Help with S.E.X 2.1

[Marcin]

Completely disconnecting RCA cables and connectors didn't help. What if I connect a wire to the grounded terminal of the power entry module and check all the parts that should be grounded? Paul, any specific way or place where I can measure the hum?

[Paul Birkeland]

Marcin,

Put one meter probe onto the short #8 screw by the power entry, then measure impedance to all the other screws on the chassis.

They should all measure very low.  If your meter has the beep function for continuity, use it.

The two that are most important are the two front screws on the tube sockets.  If you had no continuity between those points, you'd have the issues that you are describing.

-PB

[Grainger49]

Excellent idea of checking that all "grounded" points, screws and terminals, are grounded.  Someone with the 2.1 manual will need to supply the list.

Look at Paul Joppa's reply #13.  It has a link to a description of how to measure hum that I put in another thread yesterday.

To be certain that you are not picking up outside noise the inputs need to be shorted.

Look here also:

http://www.bottlehead.com/smf/index.php/topic,3145.0.html

I'll will add a picture of jumpers too.  If you don't have any you should hurry to Radio Shack and buy a dozen before they stop selling them.  Buy the larger wire, the fine wire might become a problem.

[Grainger49]

The picture is added.  It is the second picture in the "Picture Of Tools" thread in my corner

[Marcin]

Jumpers and shorting 2K RCA plugs done. It's time to do some more testing ..
   - https://plus.google.com/photos/117011424417543401185/albums/5837281958597853329?authkey=CKPsv8iOweXzNg

[Paul Birkeland]

I'd remove the resistors and use wire.

2K is not a short!

[Marcin]

I was reading online that the direct short can have a negative impact on the amp. I can try with the direct short if that's what is recommended. I'm also able to get continuity between the safety ground connection and mentioned front screws, or any other screws. As for the hum, Left: 05.2mV, Right: 15.4mV measured at the speaker leads with the 2K RCA plugs in place.

[Marcin]

One more comment regarding used resistor, I think it also helps to eliminate potential external interference?

[Paul Birkeland]

I see your problem, you received a PT-3 power transformer instead of a PT-7.

The good news is, you can retain the PT-3 and get your amp working properly quite easily.  

1.  Clip out the black wire that goes from the 5-lug terminal strip where you have your 220uF capacitors to the power transformer.
2.  Wire Terminals 6 and 9 together on the power transformer.
3.  Wire terminals 7 and 10 together on the power transformer.
4.  Jumper C1/C3 together with a piece of wire.

If you're not interested in this route, a replacement power transformer can be mailed out tomorrow.

-PB

[Doc B.]

One more comment regarding used resistor, I think it also helps to eliminate potential external interference?

The resistor is needed if you are loading an output, not an input. At the input you want a dead short. There is already a resistor across the input in the form of the volume pot. In theory turning the pot to the lowest setting shorts the input. But if it is not set up right and picking up noise because of it using a shorting jack will help to isolate the problem.

[Marcin]

I was checking every part of the documentation at least few times and I missed the power transformer type. I would prefer to replace the power transformer. Thanks!