Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

[Helmholtz]

Could you point me in the direction of coupling for the NFB loop also? Thanks.

Not sure what you mean.
Does this relate to some earlier post?

[ca7922303]

I think Jon Snell meant to lift (one end of) C10 in order to isolate the preamp from the power amp.
Instead of physically disconnecting C10 you can disconnect the treble pot wiper instead.
If still hum with C10 disconnected, it's definitely not coming from the preamp, i.e. before the PI.

Disconnected wire from center lug of treble pot and no change in hum. No controls have any effect on hum with nothing installed @inputs except for presence control.

[ca7922303]

Not sure what you mean.
Does this relate to some earlier post?

Post 19 Jon Snell asked me to check coupling at NFB loop. Looking for that location on schematic

[Helmholtz]

Post 19 Jon Snell asked me to check coupling at NFB loop. Looking for that location on schematic

Best let Jon Snell clarify. Especially as the schematic seems to be wrong here.

[ca7922303]

If you have other 12ax7's available that are known good tubes please try several in the V2 position and see if any of them makes a difference.

Installed four(4) new(and known good) JJ 12AX7s in V1-V4, no change in hum. Noticing a high pitched squeal on channel 2 on anything above 5 on volume and presence controls. Channel one now is aheavily distorted signal. Only tube that is not new, known good tube is GZ34 rectifier tube.

[Helmholtz]

Still hum wih presence at 10? Does the presence work as supposed?
Are the OT and its wiring original?

[Jon Snell]

Post 19 Jon Snell asked me to check coupling at NFB loop. Looking for that location on schematic

The Negative Feedback Loop consists of a 27k feed from the output transformer, we'll call it R nfb A, the potential divider we'll call it R nfb B and the 0.1uF shaping capacitor to reduce the frequency of nfb response. ( Make it sound brighter ).
If the feed between the 27k resistor and the 5k pot is next to a heater/mains lead, that can produce hum.

As a matter of interest,
many that change output transformers or do major rework to this type of amplifier forget to check the polarity of the speaker phasing.
If it is upside down (180° out of phase) or the anode feed to the transformer have been crossed by mistake, the nfb becomes Positive Feedback, and we have all heard the scream or droning noise from microphones or instruments too close to the loudspeakers.
Sometimes, depending on the characteristics of the transformer, it can produce a lower frequency tone.

[ca7922303]

Still hum wih presence at 10? Does the presence work as supposed?
Are the OT and its wiring original?

Yes, still hum with presence at 10, squeal beyond 5, other than that, presence works.

[Helmholtz]

Find the wire that connects from R25 (27k) to the 16 Ohm OT secondary tap and disconnect it.
Still hum? Still signal?

Please post scope pics of the hum at the output.

[ca7922303]

Find the wire that connects from R25 (27k) to the 16 Ohm OT secondary tap and disconnect it.
Still hum? Still signal?

Please post scope pics of the hum at the output.

Scope photo, no signal generator, probe is on 8 ohm speaker cable.

[Helmholtz]

Scope photo, no signal generator, probe is on 8 ohm speaker cable.

If this is without feeding a signal, the amp is oscillating.

Disconnect the NFB wire as advised above.
Can't read the readouts. What is the frequency?

[ca7922303]

If this is without feeding a signal, the amp is oscillating.

Disconnect the NFB wire as advised above.
Can't read the readouts. What is the frequency?

Ranges from 439khz to 1.4mhz

[ca7922303]

Ranges from 439khz to 1.4mhz

This is with no signal injected.

[ca7922303]

If this is without feeding a signal, the amp is oscillating.

Disconnect the NFB wire as advised above.
Can't read the readouts. What is the frequency?

R25 trace runs to purple wire that connects to speaker ohms selector. Do I disconnected there?

[Helmholtz]

R25 trace runs to purple wire that connects to speaker ohms selector. Do I disconnected there?

Yes.