I am trying to make an educated guess about the identity of a (partially) unknown zener diode and also trying to learn some fundamental theory about a solid state amplifier.
I have a Cadence car audio amplifier that pooped out on me. I opened it up and saw a large-ish resistor that had signs of heat stress.
I snipped the resistor out to test it - it read properly according to color-bands. Then i found a nearby zener diode that was a dead short in both directions.
I had been talking to a tech from Cadence meanwhile. At the time I had assumed that the resistor was the problem and he was checking to find the wattage of the resistor for me. He explained a bit of the operation of the circuit while i was on the phone with him. He said that the resistor is a part of the power supply to the preamp section. He said that the circuit developed +15v and -15v through the heat-stressed resistor (as well as its twin resistor that did not show signs of heat stress), the zeners and two capacitors.
Huh. Cool... i guess. I know all those words but don't know why you want negative volts or why zener diodes are necessary. I filed the info in the "trivia" part of my brain.
But then i found the failed zener diode. I happened to have a zener in the parts drawer - but it was 18v. I soldered it into the circuit anyways and tested the amp. Got sound. Groovy.
Now i'm guessing that i need to find the right value of zener, but i can't make out the values on this tiny little speck of glass.
It is definitely a 1n47 and it definitely ends in A, but the other two digits are screwed up. Attempting to read the other zener was attempted. Failure.
So i looked at parts for a while until i came up with 1n4744a. It has a 15v zener voltage. I don't know if that's what it actually is. My first guess at the final digits of the zener was "USA". I suppose a u and an s could result if a 44 was properly smudged?
Then i started wondering if it even mattered. Do those zeners need to be matched to each other? Then what if i just cut out the other 15v zener and replaced it with an 18v zener? Would that (voltage ratings aside) screw up anything downstream? Would it make the tone bad? How important is that +15 and -15?
I suspect that the most correct answer is to wait until monday and call the technician again, but i'm having a hard time being patient. I thought maybe someone might be willing to fill in a few of the gaps in my knowledge?
Oh and while i'm asking basic theory questions - would any of you guys ever make a permanent repair to an axial component by soldering it to the leads of the old component? I ask only because i'm not looking forward to removing the circuit board from the chassis. It's going to be a huge pita. If i can replace components without having to flip the board over it would save a lot of time.
Well thanks for reading. 🙂
I have a Cadence car audio amplifier that pooped out on me. I opened it up and saw a large-ish resistor that had signs of heat stress.
I snipped the resistor out to test it - it read properly according to color-bands. Then i found a nearby zener diode that was a dead short in both directions.
I had been talking to a tech from Cadence meanwhile. At the time I had assumed that the resistor was the problem and he was checking to find the wattage of the resistor for me. He explained a bit of the operation of the circuit while i was on the phone with him. He said that the resistor is a part of the power supply to the preamp section. He said that the circuit developed +15v and -15v through the heat-stressed resistor (as well as its twin resistor that did not show signs of heat stress), the zeners and two capacitors.
Huh. Cool... i guess. I know all those words but don't know why you want negative volts or why zener diodes are necessary. I filed the info in the "trivia" part of my brain.
But then i found the failed zener diode. I happened to have a zener in the parts drawer - but it was 18v. I soldered it into the circuit anyways and tested the amp. Got sound. Groovy.
Now i'm guessing that i need to find the right value of zener, but i can't make out the values on this tiny little speck of glass.
It is definitely a 1n47 and it definitely ends in A, but the other two digits are screwed up. Attempting to read the other zener was attempted. Failure.
So i looked at parts for a while until i came up with 1n4744a. It has a 15v zener voltage. I don't know if that's what it actually is. My first guess at the final digits of the zener was "USA". I suppose a u and an s could result if a 44 was properly smudged?
Then i started wondering if it even mattered. Do those zeners need to be matched to each other? Then what if i just cut out the other 15v zener and replaced it with an 18v zener? Would that (voltage ratings aside) screw up anything downstream? Would it make the tone bad? How important is that +15 and -15?
I suspect that the most correct answer is to wait until monday and call the technician again, but i'm having a hard time being patient. I thought maybe someone might be willing to fill in a few of the gaps in my knowledge?
Oh and while i'm asking basic theory questions - would any of you guys ever make a permanent repair to an axial component by soldering it to the leads of the old component? I ask only because i'm not looking forward to removing the circuit board from the chassis. It's going to be a huge pita. If i can replace components without having to flip the board over it would save a lot of time.
Well thanks for reading. 🙂
I suspect the zener has gone short and the extra voltage across the resistor has cooked it.
Its probably the op amp supply 15v zener.
You can be certain by removing the other zener put a 1k resistor in series with it and put it across a 20 volt supply. Measure the volts across the zener.
Its probably the op amp supply 15v zener.
You can be certain by removing the other zener put a 1k resistor in series with it and put it across a 20 volt supply. Measure the volts across the zener.
Opamps usually work with split supply´s so you need one for regulating the positive and another for the negative voltage. You say there are two capacitors too ...I presume they are connected in parallel with the supply rails and if you don´t match them you will get DC on the opamp output.
There is a device you can buy (I have many but never used them) that is basically a little spring shaped coil of solder. It's intended to be used to replace PC board components.
What you do is snip the old component out, leaving some stub of a lead. You drop the spring over the stub, then force the new component into the spring with the stub. Then heat and presumably the solder melts and makes a 'permanent' repair.
What you do is snip the old component out, leaving some stub of a lead. You drop the spring over the stub, then force the new component into the spring with the stub. Then heat and presumably the solder melts and makes a 'permanent' repair.
So you told me exactly what to do but i didn't read until it was too late. All the resistors and transformers are not at my house. :-( My loss. Thanks for the advice!
@Maaco - i thought the sound was pretty interesting - so i guess i know what it sounds like when the voltages aren't matched. It sounded kinda... boomy i guess. I guess it was kind of like the speakers were covered by blankets. Maybe the filters were screwed up? It reminded me of what my components sound like if the tweeter isn't connected, except not quite as extreme.
@bob - that sounds brilliant. I like that.
Hi, that 18V Zener might be ok just to test, but not all OPA's are designed to work on more than + & - 15V, so i would replace it with a 15V one. A 400mW - 1W type should be fine, & it won't matter what the code number is !
Make sure you replace the burnt out resistor with one that is 600mW or above.
Make sure you replace the burnt out resistor with one that is 600mW or above.
The cause of death was a shorted zener diode - Imgur
Pic of relevant circuitry. The failed zener is the one immediately below the pair of voltage dropping resistors. I was tentatively told (until the tech can get the real answer) that the resistor is probably either 3 watt or 5 watt.
Pic of relevant circuitry. The failed zener is the one immediately below the pair of voltage dropping resistors. I was tentatively told (until the tech can get the real answer) that the resistor is probably either 3 watt or 5 watt.
Pics are good !
I see the resistors are indeed a higher wattage than i envisaged, so naturally replace with same 😉
I see the resistors are indeed a higher wattage than i envisaged, so naturally replace with same 😉
Congratulations that´s how we people repair things. By sharing experience and knowledge.
I was struggling this week with a smoke machine board replacing components just to find out today that the heater relay driving transistor I replaced (S8050 ~ BC548 type) was inverted.
I was struggling this week with a smoke machine board replacing components just to find out today that the heater relay driving transistor I replaced (S8050 ~ BC548 type) was inverted.
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