Quasi-infant death - high voltage readings observed

[Dr. Exotica]

About a month ago or so, my son (16) and I completed his S.E.X. kit and he had been enjoying it quite a bit, using both speakers and headphones (great father/son project btw). A couple days ago I turned it on to see how it was sounding, and it was dead. Checked all the cables, etc.; all seemed good. Hmmm...

I then went and checked all the resistance values and found everything within the specified 10% tolerances. Voltage readings were next. Much to my surprise, both channels were quite even, but abnormally high. More specifically:

1,21  230.8 230.3
2,22  230.5 230.1
4,24  230.5 230.9
5,25  0 0
6,26  462 463
9,29  0 0
10,30 0 0
11,31 462 463

...

Pretty strange. I was not expecting the abnormally high readings, including the absense of -17VDC on 9,29.

All appears normal through visual inspection.

Ideas? Starting points?

Thanks,
Erik

 

[Paul Joppa]

The high voltage indicates that the power supply is working. Assuming your power line voltage is relatively normal, it also indicates that no significant current is flowing.

The lack of a negative voltage at T9 also indicates there is no current flowing in the amplifier circuit.

Are the tubes glowing (i.e. are they getting heater power?) If that's not it, we'll need to know the rest of the voltages. See DocB's recent post at
http://www.bottlehead.com/smf/index.php/topic,1667.0.html

 

[Dr. Exotica]

Paul,

Your comments regarding the heater and high voltqages make sense; the heaters are not working in either channel. Looking at the heater circuit, I obtained the following voltage readings:

PT-3 (4)  3.32 vac
PT-3 (5)  3.35 vac

C1  0 vdc
C2  0 vdc
C4  2.64 vac
C5  0 vac

Looking at the rectifier diodes, everything seem appropriate (but it must not be). I suppose I could rebuild this portion of the circuit ...

Erik

 

[Paul Joppa]

C1 is (or should be) connected to C1 with a piece of wire, so they should both be at zero volts.

 

[Dr. Exotica]

Paul - genius.

I removed the wire between C1 and C4, then resoldered a new one in. Heaters and amp now work like a charm.

This is a canonical example of why Bottlehead is so valuable to the community - great sounding DIY gear at affordable prices, and terrific support when something goes amiss. Left me slapping my head when Paul explained that the voltages readings were high due to issues with the heater circuit, and as a consequence, no current flowing through the system. I should have known this, but I did learn a great lesson.

Many thanks,
Erik

Also, thanks for your input too Grainger.