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Author Topic: Marshall 5203  (Read 7614 times)

Roly

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Re: Marshall 5203
« Reply #15 on: September 05, 2014, 12:34:13 PM »

Yeah, step by step, change one, test, change other, test.   :dbtu:

It's slower and boring in the short run, but it can help a lot in nailing down what exactly a generic fault is, and maybe what it's being caused by.
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Stu

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Re: Marshall 5203
« Reply #16 on: September 09, 2014, 11:48:51 AM »

OK, turned out to be IC1.  Once changed I noted that both LED1&2 start to illuminate as volume and break-up occur, I'm thinking this is what must be clipping the signal to get the distortion - I've seen it done with normal diodes in a fuzz box circuit before.

Anyhow, job done. Thanks for the input folks.
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Roly

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Re: Marshall 5203
« Reply #17 on: September 09, 2014, 12:51:32 PM »

OK, turned out to be IC1.  Once changed I noted that both LED1&2 start to illuminate as volume and break-up occur, I'm thinking this is what must be clipping the signal to get the distortion - I've seen it done with normal diodes in a fuzz box circuit before.

Anyhow, job done. Thanks for the input folks.

Good.   :dbtu:

That's right.   :dbtu:

 ;D
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Matt Downing

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Re: Marshall 5203
« Reply #18 on: December 21, 2017, 03:05:07 PM »

I have had a similar problem with my Marshall Master Reverb 30 5203 and thanks to this thread I fixed it quickly.  All the posts above are much appreciated. 

On my amp, as I turned up the preamp volume, it was fine up to about 3, then started to fiz and go quiet at 4.  The problem was one of the little IC op amps, in my case IC1, an MC1458, easily and cheaply obtainable from ebay or Farnell.  It was only a few minutes work to replace it, although desoldering the old one was a little tricky.

For future ref, I noticed that while the amp was faulty, turning up the pre-amp lit only one of the LEDs; after fixing it, they both light up.

All the best and thanks again to the original poster and answerers.
Matt


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THRobinson

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Re: Marshall 5203
« Reply #19 on: September 12, 2018, 10:42:02 AM »

Hey guys... sorry for the resurrection but, thread is 99% exactly what I have happening so made more sense to reply than start a new thread of the same thing with a link to it. :D

Basically, identical issue as the OP, except, it's the preamp volume causing issue not the master volume. Otherwise, same problem, and even the same LED1 and LED2 light situation.

Would it then likely be the same IC that needs replaced?
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Matt Downing

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Re: Marshall 5203
« Reply #20 on: September 12, 2018, 11:55:29 AM »

I have had this problem twice.  The amp is pretty old (circa 1986), I don’t know what the underlying cause was, but each time I fixed it by replacing either IC1 or IC2.  The part number can be found in tiny lettering on the IC and people sell them on ebay for just a couple of pounds. (It was a bit gutting to discover that this classic cranked Marshall sound is all coming from a dirt cheap Texas Insturments IC (insert emoji of choice here)).

The tricky bit is desolding the IC because it has 6 or 8 legs which all need removing.  My method is to pull on the top of the IC with a pair of pliers, while giving the all the legs a quick blast with a cook’s blow torch.  It’s obviously risky and I have slightly scorched the circuit board, but I was quick enough not to do too much damage and it worked.

Good luck.
Matt
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THRobinson

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Re: Marshall 5203
« Reply #21 on: September 12, 2018, 12:14:43 PM »

Thanks for the info...

I was debating, if having to do all this work, maybe soldering in IC Holders... less chance of damaging the IC with the soldering gun, and in a case like yours, if needed done again, it's a simple pull one out, pop a new one in.

Shouldn't be any downsides to doing it that way, I assume?

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Matt Downing

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Re: Marshall 5203
« Reply #22 on: September 12, 2018, 12:59:40 PM »

Very sensible idea.  If one of my ICs goes again, I’ll definitely put in IC holders because I don’t know how many times the circuit will survive being blow-torched.
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THRobinson

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Re: Marshall 5203
« Reply #23 on: September 12, 2018, 01:22:13 PM »

Unrelated (kinda) but, the pots... do they say Alpha on the back?

Just curious, had to clean them and the Bass was seized pretty tight, turned but with effort... Deoxit'd them all, no longer dry, and after 6 flushes the Bass is normal again. When opened it up saw they all said Alpha, which I knew made compatible pots for Marshalls, but the originals I suspect were replaced? Didn't originally say Alpha did they?

Part of me wants to replace those while I got it apart. Shame but I don't think Bourns fit. If I recall, the leads are staggered a bit differently on Marshall pots.

Nephew replied finally, pre-existing issue... I'm outta the dog house for breaking his amp. :D For the price, I guess there are three CA1458's and one TL701... I'll buy them all, maybe replace them all and add the holders. For the price it saves me some time... though always the risk of damage.
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THRobinson

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Re: Marshall 5203
« Reply #24 on: September 12, 2018, 01:54:37 PM »

Got pics... looks like the schematic said CA1458, but chips are Motorolla MC1458CP1. Some on eBay, cheap but China and don't have the 15-45 days it takes to ship.

Do they have to be identical? I heard they can be finicky... but, I see some on sites like digikey, mouser and tayda that are close... like LM1458, or MC1458L... those work?

https://i.imgur.com/MeDbj2y.jpg
« Last Edit: September 12, 2018, 10:47:34 PM by THRobinson »
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Matt Downing

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Re: Marshall 5203
« Reply #25 on: September 13, 2018, 02:54:24 AM »

I got my ICs from eBay seller HA5IA and the amp sounds great to me.
https://www.ebay.co.uk/usr/ha5ia?ul_ref=https%253A%252F%252Frover.ebay.com%252Frover%252F0%252Fe11401.m1842.l1181%252F7%253Feuid%253De16cf49c9615433db2b37278b60bb955%2526bu%253D43117872547%2526loc%253Dhttps%25253A%25252F%25252Fwww.ebay.co.uk%25252Fulk%25252Fusr%25252Fha5ia%2526exe%253D19027%2526ext%253D49342%2526es%253D3%2526nqc%253DACQAAAAAAAAAAgEACAAAAQAAQAAAAAAAAAAAAQIBAAAAAAEAAAAAACIAAAgCJEAAAQAAIAAAIAAAAAAEBAAAAAgAgECAAABAAAIAAAAAAAAAIYAAIAAAEAICUAICkAAIBAEEIAACBAAQBAABAAwQAAACAABAYAAKBAAIACQAAAAAAAEAIIAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEA**%2526nqt%253DACQAAAAAAAAAAgEACAAAAQAAQAAAAAAAAAAAAQIBAAAAAAEAAAAAACIAAAgCJEAAAQAAIAAAIAAAAAAEBAAAAAgAgECAAABAAAIAAAAAAAAAIYBAIAAAEAICUAICkAAIBAEEIAACBAAQBAABAAwQAAACAABAYAAKBAAIACQAAAAAAAEAIIAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEA**%2526ec%253D1%2526sojTags%253Des%253Des%252Cnqc%253Dnqc%252Cnqt%253Dnqt%252Cec%253Dec%252Cexe%253Dexe%252Cext%253Dext%252Cbu%253Dbu%2526srcrot%253De11401.m1842.l1181%2526rvr_id%253D0%2526rvr_ts%253Dd1b1f01e1650a8602960bd91fff7d1e5&ul_noapp=true
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THRobinson

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Re: Marshall 5203
« Reply #26 on: September 13, 2018, 07:54:49 AM »

Typed in 1458, MC1458 and MC1458CP1... nothing shows. Sold out I guess.
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Jazz P Bass

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Re: Marshall 5203
« Reply #27 on: September 13, 2018, 12:19:44 PM »

Why are you considering changing the op amps?

MC1458 (LM1458) was a low cost IC and it worked well in the day.

The modern equivalent would be RC4558.
A lower 'noise' IC is the NE5332.
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g1

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Re: Marshall 5203
« Reply #28 on: September 13, 2018, 03:34:23 PM »

Mouser has MC1458P.  That is what I would use.
Jazz, I think some Marshall circuits get unstable when 1458's are replaced by 4558, that's why it is suggested to stay with the original number in SS Marshall's.
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THRobinson

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Re: Marshall 5203
« Reply #29 on: September 13, 2018, 04:01:39 PM »

I was on the TAYDA site, pretty good site, saw LM1458N's listed... any chance those work? or would the MC1458P's be a better match?

On another site someone said to swap the 1458's with TL072's because lower noise... but honestly don't know enough about IC's to know if a good or bad idea.

Then again, if installing 8-Pin Sockets... would it hurt to try both? or best stick with 1458?
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