Hi I have a Marshall Master Reverb 30. It's a great little amp. It has recently developed a strange fault - when turning the amp up using the master volume the amp gets louder but as the pot goes past half way the volume decreases?!?
I have removed the mater vol pot ad checked it with my multi meter and it looks fine.
any help, advice etc would be much appreciated
The main chance is that it's a pot problem, and that cleaning with a contact cleaner, or perhaps replacement, might be the cure. But this sort of symptom has come up when one of the by-pass electros for one of the preamp sections has died and you are getting unwanted inter-stage negative feedback. Maybe. It could be one of those rare cases where one of the caps really is to blame.
Needs circuit and more diagnosis.
Hi Stu,
5302 brings up a 12/20 watt keyboard amp. I think you mean 5203?..
http://www.drtube.com/schematics/marshall/5203.gif
That's better. here it is again but in a pdf, if anyone prefers.
http://bmamps.com/Schematics/marshall/Marshall_30w_5203.pdf
:( A bit stumped here I hafta admit.
Okay, up the river; somewhere on the board there are two zener diodes ZD1 and ZD2. These should both have 15 volts across them.
Please find, check, and post (then we at least we know are starting our fault finding with good supplies).
Internal pic of the board would be helpful too.
Q. Does the position of the Treble control change how the Volume control acts at all?
{I can't see anything obvious that would cause this odd volume control behaviour. I suspect it's impedance related, the level rises and falls as the impedance of the Volume wiper, which suggests that it might be something to do with the following input of IC2a, but that is only a wild surmise at this point.}
BRAINS TRUST! - this is a curly one we got 'ere maties. Ideas?
(https://i.chzbgr.com/maxW500/5688080640/h4AF02E1F/)
Thanks for the replies folks.
Yes, primofacprima it is a 5203, my bad...
Roly, zeners checked and they both have 15 volts across them! I am also sure the pot is ok, the resistance increases smoothly when the pot is turned, no jumps or sticky points....
The Treble control has no effect on volume.
However, I have spotted that as the Master Vol reaches half way and the volume starts decreasing that LED2 begins to illuminate and gets brighter as the MV is increased (and the volume reduces) - I'm thinking this could be a clue?
the circuit board looks exactly the same as the one in this thread: http://www.ssguitar.com/index.php?topic=2851.msg21069#msg21069 (http://www.ssguitar.com/index.php?topic=2851.msg21069#msg21069)
Well the LED is certainly a clue, I just don't know what it means.
Have you given all the pots and signal sockets a good cleaning with metho or Deoxit yet?
Sounds like it may be breaking into oscillation, causing volume loss and turning on the LED?
Quote from: g1 on September 03, 2014, 11:35:32 PM
Sounds like it may be breaking into oscillation, causing volume loss and turning on the LED?
ooh .. ah ..ooh .. ah ...
We've been here before.
Something about a long trace back to an earthing contact on the input socket, having to mod the trace because the instability is inherent when the contact gets dirty, ...
... something, something?
It looks to me as tho the output of the second op-amp must have some sort of +ve bias which increases with the master volume (hence LED2 illuminating and not LED1). But I can't understand how this is happening as the MV is on the output of op-amp3, it seems strange that it is effecting the previous stage!! Oh, I wish I had a scope!
A couple thoughts:
Clip a couple extra filter caps across the 15v rails to ground, there might be insufficient decoupling from the power supply.
The schematic specifies 1458 dual op amps. ANy chance someone has put more recent op amps in the circuit, like even 4558 or newer? There are some Marshall solid state amps of that era that were sensitive to it. The newer ICs had more bandwidth or something, I forget just what.
Once I replaced a bad 1458 with a 4558, which usually is OK, but I found the amp unstable. After chasing it a while, I talked to someone at marshall who pointed out to me that these amps really did need the 1458.
Just a possibility.
Quote from: Roly on September 04, 2014, 05:40:52 AM
ooh .. ah ..ooh .. ah ...
We've been here before.
Something about a long trace back to an earthing contact on the input socket, having to mod the trace because the instability is inherent when the contact gets dirty, ...
... something, something?
Hey Roly was this the one you were thinking about?
http://www.ssguitar.com/index.php?topic=3070.30
I may have discovered the answer.... http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://www.circuitsonline.net/forum/view/31529&prev=/search%3Fq%3Dmarshall%2B5203%2BLED2%26biw%3D1118%26bih%3D584 (http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://www.circuitsonline.net/forum/view/31529&prev=/search%3Fq%3Dmarshall%2B5203%2BLED2%26biw%3D1118%26bih%3D584)
sounds like a very similar problem, he doesn't say which op-amp he changed but at 60p a pop I'll do um both.. will post my results in a few days.....
Yeah, think so Doc.
Good find Stu.
Could you please do them one at a time and see which was the problem? That way we will have a definitive solution to the issue, which may come up again.
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.
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.
Quote from: Stu 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.
Good. :dbtu:
That's right. :dbtu:
;D
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
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?
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
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?
(http://cdn.shopify.com/s/files/1/1496/7798/products/IC_Socket_grande.jpg?v=1487349080)
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.
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.
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
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
Typed in 1458, MC1458 and MC1458CP1... nothing shows. Sold out I guess.
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.
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.
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?
IC1 was indeed the culprit.
Bought from Digikey,
PART: 296-1403-5-ND
MFG : Texas Instruments / MC1458P
Soldered in an 8-pin socket, and soldering was perfect, looked like a machine did it. 0 sound. Desoldered without thinking, shoulda used a multimeter first. Went to solder it back again and a few of the traces were gone. I will mention, this board was worked on before... looks like newer jacks, the pots all replaced with cheaper ones sadly, and IC1.
Soldered in small wires to rebuild the traces, this time, multimeter like I should have done, and works fine. Bit buzzy... grounding issue, but may be in part due to this house. 100+yr old house, still needs to be rewired, no ground.
In anycase... by the time I finally got this all done, the bass pot was starting to get stiff again, so I suspect by Spring, I'll be replacing the pots with genuine Marshall. I'd use Bourns or CTS but the middle lead is offset... though still, if leads long enough, may be as simple as a small bend and it'll reach. :)