Marshall Dual Reverb 4100 amp question

[mresistor]

Hello all..  I have a question about the 1990's Marshall Dual Reverb 4100 amp. I've included the schematic.   In the schematic of the power supply, on the right side in the middle of the schematic you will find a "Star Earth".  Below it is a resistor and a capacitor that goes to earth ground.  In effect the main 50uf ps filter capacitors have had their ground elevated at a potential above earth ground.   
My questions..  Why did Marshall do this?  I suppose it is to quiet the amplifier since it is high gain.


If you look at the notes.. it says 100 ohm for the master volume amps and 22 ohm for the dual reverb amps.  Why is there a difference in the value of this resistor? 
Would a 100 ohm resistor not work in a dual reverb 4100? 


https://drtube.com/schematics/marshall/cd0192-iss7.pdf

[PRR]

> ps filter capacitors have had their ground elevated at a potential above earth ground. 

Where is earth?

I see "Earth to preamp PCB" far left. This is solid connected to "Star Earth". I see speaker jack shells far right, and they also are solid connected to "Star Earth".

The network from preamp--speaker common to chassis is to avoid signal or power currents in the chassis.

Why different for different models, I have no idea.

The "Star", if really as shown, is misbegotten. Follow the roundabout path from rectifier to main filter cap. It returns through point "Bk", which is common to both signal (at least NFB path) and the big rectifier spikes.

[mresistor]

PRR - My mistake  I meant  "Chassis Connection"  right below the resistor and cap which is right below the "Star Earth".  And I see the path to ground for the rectifier.  So what is the C16 and adjacent resistor for? Is it a filter of some sort? Is it filtering noise or ?


In the amp I have here something caused the 22 ohm resistor to fry, literally started on fire and also ruined the capacitor C16 and scorched the pcb burning off the printed R# of the resistor.  So I assume there was some current involved. I'm trying to figure out what did that. (I guess it wasn't flame-proof)  When I took the chassis out of the cabinet there was something loose inside it that rattled around and I found it to be a steel washer that looks just like the washers on the chassis bolts. There was no arcing signs on it though so I don't know if that was the cause. And one EL34 is quesitionable.  It pegs my hickock tester while the other three check fine, it doesn't register any shorts though, it just conducts like crazy.   I am waiting for a T4 250v fuse. All I have in the 5x20mm size is a 1 amp.   So far the bias circuit checks good and with no power tubes installed I can take it off standby and everything checks out.


The owner said the reverb isn't working ..  don't know if that relates ..  guess I will find out..



 

[2deaf]

Sounds like an output tube shorted from the plate to the heater. 

[mresistor]

Sounds like an output tube shorted from the plate to the heater.

My Hickok shows a very slight deflection for heater - plate short..   Could be what did it.. 


In the amp itself I cannot find the 100 ohm resistor shown in the schematic going to the heaters..   so maybe the one that cooked IS that resistor..   

[sluckey]

In the amp itself I cannot find the 100 ohm resistor shown in the schematic going to the heaters..   so maybe the one that cooked IS that resistor..

That's likely since you show signs of plate/heater short. Blown resistors rarely destroy their leads so you should be able to look at what the leads were connected to in order to determine which resistor actually blew.

[mresistor]

OK  one side of the resistor has continuity to anodes of D3 and D4 of the rectifier circuit and the speaker sockets.   The other side goes to the ground terminal of the power cord jack and then to "chassis connection" or chassis ground which is a bolt to the chassis.  So it appears to me that this resistor is the one that is in parallel with the  22 ohm which is for Dual Reverbs. Its impossible to tell if that burned up resistor was 22 ohms, bit I'll assume it was.  There is not a "star earth" as one would think one would look.. there are a couple of bolt connections near each other for the grounds and there is a ground buss run on the PCB.


Also there is no FS6 or FS7 for the heater leads as the schematic shows, the heater leads which run straight from the PT to the tubes.   The center tap goes straight to a chassis bolt. The power amp PCB has two empty areas on the PT end that are labeled FS5 and FS6 and I assume that that was where they intended to mount two fuse holders for the heater leads.. There is printing that says T6 .3A 20MM  - So I guess the factory ran out of fuse holders and just omitted them from this build.  And the schematic has them labeled wrong. There is no 100 ohm resistor in line with the center tap for the heater winding of the PT.


This JCM900 4100 is an early one,  it has inspection stickers from early 91. 

[shooter]

has continuity to anodes of D3 and D4

not the cause, but for 30 second check   , If either failed short then open, might be enough time to take out the R

have you looked under the hood, sometimes there are amazing finds under the PCB

[mresistor]

Yes I looked..   I had to take the PA pcb up to get to the backside to unsolder the bad components and solder the new ones..   there is nothing under it and the backside looks like new.  I also took a mirror and light and looked under the preamp board for stray stuff.. I scraped off the carbon on the burned part and cleaned it with denatured alcohol. 
I'm perplexed as how this resistor caught fire when it is electrically between ground and ground..  Makes no sense to me. Maybe it was that washer that was in the chassis..  who knows. 

[sluckey]

The anodes of D3 and D4 are connected to ground.

[shooter]

D3 and D4 are connected to ground.

to star earth, then through his burnt R to real ground, chassis.

[mresistor]

D3 and D4 are connected to ground.

to star earth, then through his burnt R to real ground, chassis.

well the R isn't burnt anymore..  I replaced it..  but yeah

[2deaf]

Once a power tube shorted from plate to heater, it created a path for the HT from the plate to the heater, through the heater winding to its center tap, through the 100R resistor to circuit ground.  Once the 100R burned, there was no circuit connection to the chassis and the ground in your house.  I would feel a little uneasy with a device generating 500 volts connected to my guitar while having no connection to the ground in my house. 

[mresistor]

2deaf   which 100r are you talking about..?  There is not 100r on the center tap of the filament winding on this amp.   It does not exist inside the amp.    The filament winding center tap goes directly to a chassis ground bolt..  see my picture    the filament center tap on the PT is far left.

[2deaf]

2deaf   which 100r are you talking about..?  There is not 100r on the center tap of the filament winding on this amp.   It does not exist inside the amp.    The filament winding center tap goes directly to a chassis ground bolt..  see my picture    the filament center tap on the PT is far left.

The 100r you just put in where it used to be 22r before it burned up.  That resistor is between the heater center tap and circuit ground. 

The chassis connection has nothing at all to do with the circuit.     

[mresistor]

OK

[2deaf]

Geez.  I read through post #12 and I consistently said 100R instead of 22r.  No wonder you didn't understand what I said.
It's funny the way the 100R from the heater center tap to chassis has a line through it.  Maybe that's Marshall for "sometimes it's there and sometimes it ain't"?

So, what about that reverb? 

[mresistor]

Today is a better day !    2Deaf  - I understood what you meant. 
maybe that 100R with a line through it is a Marshall "special 100 ohm" precision wire.. LOL


I am still waiting for the T4 20mm fuses..  should be here tomorrow.. I've got another EL34 quad. My new Fluke mm should be here too..
Once I get the amp working then I will look at the reverb.