ufd and 15v, but could it be the other way around?

I can't find a manual or schematic anywhere.

On another note the problem it is having is a hum, and no sound on 1 channel no matter the input. So I'm starting by replacing all Caps. Any other ideas what it might be?

 

[slimecity]

Do you have a capacitor tester you could use? Ie: pull and test one, whatever value you get closer to....that would be your uF.

 

[toxcrusadr]

Where are they in the circuit, any idea?

Tube set or SS?

 

[gslikker]

250 µF indeed, they were marked that way.

 

[gadget73]

I'll say 250uf, 15v. They aren't physically big enough for the other way round.

 

[NAD80]

They are Frako caps. Replace them. Just ask Tandberg receivers owners about frako caps.

 

[SudoSpencer]

I plan on replacing them, hence finding out what they are. I did end up just pulling them and testing and got close to 250uf. Swapping them with 250uf 50v capacitors.

It's an old SS piece. Can't figure out the hum yet, swapped the Can Cap capacitors already along with the main big caps and no luck. Every changed the left transistors to the right side and vice versa and everything stayed the same. So these caps are all next.

 

[gadget73]

Got a scope? That will tell you if its power supply, power amp, preamp, or other. PS is usually 120 hz, ground loop or poor shielding is usually 60.

 

[SudoSpencer]

I don't have a scope. Uh.. On the capacitors which side is positive and which is negative? I think the indented side is positive since it has the+ nearer to that side but could be wrong.

 

[gadget73]

Sounds right. Usually the - goes to the can, the + goes through a rubber washer on this type. Rubber washer usually on the dimpled end.

There are phone apps that will tell you the frequency of a tone. I've played with a couple different spectrum analyzers before. Easy enough to have that tell you if its 120 or 60.

 

[SudoSpencer]

Any chance you could walk me through how to do that? Just use a 60hz tone from the phone app into the aux input or something?

My guess is it has to be one of the two output transformers, but not the main power transformer since the hum is only in one channel.

 

[gadget73]

Put the phone up to the speaker and let it tell you what frequency the hum is.

Transformers very rarely go bad. Its also unlikely for them to cause hum. I'll also go out on a limb and say those are probably driver transformers, not output. Transistors are usually either direct coupled to the speaker or cap coupled. Older designs used a transformer to drive the outputs though. Those don't look big enough to handle any kind of power, likely not more than a watt or so.

 

[SudoSpencer]

So I believe it's 120hz.



So if it's the Power Supply I need to get an entirely new one? I'm well beyond my typical repair knowledge.

THANK YOU @gadget73 by the way

 

[gadget73]

120 is power supply ripple. Usually that means bad power supply filter capacitors. Generally these will be the biggest caps in the unit, probably several thousand microfarad in value, caps are probably 50-100 volt rated.

Being only in one channel is very unusual though. Is it humming in the channel that makes no sound, or no?

The caps have a fair chance of fixing the hum. I doubt thats going to fix the dead channel. That will need further troubleshooting.

 

[SudoSpencer]

I had already replaced all Caps in the unit, starting with the power supply filter caps first. The hum was only in one side, the side not playing music. After doing all the Caps I finally started swapping the transistors from left to right side again (even though I had done this at the start), and found that when I swapped two of the germanium ones back between spots the hum and music would swaps sides. Thus I found my problem. Either I wasn't paying close enough attention before, or the problem was two fold, capacitors bad and a bad transistor. Nonetheless I have two new NTE121 transistors ordered and hopefully that's it!

 

[gadget73]

Cool. I've done that for troubleshooting. Works well when you have a good channel.

 

[SudoSpencer]

Well I'm an idiot. Had the new transistors arrive today. Then I realized while swapping the old one that it had a plastic separator on it but the other channel didn't. Made a new plastic separator for the second channel and put in both old transistors and boom. Perfect. Duh. Should have noticed that long ago.

Anyone know where I can find this actual small plastic piece?

 

[gadget73]

picture of part in question?

 

[SudoSpencer]

The tiny plastic piece under the transistor here.

 

[OMGCat!]

That's a mica insulator. They are typically used to insulate the transistor from the chassis so you don't get shorts.
http://www.ebay.com/itm/Lot-of-25-5...807530&hash=item2f05e16f14:g:ss4AAOSwpONZTBTp

This is a whole lot of them but not a bad price. You'd probably want to put a bit of heat sink compound on either side of the insulator to help keep the transistors cool.