1974 PCB Restoration & Repair

matttornado

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I realize this post might not be for everyone, especially for those who are vintage purists. I might get a lot of criticism for this but it’s my amp! haha

There hasn’t much about anything that pertains to the old circuit boards Marshall started using back in late 73 or early 74 so I thought I would share this. In case anyone might be interested.

First, I must give a quick history of this amp. It’s a 1974 MKII Superlead that came stock with 6550 tubes, 4 speaker jacks & a polarity switch. I bought it back around 1992. It was in rough shape but for 600 bucks, I snagged it. Right after I purchased it, I gave it to my friend who is an E.E., Amp tech / Marshall expert. He converted it to use EL-34s and gave it a check-up and got ready for me to use. Around 2001, He replaced the Power Tranny because it took a dump. This amp been my main amp all the way up until I started using my DSL in 2018 so it has been worked HARD for many years, mostly cranked through a Marshall Powerbreak for home use, band rehearsals & gigs.

Back around 2008 maybe? I discovered the Metroamp forum & around that time, my amp tech friend started to go on tour with his band, so I started to do maintenance on my own. I also went down the EVH amp mod rabbit hole. That being said, I tried everything swapping out resistors, caps, tried all types of master volumes, etc. Over time, my circuit board took a beating here and there as you could imagine.

Also, this is the same amp I posted on another thread that has the boomy bass I can’t seem to get rid of.

So, While I had covid and was stuck home for two weeks, I decided to remove the circuit board completely and see what was going on underneath as I suspected it wouldn’t be too pretty. Lots of dirt, gunk, flux, dusty stuff & questionable solder joints. I noticed a few solder pads were gone and some components were soldered where they normally wouldn’t go to bypass the non-existing solder pads. Remember that someone had this at least 18 years before me!



So, I’d like to share some of the repairs I did, and how I did them. I tried to keep the underside of the board as original as possible, but I did replace some old components for reliability purposes & piece of mind. Spending $$$$ for original mustard caps were not in my budget so I used what I already had- a few Orange Drops, Mallaory and Bianchi caps. All good for me. I used a small piece of buss wire with a small loop on one end to repair the missing solder pads.



I used green UV solder mask to protect the traces where I did repairs and where the original mask came off or deteriorated. Turned out pretty cool!



I also cleaned all of the pots and put a new ground bus wire on as well. I’ll post more pics as the amp get done and share my final specs. If anyone cares. LOL

pic44.jpg pic45.jpg pic 13.jpg pic3.jpg pic 56.jpg pic12.jpg
 

South Park

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So what is the plan PTP or PCB . If you go PCB granger amps sells a good board that is marked and has a wire layout . I’m building a SLO 100 with a PCB it is marked but no wire layout . Keep that board you might it for the wire layout nice job on that board
 

matttornado

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So what is the plan PTP or PCB . If you go PCB granger amps sells a good board that is marked and has a wire layout . I’m building a SLO 100 with a PCB it is marked but no wire layout . Keep that board you might it for the wire layout nice job on that board
thanks! i plan on wiring it back up and start playing it again.
 

Go Loud Early

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Looks good, the amp was worked on before, so I don't see what you did as anything wrong and I'm more anal than the next 10 guys. You made the amp more reliable and that's a good thing.
 

rocker68

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That approach looks like the future when dealing with fewer and fewer unmolested examples. Great work!
 

matttornado

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I posted earlier about adding leads to old mustard caps and re-using but the more I thought I about it, it just didn't make sense. Why would I want to put 48 year old beat to hell caps back on a PCB that I'm restoring & plan on getting many more years of playing from. If I were looking to sell, It would be a whole different thing.
 

neikeel

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You can often buy the pcb radial mustards at decent prices. Worth the effort. If you use random parts no one will appreciate what you have done - except you of course!!
 

matttornado

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You can often buy the pcb radial mustards at decent prices. Worth the effort. If you use random parts no one will appreciate what you have done - except you of course!!
Yes maybe I will do that some day. It would be really cool to put all mustards on for sure. I'll keep an eye out for them.....
 

Spanngitter

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Next time you do one of these "reworks" I'll recommend to have a look either for radial wired mustards (they show up from time to time) or use NOS ERO MKT1818/1822.
Later one's do fit precisely into the PCB, are way cheaper then any Mustard, Chicklet or Tropical Fish and they do perform really good in Marshall Circuits (something I wouldn't say for WIMAs). Also I'll recommend to snip the components on the PCB side, straighten the remaining leads and then desolder to trace side. This way the traces do not get damaged or lift off the PCB if you keep the soldering time and heat reasonable. Excess solder can be removed with a solder wick...

6-x-ERO-MKT-1822-047uF-400V-470nF-Audio.jpg
 
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matttornado

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I got the circuit board wired back up. THE BOOMY BASS IS GONE! The amp sounds much better now.:cheers:
I still have a few more tweaks to do & I want to check my grounds. The bright channel seems a little noisy & when I first flip it off standby, I hear a static sound for a few seconds.


Here are the specs / mods:
V1B 2.7K changed to 820ohm
V2 added 0.68 cap across the 820 ohmR
bright cap 5000pF changed to 500pF (as of now) might remove it like it's always been.
PI 47PF fizz cap changed 100pF
Neg. Feedback 100kR / 4 ohm tap changed to 47K / speaker jack
6550 power tubes changed to EL-34

I tried to record a quick clip with my phone but the sound from my phone keeps cutting in and out. I'll try again.

marshall1974 013122.jpg
 

Matthews Guitars

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I have a late '73 Superlead that has the same early ST1 PC board. Being a single layer board with no solder mask and no plated thru holes, the traces are likely to be corroded by this point and they come off pretty easily. When cleaning my board, which was pretty crusty, one trace did crumble to dust and fall off. I reinforced a number of traces and bridged where I had to.

It's been reliable since I did that.


Now, as for component choice...orange drops just don't work in a Marshall. Oh, they're good capacitors. But tonally speaking, when I see them in a Marshall I want them OUT OF THERE. They're OK for Fender and Dumble circuits, but I'd feel like a criminal if I put orange drops in a customer's amp! They do change the tone, not for the better.

Xicon and Panasonic film capacitors do a fine job. Some Mallory types, too.

Wasn't it 1975 when the ST1 Rev 202 board was introduced?
 

matttornado

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I have a late '73 Superlead that has the same early ST1 PC board. Being a single layer board with no solder mask and no plated thru holes, the traces are likely to be corroded by this point and they come off pretty easily. When cleaning my board, which was pretty crusty, one trace did crumble to dust and fall off. I reinforced a number of traces and bridged where I had to.

It's been reliable since I did that.


Now, as for component choice...orange drops just don't work in a Marshall. Oh, they're good capacitors. But tonally speaking, when I see them in a Marshall I want them OUT OF THERE. They're OK for Fender and Dumble circuits, but I'd feel like a criminal if I put orange drops in a customer's amp! They do change the tone, not for the better.

Xicon and Panasonic film capacitors do a fine job. Some Mallory types, too.

Wasn't it 1975 when the ST1 Rev 202 board was introduced?
yeah I hear ya about the orange drops. So far the amp is sounding pretty good. i just might replace those orange drops with mustards some day!
 


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