Speedball Small Board Experiencing Low voltagse on OA and OB [resolved]

[Baskevyl]

So I'm in the process of assembling the speedball for my roughly 5-6 year old crack which was doing great this whole time. I just added the small board and performed voltage testing.

Values were approximately:

OA: 12.5V-20V (rec 60 - 90V) and it seems to fluctuate. If I start by testing OA, it climbs to 12.5 and stabilizes. If it just finished testing IA and it's stabilized at 175, quickly switching to test OA makes it go down from about 20V to 12.5 and stabilize.
IA: 175V (rec 170 - 270V)
B-A/B: 0V (rec 0V)
IB: 175V (rec 170 - 270V)
OB: 12.5V (rec 60 - 90V) and it seems to fluctuate. If I start by testing OB, it climbs to 12.5 and stabilizes. If it just finished testing IB and it's stabilized at 175, quickly switching to test OB makes it go down from about 20V to 12.5 and stabilize.

The wall outlet is also roughly 123V. The 2 LEDs on the original Crack as well as the 4 LEDs on the small speedball board light up.

Are these OA and OB values fine even though they're considerably lower then the recommended range?

[Paul Birkeland]

Can you post some build photos?

No, these voltages are not OK.  Whatever is causing this has to be resolved before you progress with installation of the other board.

[Baskevyl]

Can you post some build photos?

Hello Paul,

Here are some pics of the small board and U1-5 terminal connections. I have soldered the bigger board but have not added it to the build yet.

Edit: Hmm, pictures are best viewed in photo viewer.

[Paul Birkeland]

The only thing I see is that the wire going into A7 has way, way, way too much insulation stripped off and that could touch other terminals and cause problems.

Other than that, what I'm seeing would have me questioning the original voltage readings, can you double check?

[Baskevyl]

Alright so I am kind of baffled and miffed but at least slightly relieved.

As I said, I had already assembled the big board and removed the 3k 10W resistors connected 3U to 7U & 9U on the base crack. After unsuccessfully attempting to reattached them for a third voltage retake on the small board (I posted after a double-take since those numbers were too low but not 0V), seeing as everything else seemed to be wired correctly and no clear explanation existed, I decided to just add the big board to the crack and try my luck.

Well I might be lucky but none the wiser Here are the big board readings:

OA: 107.5 (recommended 75-100V, so within a 10% upper limit margin)
OB: 103.5 (recommended 75-100V, so within a 10% upper limit margin)
G: 0V (recommended 0V)
B+: 182.5V (recommended 170-195V)

Okay, everything is doing good on the big board, and all 10 LEDS on the Crack light up.

The small board now reads the OA and OB went from ~12.5V ---> 12V, still not close to the lower limit but still not 0V.
IA & IB changing slight from 175V -->183V and B-A/B still at 0V, so those 3 are within recommended ranges.

I also checked to make sure that original A7 solder was gingerly tucked down to avoid touching anything it shouldn't.

I'm at a loss.

Edit: Listened to a few orchestral scores. Sounds great. When it's turned up maybe 80% or higher there is a minor hiss, but that's normal with my cheap DAC. So it's seemingly working correctly.

[Doc B.]

Do you know if the voltages were in spec before you installed the upgrade? After running for 5 or 6 years it might be worth trying a different 12AU7 to see if the voltage at the small board OA/OB comes up.

[Paul Birkeland]

Here are the big board readings:
OA: 107.5 (recommended 75-100V, so within a 10% upper limit margin)
OB: 103.5 (recommended 75-100V, so within a 10% upper limit margin)
G: 0V (recommended 0V)
B+: 182.5V (recommended 170-195V)
The small board now reads the OA and OB went from ~12.5V ---> 12V, still not close to the lower limit but still not 0V.
Edit: Listened to a few orchestral scores. Sounds great. When it's turned up maybe 80% or higher there is a minor hiss, but that's normal with my cheap DAC. So it's seemingly working correctly.

I definitely would not try listening to an amp that doesn't work properly.

On the small C4S board, there are two little blue R1 resistors.  Can you provide the most precise measurement you can for the voltage on each side of each resistor?

[Baskevyl]

@ Doc B.

I think so. But I did do that test a few months ago, then life happened and I couldn't add the upgrade immediate upon getting it. Now I finally had time and decided to jump right in. I guess something could have happened in that time span even though nothing seemed off at all.

I haven't thought about getting a new 12AU7, but it has been used a long time so I'll look into it

@Paul

Thanks for the continued responses and troubleshooting. Ya I stopped with the music and checked the resistors;

R1A has a voltage of 184V on the side towards the big board, and 183V on the side away from the big board, with a voltage difference of .867V between them.
R1B has a voltage of 183V on the side towards the big board, and 182 on the side away from the big board, with a voltage difference of .873V between them.

[Baskevyl]

Alright, I got suspicious/paranoid and found another multimeter to check with, a Dawnson DDM420.

Big board:
OA: 109.8V (within 10% error range)
OB: 106.1V (within 10% error range)
G: 0V
B+: 184.4V

Small board:
OA: 77.4V
IA: 184.2V
B-A/B: 0V
IB: 183.9V
OB: 71.1V
just like that, everything is within recommended range or margin of error. I was using a FLUKE 7-600 before this, maybe the problem was with the Fluke? Anyway, everything seems fine.

[Paul Birkeland]

I'm glad you had another meter to try.  I could not for the life of me figure out how you would have 12V at the driver plates but have everything else working properly. 

[2wo]

Try replacing the battery in the Fluke I had an issue measuring a low voltage and that turned out to be the problem...John

[audioguy]

Try replacing the battery in the Fluke I had an issue measuring a low voltage and that turned out to be the problem...John

This is a good suggestion. I've been bitten by this before when using DMMs. After pulling my hair out I'll finally throw something on my scope and get the proper readings. That 9v battery in those DMMs need to be new and replaced often if you're using your DMM often.