marshall jcm 800 distortion channel not fucntioning properly

[Tiki Monster]

So Ive had this 2205 (1982) for a couple years now and never had a single issue with it. Great amps these things are. But last night while playing through it, i started hearing a strange crackle/fizzy type sound coming through the cabinet. Not too loud. Not matched with the volume of the amp. I hit the standby switch on the amp and the sound continued through so I figured there might be a ground issue somewhere in the place.
Anyways switching between channels while the sound was occurring didn't affect it either. The sound would cease for a bit, and then would return, cease again, and return once more. During this time the amp functioned fine even while the sound was occurring. After about 15 minutes of this weird sound's phase, the overdrive channel gave away. The sound is very light, clean/fuzzy, and quiet in volume. But only on the overdrive channel.
The Clean channel works fine. And master controls do as well. The issue seems strictly on the overdrive channel. The overdrive channel sounds weak basically. I thought the issue might be a bad preamp tube perhaps, but that doesn't add up to why the noise occurred evenly thought all functions and modes of the amp, and only affect one channel.

I am no expert tech so any help help would be awesome.
Thanks

[Jazz P Bass]

Marshall 2205 pre schematic

You are going to have to pop her open.
On the second tube, V2, check the plate resistor R17, 100K, 1/2 watt.

[g1]

If it's the older version with transistor switching instead of the chip, here's another schematic:
http://www.drtube.com/schematics/marshall/4210.gif

[SWEINC]

At first I thought I remembered seeing several units with dirty shorting contacts on the footswitch jack causing this problem. But in looking at the schematic now I don't see how this could be because the part of the jack with the shorting contact only controls the reverb. I would suspect the CA3046. It serves to route parts of the circuit for the channel switch function and if not operating correctly it could cause this type of problem. And there are a few caps thoughout this part of the circuit that could possibly be bad also.

[Tiki Monster]

Hey Jazz, fisrt off sorry to you and the other fellas on here who replyed to me, for I didnt reply soon. My appologies.

But I did open her up, Checked resistor 17 and its fine. Everything actually looks really good. Joints are well soldered. The only thing I noticed a bit iffy about this monsters guts were R1 and C48- (or it might be C40?) I cant tell weather its 48 or 40 because the cap is right over the lableling. But It is in the same circuit trace. Anyways R1 and C48 or 40 are right next to eachother and they seem to have experienced a bit of a current overload. They look very slightly battered its hard to tell weather this is normal. I mean, really other than that everthing looks fine. I am going detach the board so I can check the joints under it. Perhaps an issue can be there.
But any further guide you could give me here on would really help. Thanks.
Oh! and one thing I forgot to mention. Before this problem occured, I was having problems with the Mid trim pot in the distortion channel as well. It wouldnt crackle like when its dirty but rather it would cut in and out of phase. Affecting the who channel. It would cutt off all the Eq's on that channel and only the volume would work. Think this may be a lead?
Again, any help would be much appreciated, Thanks,
Joel-

[Tiki Monster]

Hey Sweinc, Thanks for your respond and my appologies for the late reply. But I opened up my amp and everything seems to be fine. C30 looks fine by as I mentioned to Jazz up top, C48? or 40? or maybe even 46 isnt so charming. I can't tell weather is Ca-48, 46, or 40 but its in the same trace as R1 and R2, and R1 and C4_? seem to have encoutered some current missfortune. Although I would also add that they don't appear very severe, to the point were I can't tell weather the missfortune was there, or if its rather a matter of aged components. Also Im only very basically relative with schematics and I dont see any trace between R1 and C 4_?. I did notice C46 being in the footswitch trace though. Dont really know if it connects with R1. I havent yet checked the joints under the board but thats what Im gonna do next. If you conjure any ideas as to what is the what Id be greatly appreciative. If picures would help I could do that aswell.
Thanks Much,
Joel-

[Tiki Monster]

Hey Sweinc,
I just noticed a but of brown gooey gunk on C46 and C45. Can this be a lead?
Thanks,

[Jazz P Bass]

I see that you are "checking" this & that.
What I do not see is any troubleshooting skills being applied.
There are various voltages that have to be.
You have posted none of them.
Measured dc voltages directly at the plate & cathode pins of all 12AX7 tubes will tell if the tubes are conducting or not. (in this case, specifically V2)

[km6xz]

This sort of 50 post thread could get to the point a lot faster if a standard preface is used for those requesting help. For example:

Model:
Specific symptoms and procedure to reproduce it:
Training level:
Test instruments available:
Supply readings:
Any modifications done in process of trying to repair it:
.....etc.


Even this basic info would cut the time and tinkering down dramatically and get gear working faster and cheaper than poking around and swapping out things or pulling parts to test. Like a detective, I prefer working on cases where the evidence is obscured by people tracking muddy footprints all over the crime scene, it wastes everyone's time and the culprit is often hidden by the evidence becoming ambiguous and stale.