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Old 19th May 2013, 12:38 AM   #1
Nelec is offline Nelec  Australia
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Default Marshall VS 8080 IC1 Blowing.

I repaired a VS 8080 for a fellow awhile back. It was in fantastic condition, just repaired a few dry solders joints, and it worked well after that.

About 3 or so months later he called back saying it had stopped working.

I loaned him my 8080 test amp, and grabbed his back for a look. I traced it down to IC1, popped a socket in, and replaced the IC, and it was all good again. I gave it a thorough playing with a couple of guitars to test. I also replaced all the power section electrolytics for good measure.

In the meantime the loaner amp failed. This amp is a little modified, so I wondered if my mods had taken something out in the preamp, but when I got it back it had the same symptoms. I replaced IC1 and it was fine again.

Then his amp failed again with the same symptoms, within a short timeframe.

My gut feeling is that something being plugged into it is taking it out. Given that the exact same fault has occurred on two different 8080's, it seems unlikely that it is a pure coincidence.

He is a little concerned, which is understandable, as his small Peavy practice amp (I'll chase its details up) works with the same setup without failing. I'm thinking it has a more robust input section for whatever reason.

I asked him if anything was changed gear wise around the time of the first failure, and he had started using a loop pedal, and something else I can't recall. I'll chase these details up also.

Preamp - http://www.drtube.com/schematics/marshall/pc0689.pdf

Power Amp - http://www.drtube.com/schematics/marshall/pc0689p.pdf

My questions are as follows:

1/ Has anyone else come across this fault? Is it common? I haven't found it in any of the other 8080 posts that I've read.

2/ If this is a weakness, any suggestions as to how I can make this section more robust without affecting the signal? Some kind of buffer pre IC1 perhaps?

3/ I was going to take the loaner back, tell him to plug his pedals into the FX loop instead of the front end, and see how it stands up after that. Is it fair to say that the FX loop would be more robust as this is its intent, or will IC8 be the next candidate to start blowing (both IC1 & IC8 are TL072 or similar), or I barking up the wrong tree here?

Thanks.
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Old 19th May 2013, 03:57 AM   #2
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Nelec View Post
I repaired a VS 8080 for a fellow awhile back. It was in fantastic condition, just repaired a few dry solders joints, and it worked well after that.
Only that?.
Didn't you mod it or something?.

Quote:
About 3 or so months later he called back saying it had stopped working.

I loaned him my 8080 test amp, and grabbed his back for a look. I traced it down to IC1, popped a socket in, and replaced the IC, and it was all good again.
Personally, I avoid IC sockets if possible, specially in Guitar amps.
Both because it's one extra possibility of poor contact, and it dissipates less.
Maybe not important in a preamp but more so in a power amp driver, so definitely no socket for IC8.

Quote:
In the meantime the loaner amp failed. This amp is a little modified, so I wondered if my mods had taken something out in the preamp, but when I got it back it had the same symptoms. I replaced IC1 and it was fine again.
Ok, please state your Mods.
And I would resolder that IC properly.
VS8080 are known for years long faithful service, with the exception of "mechanical" problems: dirty/worn/crackling pots/switches/jacks and the occasional power transistors blown but hey, they are 20 years old by now !!
That counts as "reliable" in my book.

Quote:
Then his amp failed again with the same symptoms, within a short timeframe.

My gut feeling is that something being plugged into it is taking it out. Given that the exact same fault has occurred on two different 8080's, it seems unlikely that it is a pure coincidence.
Same gut feeling here.
There's 3 common elements in this problem: him, the amp setup and you.
Please state what the Mods were.
Not dissing you, just in case.
As in "leave no stone unturned".

Quote:
He is a little concerned, which is understandable, as his small Peavy practice amp (I'll chase its details up) works with the same setup without failing. I'm thinking it has a more robust input section for whatever reason.
You bet, Peaveys are strong .
Among other measures, they usually protect all inputs (including Loop, etc.) *and* outputs with clamping diodes.
Unjustly despised Crate does the same.
You know, they don't have Marshall's huge reputation backing them, so they are forced to do things right .
I'm attaching examples of Peavey liberal use of clamping diodes; in this case in a trusty old Bandit 75.

Quote:
I asked him if anything was changed gear wise around the time of the first failure, and he had started using a loop pedal, and something else I can't recall. I'll chase these details up also.
As they say, the Devil is in the details.
Please mention them , as well as any Mods and any other unusual circumstances.
My hunch is that some pedal or the looper or poor switching (voltage spikes) is frying IC1.
Or some pedal turns itself on with an extra contact at an output jack.
With the plug halfway in you are applying a 9V DC burst to IC1 input .

As of "details" seldom mentioned but vitally important:
Once I had a long string of recalls from a customer .
Only on the 3rd or 4th visit he mentioned they run everything out of a small gasoline/petrol(UK) generator.
Unloaded voltage?: 240/260VAC, with occasional bursts of 280V
Thanks for mentioning it 3 months later, moron

So in a nutshell, ghosts do not exist, but unacknowledged circunstances ..... which can drive you crazy.

Quote:
My questions are as follows:
1/ Has anyone else come across this fault? Is it common? I haven't found it in any of the other 8080 posts that I've read.
Nope .

Quote:
2/ If this is a weakness, any suggestions as to how I can make this section more robust without affecting the signal? Some kind of buffer pre IC1 perhaps?
Skip the socket and add clamping diodes.

Quote:
3/ I was going to take the loaner back, tell him to plug his pedals into the FX loop instead of the front end, and see how it stands up after that. Is it fair to say that the FX loop would be more robust as this is its intent, or will IC8 be the next candidate to start blowing (both IC1 & IC8 are TL072 or similar), or I barking up the wrong tree here?
Add protective diodes.
Or next he'll kill IC8.

Worst case, they don't hurt, and can be useful if somebody connects some speaker output into a loop in or something.
Attached Images
File Type: gif Pvy01.gif (13.1 KB, 48 views)
File Type: gif Pvy02.gif (12.5 KB, 46 views)
File Type: gif Pvy03.gif (12.5 KB, 46 views)

Last edited by JMFahey; 19th May 2013 at 04:01 AM. Reason: Oops, forgot attachments.
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Old 19th May 2013, 04:45 AM   #3
brig001 is offline brig001  United Kingdom
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I agree with JMFahey about protection diodes, but would also like to see some resistance in series with the input jack too. The schematics he provided show 22k, but 10 to 47k in series with C1 on the preamp schematic the OP linked to would be good.

HTH,
Brian
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Old 19th May 2013, 04:47 AM   #4
Nelec is offline Nelec  Australia
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Quote:
Originally Posted by Nelec
I repaired a VS 8080 for a fellow awhile back. It was in fantastic condition, just repaired a few dry solders joints, and it worked well after that.
Only that?.
Quote:
Didn't you mod it or something?.
Hi, and thanks for response. No, His amp was stock standard. I reflowed a bunch of solder joints in the usual problem areas. Pots, caps etc. Stock Standard 8080.


Quote:
About 3 or so months later he called back saying it had stopped working.

I loaned him my 8080 test amp, and grabbed his back for a look. I traced it down to IC1, popped a socket in, and replaced the IC, and it was all good again.
Quote:
Personally, I avoid IC sockets if possible, specially in Guitar amps.
Both because it's one extra possibility of poor contact, and it dissipates less.
Maybe not important in a preamp but more so in a power amp driver, so definitely no socket for IC8.
Fair point on the IC sockets. I have used the quality machined ones, not the cheap nasty spring loaded jobbys, but point taken none the less. And so far I have only ever used them in the preamp or reverb circuits.


Quote:
Quote:
In the meantime the loaner amp failed. This amp is a little modified, so I wondered if my mods had taken something out in the preamp, but when I got it back it had the same symptoms. I replaced IC1 and it was fine again.
Quote:
Ok, please state your Mods.
And I would resolder that IC properly.
VS8080 are known for years long faithful service, with the exception of "mechanical" problems: dirty/worn/crackling pots/switches/jacks and the occasional power transistors blown but hey, they are 20 years old by now !!
That counts as "reliable" in my book.
Quote:
Then his amp failed again with the same symptoms, within a short timeframe.

My gut feeling is that something being plugged into it is taking it out. Given that the exact same fault has occurred on two different 8080's, it seems unlikely that it is a pure coincidence.
Same gut feeling here.
Quote:
There's 3 common elements in this problem: him, the amp setup and you.
Please state what the Mods were.
Not dissing you, just in case.
As in "leave no stone unturned".
Yes, I have a soft spot for the Gen 1 valvestates. They tend to polarise people in regards to their opinion, but I had an 8040 for many years that served me well. I plugged all sorts of pedals into that amp and never had an issue. The mods weren't anything unusual, basically just upped the annode resistor a little and put a bypass cap on the cathode resistor. However this was only on my amp which I hadn't personally had any issues with.


Quote:
He is a little concerned, which is understandable, as his small Peavy practice amp (I'll chase its details up) works with the same setup without failing. I'm thinking it has a more robust input section for whatever reason.
Quote:
You bet, Peaveys are strong .
Among other measures, they usually protect all inputs (including Loop, etc.) *and* outputs with clamping diodes.
Unjustly despised Crate does the same.
You know, they don't have Marshall's huge reputation backing them, so they are forced to do things right .
I'm attaching examples of Peavey liberal use of clamping diodes; in this case in a trusty old Bandit 75.
Awesome, I had a look at some Peavy Schematics and some higher end Valvestates, and they all had some form of Input protection. Not all had the same on the FX return/s which was going to be one of my next questions after seeing this.


Quote:
I asked him if anything was changed gear wise around the time of the first failure, and he had started using a loop pedal, and something else I can't recall. I'll chase these details up also.
Quote:
As they say, the Devil is in the details.
Please mention them , as well as any Mods and any other unusual circumstances.
My hunch is that some pedal or the looper or poor switching (voltage spikes) is frying IC1.
Or some pedal turns itself on with an extra contact at an output jack.
With the plug halfway in you are applying a 9V DC burst to IC1 input .
Yes, I couldn't agree more in regards to details, I was mindfull of posting prematurely, but I wanted to sort this problem today. I have contacted him chasing the info but won't get a reply till later this afternoon as he is working. I'm fairly confident the clamping diodes are the fix, however I would like to confirm what the input of his practice amp has, and the various pedals he is using.


Quote:
Quote:
As of "details" seldom mentioned but vitally important:
Once I had a long string of recalls from a customer .
Only on the 3rd or 4th visit he mentioned they run everything out of a small gasoline/petrol(UK) generator.
Unloaded voltage?: 240/260VAC, with occasional bursts of 280V
Thanks for mentioning it 3 months later, moron
Ouch, gotta hate that. I had intended to take my meter around and check his outlet. I haven't come across any recently in town that are out of 230VAC+-10% though.


Quote:
Quote:
So in a nutshell, ghosts do not exist, but unacknowledged circunstances ..... which can drive you crazy.
Agreed.


Quote:
My questions are as follows:
1/ Has anyone else come across this fault? Is it common? I haven't found it in any of the other 8080 posts that I've read.
Quote:
Nope .
Cool.


Quote:
2/ If this is a weakness, any suggestions as to how I can make this section more robust without affecting the signal? Some kind of buffer pre IC1 perhaps?
Quote:
Skip the socket and add clamping diodes.
Righto, I'll just add the clamp diodes and resistor in my loaner amp. Once proven, I'll resolder the IC to the board and do the same input & fx return protection mod on his.


Quote:
3/ I was going to take the loaner back, tell him to plug his pedals into the FX loop instead of the front end, and see how it stands up after that. Is it fair to say that the FX loop would be more robust as this is its intent, or will IC8 be the next candidate to start blowing (both IC1 & IC8 are TL072 or similar), or I barking up the wrong tree here?
Quote:
Add protective diodes.
Or next he'll kill IC8.
ok, yeah that's what I was worried about.


Quote:
Quote:
Worst case, they don't hurt, and can be useful if somebody connects some speaker output into a loop in or something.
Cool, in regards to the 10K resistor added in series after the input coupling capacitor, will it be "transparent" to the ear?
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Old 19th May 2013, 04:52 AM   #5
Nelec is offline Nelec  Australia
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Quote:
Originally Posted by brig001 View Post
I agree with JMFahey about protection diodes, but would also like to see some resistance in series with the input jack too. The schematics he provided show 22k, but 10 to 47k in series with C1 on the preamp schematic the OP linked to would be good.

HTH,
Brian
Hi Brian, thanks also for your input. Much appreciated.
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Old 19th May 2013, 03:34 PM   #6
JMFahey is offline JMFahey  Argentina
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Yes, a 10K to 47K (or Fender mythical "68k") in series with a high impedance (220K to 1M) are transparent.
Won't re-download the preamp schematic just to check, but in general, in any input or output where you add clamping diodes, you need *some* resistor in series to limit current or you'll fry the diodes themselves.
So 10K to 68K on inputs, 1K in series on outputs usually works well.
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Old 19th May 2013, 11:50 PM   #7
Nelec is offline Nelec  Australia
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Thanks JM.

I have soldered up the diodes on the Input, with a 10K resistor prior to the coupling cap.

On the FX return I have also added the diodes and a 1k resistor as I was copying the diagram above. I'll change that to a 10K.

In regards to the FX-out, how necessary is protection here? In a number of schematics I have looked at that had FX-return protection, I've found some that have it and others without.
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Old 20th May 2013, 01:32 AM   #8
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It might be worth putting a small resistor in series with the input to soak up high voltages coming in.

Another trick as well as adding a resistor is diodes to the power rails to absorb the high voltage.
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Old 20th May 2013, 03:16 AM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
In regards to the FX-out, how necessary is protection here? In a number of schematics I have looked at that had FX-return protection, I've found some that have it and others without.
As far as what *an effect* can do to it, it's unnecessary.
But the main idea is to protect that *output* from a much larger one: the *speaker* output.

And who would be so dumb as to connect the speaker output from a head into any preamp output or input?

Well, I have seen worse onstage, a dark and sometimes smoky habitat where you often find drunk/stoned people.
Or some 4ssh0le who shouldn't be there to begin with, who steps on a cable, unplugging it, and tries to re-plug it "somewhere".

Probably quite unlikely in your situation, but Peavey/Crate/etc. design for the masses, so ......
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Old 20th May 2013, 03:51 AM   #10
Nelec is offline Nelec  Australia
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Quote:
Originally Posted by JMFahey View Post
As far as what *an effect* can do to it, it's unnecessary.
But the main idea is to protect that *output* from a much larger one: the *speaker* output.

And who would be so dumb as to connect the speaker output from a head into any preamp output or input?

Well, I have seen worse onstage, a dark and sometimes smoky habitat where you often find drunk/stoned people.
Or some 4ssh0le who shouldn't be there to begin with, who steps on a cable, unplugging it, and tries to re-plug it "somewhere".

Probably quite unlikely in your situation, but Peavey/Crate/etc. design for the masses, so ......
Thanks JM, interesting point you make, certainly hadn't considered that!

Also thanks for your input Nigel.

The Peavey amp in question is apparently a Rage158. It has a pair of 1N4148's tied to rails, and a 1K resistor in series. So this would certainly explain why it has stood up to more than poor old unprotected IC1 in the 8080.

The Free Information Society - Peavey Rage 158 Electronic Circuit Schematic
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File Type: jpg Rage158 Input.jpg (52.9 KB, 31 views)
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