Jtm50 black flag kit standby hum

[Alshaheen]

I have a modulus amplification kit clone jtm50 1967 combo second hand about a year old with a hum 120hz ish problem... Hums with everything on 0 if i put on first then standby on.... But if i put standby on first then the on switch no hum please help

[martin manning]

Hums with everything on 0 if i put on first then standby on.... But if i put standby on first then the on switch no hum

Does this happen every time or intermittently?

[Alshaheen]

https://youtube.com/channel/UCVA48m6BARdcOZLWdPhgflw

[Stevem]

I did not look at a schematic, but that’s likely because it’s built with the flaw that the originals had, that being due to the bias voltage not being on until the standby is engaged.

It needs a simple rewiring to get the bias voltage on when the on switch is closed.

The modification is easy!

There is a wire on the normally open side of the standby switch that feeds the 100k dropping resistor for the basis supply.

Unsolder this wire and move it to the other side of the standby switch that is always hot when the on/ off switch is engaged.

If I am wrong then I would say that you should replace the power supply filter can on the second node, since to me that could be the only other source of this issue.

[Alshaheen]

Screenshot_20210619_123048_com.ivilamobie.pdfreader.pdfeditor.jpg

IMG_20210609_113907.jpg

[pdf64]

Is it intermittent or the same every time?

Does your amp exactly match the Modulus layout?

[Alshaheen]

Not intermittent it will stay not go away
It seems to match the layout.
Check this vid it is louder than in the vid

https://youtube.com/channel/UCVA48m6BARdcOZLWdPhgflw

[pdf64]

What mV do you measure across the 1 ohm output valve cathode resistors in both modes (hum and no hum)?

[Alshaheen]

Will have to check after not home atm

[Alshaheen]

PDF64 could you mark where i need to take red and black mv from please im a noob but understand the dangers i will use a fluke177 with mv thanks in advance

[Alshaheen]

IMG_20210619_165327.jpg

[pdf64]

Black probe to one leg of the resistor, eg pin 1 of the socket, red probe to the other, eg pin 8.
Probe clips are a very good idea, as they avoid the risk if the probe tip slipping off the terminal and shorting adjacent terminals together.

[Alshaheen]

Thanks PDF64 will do....
Would i red probe to 8 and black to chassis to get bias value just for future reference
many thanks

[Alshaheen]

Ok the one on the left in the pic shows 33 the one on the right shows 28 with the hum but while i have the probes on the right one after a few seconds it low clicks and the hum goes off

[Alshaheen]

With the hum shows 36 then when it clicks and the hum goes off it shows 28