Marshall JCM800 "4210" Combo Repair Part 2

[@butchlauer]

I miss my old 2205, for all its quirkiness it rocked like no other.

[@DATT]

Yeah, I did quite like the overall sound of this one.

[@paper-boy4566]

Great video. I notice that the black wire that you desoldered in Part 1 was reconnected in a different place (on top of one of the big caps). I don't know of that makes a difference but I thought it was worth mentioning.

[@DATT]

Hmm, not sure. I don't have time to review the videos. There can be more than one way to correctly wire a circuit in a way that doesn't make a difference.

[@dwoolley2243]

Nice switching analysis. About to tackle one myself. Now with relays you have a ton of to-92 packaged muting jfets you gotta check.

[@DATT]

Thanks ! With Relays ?

[@toastedphantom3007]

May have ghosts in the blood and the designers at Marshall sure did cocaine about it.. I read from multiple sources so far that the power supply design on a fair few models is very iffy. Basically, when turned off, V-bias discharges faster than B+. If you turn it back on too fast or have a power outage, the bias voltage is way too low for the B+ at that moment. So until the voltages are stabilized the output tubes conduct hard and essentially self destruct by red-plating. Dunno if this model is one of those but better be safe than sorry.

[@DATT]

Heh, I wonder if this is another incentive to use the standby switch. I do recall commenting that this PSU didn't have the usual discharge resistors on the caps that I'm used to seeing. I guess that might be another reason for those resistors.

[@PhuketMyMac]

Very instructive. I wish you would develop more the reasoning behind why if the preamp ground through the shielded cable is lost, it is grounded hot and becomes a shock hazard for example. Or the others ground reasoning. Learning amp circuits through your videos and other’s. Thanks bud.

[@DATT]

Well, a circuit needs both + and - power to work right, this we know. The - Power is being passed thru that shield, but also, internally - is tied to ground, and therefor to the chassis, therefor any metal surface of the amp. If the wire breaks, the preamp will loose - power, but you are connected to the preamp ground via your guitar, so if you are holding the guitar, and touch a metal surface of the amp, or, any other ground source, like a microphone, you become the wire, and the preamp will try to power up via that path.

[@stuartrojas9582]

😬 P r o m o s m

[@DATT]

What ?