[BUILD] New FET/RACK Official Help Thread - Please read first post!

[Echo North]

Isn't that what we wanna know how things are interacting?

majority of those readings are more or less spot on with the valve of the resistor, there is only a few that aren't their stock value.

Google “parallel resistors”.  Easy enough, but toss in some C and L and you’re at a different level. Good luck.

Is your voltage low with Q5 in and Q6 out?

 

[SunkenCity]

I'll give that a shot and see what happens.

Just figured you could cross reference some of those readings with a working unit since there are only four maybe five that are not the stock valve of the component.

 

[Echo North]

I'll give that a shot and see what happens.

Just figured you could cross reference some of those readings with a working unit since there are only four maybe five that are not the stock valve of the component.

They seem fine. Reading about parallel resistances would help you sort what's going on.  We don't guarantee support online, I do it because I want to help and I want people to be happy with their builds.  At some point I have to tap out because I am stumped and wasting both of our time. I've probably spent an hour hour looking at the schematic trying to sort what your issue is, not knowing if it's simply a bad joint, misplaced component, or lifted trace. At some point I need to give up, that's why I offer the dirt cheap repair, because I only have so much time I can spend looking at the schematic no knowing what the build even looks like. I hope that makes sense.  I don't give up often, but this is one of those times.

Maybe your Q6 is testing ok with a diode tester but way out of spec.  It is the output transistor so it could sag your rail if damaged or operating oddly. Removing Q5 would turn it off.

Mike

 

[saqeef]

You're sure R84 is correct? 75Ω/3W?

This is going to be super hard since you didn't test the supply initially.  Maybe CR5 is weird, but it also could be any of the semis on the rail pulling too much current. Q8 and Q9 are in the correct spots?  Correct orientation.  Also check CR1 and CR2.

Any transistors getting hot?

Mike

Got a bit discouraged for a while, but coming back at it again.

R84 is 75Ω, yes.

So Q8 and Q9 are definitely heating up. And although you mentioned before a slight burning odor is normal, I'm pretty sure I also saw a tiny sliver of white smoke for a brief moment, and that has me a little concerned.

CR1 and CR2 are in the correct orientation.

 

[Echo North]

Got a bit discouraged for a while, but coming back at it again.

R84 is 75Ω, yes.

So Q8 and Q9 are definitely heating up. And although you mentioned before a slight burning odor is normal, I'm pretty sure I also saw a tiny sliver of white smoke for a brief moment, and that has me a little concerned.

CR1 and CR2 are in the correct orientation.

So IIRC you have a Rev F.  Let's start from the top.

Q8 and Q9 are the push pull output transistors.  They shouldn't get more than warm.  If they are heating up....

1. They are swapped or inserted in reverse
2. One or both are DOA.

If one or both are DOA, that could cause R40/41 to heat up and smoke. Do those look brown are darkened at all?

Are your rail voltages ok?

Mike

FYI: I will not be online all day tomorrow.

 

[saqeef]

So IIRC you have a Rev F.  Let's start from the top.

Q8 and Q9 are the push pull output transistors.  They shouldn't get more than warm.  If they are heating up....

1. They are swapped or inserted in reverse
2. One or both are DOA.

If one or both are DOA, that could cause R40/41 to heat up and smoke. Do those look brown are darkened at all?

Are your rail voltages ok?

Mike

FYI: I will not be online all day tomorrow.

Checked the transistors: Q8 is MPSW06 and Q9 is MPSW56, and double-checked it to the build guide. The orientation for both of those is also correct.

R40/R41 don't appear to be browned, but they do seem to have a bit of a darker blue color compared to the other resistors...

For the rail voltages, I hope I'm answering right, but I got .01-02Ω at the ground tab on the IEC. For CR8, I get about 12.55ish V DC, and for the anode on CR9, I get like -9.56V DC.

Thanks for the help and the heads up! I'm in no rush.

 

[SunkenCity]

So I soldered in another Q5 and left Q6 out now the voltage is correct at the cathode.

 

[Echo North]

So I soldered in another Q5 and left Q6 out now the voltage is correct at the cathode.

You have another Q6 you can try?

 

[SunkenCity]

There's one in the other unit I could give a shot but the unit wasn't putting out the right voltage at the cathode either. So If swapping it's Q6 in doesn't fix it could be that it's Q6 one isn't functioning proper either.

I'll pull it from the other unit and see if the voltage returns on that unit

diode test it and solder it in and see what happens

 

[Echo North]

There's one in the other unit I could give a shot but the unit wasn't putting out the right voltage at the cathode either. So If swapping it's Q6 in doesn't fix it could be that it's Q6 one isn't functioning proper either.

I'll pull it from the other unit and see if the voltage returns on that unit

diode test it and solder it in and see what happens

Two DOA Q6’s would be super weird, but not impossible.  Make sure you use the spacer and are not soldering them right against th PCB.  The can would cause a short.

Something really weird is going on with your builds.

 

[Echo North]

Also triple check your output transformer wiring.

 

[SunkenCity]

I used the Q6 spacer on both units when originally assembled and when I swapped it.

I removed the Q6 from the other unit and the voltage returned to 31.26 at the cathode but CR9 didn't have a -10 the reading was -0.5 vdc.  None of the resistors are lifted on the rail like the other unit tho.

Soldered in the Q6 and the voltage dropped back to 20 vdc.

 

[Echo North]

I used the Q6 spacer on both units when originally assembled and when I swapped it.

I removed the Q6 from the other unit and the voltage returned to 31.26 at the cathode but CR9 didn't have a -10 the reading was -0.5 vdc.  None of the resistors are lifted on the rail like the other unit tho.

Soldered in the Q6 and the voltage dropped back to 20 vdc.

This is all so weird.  The negative rail should not be altered by Q6. There is something really weird going on. Could be any number of things and I'd just be pissing in the wind at this point.

Strongly suggest you send at lease one in for repair. You could try new Q6's but that would be a hail mary at this point.

Mike

 

[Echo North]

Got a bit discouraged for a while, but coming back at it again.

R84 is 75Ω, yes.

So Q8 and Q9 are definitely heating up. And although you mentioned before a slight burning odor is normal, I'm pretty sure I also saw a tiny sliver of white smoke for a brief moment, and that has me a little concerned.

CR1 and CR2 are in the correct orientation.

I meant you +30V rail. Is that ok?

Seems like either Q8/Q9 or one of the two might be shorted.  I would start by replacing those.  Send us an email.

Mike

 

[SunkenCity]

This is all so weird.  The negative rail should not be altered by Q6. There is something really weird going on. Could be any number of things and I'd just be pissing in the wind at this point.

Strongly suggest you send at lease one in for repair. You could try new Q6's but that would be a hail mary at this point.

Mike

Something else is off with that unit probably unrelated to Q6. The Shorting out/malfunctioning off switch probably didn't help anything any.

If the voltage drops when Q6 is in the circuit does that mean that Q6 is bad?

I thought we had traced the problem to Q6 but now your saying replacing it isn't the solution?

 

[Echo North]

Something else is off with that unit probably unrelated to Q6. The Shorting out/malfunctioning off switch probably didn't help anything any.

If the voltage drops when Q6 is in the circuit does that mean that Q6 is bad?

I thought we had traced the problem to Q6 but now your saying replacing it isn't the solution?

If the issue was with just one unit, I’d say Q6 is bad.  But having the same issue on both units, in addition to the dropping negative rail (which is isolated from Q6) and shorting switch (which I’ve never heard a report of in a decade) makes me think there is some fundamentally wrong that we are missing.

Could be a string of weird bad luck but I can’t tell you without looking at it deeper.

Mike

 

[Echo North]

I’m happy to send you a new set of Q6’s if you want try that.

Mike

 

[SunkenCity]

Yeah that would be great thank you.

I built another meter switch assembly with the parts you sent and it worked fine. I looked over the old one pretty thoroughly couldn't find a problem with it and assumed in was a short inside of the off button.

on the unit that came with a good switch:

I've been over every component in the line amp section to make sure it's the correct valve, color code, orientation. 

I checked continuity from component to component marking off the schem as I went along which eliminates a bad solder joint as the problem.

output transformer is wired correctly.

with the resistor legs lifted it removes those sections for the power, right?

does that mean that other sections wouldn't be effecting the line amp or power supply?

that leaves Q6, the output transformer, and the caps to check right?

 

[saqeef]

Hey Mike,

Not sure you got my last response, but here's a link to the post:

https://groupdiy.com/index.php?topic=63600.msg890396#msg890396

Thanks.

 

[Echo North]

Hey Mike,

Not sure you got my last response, but here's a link to the post:

https://groupdiy.com/index.php?topic=63600.msg890396#msg890396

Thanks.

Are sure you have the voltage select switch set to your correct voltage?  120v for USA?