HELP With VOX AC100CPH

[angelothewolf]

http://www.ampix.org/albums/userpics...x_AC100CPH.pdf

Hi everyone!! ive got this VOX AC100CPH wich suffer some of those ^worst than before^ repairs.

the amp came up with a very low output and stuck in the dirt channel. some hours later i get the schem and i realise that the TCN207 - CON7 / TCN9 - CON9 connections were inverted, so, the TCN9 with the TCN207 & the CON7 with CON9.

after connecting everything in the right way the amp came to life again, sounds loud and the channel switch does his job too. BUT! an intermitent loud poping sounds appeared. so, its being difficult to get to the source of the POP. i try with new valves and the problem persist, all the voltages on the schem are normal. i think its an preamp issue couse trhough the return the pops goes away.
today i got an oscilloscope so a little tips would be very appreciated!!

http://s11.postimg.org/d34rthpar/DSCN2701.jpg
http://s11.postimg.org/8deu4vyab/DSCN2703.jpg
http://s11.postimg.org/raz1vvrdf/DSCN2705.jpg
http://s11.postimg.org/gcnsdp2s3/DSCN2706.jpg

[drewl]

Does it "pop" with nothing plugged in?
The input jack grounds the preamp up to like the 3rd preamp tube I think, so that might help isolate which stage.
Pull the preamp tubes one at a time starting from the left or first stage and that may tell you which stage is popping.

[angelothewolf]

Does it "pop" with nothing plugged in?
The input jack grounds the preamp up to like the 3rd preamp tube I think, so that might help isolate which stage.
Pull the preamp tubes one at a time starting from the left or first stage and that may tell you which stage is popping.

disconnect con7 and the noise remains the same, disconnect con7 and con9 and the noise still there. that means that the problem is not in the preamp stage>? when i disconnect CON16 the noise goes away!... when i pull out the first tube the noise goes away too. aaaaa

thanks for the replies!

[nickb]

I wonder if the base of V1 might have bad contacts or solder joints. Try gently rocking V1 in it's base so see if it's mechanical.

[angelothewolf]

I wonder if the base of V1 might have bad contacts or solder joints. Try gently rocking V1 in it's base so see if it's mechanical.

its no mechanical...

today i try to trace the noise, starting from the output and i end up in V1. the DC poping noise noise is pasing trhough C32, and right after that cap im measuring up to 200mV DC peaks. i think it could be the mosfet Q6 but i cant find a replacement. what y think?

[nickb]

its no mechanical...

today i try to trace the noise, starting from the output and i end up in V1. the DC poping noise noise is pasing trhough C32, and right after that cap im measuring up to 200mV DC peaks. i think it could be the mosfet Q6 but i cant find a replacement. what y think?

Does the channel 1 gain pot change the level of the pops? They might be coming from the first section of V1. You need to rule that out before assuming it's the P-channel JFET (not a MOSFET). The J174 is obsolete but it's not critical. a 2N5460 springs to mind but the pinout is different so be warned.

Also, have you probed the gate of the JEFT to see if the noise is coming in there?

[angelothewolf]

Does the channel 1 gain pot change the level of the pops? They might be coming from the first section of V1. You need to rule that out before assuming it's the P-channel JFET (not a MOSFET). The J174 is obsolete but it's not critical. a 2N5460 springs to mind but the pinout is different so be warned.

Also, have you probed the gate of the JEFT to see if the noise is coming in there?

Ok, the noise its not in the gate of the jfet, when channel select switch is on, 15vdc goes to the gate. i think its not the problem. but its getting difficult to keep tracing the noise. So far the noise is loud on pin 6 of V1, its present in pin 8 too, but it is not in pin2.....

[nickb]

You seem to be skipping steps. Don't.

Did waggling V1 in it's socket affect the pops? If yes the problem is the socket and or solder joints.

If no,
Try swapping V1 with V2 to see if it's tube related - if it's the tube the pops will swap channels.

If no,
Does changing the channel one gain control affect the noise? If it does than the problem is prior to the gain control i.e the first half of V1.


Turning to your observations, if you have a pops on the plate pin 6 then since the same current flows through cathode pin 8 so you would expect them there too, but the voltage change will be much less.

Now, remember the signal on pin 2 will be very, very small so you could easily miss it, if indeed it's there, especially if you're not really careful how you connect the ground of the 'scope lead. It's easier just to pull the the input plug since that shorts the input out and kills any signals.

It's great to have a 'scope but the trick is knowing when to use it - less often than you might think.

[angelothewolf]

You seem to be skipping steps. Don't.

Did waggling V1 in it's socket affect the pops? If yes the problem is the socket and or solder joints.

If no,
Try swapping V1 with V2 to see if it's tube related - if it's the tube the pops will swap channels.

If no,
Does changing the channel one gain control affect the noise? If it does than the problem is prior to the gain control i.e the first half of V1.


Turning to your observations, if you have a pops on the plate pin 6 then since the same current flows through cathode pin 8 so you would expect them there too, but the voltage change will be much less.

Now, remember the signal on pin 2 will be very, very small so you could easily miss it, if indeed it's there, especially if you're not really careful how you connect the ground of the 'scope lead. It's easier just to pull the the input plug since that shorts the input out and kills any signals.

It's great to have a 'scope but the trick is knowing when to use it - less often than you might think.

Ok,first thanks for the help...

swapping or waggling the tube makes no difference, the noise still there.

channel 1 gain control affect the noise.

without V1 in his socket the noise still in C32.

im tracing the noise with a BKPrecision 1422 vintage scope, AND with my ears with a signal tracer to the input of a mixer. i do not understand very well valve stuff so its good to get some help to learn about it.

so the next step would be check that components of the first half of V1???

[nickb]

Ok,first thanks for the help...

without V1 in his socket the noise still in C32.

Are you sure about that? If V1 is out and noise is present on C32 then you would hear it on the output I would think. You said earlier that when you pull V1 the noise goes away. Further, you also said that the gain control reduces the noise which points to the first section of V1.

I would suspect C21 or C17.

[angelothewolf]

Are you sure about that? If V1 is out and noise is present on C32 then you would hear it on the output I would think. You said earlier that when you pull V1 the noise goes away. Further, you also said that the gain control reduces the noise which points to the first section of V1.

I would suspect C21 or C17.

confirmed,,, gain control affects the noise just when V1 is in place. when i pull out V1 there's no output, but the noise still in C32 (i can hear it with the tracer) not affected by the gain control. C21 & C17 are ok. so i think its not that section...
May be related with the PSU_?? with the high tension supply?

[nickb]

confirmed,,, gain control affects the noise just when V1 is in place. when i pull out V1 there's no output, but the noise still in C32 (i can hear it with the tracer)

I believe you are being led astray by your tracer. Affected proportionally by CH 1 gain control implies before gain control.
You don't happen to have a cell phone near this when it happens?

[angelothewolf]

I believe you are being led astray by your tracer. Affected proportionally by CH 1 gain control implies before gain control.
You don't happen to have a cell phone near this when it happens?

with the scope on C32 i can see the voltage fluctuations with intermitent peaks corresponding with the noise i hear with the tracer, whether or not V1 its in the socket. this is making me crazy.
its possible that some cap on psu is making that fluctuations in HT voltage_?? my scope is max 300v so i dont know if its good idea to keep tracing high tension voltage with that. got some idea>>?????

[nickb]

with the scope on C32 i can see the voltage fluctuations with intermitent peaks corresponding with the noise i hear with the tracer, whether or not V1 its in the socket. this is making me crazy.
its possible that some cap on psu is making that fluctuations in HT voltage_?? my scope is max 300v so i dont know if its good idea to keep tracing high tension voltage with that. got some idea>>?????

Anything on the power would have to come via R15 and C17 and you already checked C17 so it can't be that.

The fact remains that if the noise is present on C32 it must be present on the output. Put V1 back in and look at C32 and see what the noise really looks like with the scope. Trace it back from there to V1 pin 7 (about 1/10 the amplitude of pin 6) and V1 pin 1.