Thanks Phil,

This is the method I have been using so far, I thought there might be other ways to test in circuit.

Thanks Enzo,

Where would be the next logical points to test?

I am reading 30.00vac at r82. Does this   suggest the rectifiers are faulty? You cannot view this attachment.

Just ran the preamp out into another amp. The buzz is there but it is quieter now and does not sound like the previous 100hz hum.
With the main filter caps replaced, could the next suspects be diodes breaking down under load in the power supply?

Checked the ac on speaker 3.59vac
It drops to 1.32 wiith a jumper from cr3 anode to ground.

I also checked for broken pads/cracked solder and all checked out ok.

Thanks,
Andy

Hi all,

I finally got hold of the new filter caps and had time to install them. I also replaced q9 while the board was out.

Long story short - I still have the same or similar buzz happening.

After cap replacement I checked for dc on speaker 0.023vdc

Tp1 30.05vac
Tp2 30.00vac
Tp3 39.02vdc
Tp4 -39.03vdc

Ac voltages

Tp3 0.066vac
Tp4 0.209vac

You cannot view this attachment.

Thanks g1,

I guess either I misread or recorded the ac voltages incorrectly, or something changed after I replaced parts.

I reflowed the solder on the main filter caps and checked resistance to all connections when I originally pulled the board so will assume c48 is bad. Will order new filter caps!

Thanks again

Just double checked ac on collectors of both tip147s
Q11 and Q2
2.34vac 2.35vac respectively.

Tp1 30.31 vac 
Tp2 30.31 vac

Tp3 30.12vdc    00.04vac
Tp4 - 30.20vdc  02.45 vac

Tp5 16.40 vdc    00.02vac
Tp6 -16.42 vdc  00.01vac

Have got the 100hz tone coming from the preamp out.

Yes, am on 50hz 230v mains in the U.K.
I looked up 100hz tones on youtube and thought it sounded close.

I don't have another amp here to test it at the moment. I might be able to dig out a sampler to test it though.

Initial ac voltages
1.72vac with jumper
4.5vac no jumper

1.48vac with jumper to ground.
3.90vac no jumper

These are the readings that it settled at. Both readings started higher then dropped.

Similar to plugging guitar into power amp in.