new build - most voltage reading are spot on - except for a few

[FS]

Hello,
I have finally finished my Stereomour. Thanks to Josh for the fast shipment of some missing resistors!
The resistance checks came out very well, so I proceeded to the voltage checks and got most voltages within the 10% range. However, the following readings seem a little off and I would like to know if there is cause for concern:

Terminal  AC Voltage expected  AC Voltage measured

A1/A4            2.5V                              1.7V
C1/C4            2.5V                              1.7V

These values are off by over 30%. Should I be concerned?

The DC voltages are somewhat different for the two 2A3s:

Terminal  DC Voltage expected  DC Voltage measured

2                230                            197
14              230                            214

and the active load board:

Terminal  DC Voltage expected  DC Voltage measured

0A              230                            196
0B              230                            216

I would appreciate any help with this as I have re-checked all wires and they seem OK to me. Shouldn't the two 2A3 perform very close to each other in specs if they are a matched pair?
Or could the culprit be the board that shows the same trend in voltages?
Thanks very much in advance.
Falko

 

[Paul Birkeland]

The OA/OB readings are within specification. 

What are the "K" voltages on your power supply board?

(I wouldn't run this amp for more than 2-3 minutes at a time until this is sorted)

-PB

 

[FS]

Hi Paul,

Thanks for the speedy reply!
The Kreg voltages on the board are 2.4V at the A-side and 2.41V on the C-side.
Falko

 

[Paul Birkeland]

Hello Falko,

The KS for getting back to me. The "K"  pads are on the larger green PC board mounted above the power transformer, near the front edge.

-PB

 

[FS]

Hi Paul,

Sorry for the misunderstanding. I checked the two "K" positions on the board. One (C-side) gives a reading of 55V, the other (A-side) 54V. Thanks again.
Falko

 

[Paul Joppa]

Those all look good, except the 1.7 volts AC on the filaments of the 2A3, which should be closer to 2.5 volts.

Can you see the filament glow inside the 2A3? It is always fairly dim and usually requires that you dim the lights, but it should be sort of orange. It's on the very inside of the tube structure.

If it's visibly glowing, than I am wondering whether the meter is accurate at that low a voltage. What is the meter scale? Maybe a picture of the meter would help as well.

 

[FS]

Thanks, Paul for your reply. I can see both the 2A3 tubes glow. The meter that I had used is the Craftsman 82140:

http://www.craftsman.com/craftsman-digital-multimeter-with-ac-voltage-detector/p-03482146000P?prdNo=3&blockNo=3&blockType=G3

You may be right in that the meter is imprecise, as the range for AC voltage has only the 600V and the 200V settings, unlike DC, where it goes down to 200 mV.
I checked with another, simpler multimeter (Radioshack 22-810), and that gave me a reading of 2.1 V between A1 and A4. The range is not much better for that meter.
I used that meter to read the battery of the craftsman multimeter, which showed 9.4 V DC, so the battery was OK.
The 2.1 V are still 26% off the expected voltage though...
Thanks again for your help.
Falko

 

[Paul Joppa]

The Craftsman specifications are up to 10 digits error on the AC scales, which means +/-1.0 volts on the 200v scale. So your meter does not have the ability to accurately measure 2.5vAC. If the tubes glow and it sounds good, don't worry.

What is happening is that there is a rectifier in the meter, which causes a small error, perhaps around 1 volt. It will always read low on small voltages. At 200 volts, a 1-v error is very small, but at 2 volts it is not small at all! It is significantly more complicated to make a meter that will read low AC voltages accurately.

 

[FS]

Thanks, Paul. The meter seemed to work well for the low DC voltages at Kreg/-reg, but that could be due to the better meter ranges offered for DC voltage readings, correct? I will give it a try today in the evening to see how it sounds  . Thanks again, and have a great day.
Falko

 

[NishBtl]

I also have a multimeter that doesn't have a mV scale for AC voltage measurement. I was wondering what the best way to do the hum balance adjustment is, since my meter doesn't have the precision down in the mV range.

 

[galyons]

I also have a multimeter that doesn't have a mV scale for AC voltage measurement. I was wondering what the best way to do the hum balance adjustment is, since my meter doesn't have the precision down in the mV range.

Yep, short the inputs and turn the pot to get lowest audible hum.  Not as precise as using a good meter, but hum balancing by ear is as old as the circuit.

Cheers,
Geary