60 Cycle Hum in '74 50watt JMP

[maxwedge572]

This is supposedly a 73 JMP 50 watt.
It is not mine but you can see how the wiring is done like mine and it has the same terminals.
So the same guy modded this one too.

Maybe as some of the wire insulation is melted with an iron and there are blobby bits of solder

So wtf with the rolling eyes shitty sarcasm? I'm not trying to show you the hack soldering on this amp but how is has the same wire terminals and wires that don't run through the holes like my amp does! I guess you've seen every Marshall that left the factory!

Hey thanks for the help!

[neikeel]

So wtf with the rolling eyes shitty sarcasm? I'm not trying to show you the hack soldering on this amp but how is has the same wire terminals and wires that don't run through the holes like my amp does! I guess you've seen every Marshall that left the factory!
Hey thanks for the help!

Whoa there we are touchy!
The rolling eyes were purely addressed at the soldering skills of whoever did the work on the amp, so unless it was you I would not be wound up. No, I do not pretend to have seen every amp that Marshall has made but merely pointing out that the standard of workmanship on both amps was pragmatic to say the least. We have to bear in mind that these were working amps for many years and many repairs and mods were done in a hurry to keep these things working between gigs/mid tour.
Both amp have been worked on extensively.

I have suggested that your heater mod was only necessary as the wiring could probably tidied up significantly without recourse to such mods.

If it works for you that is cool, now chill

[maxwedge572]

So wtf with the rolling eyes shitty sarcasm? I'm not trying to show you the hack soldering on this amp but how is has the same wire terminals and wires that don't run through the holes like my amp does! I guess you've seen every Marshall that left the factory!
Hey thanks for the help!

Whoa there we are touchy!
The rolling eyes were purely addressed at the soldering skills of whoever did the work on the amp, so unless it was you I would not be wound up. No, I do not pretend to have seen every amp that Marshall has made but merely pointing out that the standard of workmanship on both amps was pragmatic to say the least. We have to bear in mind that these were working amps for many years and many repairs and mods were done in a hurry to keep these things working between gigs/mid tour.
Both amp have been worked on extensively.

I have suggested that your heater mod was only necessary as the wiring could probably tidied up significantly without recourse to such mods.

If it works for you that is cool, now chill

Ok we're cool and I'm sorry.
It appeared to me that you were applying sarcasm at what I was trying to prove with the pictures, that the circuit board and terminals are original.
No, that crappy soldering wasn't mine.

[demonufo]

Well, look at this picture here and see how that the red dye on the board mounting posts have not been torn up. How would someone install these supposed turrets, which are in fact not turrets but snap on terminals, without removing the board and damaging the dye? I have some pictures of one other marshall that has the same type of terminals and the wires don't run through the holes. I've had this amp since about 1980 and if anything major was done to it, like you say, then it was done before I bought it. I'm fairly certain it's a 1974. The serial # is 5024F

One thing I'm absolutely certain about.
That amp never left the factory new with that one 'hot pink' wire installed.
I'm with Neil though, that's had extensive work done, and they both could've been done better.

[maxwedge572]

Well, look at this picture here and see how that the red dye on the board mounting posts have not been torn up. How would someone install these supposed turrets, which are in fact not turrets but snap on terminals, without removing the board and damaging the dye? I have some pictures of one other marshall that has the same type of terminals and the wires don't run through the holes. I've had this amp since about 1980 and if anything major was done to it, like you say, then it was done before I bought it. I'm fairly certain it's a 1974. The serial # is 5024F

One thing I'm absolutely certain about.
That amp never left the factory new with that one 'hot pink' wire installed.
I'm with Neil though, that's had extensive work done, and they both could've been done better.

Hi demonufo,
I agree with you on this one. My amp had had a master volume mod and the presence was moved to the rear of the amp. I took the mst vol out and moved the presence back to the front and shortened the 'hot pink' wire. The other things you see in this pic is the 150k carbon comp resistors that replace the original 220k. My guess on this is that they may have been added when the amps were shipped to the USA to accommodate the 6550 tubes. My understanding is that most of the UK amps had EL34 and the USA amps used 6550's.

Also some one soldered the green and orange wires for the bias supply directly to the pins. I guess there was a loose connection but I was still having a problem with the orange wire and when I tapped it with a chop stick the amp would go wild. That pin is loose. I ended up soldering the wire directly to the .022 cap and this solved that problem. I don't want to lift that board up unless I really need to.

Scott

[neikeel]

Something else worth noting on the last pic is that the PI output caps have probably been replaced too. They appear to be the correct value of 22nF but the one I can read is only 160v which is likely to be operating beyond its spec. You can use 160v on cathodes as bypass caps or on the presence circuit but in the main preamp 400v is preferred.
On looking carefully they could be original but Marshall must have great faith in those caps as they typically see 200v.

What are your PI voltages?

[demonufo]

Whoa! Well spotted. They've clearly been in there a very long time judging by the corona stain (or at least in one amp or another), but I've never seen 160V caps used before. Well, not .022uF's I mean.

[maxwedge572]

Man, this is pissin me off now! I agree, those 2 caps should be 400v or more.
I'm going to have to lift that board up and have a look-see underneath it. Not going to do it today but I'll check the PI voltages.
It has lower plate voltages at 380v which I understand is normal for a 74 50 watt.

Thanks

[maxwedge572]

Something else worth noting on the last pic is that the PI output caps have probably been replaced too. They appear to be the correct value of 22nF but the one I can read is only 160v which is likely to be operating beyond its spec. You can use 160v on cathodes as bypass caps or on the presence circuit but in the main preamp 400v is preferred.
On looking carefully they could be original but Marshall must have great faith in those caps as they typically see 200v.

What are your PI voltages?

155v and 167v so they are definitely running maxed out, lol. They are both 160v caps so I guess someone pulled from the wrong parts bin.
I can't see all of the voltage markings on the caps but it looks like one .022uf is 400v and the rest are 160v. The 400v .022uf is the bright channel coupling cap.
This amp is definitely screwy
It sounds good though.

[demonufo]

This amp is definitely screwy
It sounds good though.

And that my friends, is all that matters.


Well, that and doing you're best to make sure it doesn't go boom that is.
167V isn't pushing your luck too much though. Depending on what happens with the variations at the wall voltage that is. Of course if it was an electrolytic I'd definitely consider that overstressed.

[maxwedge572]

This amp is definitely screwy
It sounds good though.

And that my friends, is all that matters.


Well, that and doing you're best to make sure it doesn't go boom that is.
167V isn't pushing your luck too much though. Depending on what happens with the variations at the wall voltage that is. Of course if it was an electrolytic I'd definitely consider that overstressed.

I think if it was going to go boom it would have done it a long time ago because I've been playing this amp about 30 years now, on and off.
I'm on a mission to find more examples that have those turrets/pin connectors on the circuit board but its looking bleak. I just don't understand why someone would go through the trouble to add those, that's a lot of work! I have to find out if they are original or not and it's looking like not!

[maxwedge572]

Looks like I've found another that's wired like mine and it's a 74 100 watt. I'm going to ask for some close ups.
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... ink:top:en" onclick="window.open(this.href);return false;

[maxwedge572]

Looks like I've found another that's wired like mine and it's a 74 100 watt. I'm going to ask for some close ups.
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... ink:top:en" onclick="window.open(this.href);return false;

He sent me the pics. This is the left side of the board and then I cut out a close up of pot side. It has the same pins that mine has.
The original pic is 3072x2304. Sure it's had a few mods and repairs but I'd say this is the way it came wired from the factory, like mine.

So I have to conclude, at this point is:
1) My amp is original and came from the factory this way.
2) It has not had "extensive work" and the circuit board has never been out.
3) It has had some minor repair work and some minor mods.
4) Is the wiring sloppy? Yes! And probably why it was humming? Yes!
5) Do I want to go and change it if it's original? No.

Then there is the question as to why.
I bet that they tried this for a VERY SHORT time in an effort to speed up production and then gave it up. Just guessing.

PS: No hard feelings to any of you guys, this just something I need(ed) to research.
Scott


Look at the bare pin lined up with the 1 watt resistor and then go back and look at the picture of my circuit board and you can clearly see that they are the same.

Well, look at this picture here and see how that the red dye on the board mounting posts have not been torn up. How would someone install these supposed turrets, which are in fact not turrets but snap on terminals, without removing the board and damaging the dye? I have some pictures of one other marshall that has the same type of terminals and the wires don't run through the holes. I've had this amp since about 1980 and if anything major was done to it, like you say, then it was done before I bought it. I'm fairly certain it's a 1974. The serial # is 5024F

One thing I'm absolutely certain about.
That amp never left the factory new with that one 'hot pink' wire installed.
I'm with Neil though, that's had extensive work done, and they both could've been done better.

[neikeel]

Not a problem, it is useful to follow these things up.

The amp you show does have a Fischer type PPIMV added, addded V2a 0.68uF cap, replaced V1 plate resistor, piggybacked NFB resistor and piggy backed bias resistor so seen some work but the board will not necessarily have been lifted.

On both amps (or all three!) you can see where the dye is intact on some of the pot wires. On the first two you can see where some of the wires were looped through the board holes at some time, but not on the latest amp.

Question is did Marshall make them that way or did a tech working on these add them to the pcb boards that he/she was sent to work on...........................................

If we could find a completely un modded/repaired example that does not have an obviously touched solder joint anywhere I would more easily be able to believe Marshall sent some out like this

[maxwedge572]

On both amps (or all three!) you can see where the dye is intact on some of the pot wires. On the first two you can see where some of the wires were looped through the board holes at some time, but not on the latest amp.

Question is did Marshall make them that way or did a tech working on these add them to the pcb boards that he/she was sent to work on...........................................

If we could find a completely un modded/repaired example that does not have an obviously touched solder joint anywhere I would more easily be able to believe Marshall sent some out like this

What ever happened it seems that very few were done this way and so far only 73 and 74 USA amps. I've still been looking for pictures and have only found the one's in this thread. Need an old time Marshall employee to speak up.