Please help: High resistance readings on large board for speedball [resolved]

[CuriousMister]

Hi,
This is my first post.

The initial build and small speedball board went great.  I hit a potential snag right at the end.  Below are the readings for the final resistance check of the large board.  Both tubes are glowing.  Is this alright?  If not, where do you think the issue may be occurring?

Here are my readings.
192 at OA
185 at OB
G at 0.2 (okay)
193 at B+ (okay)

Thanks you so much!  I appreciate your guidance.

Tom Guyer

 

[Paul Birkeland]

Can you post some photos of the large board?  You may just need to reflow the joints on the board.

 

[CuriousMister]

It also turns out two sets of LEDs are not actually lighting. The ones farthest from the TOP of large board and the ones farthest from the TOP on the small board. Oh boy.

Here are a few photos.

 

[Paul Birkeland]

If there are LEDs that don't light up on the small board, it makes me wonder if they were illuminating when you first installed the small board?

Your photos are all 0kB.  There's nothing uploaded.

 

[CuriousMister]

Sorry. iPad issues on the photos.  I’ll have to send them when I can get to a computer.

You’re right. I was thinking the same thing about the LEDs on the small board.  Should I retrace my steps to see if something went wrong there first?  It seems logical.

Also, thanks for your speedy replies.

 

[Paul Birkeland]

I would post the photos and post the OA/OB voltages on the small board.

 

[CuriousMister]

Small board readings.  I want to say they were reading around 87 originally (I think).  Also, I looked at the small board again and everything looks to be oriented correctly. 
OA: 185
OB: 190

Second try at the larger board photos.

 

[Paul Birkeland]

Those pictures aren't useful in any way.  They are super pixelated.

You need to debug the small board first and figure out why OA and OB have gone high.  I would guess that the LEDs on the 9 pin socket also aren't lighting up, but I would encourage you to ignore that and not touch them for now, as they aren't the problem.

Is the 12AU7 tube glowing when you turn the amp on?

 

[CuriousMister]

Alright.  Here's another attempt at photos.

Update.  Both tubes are glowing and the the small board is fully back in business.  All LEDs are lit on the small board and socket.  The large board is still way off though.  None of the LEDs are lighting there and all the voltage readings are super high now.  This is what I am getting.  I simply tried to reflow or make any connections better.
OA: 211
OB: 215
G: 214
B+: 217

Any thoughts now?

 

[Paul Birkeland]

The thing that really jumps out at me is all the solder balled up on the top side of the big PC board around the TIP50 transistors.  That has me wondering if you tried to solder both sides of them.  Then on the bottom it looks like possibly a trace is missing?  If you have an adjustable soldering iron/station, it needs to be turned up all the way and these joints need to be heated till the solder flows through the board.

 

[CuriousMister]

Thanks for the additional idea Paul. I really appreciate the comments and am up for anything to help this thing get back on track.

I didn’t solder both sides of the board on the TIP-50 resistors, but solder bled through and created those balls on the top side.  Like you suggested, I put my soldering iron up to 850 to try to get the extra solder to run through the board. It didn’t.  I ended up just using wick to clear it out. That ended up pulling some of the solder on the bottom board side too. You suggested trying adding more on the bottom there. When I did, the solder just did the same thing and ran to the other side like before - multiple times.  It just runs through to the other side immediately.  I tried to clean it up as best I could and reattached the board. I saw the faintest of lighting on one LED only. It could have been there before.  I’m not sure.  It’s hard to catch. The voltage readings remained way off.

I took the board back off and am open to more suggestions.

Do you think the issue is with the TIP-50s, or is it somewhere else?

 

[Paul Birkeland]

You can check for continuity between pairs of transistor legs with your meter. A shorted transistor would tend to pass a lot more current than is desirable, but your board isn't passing any.

 

[CuriousMister]

Great idea to check the board readings. The TIP-50s passed the continuity test. I tested the 31.6ohm resistors to see if they read the same while I was at it.  I’m getting one measuring correctly and one reading high at 56.  The 46ohm ones are reading correctly. Additionally, there is some solder bleed through on the LEDs on the top of the board too.  Since they’re sensitive, I haven’t touched them.  I could try to pull it back through with some solder braid.

Could the 31.6ohm resistor reading high be the culprit? 

Thank you so much for your continued guidance (and patience).  I am an extreme newbie. 

 

[Paul Birkeland]

No, that would raise the cathode voltage only by a few volts. 

If you've reheated the LEDs a bunch, you can overheat them and break them.  Some meters will light the LEDs when on the diode setting, but I know that the meter I use will only light one of the diodes in the C4S circuit but not the other, even if both work.

 

[CuriousMister]

I think we’re almost there.  The issue was likely due to an overheated LED.  I didn’t see any light with a meter diode test, so I replaced them all. Luckily, I had exactly four from a replacement part order (busted one initially on the socket install). All LEDs are now lighting up!  That’s a first. Here are my current voltage readings.  OA and OB are a little high.  Are they good enough?

B+ 186
OA 125
G 0.3
OB 115

 

[Paul Birkeland]

That looks a lot more like a working set of voltages.

 

[CuriousMister]

I really hoped I was done based on the last post.  When I plugged in my headphones last week, the left channel was much lower in volume than the right - significantly.  That wasn’t there pre-large board.  It isn’t the slight channel imbalance that’s all over the forum either.  After looking at the forum for suggestions, I reflowed all the joints in the crack and fired it back up (since that seems to be most logical problem).  The same channel issue is still persisting.  All LEDs lit back up and my large board readings are now OA: 126, OB:116, G: 0.3, and B+: 189.

Also, I’ve done a fair amount of reflowing and wicking over the course of build too.  When I didn’t know I had overheated at least one LED on the large board, that’s how I went about trouble shooting.  Could I have damaged something?  Or, what else would you suggest trying?

 

[Paul Birkeland]

Put the 3K resistors in and e-mail replacementparts(at)bottlehead(dot)com to get a new big board and parts.

The LEDs don't need much heat to be soldered fully, as they are small and don't really absorb much heat.  The transistors and resistors need to be soldered till the solder sucks into the PC board and flows to the other side.

 

[CuriousMister]

A new large board and parts did the trick. Thanks Paul! 

I am months late in my reply because I just can’t stop enjoying the crack. It sounds amazing.  I’ve already started experimenting with different tubes and can see why people love to do it. Also, I have almost stopped listening to my solid state amp completely. Tubes are for me and for life. They’ll be another build in my future for sure.

This experience has been amazing for someone with no soldering skills at the beginning. Thanks for providing great instructions, helpful advice, and for cultivating a truly one-of-a-kind online community.