Wowbagger: I measured across R11 and R12 and saw no voltage.
john65b: I'm measuring the ends. The center isn't accessible to me without unmounting the board.
john65b: I'm measuring the ends. The center isn't accessible to me without unmounting the board.
You wont get any resistance change measuring from the ends...that explains that - the pot you removed - measure to see if the resistance changes?
Its just one less thing to check if the pot is functioning correctly...
Its just one less thing to check if the pot is functioning correctly...
John, when in the PCB, the wiper is shorted to an outer pin. Out of the circuit, you're right, it has to be measured between the middle and outer pin.
john65b: I don't remember how I measured it when I removed it yesterday. I measured again just now. One side has ~2R, regardless of pot position (center and left pins). The other side has ~1.8k, regardless of pot position (center and right pins).
So the pot is definitely a problem, but what leaves me stumped is why replacing the pot didn't fix it, since adjusting the new pot does indeed change the resistance to some degree.
I'm out of ideas - hopefully replacing Q2 helps, but that doesn't explain the pot behavior.
I'm out of ideas - hopefully replacing Q2 helps, but that doesn't explain the pot behavior.
Wowbagger: Thanks for all the help. The new ZTX parts should be here tonight. I'll let you know how it goes!
Don't thank me for asking a ton of questions that didn't solve your problem! I look forward to your updates.
Ugh. I replaced Q2, Q3, and Q4 - and the results haven't changed. I have replacements for the zener diode and the MOSFETS if anyone thinks that's where the problem may be.
This may be unrelated, but how is your input wired? It looks like you have two wires soldered to IN+
I do. It was the only shielded cable I had - so I used both wires for the center terminal on the RCA.
If you take out Q7 and Q8, you can test them: How can you conduct a simple test for a MOSFET? - Electrical Engineering Stack Exchange
I couldn't remove them without cutting the leads. I have a solder-sucker but no wick. One of them passes current between the source and drain in both directions. None of the others do. I think we found it!
ZenMod: I just checked (because I forgot every detail from yesterday). Either direction between source and drain presents too much resistance for the meter to beep. I don't remember what the meter actually read yesterday. I'm hoping this works!
Okay - I'm a bit discouraged now. Same result. There's +19v on the speaker output upon power up, and it seems to climb; but I don't want to keep the power applied because I don't want to burn anything out.
I tested the MOSFETs in place, and all of them are passing in one direction, but not the other (between source and drain).
Could the bad MOSFET have effected the ZTX parts?
I tested the MOSFETs in place, and all of them are passing in one direction, but not the other (between source and drain).
Could the bad MOSFET have effected the ZTX parts?
if you can't observe any heating, you're probably in good position , just needing proper setting
put one DVM across output posts, goal to set DC offset to 0
put second DVM across any mosfet source resistor , to observe Iq, goal is to set to prescribed value (was it 400mV?)
your positive DC offset now means that lower mosfets are completely shut off, thus representing too much resistance , so output noe voltage is maxed
now fiddle with R7 trimpot, having a goal to increase gate voltage for lower mosfets , to open them for current .....and finally set DC offset
write what you got
put one DVM across output posts, goal to set DC offset to 0
put second DVM across any mosfet source resistor , to observe Iq, goal is to set to prescribed value (was it 400mV?)
your positive DC offset now means that lower mosfets are completely shut off, thus representing too much resistance , so output noe voltage is maxed
now fiddle with R7 trimpot, having a goal to increase gate voltage for lower mosfets , to open them for current .....and finally set DC offset
write what you got
Zen Mod: I tested across the .47R resistors at either end of the board (I think they're R16 and R19). I get 0V across both of them with my meter on the 200mv setting. Ugh
there can't be current if lower mosfets are closed shut
you need to open them to enable current path
try what I wrote
you need to open them to enable current path
try what I wrote