Need help fixing a Marshall Lead 12 Combo

[steve]

Since I'm more of a tube guy, I don't even begin to know where to start fixing this amp.

I have a Marshall Lead 12 combo. I bought it used a few years ago to gut and build into something else, but it sounds pretty good as is and makes a nice light practice amp. The stock speaker really sucks and makes the amp sound crappy. When the crappy stock speaker is replaced with a good Celestion or similar, it really opens up the amp's tone and makes it sound much better. The mini stack head was used by a lot of the LA guys in the mid to late 80s in rack setups as the tone generating machine, IIRC.

It has never really worked right. When I got it the low input channel was intermittent, it would cut in and out while playing the amp. The high input is fine. I tried reflowing the solder on the low input jack, but this made the problem worse. Now it squeals when the gain is dimed and the volume is turned up.

Any suggestions where to begin? There aren't that many parts connected to the low gain input: a jack, a resistor and an opamp.

Any ideas where to start to fix this thing?

Schematic is here:

http://www.drtube.com/schematics/marshall/3005.gif

The combo is model# 5005, the mini stack is model# 3005.

TIA,

steve

[Ray Ivers]

Steve,

If it were my amp, I would replace both input jacks. There's a muting function on the op-amp output being performed by the 'low' input jack, and a voltage-divider function by the 'high' jack; I don't know precisely what's going wrong in your amp - possibly some kind of positive feedback or the op-amp + input ground reference disappearing - but I'm pretty sure that replacing both jacks will make it go away. I would also recommend resoldering all the component connections around IC1, and in my Lead 12 I pulled the 1458 and put in a socket and LF353 op-amp, which improved the overdrive quite a bit IMO.

You could also try cleaning and retensioning the jack contacts; IME the effect of this will last anywhere from a week to 3 months, but YMMV.

Ray

[Mark Black]

I agree with the jack replacement but would like to add that every one of those I have seen and/or worked on did the same squeal thing - I think it may be inherent in the design or (more likely) layout.

Good luck!
Mark

[steve]

Mark and Ray,

Thanks for the replies. I will replace both jacks.

I planned to remove the op amp that is in the amp and put in a socket. Thanks for the tip on the LF353 op amp.

What is the best place to get real Cliff jacks? AES has them, but have a handling charge in addition to the shipping.

This is the third Lead 12 amp that I've owned. I owned a combo and a head previously, and both of them worked flawlessly; no noise issues whatsovever. Made me wish I'd kept them

Regards,

steve

[Enzo]

The handling charge applies to orders below the minimum. WHen I face a low order, I usually look for somehting else I need there. How about fleshing out the order with tubes or a reverb pan or something you might order elsewhere.

Otherwise Mouser has no minimum:

Page 891 on curent catalog - neutrik
stock # 550-20301 58 cents each TRS (stereo)
550-10201 70 cents each TS (mono)
568-NYS216 69 cents each TRS

Page 903 - Pomona
565-7158 74 cents each TRS


I am sure there are other sources.

[steve]

Enzo,

As always, your input is greatly appreciated.

Regards,

steve

[steve]

Update:

I got three new Cliff jacks from AES. I replaced all three jacks; low & high inputs and headphone/line out jacks. I also replaced all three resistors on the input: 68k, 33k, 330k. Oddly enough the 300k resistor measured 270k as did all "330k" resistors in my parts bin.

Anyway, the squeal is gone. Although there is a split second of squeal when plugging into the low gain input due to the shorting jack opening and then closing to make contact.

BTW, the original jacks were Rean brand. Are these any good? They appear to have been made in England.

The low gain input does seem louder than the high gain input with the same master and gain levels.

Thanks for all the replies.

steve

[Rick Erickson]

Oddly enough the 300k resistor measured 270k as did all "330k" resistors in my parts bin.

Were they carbon comp. resistors by chance?
Rean jacks are O.K., but I prefer Switchcraft #11 through #14B, depending on the application. Retro-fitting them in an amp with plastic jacks can cause some grounding issues, but that can usually be worked around by rearranging grounds or using isolation washers. The only other issue is space - the Switchcraft jacks are typically larger than the Rean jacks.

RE

[steve]

Rick,

Were they carbon comp. resistors by chance?

Oddly enough, they are 1/2w carbon film resistors I got from Mouser about 6 months ago. They are marked 330k but the 20 or so that I tested of the 100 I got all tested at 250k or below.

Rean jacks are O.K., but I prefer Switchcraft #11 through #14B, depending on the application. Retro-fitting them in an amp with plastic jacks can cause some grounding issues, but that can usually be worked around by rearranging grounds or using isolation washers. The only other issue is space - the Switchcraft jacks are typically larger than the Rean jacks.
RE

The Cliff jacks that replaced the Rean jacks looked like they were about the same quality.

Does Switchcraft make PCB mount jacks?

I like the quality and feel of the Switchcraft jacks over the Rean and Cliff jacks. The Switchcrafts just feel more substantial to me.

Thanks for your reply.

steve