|earache||6/4/2018 3:41 PM|
|Marshall JCM800 50W Combo|
I have this Marshall JCM800 50W combo that isn't as loud on the "Boost" channel as it should be.
I also had an issue with the reverb being static-y, buy solved it with a spray of DeOxit to the footswitch jack. I cleaned all the other jacks and pots with that stuff while I was at it.
I thought the issue might be with a weak preamp tube, but I swapped in a known-good one at each location with no difference.
What gives? Schematic attached
|g1||6/4/2018 4:13 PM|
|Did you check DC voltages at pins 1,3,6,8 of V1 and V2? Please post.|
|m1989jmp||6/4/2018 4:21 PM|
Clipping diodes in that circuit really mess up the tone and cut some volume too.
|earache||6/4/2018 4:57 PM|
|G1 The voltages as follows:|
m1989jmp Re: "Isn't as loud as it was before or that's your opinion?"
It isn't my amp but it seems that the owner's claim is plausible, as similar loudness settings on the volume knobs on both channels have the Boost channel being quieter. Otherwise, the amp isn't noisy as far as crackles or static.
|m1989jmp||6/4/2018 5:41 PM|
I may be wrong, but there's a possibility you'll find nothing wrong with the amp. Try bypassing the diodes with alligator clips and something like 470k to ground, the channel will become much louder.
|earache||6/4/2018 5:44 PM|
|So these voltages also look ok?|
|The Dude||6/4/2018 5:52 PM|
|They look "OK", but the amp works, "kind of" so we are looking for something "kind of" off. If you follow that strange logic, I think V1 P6 is slightly low. I could be wrong, but I'd check the plate resistor for that plate.|
|m1989jmp||6/4/2018 5:52 PM|
|Pedro Vecino||6/4/2018 5:52 PM|
|That amp is the first version of Split Channel. His lead channel is probably the worst in Marshall's history and can be confused with a broken amp. Marshall stopped making it after a year and a half and redesigned it completely.|
|g1||6/4/2018 5:56 PM|
m1989jmp asked a good question earlier, did something change for the owner, or was it always this way? The channels are different, so you can't expect to get the same volume by setting them up to the same numbers. Can the boost channel get loud enough using different setttings?
Now, about the voltages. V1 pin6 & 8 look suspicious. But I'm not sure you have the correct schematic. Does it have the IC for switching like this schematic?
(I'm suspicious about the first schematic because I don't see any measured plate voltage resembling what should be seen for a cathode follower)
|J M Fahey||6/4/2018 9:01 PM|
similar loudness settings on the volume knobs on both channels have the Boost channel being quieter.
"Knob position does not mean any specific power output"
Feed 100mV 1kHz to input, set all tone controls to 7 , master to 10, and slowly rise channel volume ... can you make power amp clip?
If you can (and I bet you can) there is nothing "broken" .
When I looked at schematic found it goofy clumsy, and those diodes adter the cathode follower are a mistake, but too sleepy now (and doubly stupid because of the flu and fever, Winter hit us hard and suddenly) to suggest corrections.
But please check sensitivity.
|earache||6/5/2018 2:39 AM|
|If this channel is a known poor design, are there common mods to “fix” it?|
|earache||6/5/2018 2:45 AM|
|G1 - the amp has the chip, i believe a CA3046|
|J M Fahey||6/5/2018 2:59 AM|
Personally it´s the first time I see it and am amazed by its kludginess.
When I saw "JCM800" in the thread title I expected the classic "1 trick pony" we all know, a very simple amp, which does just 1 thing, that 80´s metal tone, VERY well.
But when I enlarged the schematic, I saw without much analysis a couple WTF? things; the main one adding diodes after the classic cathode follower stage, the core of the most Marshalls distortion, so they negated its action. WTF?
And in general they tried to do very many things at the same time, which in general is not good.
Let me see it while I sip my morning Tea, might suggest something.
Is this for you or for a customer?
For personal use it might pay to spend a lot of time modding it all over the place and, worst case, ripping the board off and building something else, nof for a customer you´ll never be able to charge for all the time you´ll soend on it, in nthat case address his specific request and be done with it.
Or it will be an endless time, money and effort sink.
Ok, the kettle is hissing, see you later.
NOTE: maybe somebody knows a specific set of Mods, or some discussion page or Forum about it; in that case, please suggest that.
I will only *look* at the schematic , can´t promise anything.
|Pedro Vecino||6/5/2018 3:12 AM|
Check it with the scheme of the new version.
|J M Fahey||6/5/2018 3:57 AM|
|Agree, it must be the "good" one |
Voltages now make sense, specially around V1b.
I guess they wanted "just a little more gain" in the dirt channel but without using an extra valve, so they modded V1b gain stage into a "starved plate" design: as large a plate resistor as can be, accept huge voltage drop across it so low plate voltage, problem is that stage is anemic, dull, all for very little extra gain.
I suggest you turn *that* gain stage into a normal one: 100k plate, 1k5 cathode, bypassed with .47 or 1uF electrolytic.
Ditch the cathode diode of course.
In any case that channel is a fraud: being a gain block followed by clipping diodes (W005+1N4007) , it´s not more than a glorified distortion pedal.
Going any further would mean ripping entire sections, full rebuilds, way beyond the call of duty.
Try turning the starved plate stage into a regular one, it should improve sound, not sure about that channel "lower volume at the same setting" but it might improve that too.
I don´t suggest pulling the clipping diodes, because switching here is done by very weak transistors inside the chip, which is just a transistor array, faintly remember they stand 20V or so.
Clipping diodes keep peaks at 2.1V ; without them you could easily have 30 or 40V RMS, not good.
Ok, try it and post results.
|Mick Bailey||6/5/2018 4:23 AM|
|FWIW I had one of these in earlier this year and noticed the Boost volume needed turning up a lot more than the Normal to give an appreciable boost. The owner had it from new and said it was always like that so I didn't pursue it as a problem. So either there was a problem and it was faulty from new, or it was fine. I can't recall having another of this specific design for comparison.|
|Helmholtz||6/5/2018 7:33 AM|
|Maybe the boost volume knob has shifted and needs re-adjusting/aligning to the scale?|
|earache||6/7/2018 2:56 PM|
|I'm returning this to it's owner after a regular service (clean up and check out). The reverb and master volume which were noisy have been fixed with some DeOxit. As far as it's other "issues"....it is what it is. I sent the amp's owner to read these posts and he now understands what he has.|
Thanks for your help everyone!