Votage Check Errors

[deserat]

Crack + SB

Hey all, re soldered and rewired some for the hookup wire.  Now my voltages are off, everything appear to be hooked up right and I've even taken a loupe to the joints to make sure they are clean.

All LED light up fine except A-Center and A-8.  That LED only lights up as I turn the volume pot up.

Reistance Checks

Only Abnormals listed

7 - 0
9 - 0

B3 - 0
B6 - 0

Voltage Checks

Excludes 0's which all check out fine.

1 - 31
2 - 178
4 - 178
5 - 67
7 - 56
9 - 81
13 - 170
15 - 195
22  - 221

A1 - 63
A6 - 32

B1 - 32
B3 - 178
B4 - 63
B5 - 177
B6 - 81

Speed Ball Voltages

OA - 32
OB - 63
B+ - 178
G - 0

I see the patter of 32 appearing and cant trace the wires but don't really know what could be bad in the path.

Thank you.

 

[deserat]

The thing that stands out to me is that 1, A6, B1, and OA  are all 32 volt in the voltage test. 

Those terminals are all connected to each other.  I've tried swapping tubes, so I know that's not it.  Maybe a fried transistor?  I miss listening to my crack.

 

[Paul Birkeland]

If 7 and 9 are both 0 ohms,  you absolutely must not use power on your Crack. Please see the instructions in the manual for mounting the tip50c,  you have at least that as an issue.

The LED moving with the volume pot makes me wonder if you built the amp stock first.

 

[deserat]

Cool.  I did have a working Crack for about 2 months and then a working Crack + Speedball for about 3 months.  So I guess what you are telling me is that despite what I believe looks like correct wiring.... it is very not.  Or at least it wasn't at some point.

Unfortunately after some of the rewiring I found one of the  wires between the small Speed ball board and the first set of terminals were broken... this occurred after having powered  up the crack to check the voltages, as I didn't check the resistances before doing so. Unfortunately my dumb self checked the speed ball manual not the crack manual first. Perhaps that burnt out the transistors on the large board?

Anyway I'll find replacements for those. Appreciate the pointer to the  TIP-50 transistors.

 

[Paul Birkeland]

If you have 0 Ohms at 7 and 9, the shoulder washers or the order of the hardware used to install the TIP50C transistors to the heatsink is incorrect.

 

[deserat]

Hey I removed and reconnected all the wires to the big speedball board  and desoldered and resoldered the transistors  Resistance and Voltages all check out now.  Though I get nothing but static from the left channel. The static is constant in and  does not change volume with the pot, it is also unaffected by removing the RCA in.  I've rolled the tubes and switch out headphones to make sure those components not part of the issue.  Any chance I burned up one of the transistors or is there something else I should look at.

I did fine out the reason the LED was changing brightness with the pot was a small bridge of solder  from A7 and the A8. Thank you.

 

[Paul Birkeland]

Can you post your voltages for terminals 1-10 just so I can have a look.

Most of the time a dead left channel is because of a solder bridge from the center pin of the RCA jack to the shell of the RCA jack, but since your Crack was working, this seems unlikely.

 

[deserat]

Thanks I did rewire from the rca jacks to the pot so the guess of a solder bridge was totally reasonable. I double checked the resistance on on the shell vs wire on the rca.

1 - 64
2 - 174
3 - 0
4 - 174
5 - 63.5
6 - 0
7 - 81
8 - 0
9 - 80
10 - 0 

 

[Paul Birkeland]

OK, well, your amp is working but not passing signal on one channel.  What you want to do is to download a 60Hz tone or download a signal generator onto our phone, then put 60Hz signal into your amplifier.  Set your meter to AC volts (the 2V or 5V scale is good to start with, if you have an autoranging meter, this doesn't apply).  Now with the tone playing, measure the AC voltage between ground and each center pin of each RCA jack.

Next, turn the volume pot all the way up and measure the AC voltage between the center lug of each level of the volume pot and ground.

Let us know what these voltages are.  Do also note that these two tests can be done with the amplifier powered off. 

-PB

 

[deserat]

Very cool.  Will do that tonight. Thank you.

 

[deserat]

I'm sorry to have disappeared. I had to go take care of a friend who had major surgery and haven't been home in 3 weeks.

I finally got a chance to run the test running a 60hz tone from here: https://www.youtube.com/watch?v=E_Sf7rSOU78

I am using 12 as ground.

RCA Jacks with level at 0

R - .45 V
L - 0V

Pot center lugs with level at 10 ( maximum )

R (top) - .45V
L - 0V

 

[Paul Birkeland]

No signal is making it through the left jack.  The most common cause of this is when soldering the center post of the jack, too much solder is used and it bridge the center of the RCA jack with the shell, causing a short.

 

[deserat]

Cool.  Well I got the jewlers loop out and it does look like there is some solder in the gap between the post and ground.  Couldn't get it cleared, out so I ordered new rca plugs.  Thanks for the help. I appreciate your patience.

 

[deserat]

Alright new RCA  Jacks installed and resoldered in.  I was very carefull not to over apply solder.

RCA Jacks with level at 0

R - 1.95 V
L - 1.95 V

Pot center lugs with level at 10 ( maximum )

R (top) - 1.95 V
L - 1.95 V


The left channel now has a slow steady hum like it's dead. Shortly after plugging in headphones  and turning up the volume I heard a loud pop in the left channel.

I'm also noting that if I disconect the right channel and turn up the volume a small amount of sound comes out of both channels.

I retested voltage.

Large speedball board everything is in spec except OB which is 150 V
Small speedball board everything is in spec except OB which is 250 mV

LED on 12au7 pin A3 to A center no longer lights.

Am just about at the point of packing it up and paying the fix-it fee.

 

[Paul Birkeland]

You have a loose connection elsewhere in the amp that has been disturbed by the work that you have done with the RCA jack.

Since the RCA jack solder bridge would have prevented the amplifier from working properly in the first place, I would suggest removing the Speedball and returning the amplifier to its original form before continuing to do any debugging.

-PB

 

[deserat]

Thanks, and thank you again for your patience.