I am trying to find a 350 ohm 3 1/2 watt fusible resistor. It is used in a 1971 Zenith table radio I just found. The fusible resistor is ceramic, about 3/4 inch long and 1/4 inch square. Any Ideas?
Hi @jkossuth ,
Thank you for contacting DigiKey. Unfortunately not an exact replacement, the closest offering we have is 360 ohm @ 3W BC360W-3JCT-ND.
Is that a Zenith Trans-Oceanic?
I suspect this resistor is placed in the power supply just after the rectifier.
Please allow me to second @Ryan_2724 solutions. Given the highly variable nature of the line voltage, a few percentages difference on the resistor should be fine. If my assumptions are correct, changing this resistor would be less extreme than changing the line voltage from 115 to 120 VAC.
The important part is that the fusible resistor be flame retardant. After all, every resistor operates as a fusible link - as they overheat, they will fail. The problem is that some do so with angry flames.
I do have one additional caution for our readers regarding the fusible resistor:
DO NOT oversize a fusible resistor.
The physical size and overall ability to dissipate heat is an important consideration. The fuse-like protection will be lost if the resistor is oversized.
P.S. Kindly post a few pictures of the radio and the resistor.
This Zenith radio was called the circle of sound. I’ll try to send the schematic of the radio along with the picture from the manual. The fusible resistor is R501.
Thank you for the picture and schematic.
I see it now, Jim.
The resistor is before the primary rectifier, not after.
A word of caution: This radio was built in another era. It lacks the safety isolation transformer found in nearly every modern device. The problem is that this set can have a hot ground. A miswired electrical outlet in your home, or a leaking capacitor can result in 120 VAC appearing on the chassis. This is very important as you troubleshoot as what we think is ground may actually be 120 VAC. This statement extends all the way to the speaker connection which could potentially be at 120 VAC relative to the floor you are standing upon.
Out of curiosity, is the resistor damaged (open) or just charred. If it is open, there may be other problems in the radio. For example, the output transistor Q402 may have developed a short.
Again, thank you for the update. Please let us know if you were successful in your repair.
P.S. All readers are encouraged to purchase an isolation transformer to mitigate against the electrical hazard. Also, it’s important to use a ground fault interrupter such as found in your kitchen or bathroom - the kind with a test button.
Thanks for your suggestions. The fusible resistor showed no signs of overheating only that it was open. I’ll pass along updates.
Here’s a close up of the fusible resistor that is open. No burn marks on the resistor but the right side of the lead where it’s connected to the terminal strip shows it was overheated.
Thank you, Jim.
I truly appreciate you taking the time to upload the picture.
Out of curiosity, did you try reflowing (resoldering) the resistor? While it’s impossible to tell from a picture, it looks like a poor solder joint especially on the right-side connection.
It’s possible for a bad solder connections to work for decades only to fail after many thousand thermal cycles. This cracked solder joint problem is often encountered in old stereo equipment where the output transistors meet the PCB.
Then again, the same can be said for the resistor itself. How many times has it transitioned from cold to hot?
Checked the solder connection and it was fine. The fusible resistor is definitely open. Just to check the radio out, I soldered in a fuse and powered up the radio and it worked! I didn’t leave it on for long. Nothing was smoking or heating up. I will look into it more. I know this isn’t a permanent fix.
Not recommended, as the voltage will be excessive without the resistor in place. You risk damaging downstream components such as the audio output transistor.
Thank you for the update.
Thank you for all you help. I appreciate it.
I put in a 330 ohm 3 watt 5% flameproof resistor in place of the fusible resistor and put a 2 amp pigtail fuse in series with the resistor. The radio is functioning and sounds great.
Thank you so much for sharing your experience and success.
Kindly share a picture of the radio.
Have a good weekend,
Thank you, Jim.