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September 18, 2021, 02:53:34 AM

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Author Topic: Marshall 3005/Lead 12 Power Trans Question  (Read 20550 times)

DrGonz78

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #30 on: August 31, 2021, 03:46:55 PM »
I am not saying the 1458 is healthy by any means. I would remove that 1458 opamp and then measure for voltages at the pads for pins 4 & 8. If the opamp was to blame for missing voltages then with it removed you would still see voltage present at those points in the circuit. If the voltages are missing then we must figure out why.
“A person who never made a mistake never tried anything new.” -Albert Einstein

pinkjimiphoton

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #31 on: August 31, 2021, 04:11:39 PM »
well, its about the only thing i would expect to drain enough current there.
it looks like the "replacement" xformer was installed, but the 2 1k resistors that feed the +/- rails to pins 8 and 4 were never adjusted to account for the voltage change.

gonna go pull it before i leave for tonite's gig. thanks for the advice, sir! :dbtu:

Enzo

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #32 on: August 31, 2021, 05:27:01 PM »
Hi Jimi
You will find in most amp forums it is confusing to mix repairs in threads.  Someone coming upon this one might skim along and look at your post and assume the readings and findings in the earlier posts were from YOUR amp.

pinkjimiphoton

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #33 on: August 31, 2021, 06:04:06 PM »
gotcha. my bad. and my apologies to the forum.  :dbtu: :dbtu:

well to finish up, yeah the 1458 was the issue, but not the one i thought it was.
previous owner had tried to replace the chip apparently, and killed the pad for pin one of the opamp.
so i desoldered it, replaced with a socket and ran a piece of wire from pin 1 of the socket to r7/8.
took some voltages, now i was reading a lower voltage than i expected, +/- 5vdc.
close enough for rocknroll to not fry the chip. replaced it. dead. put in a fresh 1458n out of my junk drawer and fired right up.
thanks for the support and patience.
rock on!
PjP

Enzo

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #34 on: August 31, 2021, 09:42:28 PM »
Glad you got it.   Good you had a 1458.

Just a note.   Most times a 4558 will work fine in place of a 1458.   But in the Marshall preamps of that era, it won't.  a 4558 might function, but becomes unstable and you get squeals and can't turn it up past whatever without howls.   Oh not every time, but I always stocked old 1458s in my shop just to have them for Marshall repairs.

And yes, I saved good pulls from working equipment

pinkjimiphoton

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #35 on: September 01, 2021, 11:53:15 AM »
i build guitar effects for a living. the 1458 is my favorite chip in many distortions, it really sounds different from a lot of the others, i mean for all intents its a dual 741, but to my ear, it sounds great. so i tend to keep a couple hundred kicking around in the semi conductor junk drawer.
i DID socket it, so i could mess around with it if i wanted to, but i always go back to the 1458 in all of my own personal circuits. i figure if it was good enough for the guv'nor, its good enough for me ;)
i've tried just about every dual opamp available in distortion circuits. most folks prefer the jfet based ones, but to me, the '58 is the only way to fly.
proper transformer arrives tomorrow, so i'll install that and button it up.
thanks for the support!