F5 build power on with smoke from R19 and R20

[kj123]

Hi there,

Beginner here, so I could use all the help I could get

I started building my F5.. Power supply tested good. So I fired on with a dim bulb attached. The bulb lit for a second or 2 and dimmed completely but I saw some smoke coming out of R19 and R20... Here is a picture where you see R20 discolored ... any advice before I do more damage

Edit: One channel attached, I have not yet assembled the other board. The short ti,e it was up, I read 0V across R11 R12

Thanks

KJ

 

[Zen Mod]

bjt orientation ?

however - post or link ref. schmtc

 

[kj123]

Hi Zen Mod,

I am using the "official" F5 build (schematics) with 2SK170, 2SJ74, IRFP9240, IRFP240, ZTX550 and ZTX450 ... I followed the pinout on the board using the build guide as a reference - on page 13, they gave the pin orientation of the different parts that can be used for the build.

Here is another picture, maybe someone more knowledgeable like yourself will spot what I am doing wrong

Thanks

KJ

 

[Zen Mod]

why JFets are backwards ?

also bjts are looking backwards

and left NTC is dangerously close to mosfet washer , with naked legs

 

[e_fortier]

Reverse Q6 like ZM mentioned. Can't see Q5 but it's probably the same.

Q1, Q2 are also reversed, why ?

Rgds,
Eric

 

[Zen Mod]

well - OP didn't read build guide enough times

check Jfets just in case ( common Idss test with 9V battery and mAmeter)

replace them if needed , even if I believe that they're good

replace small bjts without hesitation

replace R19 and R20

triple check pinout of parts prior to powering up ....... post pic here , so we can check together

 

[kj123]

Thank you Eric and ZM,

I will recheck/reposition Q5 Q6 and report back... also push NTC away from the washer.

Thanks again

 

[kj123]

As FYI to other extreme newbies like me, here are the pinouts for Q1 Q2 Q5 and Q6 (2SK170, 2SJ74, ZTX550, ZTX450)

I am going to fix the mess and will post the picture here before powering as ZM suggested in case I do another monumental mistake

Thank you again gentlemen for your help

KJ

 

[kj123]

Hi,

I turned around the transistors (replaced ZTXs and resistors). Does the board look ok now? Can I go ahead and power it on?

PS. I will probably replace the whole board but I want to make sure that it's fine the way it is so I do not waste another pcb/parts

Many thanks

KJ

 

[e_fortier]

Based on your latest pics I would give it a try.

Keep us posted.

BR,
Eric

 

[kj123]

Thanks Eric... before I do that, I did some googling and double checked the resistors value and the ones I am using (from the BOM - for board rev 2, I am using rev 3 - are 100R) but the ones on this schematics posted by 6l6 on the F5 board thread are 10k? Actually there are other differences, for instance the schematic shows R21 and R22 as 2.21k while in the BOM they are 10k. Are versions 2 and 3 of the board using different schematics/versions of the F5?

If someone could clarify this to me, I would greatly appreciate it.

Thanks

KJ

 

[Zen Mod]

pinout looks good now

regarding resistors :

resistor going from ztx base to rail - 100R

resistor going from ztx base to output - 10K

I'm sure you can decipher which ones they are

 

[WalterW]

No don't fire it up ! You have all the resistors in the wrong places. I see they have renumbered all the resistors on the version 3 .0 boards.
i.e. R5 - R8 from V2.0 is now R9-R12 on V3.0 boards

 

[kj123]

Well I have bigger problems then... the resistors are not the right value...R19 and R20 should be 10k and I am using 100R ! I was following the wrong schematic all along. In the attached schematic from the pdf guide, R19/R20are 100R ... like I said in my other post, there are other differences in the values... can you please confirm that 6l6 schematic I linked to is the right one for the store board version 3? Many thanks

 

[Zen Mod]

that without me ...... I'm not having xperience with boards in question

 

[WalterW]

I think you should use these.... I'll check it later, when kids are to bed

 

[jwilhelm]

Looks like a cold solder joint on the output wire.

 

[kj123]

Thanks WalterW, I think you are right. I should use the schematics from the 5FT article where Nelson Pass talks about the F5 first before talking about the changes for the Turbo version.

Actually it's all written on the board page in the store... I should have paid more attention to the change log... unfortunately the Schematics button on the same page links to the one for board version 2!

V3.0
Change to F5 schematics from F5 Turbo article

Minor diagram change in NTC and limiter sections
P3 added to adjust N/P variations to reduce distortion
Relabelling of resistors to match schematics

 

[WalterW]

yep, I checked. The schematic I posted is the right one for the V3.0 boards.
It is from the F5 turbo article with the addition of P3. I have both v2.0 and v3.0 boards here so it was easy to check.
So all resistors, except R13 and R14, seems to be wrong...

 

[WalterW]

Ah, same time posting.... yes you are right, they should remove the old v2 schematic from the store! It's confusing....