music angel 845 SET

[Dietervsw]

That's good, have you verified the power supply voltages are close to what the schematic shows? If so, with only the 845 plugged in, is there still a hum? If not, plug in V2, is there a hum? If not, then the problem is with V1. It's just a process of elimination, you need to isolate the spot where the problem occurs.

I will try that and let you know. Thanks!

 

[Dietervsw]

2) Measure the AC components on top of OPT, ie B+920V, is it only one cap 0.33uF there?

Hello,

actually there are 2 large electrytic caps in parallell with the 0.33uF.
I could not read the value, nor it is written on the original schmatic.
As a matter of fact, using a 75w@220v bulb to discharge it takes around 2s for the bulb to fade.

B1+ is filtered by 2 47uF in paralell with 0.22uF, just like in the diagram in the link.

I just bought 2x 8 Ohms@50w resistor to attach to the OPTs and power the amp on. Verify the voltage the comming out through the resistor and than start removing the tubes.

I noticed that oscilloscopes are so cheep in China nowadays, I thik is a good investment.

Thanks

Regards,
Dieter

 

[jazbo8]

Please be very careful when measuring the B+ voltages, especially at the first node, i.e., at one end of the OPT and always follow the "one-hand-behind-the-back" rule to avoid electrical shock.

While troubleshooting the hum problem, do not apply any input to the amplifier, i.e., the input pin should be shorted to ground, with the output of the OPT loaded with the 8R dummy load. Only after you have gone through finding the source of the hum, should you apply signal to the input.

A oscilloscope is always good to have, but please bear in mind that most of them, especially the new ones, can not handle high voltage, you need to get 100:1 probes or build a simple resistor voltage divider to use the scope when working on tube amplifiers.

 

[Dietervsw]

Guys,

I took the amp appart and found following issues:

1) I did not connect the ground of the power supply PCB to the GND Bar where the 4 small tubes are. This is problably the cause of the 50hz hum (please check the photo) I measures 10vpp@50hz.

2) 1 47uF E-Cap was measuring 0,11nF, the other 3 were fine.

An externally hosted image should be here but it was not working when we last tested it.

Regards,
Dieter