Cement Resistor replacement

[ValveStandard]

I'm working on repairing a Carvin X-Series amp which has a burnt 350 ohm 10 W cement resistor that I believe is part of the power supply. I haven't been able to find a replacement part online. I was wondering if anyone knew of a suitable replacement for this type of resistor.

[Jon Snell]

350 Ohms at 10 Watts are available but only in large quantity. You may need to make up 350 Ohms at 10Watts with two or even three resistors.

Three 1k 5Watt resistors, in parallel, will give you 333 Ohms at 15Watts.

[glebert]

350 ohm 10 watt ceramic resistors are available in pairs on ebay. Unlike transistors I don't think there is too much concern about counterfeit or subperforming resistors, but maybe others have a different opinion.

[Helmholtz]

Does the resistor measure open?

I wonder what could have blown it as these can take a lot of abuse.

[Tom Phillips]

As I look at the posted photo, I am not convinced that the resistor is bad so I suggest that further diagnosis is in order as suggested by Helmholtz. The black grunge shown doesn't look like something that type of resistor would spew out.

[Jon Snell]

If that resistor is R121 then I suspect it is probably not open circuit at all as it feeds the screen grid resistors in the output stage and the rest of the circuit.
Is R121 open circuit?
What is that burnt component next to it?
The manual calls for a 350R 5Watt resistor, not 10Watt.
The value is not critical; 330R is fine.

[xtian]

Also check to see if the green PCB material near the resistor is burned. The carbonized fiberglass will become conductive if dark enough, and this issue must be mitigated.

[xtian]

360-ohms is a far more common resistor value, easier to source, and should work fine there.

[Leo_Gnardo]

Also check to see if the green PCB material near the resistor is burned. The carbonized fiberglass will become conductive if dark enough, and this issue must be mitigated.

Good observation. When I encounter boards that are toasted like this, break out the Dremel and grind the blackened section into dust. With a powerful vacuum cleaner hose in close proximity.

Agree, 330 ohm fairly common and perfectly good for this application. Stand it up on its legs a little, so it doesn't continue to burn the board.

[Enzo]

Before I Dremel, I wipe with a Q-tip and isopropyl alcohol. Often when something burns, the soot makes it look much more extensive than it really is. Get a mote clear idea what you need to Dremel.

[g1]

As I look at the posted photo, I am not convinced that the resistor is bad so I suggest that further diagnosis is in order as suggested by Helmholtz. The black grunge shown doesn't look like something that type of resistor would spew out.

My thoughts as well. Either a previous cement resistor, or a resistor along side that one burnt. Not sure if that is carbon, or some kind of goop to repair a burnt board there. But whatever made that mess is not the 350R 10W that is currently in place there.

[g1]

Is R121 open circuit?
What is that burnt component next to it?
The manual calls for a 350R 5Watt resistor, not 10Watt.

There is a later schematic that shows the R121 part upgraded to 10W, see attached.
From this photo of another unit, there is no other component where the bulk of the damage is. I wonder if it is the board itself and there is some arcing under the resistor?

[Tom Phillips]

... there is no other component where the bulk of the damage is. I wonder if it is the board itself and there is some arcing under the resistor?

Perhaps there was a previous C68 electrolytic or, the existing C68, that leaked its electrolyte.

[g1]

Perhaps there was a previous C68 electrolytic or, the existing C68, that leaked its electrolyte.

Maybe, but it looks to me like carbon burn residue on the neighbouring R120.

[Jon Snell]

We still don't know if R121 is OK or not. Once we know, we will have a better idea.
The PCB will clean up with a fibreglass pen so no worries there.