Update. I changed C5 for the 2.2 electrolytic and now the amp sounds so much better and takes all my pedals really well! Thanks for the suggestion J M Fahey!

Quote from: J M Fahey on July 31, 2013, 01:08:44 AM
You were very lucky that the diodes in the power amp were (just by sheer chance) NOT the bias ones.

To have an idea of what you did and to understand the reactions around here, imagine you told us you poured a bucket of gasoline over yourself , just to clean some paint stains in your clothes, and then , while you were waiting, lighted a cigarette to pass the time. :loco

You were lucky that you misread the label and it was actually water .

OK, back to the amp.

No "magic" parts will help you here, what matters is parts *value*, not brand, colour, who-used-it, etc.

That amp has a seriously compromised "bright" channel, which should be called "Zero bass -  zero mids" instead.

The culprit?: what on the schematic is called C5 (0.022uF) , which must be replaced by a 2.2uF x 25V electrolytic, with the "+" side towards the Tr2 transistor collector and the "-" terminal (usually indicated by a stripe) towards the R9 resistor.

This will make that channel "flat - slightly bright" , quite "normal", but most important, will make distortion pedals usable.

As of

QuoteI want to mod this amp with a bluesbreaker or old JMP type crunch, if possible.

...     ... if it were easy somebody would already have 

Get a good distortion pedal, now it will be usable, that's more than you bargained for.

The amp by itself is not bad at all (after modding), but Marshall in the 70's was just starting the SS path .... an quite half heartedly.

Please post results.

First of all, I would like to thank everyone who replied to my post! I can't express enough how grateful I am with sharing your knowledge and expertise on this forum. I will be looking to take a serious study into electronics, as I have realized the potential for great error.

Thanks for the capacitor info. I will follow your suggestion.

Quote from: Enzo on July 29, 2013, 11:42:28 AM
Oh, please don't just throw parts at it.   If it is humming really bad, you might need new filter caps, but really, solve the problem first, THEN once the amp is working think about wholesale cap swaps and other upgrades.   Otherwise, making a lot of extra uninvolved parts changes is just that many more opportunities to confuse the troubleshooting.

Schematic:  http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jmp_masterlead__30w_2199.pdf

Before any mods, have you disconnected the amp's own speaker and played the amp chassis through some other speaker you know the sound of?  Do that, it might be the speaker you hate.

You tried different diodes and that improved gain?   What does that mean??  I see only two diodes in the power amp plus a zener there.    Those plus the main bridge is all I see, none of which would affect gain. 

And has the bias been adjusted?  Too cold bias results in crossover distortion.

Yes, I tried the amp through a set of Celestion G12-80s with the vented magnets and the amp still had that same character I described.

I changed the two diodes in the power amp with leds. This gave the amp more perceived gain to my ears. I changed the diodes back though. I tried adjusting the bias pot with gradual adjustments and played the amp with each small adjustment, but it still didn't smooth out the sound. The amp doesn't hum at all, so I would imagine the filter caps are fine then.

I'll hold off on changing out the caps. Anything else that could be a culprit?

I've had this amp for several months and am finding that the overdrive is on the fuzzy, buzzy side. I've ran some pedals through all 3 inputs and the bright channel is extremely trebly to the point of no bottom end what so ever, which sounded horrible. I've opened the amp recently and found that the original Daly brand filter caps are still there and appear to not be leaking or bulging, but I am wondering about just replacing them anyway. I placed an order for Mallory 150 caps through Tubedepot.com to see if replacing the original cap values in the amp will help to smooth out the tone a bit. I tried different diode combinations which did boost the gain, but it still had that terrible quality to it. For example, when bending a string it just did not have a pleasant dynamic even for a solid state amp. The character was like a combination of ping and buzz ringing, if that makes any sense. There is potential in this amp, as turning down the gain on the amp or guitar volume yields a usable clean sound. I want to mod this amp with a bluesbreaker or old JMP type crunch, if possible. Any suggestions or thoughts on eliminating that wierd buzz quality would be immensely appreciated.

I wanted to share my experience with the G20, 20 watt solid state combo from the new Acoustic Lead Series of guitar amps. I tried this model out at a local Guitar Center using my Washburn G10V with Seymour Duncan JB bridge. Immediately when I plugged in using the gain channel it blew me away as to how fat and pleasant sounding the amp was, as it didn't have that harsh solid-state character. I bought the amp on the spot after 30 minutes of playing for $69. The EQ section consists of Gain & Volume for the Lead (gain) channel and Volume (rhythm), Bass, Middle,Treble for the clean channel. There is also a push button to scoop the mids and this gives the amp a Mesa Boogie kind of vibe which is cool for a different flavor of tone to work from the amp. Overall, when the amp is operated without the mid scoop it can be dialed in to achieve a nice Marshall sound for blues, hard rock and 70s/80's metal. Personally I would use the scoop feature to capture the modern metal sound, as it works perfectly for that. Being somewhat of a tone snob, I upgraded the 10" 6 ohm speaker with a new Celestion G10 Vintage. As I unscrewed the stock speaker, I was impressed to find that Acoustic used machine screws that thread into a metal inset within the speaker baffle. Usually with cheaper amps the speaker is attached to the baffle by wood screws that are simply screwed into the wood baffle. Once I installed the Celestion speaker, which was much heavier due to a larger magnet than the stock speaker the amp came to life with even more dynamics. The bottom end was more pronounced and not as muddy and slightly fuzzy as before. The mids and highs were rounded off and accentuated the amps already great sonic assault and amp was a bit louder than before. I'm more of a rock guy, so I mainly play on the lead channel, but the clean channel also had a more complex clarity and dare I say shimmer than stock. Anyway, I just wanted to share this experience and also let people know that the Acoustic Lead Series of amps sound awesome for economical musician.

I checked all the ICs using compressed air. I haven't tried changing any of them yet.

Well guys, I checked the ICs and now the amp is working fine and I've played it several days through both the high and low inputs on the gain channel and the issue seems to have gone away. It's puzzling, but I may eventually solder in sockets and try different ICs in place of the current ones. Thanks again for all the help!

Thanks for the info guys! I'll check the ICs and report back later.

I have ruled out problems associated to a bad guitar cable or guitar itself by using different cables and guitars. I also sprayed tuner cleaner on the 1/4 guitar jack and inserted and removed several times in low and high input jacks. I also cleaned all pots with tuner cleaner as well. Speaker connections are solid as well.

First, I really appreciate the people on this forum and greatly value the suggestions and assistance you provide. I've had a Rocktron R120C amp combo for several weeks that I bought used and recently a problem has surfaced. When I play on the gain (dirty) channel, after about 10 minutes of playing the amp will briefly cut out completely with no sound and within 2-3 seconds the sound returns. This will happen intermittently once the amp has been running 10+ minutes and continues to cut out in the same way if you remain on the dirty channel going through either the high or low input. This problem doesn't occur on the clean channel at all through either of the inputs. I opened up the amp and tried resoldering the switching components and input jacks, but the problem continues. I'm not sure if this is related to the preamp section or power section at this point? Any help would be immensely appreciated.

Thanks for the reply! I've played through the Rocktron for several days and I am impressed with the clean channel and the fatness and clarity of the gain channel. I removed the amp chassis and it looks to be well put together. It has a reverb tank and the power transformer is quite large and beefy for a solid state amp, which reinforces the quality of Rocktron. I am lucky to have found this amp.  :dbtu:

I just picked up this guitar combo yesterday afternoon from a local Music Go Round and was surprised at some of the cool tones I got through both the clean and gain channels. I tried the high gain input first and felt that this would be great for covering a more extreme side of metal, as it has way more gain than needed. I favor going through the low input more, as the gain channel was very usable and still could achieve more than enough gain to get me a modded Marshall-like tone for heavy blues, classic rock and 80's heavy metal. This amp is rated at 120 watts, has a huge amount of bottom end from the 2x10 speaker setup and also has a built-in chorus and I believe a digital reverb, as I didn't see a reverb tank, but will pull the amp chasis later today and take a look at the circuit board and electronics. Anyone else heard of this amp or tried one? I haven't found too much info or even a review about the amp.

Quote from: J M Fahey on December 23, 2011, 02:27:37 PM
You´ll have to get the same values, of course.
Don´t know whether you will have space for new, non-board-mounted ones though, because I guess the board is flush with the front panel.
Maybe you´d better leave it as-new.
Remember these were made in the early 80´s and gave long years of service, not bad at all.

Thanks for the reply. I haven't accessed the circuit board, but I agree with you. I'll stick with the same pots Marshall used. I'll just replace the reverb pot with the exact value and style. 

First of all, this is my first post and I would like to say 'Hello' to everyone on this site.
I have two Marshall Mosfet heads. One is is mint condition from 1989 and the other is from 1987 that appears to have had some replaced electrolytics and the input jack has also been replaced with a Switchcraft and this amp is going to be my test bed for mods and tweaks. I've cleaned the pots with contact cleaner and turned each pot many times to work in the cleaner, but the reverb knob is still scratchy and looks like it needs to be replaced. I want to replace all the pots with higher quality ones using flying leads instead of board mounted. Also what value/style pots would I be looking for and what website would be best to find them?

I found these pots:
http://angela.com/100kaudiotapersolidshaftguitaramppot.aspx

http://angela.com/250kaudiotapersolidshaftguitaramppot.aspx