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Marshall 2199 cap replacement oscillation issue

Started by solidstate2199, July 22, 2016, 12:37:10 PM

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solidstate2199

Hello
i know there are one or 2 threads for the 2199 already, but i have some mod questions in
the future where i will use this thread..

I bought the 2199 some days ago, mainly for the speakers (G12K-85) and to use as a 2x12, because
i previously knew that these amps dont sound good. They are playable clean, and even then these models seriously lack presence. The build in overdrive...well i come to that later.

I played the Amp a little for testing purposes and there is an electronic high frequency oscillation.
Bassically on all 3 channels, on the normal not that much but just because this channel doesnt have much highs to start with. When i plug in the mix channel and turn the treble up past 6 and the bright gain up the amp starts to squeal like a dirty pig, the oscillation changes pitch and attitude when i change eq or gain settings. When nothing is plugged in , theres no oscillation, i read somewhere thats because the cliff jacks shunt the signal to ground.

Alright, i opened the piggy up today to find a siemens capacitor roaming free inside the amp housing.
Its C17. Question: on the photos you can see white gunk on 2 green electrolytics and the free space where C 17 belongs. Is this Marshall Factory glue or innards from C17? Why is the Amp still working
without that cap, what is it supposed to do, please understand i have no clue about electronics.

There is some circuit changing on the right side of c17, the leg of one resistor is connected to the one
beneath it and a tantal cap is piggybacked on top of it. I believe this mod is factory  because i found 2
pictures of 2199 innards on the web and there is the same modification made.

There must have been someone in there before me because 2 speaker wires which where
originally hardwired to the speakers where modified with a cliff jack mounted inside the chassis.
But i doubt that the cap was loose at this point. I dont really see any burnmarks at spot C17, maybe
on one side i dont know. Could the Cap be torn off while shipping? because the seller typically for
old stinky Marshalls only wrapped a little cardboard around and called it a day :duh

Also the schematic says C17 1000microfarad/ 25volt and the one found in the Amp saz 2500/35
but i suspect this is the Marshall factory C17 because i have other 2199s with the same cap.











Regards

Enzo

It is simply a filter cap on the power supply, and with it out, the preamp has no decoupling and instability could easily result.  Replace the cap.  If the schematic says 1000uf, and you found 2200uf, I'd imagine the schematic is an earlier revision level.  I can see the factory deciding 2200uf worked better.  But even if they didn't do it, it is a perfectly reasonable change for someone to make, so install a new 2200uf and move on.

The white stuff is just glue.

If you don't understand electronics, that cap is like a shock absorber in your car.  If one of your shocks fell off, the car would still drive, but that wheel would become less stable.  Without this cap, the circuit still has all the voltages, but there is no longer that cap to stabilize the positive voltage for the preamp circuit.

The cap probably just vibrated enough that the wires snapped.  Like if you bend a paper clip enough times it will break in two.   That is why they glue some parts down on boards, to prevent them vibrating enough to break their wires.

solidstate2199

Thanks for your help, i will replace the cap with a 2200uf/63v and hope
that it remedies the oscillation.

Since i thought about modding the amp to get an overall brighter and more present sound,
should i start with a different cap value at c17 or ist this strictly a decoupling cap
which hasnt much impact on the overall sound?

I know at the moment that i should up the c5 value from 0.22uf to at least 2.2uf to thicken up the
bright channel. Bassically it will become exact like the normal channel...this is not much of an
improvement on these amps. What i want to achieve is that more highs from my guitar reach
the speaker, these amps are dark, dull and somewhat nonpresent sounding.
The Distortion sounds ok if you set the gain on the verge of breakup...more than that and
it gets an unpleasant mess if you play more than 1 note...but i will explain that with soundfiles
if i get into the modding thing.

The other 2299(same with reverb and much more gain) has a problem with radio interference and overall loud noise floor...
but only on the normal or bright input...the mix input is reasonably quite and sounds not too bad up
to a certain preamp-gain setting where it gets harsh and muddy.
Unfortunately no schematics available for the reverb version.

DrGonz78

I would curious why R29 was modified and it might be best to put amp back to stock before any modding. The idea is to get it back to how it was designed and then think about modding at that point. Also, TR3 looks like a chunk of it has blown out by the looks of the pic.
"A person who never made a mistake never tried anything new." -Albert Einstein

g1

R29 has not been modified, it matches what the schematic shows.
I would guess there was an error or change in the board layout that had to be corrected by moving one end of R29.  If R29 was placed as the silkscreening shows, it would connect to TR8, rather than TR7.
So no problem with how it is now.
The little cap that has been added across R28 is probably dealing with an oscillation issue, so that should be left alone also.

Enzo

Agree^^^^^

"Back to stock" is a HUGE assumption on your part.  Presumably the amp worked fine until whatever failure, so leave the change alone.

C17 is a filter cap and is not going to make the amp brighter or darker.

Loudthud

Tone stack looks a little odd for a Marshall. Try C13:470pF, R24:33K, C14:.022uF, C15:.022uF.

With 25V rails I would expect an 8 Ohm total load for 30W. Speakers should be 2X 16 Ohm.

DrGonz78

Quote from: g1 on July 23, 2016, 03:55:06 PMR29 has not been modified, it matches what the schematic shows.
I would guess there was an error or change in the board layout that had to be corrected by moving one end of R29.  If R29 was placed as the silkscreening shows, it would connect to TR8, rather than TR7.

Sorry about that one. It was late when I was looking at the picture and trying to match it up to the schematic. I should not have assumed it was not stock that way and thanks for setting my comment straight.
"A person who never made a mistake never tried anything new." -Albert Einstein

solidstate2199

#8
Thank you all for the good tips, i will report back when the cap is installed.
At the moment i still test the sound of these amp, today i rewired the speakers on
one of the 2199 combo with an speaker output jack so i can use it with other amps.
i found out that the to my ears kind of unpleasant sound is partly due to the 2x12 construction...maybe im just not used to it but it sounds kind of muddy to me.Maybe they need more Volume to open up the treble frequencies like my 4x10 Marshalls, they have a good treble even at low volumes...i have more testing to do before i decide to mod these 2199 amps, i have to try the 2199 with a different cab to really decide.
plus they all sound different to begin with( i have 3 , got them cheap)

At the meantime has anyone tips on the grounding issue (remember the radio signal and the higher noise floor is only on the normal input and the bright input..not on the mix input, this one has no unexpected noise and or radio interference)

yes the resistor mod is bassicaly like its in the schematic, i saw some of these amps who had the mod and some dont i guess its marshall factory, different revision.

i discovered a picture of one with a blue circuit board (made for us market)...never seen this in a marshall. i guess its a blues amp after all 8).



Also i would like to understand how the overdrive in this amp is achieved.

i read an article from rg keen on different ways to create distortion, but to be honest
even this was hard for me to understand even a little, im no technical guy.
There is no Led diodes or op-amps in these amps like later marshalls, so is it like overloading
transistors which create a cutoff for the signal or compression?
i like the sort of "crispy, krackly crinkly not really sustaining type of distortion", sounds like ripping
or crumbling paper with your hands, but on these amps it gets too harsh
at a certain pre-gain setting.

phatt

#9
Quote from: solidstate2199 on July 25, 2016, 05:44:41 PM
(remember the radio signal and the higher noise floor is only on the normal input and the bright input..not on the mix input, this one has no unexpected noise and or radio interference)

Might be because the mix channel is the only input that has some series resistance, those two 68k resistors (R1&R2), they help to stop RF interference.

Also might be a design issue lurking,, this circuit might benefit from more decoupling. (See My Edit of the circuit) I recall our dearly departed friend Roly noting that when you have many stages in series like this you can have instability problems.
The signal bleeds through the supply if it is not decoupled enough.

You may have to cut a track to do it.
Better minds might like to add comment about that. 8|

Regarding the OD or lack of.  :'(
This amp will never do Valve amp crunch, you will spend forever trying to get that effect from this circuit ,, it's not designed for that.
If you want OD then buy an OD pedal,, they come in many flavors. :cheesy:

Other observations,, if the amp is too muddy then C1 and C4 could be replaced with much lower values,, try 22nF or even 10nF.

If too bright then a small value cap across the signal might help to tame the edge? I'll have to research that problem later.

Phil.

solidstate2199

#10
Hi Phil
thanks for the thoughts , keep them coming i will refer to all of them when (if) modding
time comes. Still evaluating the Speakers/cabinet size.They are sized like the 1936 series
minus the Amp on top.Idk they just dont sound right no matter what amp i try, something happens in the upper mids to highs which mud up the sound.

As for design flaw, its only one of the amps which has the noise and radiofrequency interference,
on the other 2 every inputs work fine.

I think these amps where designed for giving the nice overdriven crunch. It was the mid 70s,
nobody would have böught a clean only amp these days, Marshall knew exactly what they wanted,
and to my ears the got pretty close, these amps sound definately like ripping Marshalls, it just needs fine tuning. Marshall ad of one of the 70s transistor heads said " transistorized Head specially
designed to give you the sound Marshall is known for" something like that. They were going for the sound. Off course, it wont be transformed into a JMP2203, mostly the bottom end will
lack, but the basic sound is there,the roundness of the notes and such, just the "whiplashing" has to go.

Unfortunately, im no pedal guy-at all. I noticed that the physically smaller everything gets, the
more small and compressed squished it sounds... i know lots of people dont believe this and say
only component value counts...to my ears if you want big sound you need bigger sized components.I dont like fiddling with other stuff than the amp itself while playing.

Second this Amp hates pedals. I really have to put up some soundfiles so you can hear what im talking about, if you turn the preamp gain up to high it sounds " whiplashing ".

I wonder if the 2195 and 2098 heads have the same problem, i heard one demo on youtube and
there the problem doesnt exist,but its always different if one sits in front the amp. maybe checking the 2199 schematic against the 2195 will help.

https://www.youtube.com/watch?v=Xu3FtoG3Vyo

Thanks guys i appreciate the help.

phatt


If by *whip-lashing* you mean very harsh hifreq hash then check C11 (100pF) it rolls off a lot of excess bandwidth. If it's gone open then that would wreak havoc on instability.  :duh
The schematic is hard to read values so my simulations maybe way off.

Also make sure the switch contacts are actually grounding when cable is removed other wise that will cause issues.

Phil.

solidstate2199

Played around with the 2199 a little.
I now think that the lacking presence of the
Amp is a design "fault" in the preamp section, and
not related with the cabinet or speakers.

I plugged in a Valvestate Head via Effects Output into
the Mix Input of the 2199.
Kept the preamp volume on the 2199 very low to not
overload the Input.
Well now there are enough highs and presence delivered
over the Speakers.

You guys gave me some mod tips on the preamp already, i guess i
will try some.

Bright cap on the Input first and go from there.

J M Fahey

If you want to add bright caps the proper way is to connect a .01uF ceramic cap from middle leg of volume pots to corresponding coupling capacitors feeding them, namely C3 and C7 .
If way too much, or amp starts oscillating incontrollably, you can tame them by ading, say, 1k or 2k2 in series with each.

An easy Mod, try it and post results.


solidstate2199

Just a thought...if i wanted to go ape crazy on this amp
and had a complete preamp makeover in mind...how
hard would it be?
Following preamp would be the final goal...Marshall 5010
Master Lead from the 1980s.
As opposed to the 70s Master Lead the 5010 is a transistorized
version of a JCM2203, at least it really sounds close, without the
big bottom and with a little more fizz, but its really good,
not like the 2199 Master Leads (hard bag of sh...es).
I guess the power section is almost exactly the same...
could somebody have a look over both schematics?
I only know that the 5010 use Ic-Opamps which make a very
good distortion sound.

5010 schematic
http://www.drtube.com/schematics/marshall/5010.gif

2199 schematic
http://www.drtube.com/schematics/marshall/2199.gif