Zenith SE Troubleshooting Help
- Thread starter rave0035
- Start date
rave0035
Disassemble first, ask questions later.
OK, back at it. At some point, my fiddling around introduced a strong hum into both channels, independent of the volume control. Bah, humbug.
I shotgunned all 4 filter caps to rule that out. No change in voltages or the buzz.
I've also traced the entire circuit looking for miswired anything or solder blobs, etc. I didn't find any.
Here's where I'm at - below is the schematic with all my V readings next to expected readings. I also put them in a table for easy comparison. The VAC on the secondary of the PT were measured to ground.
I've got a scope and can trace but am unsure of exactly what to look for until I get the buzz figured out. This thing is perplexing me.
Mike
I shotgunned all 4 filter caps to rule that out. No change in voltages or the buzz.
I've also traced the entire circuit looking for miswired anything or solder blobs, etc. I didn't find any.
Here's where I'm at - below is the schematic with all my V readings next to expected readings. I also put them in a table for easy comparison. The VAC on the secondary of the PT were measured to ground.
I've got a scope and can trace but am unsure of exactly what to look for until I get the buzz figured out. This thing is perplexing me.
Mike
PT is OK it would seem...
There does seem to be some sort of a load, but it doesn't appear to be anything that the power supply really connects to. I'm seeing 6 volts of drop across a 100 ohm resistor, which is only 60 ma of current being eaten by the output tubes and the drivers combined. The stock voltages call for 100 ma of current. The only thing that doesn't flow through that resistor is the first cap, and any possible problem with the rectifier or it's socket.
Is that first cap actually grounded? If its not, it'll throw all of this off. Would be worth confirming how good the transformer center tap connection is to the capacitor negative and to chassis. Resistance there also makes all of this go bogus. Usually the trafo CT will connect directly to the can of the first capacitor, which grounds to chassis through it's mounting ears.
Is that first cap actually grounded? If its not, it'll throw all of this off. Would be worth confirming how good the transformer center tap connection is to the capacitor negative and to chassis. Resistance there also makes all of this go bogus. Usually the trafo CT will connect directly to the can of the first capacitor, which grounds to chassis through it's mounting ears.
Mike Stehr
Poverty Audio
Usually...
rave0035
Disassemble first, ask questions later.
The CT is not grounded to the same spot as the filters. The PT is grounded directly to the chassis, and all four caps are grounded using a terminal strip and screw. The PT was never grounded to the can ears even when it the can was in place.
I checked the negative side of all four caps for continuity with the chassis and center tap - all checked out.
I'll start poking around with the rectifier connections today to see if I can find anything out of sorts.
Mike
I checked the negative side of all four caps for continuity with the chassis and center tap - all checked out.
I'll start poking around with the rectifier connections today to see if I can find anything out of sorts.
Mike
knockbill
Lunatic Member
If the terminal strip and screw are mounting replacement caps, is it located where the cap ground terminated? Connecting the PS cap grounds directly to the CT lug is a good way to eliminate hum...
Do you have a under chassis shot? if the caps aren't floating, try running a jumper from their grd leads to the CT lug, if they are floating, put a cap in series with the jumper...
Do you have a under chassis shot? if the caps aren't floating, try running a jumper from their grd leads to the CT lug, if they are floating, put a cap in series with the jumper...
rave0035
Disassemble first, ask questions later.
Thanks John... I tried running a jumper to the CT ground with no change. It's also worth noting that this amp has functioned well for a few years in this configuration, and the intermittent R channel issue is a recent development. The hum only appeared after I started farting around with it.
Here's the money shot:
...and here's a closeup of the rectifier socket. That 100ohm resistor does not appear healthy, but it's checked out on all of my tests. I don't have another 2w resistor handy to replace it.
Here is what the hum looks like on my scope with the volume at 0. Channel 1 is the right channel - it's a lot stronger on that side.
Mike
Here's the money shot:
...and here's a closeup of the rectifier socket. That 100ohm resistor does not appear healthy, but it's checked out on all of my tests. I don't have another 2w resistor handy to replace it.
Here is what the hum looks like on my scope with the volume at 0. Channel 1 is the right channel - it's a lot stronger on that side.
Mike
rave0035
Disassemble first, ask questions later.
Sorry; just seeing these.
6DZ7 - the switch you are referencing is still in my system - it's a bass boost and switches that network in/out of circuit depending on switch position. I believe the 'rear' just references the orientation of the switch diagram. All my tests are with the switch in the 'normal' (grounded) position.
Gadget - what do you mean by NFB?
The only thing I've eliminated in this circuit is the "External Speaker" plugs and jumpers. I have the Right output going to a dummy load, the Left output to another dummy load, and both of them share a connection with the grounded output.
I've also added a 1/4" TRS jack for the input, in place of a hardboard connector that was a PITA to work with.
Mike
rave0035
Disassemble first, ask questions later.
OK, guys - I'm about ready to throw this thing in the garbage. We know something's drawing excess current and throwing hum into the signal, but I can't find it.
I've checked every resistor and VR in this circuit - everything seems to be within tolerance.
I traced the circuit for solder blobs or touching wires - nothing apparent.
I've checked all grounds for continuity - nothing lifted.
I've tried three different rectifiers (all of which tested fine) - no change in PS voltages.
EDIT: the weird OT stuff I noticed was due to some gunk on the wires. Bad readings.
Based on the voltage readings on the prior page, where would you look? I'm ready willing and able to shotgun anything even remotely suspect at this point.
Mike
I've checked every resistor and VR in this circuit - everything seems to be within tolerance.
I traced the circuit for solder blobs or touching wires - nothing apparent.
I've checked all grounds for continuity - nothing lifted.
I've tried three different rectifiers (all of which tested fine) - no change in PS voltages.
EDIT: the weird OT stuff I noticed was due to some gunk on the wires. Bad readings.
Based on the voltage readings on the prior page, where would you look? I'm ready willing and able to shotgun anything even remotely suspect at this point.
Mike
Last edited:
s-petersen
Scott
I think some good pics of the underside will help...
Have you checked R121 and 122 the screen voltage source (310v)is down much more than the rest of the amp.
Have you checked R121 and 122 the screen voltage source (310v)is down much more than the rest of the amp.
Last edited:
s-petersen
Scott
6DZ7 I think you found it, C103 the coupling cap is likely bad,giving the 10v at the grid, and dropping the 310v
