Marshall looses gain after 2 mimutes

[Rod]

http://musiciansroadhouse.com/viewtopic.php?t=21769
Marshall JVM 410HJS

vintagekiki, what do you think of the filter caps C47 and C49 sensitivity that i have posted in my last message? The amp seems for a long time that has lost some tone that it at had when i bought it.

The amp has power, the amp looses gain slowly and the tone becames very squashy after some minutes.

[vintagekiki]

I have the standard JVM 410H not the satriani model JVM410HJS.

http://musiciansroadhouse.com/viewtopic.php?f=54&t=18191
JVM 410H vs. JVM 410HJS

[Rod]

all the schematics are here in this link :

the schematics are the ones that start with JVM410

Marshall JVM Schematics - Musicians Roadhouse

[Rod]

I have attached the schematics bellow

[vintagekiki]

... what do you think of the filter caps C47 and C49 sensitivity that i have posted in my last message? The amp seems for a long time that has lost some tone that it at had when i bought it.

The amp has power, the amp looses gain slowly and the tone becames very squashy after some minutes.

When sound changes whether the voltage changes on HT2 or HT3.
Check on the copper side of pcb, that it is not cold solder at C47, C49, R69.

If there is a gradual voltage drop at points HT2 ... HT5, resolder copper side of pcb at those points.

[Rod]

Ok , i had some time today to check the amp and i measure the voltages and they are in the attached part of the schematic bellow.

What do you think of them?, i have changed tubes, and still have the same problem and sensitivity in the C45,C47,C49,C41,C42 filter caps, and in the PCB when tapped with a pencil around the caps and tube sockets.

The problem that im having is that volume and gain starts to fade away after some minutes, also when i measuring the tension around R73 i have alot of sensitivity.

What can the problems be, is this a consequence of bad filter caps?

[Rod]

Hello everyone same help here were be very appreciated, whar should I do next?

[vintagekiki]

Ok , i had some time today to check the amp and i measure the voltages and they are in the attached part of the schematic bellow.

Low anode voltage on tube V6 (79 / 66V) may be due to inappropriate voltage + HT5, R67 (10k), faulty tube V6
If it is not difficult, measure voltages HT2, HT3, HT4, HT5.
The problem when tapped with a pencil is microphony in some tube.
How do V1 - V4 (EL34) react when tapped with a penci?l

[Rod]

Hello VintageKiki, i allready change a lot of preamp tubes and the last ones i put in seemed fine in terms of microphonics but he amp has the same problem, im going to check the powertubes like you are saying.

Today i have seen a schematic of a JCM800, with the voltages in the anodes of the preamp tubes around 250Volts in the v1, so the voltages i measure are too low. Can electrolitic caps became microphonics when they turn bad?

[vintagekiki]

Voltages HT2, HT3, HT4, HT5 ?

[Rod]

vintagekiki here are the voltages:

HT5- started at 214v and lowered after some time to 208V
HT4-started at 240v and lowered after sometime to 230v
HT3 -276v
HT2-336V

[vintagekiki]

HT5- started at 214v and lowered after some time to 208V
HT4-started at 240v and lowered after sometime to 230v
HT3 -276v
HT2-336V

Check on the copper side of pcb, that it is not cold solder at C42, C46, R67, R68.
For voltage drop HT4 and HT5 check (replace) R67, R68 (10k/ 1W), C42, C46 (10uF/ 350V)