Crate Voodoo 60 combo Blue

[cjenrick]

wish i had a dollar for every Fairchild BS170 i have replaced, popped like popcorn,

here is a fet popper circuit right here on the mesa 5-50>

[Enzo]

And that is on the face of it a poor design. No diode across the coil. None of the coils. There is nothing like that in the Crate circuit. It is hardly the JFET's fault.

[Mick Bailey]

wish i had a dollar for every Fairchild BS170 i have replaced, popped like popcorn....

Out of interest, what amp were these used in?

[bsco]

Hey guys.....Sorry for the delay.....I have a funeral to go to so I'll get back at this amp early next week and g1, I'll pull that V4 tube and let you know what happens.....I was thinking that the problem was in the V3 stage but will have to double check....as Enzo suggested this noise is being masked when the volume is turned up...even with no guitar plugged in......

[bsco]

Ok. To g1...I pulled that V4 tube....a 12AU7......quiet as a mouse......Pulling V3 does the same thing....If I pull V1 or V2 that noise is still there........If I plug into the return the noise stops....reinstall all the tubes and run a send out to another amp...the noise is there....so it seems that it is in the pre-amp section.....I just have to find it.....The tubes are fine.......I am pretty confident that the noise is not being generated by a bad pre-amp tube......I did hit the bypass caps and cathode resistors with a shot of cold spray...nothing happened there......open to suggestions.....
Cheers

[g1]

If the problem is still heard with V1 & V2 pulled, then I would think it should be there in the gain channel also. Maybe only audible with the master full up?

[bsco]

If the problem is still heard with V1 & V2 pulled, then I would think it should be there in the gain channel also. Maybe only audible with the master full up?

That I did not hear....I will check that again for you.......
Cheers

[bsco]

Ok. g1....I pulled both V1 and V2........I can hear the noise in Channel 1 very plain.....in the gain channel...you are right...but it is soooo low, that you pretty much have to strain your ear in order to hear it........If I remove V3, then all the noise goes away.....I am going to connect the scope to this again to see if I can see the noise on the screen....so far I haven't been able to visually see it.....

Just did some scope checking....I was being fooled with the fact that I could turn the clean channel volume down and could hear the noise....and when I turned up the volume the noise would go away.....because it was being masked by the normal sound of normal electronic noise.......That is why I couldn't find any noise with the scope.....I found the general area this time..........looks like it is visable at Pin 6 of V4.......it is very small but it is there and every time a crackling sound occurs I can see noise spikes on the scope.....so...it is in this general area.....now to dig a little deeper........I shall be back.......

[bsco]

OK......I discovered that I also have a bad scope probe.....so I switched to another one......anyway, I can not find the source of this noise.....I have no idea of what is generating this......it varies slightly in loudness...sometimes going low and other times being more audiable.....and when you turn up the main volume control in the clean channel you can mask it..... the original problem was that R115 was open...a 630 ohm 3Watt resistor....and it needed a clean up....lots of bad connections so I fixed all that up.....I will have to take a break from this one and go do something else.......when you plug in a guitar the amp sounds great.....this has me stumped......

[bsco]

Ok. back at this again....I checked the voltage on the dropping resistors in the power supply.....I used the scope set to AC coupling and scoped the voltages in the power supply......R115, R100, etc.....the trace on the scope moves or jumps up and down on the screen......I am using a setting of 50mV/div and the probe is set to x10....the reason being is that if I set the scope to X1, the trace goes off the screen........I did measure the dc voltages and the 320V is fine.....but the other voltages before that are about 20V higher.....490V line is reading 525V....I also noticed that there is 3.3VAC on the dc voltages when measured with a DMM....this static noise will increase slightly and then go fairly quiet and it does this at random.....so it looks like I might have a problem with the power supply.....maybe a noisy component somewhere......I don't know if Filter caps would produce this type of noise but I am thinking it might be one of the bridge rectifier diodes.....it uses 1N4007 and I think I might have a few of them here......I can just cut one of the leads on D3 and then D20, and just hook up a temporary replacement with a couple of Crock clips.......if anybody has other ideas fire away please....
Cheers

[Enzo]

Yes, I have other ideas.

First, monitor your mains voltage. I bet it moves around. If it is more or less 120v, I bet it moves up to 122 or down to 117 ongoing. Very common for line voltage to dance around. Ever hear your furnace kick on and the room lights dim a little for a moment when it starts? That sort of thing happens in your home, but on a larger scale, it happens in the power grid. You can read about the powr company watching usage during the SUper Bowl. They know when commercials come on because everyone runs to the kitchen for a fresh beer or some snacks, and the power use spikes. Industrial users affect voltage, etc etc.

If your scope is on half a volt a division, (50mv and x10) that means it only takes a couple volts to move the trace off the screen. In an amp with 480v B+, every 1 volt change in the 120v mains makes a 4v change in B+. SO if the mains shifts a volt, your B+ scope on AC coupling will shoot off the screen until its AC coupling cap can charge to the new voltage. And then it bounces back into view. This isn't a problem, this is perfectly normal.

On the DMM, I use that method myself, my meter on ACV will ignore the DC. To me 3v of AC - which is your ripple - on a 500v supply sounds pretty good. I bet by the screen node, that 3v is down to a fraction of a volt.

[bsco]

Ok..... Thanks Enzo.....I will check the AC line voltage....when I measured the AC on the other side of D3, I did not see the scope screen jump around....but I will double check.......but that still don't explain the static crackling noises I am getting.....sounds like bacon frying or a fire crackling if you were standing a couple of yards away from it.....sort of low in volume but can be heard until you turn the clean volume up.....then once it reaches 2 or 3 on the control the inherent electrical noise will mask the crackling sound......you only hear that with the volume control turned down to zero....I will check on that AC and let you know........
Cheers

[Enzo]

The AC thing was to explain the scope trace hopping around, which I doubt has anything to do with your noise. If the noise is not really affected by the volume control (other than it masks it), then it is after the control. We discussed isolating the noise by tube pulling and such.

That is the job - isolate the source.

[bsco]

I think I got it now......I am going to run it tomorrow for awhile and test it before it is assembled...and do some more checking.....I will let you know what happens tomorrow afternoon......as it is 9:30 PM here now........by the way, I measured the AC line voltage and started to turn on various pieces of equipment including stuff like heat guns, etc and watched the meter....depending on what I turned on, the voltage dropped about 1 volt then cam e back to the original AC voltage of 123VAC....that is what my line voltage is here tonight....I am going to check it again tomorrow to see if it changed any........
Cheers

[Enzo]

Things in your home can affect it as you see, but also, a guy a half mile down the road could start up his table saw or an air compressor and bounce your voltage. And I was serious about highly watched TV shows, when significant percentages of the population all watch some event, the cadence of the show often affects the mains.