Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on January 19, 2015, 04:12:59 pm
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.....this 1971 SL 1959.
Seems like it follows the schem.
http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_superlead_100w_1959.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_superlead_100w_1959.pdf)
The amp has worked properly in the past
It has sat in an ATA for the last year. Plugged it in to check it...and now it has this issue.
On the bright channel (with the cap across the volume pot) I have a "noise problem".
This is with No Guitar plugged in.
Volume on Zero...there is no noise.
Turn up volume and there is a loud hum. MY DMM says it is 28kHz at the speaker jack.
The other volume pot makes no noise.
There is no scratching from DC (like on the Presence Pot). The meter stays at 28kHz...it just gets louder.
I assume this is at or before the volume pot, but I am not sure how to proceed with the trouble shooting.
I have cleaned Pots...Jacks...and tube sockets...and I have tried several different pre tubes. None of that helped. I have tried chop-sticking, but can find nothing that is sensitive.
How should I proceed to find this problem.?
Thank You
V1 VDC
plates 157 and 209
cats 0.95 and 1.97
V2
plates 161 and 293
http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL005.jpg (http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL005.jpg)
http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL006.jpg (http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL006.jpg)
http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL007.jpg (http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL007.jpg)
http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL009.jpg (http://i111.photobucket.com/albums/n143/zzmoore/1971MarshallSL009.jpg)
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If you have already cleaned the pot with something like Deoxit or some other electronic contact cleaner, replace the pot. Does that help?
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Hey sluckey -
Yep...gave it a shot of De-Oxit...worked the pot several times...let it sit a few minutes...then tried the amp again, and there was no improvement.
Then I contacted "you guys".
So, OK.....I will try replacing that volume pot and report back
Thanks
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I jumpered in another 1M pot, and there is no change.
I disconnected the existing pot at the coupling cap and the Cap Resistor parallel into the next grid and I jumped the new pot in there...and to ground.
It sounds, pretty much, the same...and reads 28kHz across the speaker jack when you turn up the pot.
There are not that many parts really. (before the pot)
Is it "possible" the problem is After the pot, and does not appear as noise until it sees signal from that pot.?
(if you know what I am trying to say)
Thank You
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Try removing the cap across the pot, she looks like an old one.
Try disconnecting the bright channel at the grid of V1 and tacking it to the grid of normal channel.
otherwise, follow Sluckeys lead, he will get you there
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Turn up volume and there is a loud hum. MY DMM says it is 28kHz at the speaker jack.
28 K hz would be out of the range of human hearing (on the really high side).
Just don't want you to focus on that possible mis-read.
Wish I could help more than that.
**Unless, of course, you are near the Bosnian pyramids?? :dontknow:
http://truthseeker1313.com/tag/28-khz/ (http://truthseeker1313.com/tag/28-khz/)
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It has sat in an ATA for the last year...
...The other volume pot makes no noise.
Couple questions... An ATA is a travel case? Was it in a climate controlled facility?
The other volume pot makes no noise, but can you play guitar thru that channel?
Pull V1. Does the noise stop? If not,,,
Leave V1 out and pull V2. Does the noise stop? If not,,,
Leave V1 and V2 out and pull V3. Does the noise stop?
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Yeah...sorry...should have done that already.
V1 has no effect...V2 stops it...the good tube from V1 does not help the problem if installed in V2.
So...it is somewhere between the Vol Pot and V2.
I guess, it must be one of the two components that make up the "bright circuit"...or maybe the components of the V2 cat follower.?
I have a homemade audio probe with a little speaker, and I have a scope. I imagine I should be able to follow a clean sine wave, or a clean audio sound until I reach the bad part(s).?
Thanks
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If you have a scope you should be golden, pick your fav freq, I like 880hz, inject it in both sides, that way when you hit "noise" you should be able to easily compare , probably be able to "see it" without a signal injected. Be carful not to "assume" because you see it at the plate, it is the plate circuit. I think noise was and is the original "gremlin!!"
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Before breaking out the test equipment, resolder all three lugs on the suspect pot. Make sure the pot ground lug is firmly soldered to the back of the pot or the ground buss wire. Then resolder the 470K mixing resistor and 500pF cap between that pot and the tube grid. And resolder both ends of the wire that connects the tube grid to the board.
If none of that resoldering cures the problem then disconnect the wire from the right lug of the pot. This will disconnect the pot from the .0022µF coupling cap on the board. You won't get any guitar signal thru the bright channel with this wire disconnected, but, does the noise problem disappear?
BTW, you never said if you could play guitar thru the normal channel. Need to know that answer.
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OK...this will take me a bit.
Yes...guitar plays fine through both channels.
The "noise" is just on the bright channel
Thank You
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OK.....I'm back.
Had a cold that lasted 3 weeks, and it looks like my right wrist (that has had 5 surgeries) will have to have the fusion hardware removed.....so I have been a mess.
LOTS of people are Much Worse off than myself...I have all my limbs (whats left of them, I can see and hear, and I am not in a wheel chair).
I say this, because I have had 11 surgeries in the last 9 years...3 on my back...and I have met some people in the hospital that were in REAL bad states. The were beautiful and humble about THEIR horrible problems, so I will never complain again about MINE. Except for here and now of course. :BangHead:
Anyway.....both channels look the same until you get to that little bright circuit of the 470k-500pF in parallel.....right before the grid of V2.
Going in the signal looks pretty clean.
Going out, the signal is reduced, but it also looks clipped on the top half of the wave, doesn't it.?
I could have just replaced that cap (or resistor, or both) but I just wanted to see what kind of Noise/Issue ...In Your Guys Opinion... this shape wave might generate.
I mean, that is not "normal"...right.?
Thank You
http://i.imgur.com/A2NMvy9.jpg (http://i.imgur.com/A2NMvy9.jpg) (at the pot wiper)
([url=http://i.imgur.com/F3BXb1W.jpg]http://i.imgur.com/F3BXb1W.jpg (http://i.imgur.com/F3BXb1W.jpg)[/url] (at grid of V2)
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Guessing that's a 1k signal, can't see the scope settings, anyway, if your problem is still "noise"(@28k?) I would guess you'd see it "riding" on your test signal, maybe spikes off your test signal. The slight compression might be due to RC components between the 2 waveforms. I would keep running that signal to the speaker and look for something more distinct. If it is high frequency dial your scope out to say 20khz, leaving the 1k signal there and see what you see, but test that with your sig-gen(1khz) into the scope 1st and the time base looking at 20khz, that way you know what "good" looks like.
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J Rindt,
You mentioned that you cleaned the jacks and this is making the noise with nothing plugged in. Did you happen re-tention the shorting tab to make sure there is good contact?
Jim
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You may also want to get rid of that Erie death cap on the front end.
Jim
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Anyway.....both channels look the same until you get to that little bright circuit of the 470k-500pF in parallel.....right before the grid of V2.
Going in the signal looks pretty clean.
Going out, the signal is reduced, but it also looks clipped on the top half of the wave, doesn't it.?
The grid of V2 starts to conduct as the input signal gets more positive which bucks the input signal causing the soft clip you are seeing.
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I mean, that is not "normal"...right.?
Impossible to say without knowing the amplitude and dc reference of the waveforms.
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Yeah...that death cap. It has 4-5-6 wires tied up on it.
I need to trace everything...I imagine it is all bundled up with the 3 way ground switch.?
I will just have to check and see. It is not on a schem that I know of.
So the gird of the Cathode Follower (V2) is responsible for that clipped sine wave.?
As I said earlier, pulling V1 has no effect, but pulling V2 does.
I will carefully re-read all of the suggestions so far.
This must be a coincidence and unrelated .......2 power tubes just red-plated like a Super Nova.
With the amp upside down, and looking at the chassis from the rear, they are the two power tubes on the left. I will have to pull the power tubes
and see where the bias stands.....that is both sides of the phase inverter.
Both of those original caps were leaking DC so I replaced them. But this is just one power tube from each half of the push-pull set up, that lit up.
Anyway, I will have to see if any voltages look crazy in that area.
This (again) will take me awhile.
I REALLY do Appreciate all the help so far.
Thank You