Wirewound Resistor Question(s)

Hey everyone. I'm working on restoring/repairing the amplifier on an old Fender Rhodes Suitcase piano. I'm in the process of procuring components I need.

I found 2 out-of-spec wire-wound resistors. At least I think they're wire-wound resistors. These are those Rockwood, square, sandblock style resistors, 0.5 ohm 5W +/-10%. They both read high out of tolerance.

My question is can I just get any wire-wound resistor of the same resistance and wattage? Or, do I need to find one that also has the sand/cement coating the same as what I'm replacing? I'm asking because I'm having trouble finding a sandblock type resistor with the same resistance and wattage. I found some with higher wattage, but they're way too big and won't fit on the board

Another question is do I have to use one of those "boutique", audio-grade resistors? Or, can it just be a general purpose type?

If any other info is needed, just ask.

Thanks for any help.
 
It is just a resistor, and 5 watt "sandblock" types are common.

0.5 ohms might not be a standard value anymore today, but 0.47 ohms is, look for that at Mouser or your favorite supplier instead of 0.5.

No it doesn't have to be a sand block, but those will fit comfortably and be stable mechanically. And they are common and cheap.

Since they are really pieces of wire inside a cement housing, it is unusual for them to change value. What does your meter read if you just touch its probes together? That will be the resistance of the probe wires, and must be subtracted from your reading of a resistor. In my experience it is very unlikely the parts are out of tolerance. It is difficult to read accurate fractional ohm resistances with a hand meter.

Those wirewounds are almost always either open or OK.

If this is an old one, did it have germanium outputs? And are they bad? If so, instead of looking for new germaniums, I highly recommend converting them to silicon. MJ15003s would be great, and the only real change is the bias resistors. Working from memory, the base resistors are maybe 2 or 3 ohms? Changing them to something in the 10-15 ohm range is usually the necessary change.
 

wdecho

Member
2014-10-28 11:45 am
I also agree with the other 2 post. It takes a very good ohmmeter to measure that low a resistance accurately. We are talking lab equipment. If you were to buy a new resistor I am 99% sure it will measure the same way.

To answer the other question, just use metal film resistors 1% generally 1/2 watt for most applications. They are reasonably priced nowadays.
 
Since they are really pieces of wire inside a cement housing, it is unusual for them to change value. What does your meter read if you just touch its probes together?

00.4

That will be the resistance of the probe wires, and must be subtracted from your reading of a resistor. In my experience it is very unlikely the parts are out of tolerance. It is difficult to read accurate fractional ohm resistances with a hand meter.

I checked them again and they measure high. I also checked some new resistors that I have on hand and they measure correctly within tolerance according to their color code. I checked some more resistors on the board and they check fine. Even subtracting the 0.4 from the measured values does not put the resistor measurement in tolerance range.

Those wirewounds are almost always either open or OK.

Not arguing or disagreeing, but these measure high.

If this is an old one, did it have germanium outputs? And are they bad?

It's from the 1970s, but the outputs are Si. They're the originals, but I don't know if they're bad.

I'll look around some more for some .47 ohm resistors. Thanks for the advice.
 
They cost under a dollar, so if they make you uncomfortable, just change them.

wdecho - he is talking about the 5 watt ballast resistors for the output transistors, they need to be larger than 1/2 watt. Precision of course couldn;t hurt, but is not necessary here.

At least you know about probe resistance. Also at such low resistances, if you are measuring them in circuit, even the resistance of the probe touching the solder pad matters. I like needle sharp probe tips so I can pierce the surface of the solder when taking such readings.

And when you say tolerance on a 0.5 ohm resistor I am imagining they read like an ohm or two maybe, not something like 100 ohms. Since you plan to replace the part, pull one off the board and measure it that way.
 
They cost under a dollar, so if they make you uncomfortable, just change them.

wdecho - he is talking about the 5 watt ballast resistors for the output transistors, they need to be larger than 1/2 watt. Precision of course couldn;t hurt, but is not necessary here.

At least you know about probe resistance. Also at such low resistances, if you are measuring them in circuit, even the resistance of the probe touching the solder pad matters. I like needle sharp probe tips so I can pierce the surface of the solder when taking such readings.

And when you say tolerance on a 0.5 ohm resistor I am imagining they read like an ohm or two maybe, not something like 100 ohms. Since you plan to replace the part, pull one off the board and measure it that way.

I know, I'm messing with pennies here really. They are cheap. I probably will just replace them. Heck, I'm replacing a bunch of other stiff anyway. I did desolder one end before I measured them, though.

I found some 5% Xicon ones for about $0.60. I was probably never going to buy some 1% ones, but wanted opinions nevertheless. 5% is still better than the 10% OEM ones.
 
Better here is a relative term. is a Mercedes Benz better for delivering packages than a Ford? Chances are if the value is off, within a given lot of parts, it is probably more likely they are all off the same direction as not. If we care about precision - and we usually don't - then more important that they both be 8% high than one 5% high and the other 5% low. The sameness rather than the value being the issue.

Remember, these 10% parts were working within the greater context of the whole system.
 
Better here is a relative term. is a Mercedes Benz better for delivering packages than a Ford? Chances are if the value is off, within a given lot of parts, it is probably more likely they are all off the same direction as not. If we care about precision - and we usually don't - then more important that they both be 8% high than one 5% high and the other 5% low. The sameness rather than the value being the issue.

Remember, these 10% parts were working within the greater context of the whole system.

Yes, that makes sense.