Ampeg V4.com

forum

You are not logged in.

#26 2011-12-09 19:21:25

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Ok. Here are my findings....

Wall is putting out 124vac at 2pm in my space.

R40 45 46 39-568v

R41 42 47 48-571v

Pt A-571
Pt B- 571
Pt C- 462
Pt D- 401
Pt E- 371

Im using a Tenma 72-7735. Dont have another meter, so not sure why im getting such a small drop across 10ohm.

Any ideas?  Any bridges i should look for?

Thanks

Daryl

Offline

 

#27 2011-12-09 19:32:43

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Yout voltage on the screens is higher than the plates.  That's not good.  Check those resistor values.

R40/R45/R46/R39 (Measure AT pin 3)
R41/R42/R47/R48 (Measure AT pin 4)
Again, R55 should be replaced (why not?) with a 10W 470ohm.  Has that resistor been replaced?  Do you know if it is currently measuring to spec?

Last edited by Liquids (2011-12-09 19:34:22)


Matthew

Offline

 

#28 2011-12-09 19:40:57

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

http://i92.photobucket.com/albums/l19/strangewaysrecording/Mobile%20Uploads/IMG_8094.jpg

http://i92.photobucket.com/albums/l19/strangewaysrecording/Mobile%20Uploads/IMG_4060.jpg

http://i92.photobucket.com/albums/l19/strangewaysrecording/Mobile%20Uploads/IMG_7891.jpg

http://i92.photobucket.com/albums/l19/strangewaysrecording/Mobile%20Uploads/IMG_5654.jpg

http://i92.photobucket.com/albums/l19/strangewaysrecording/Mobile%20Uploads/IMG_8703.jpg

Offline

 

#29 2011-12-09 20:05:50

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Liquids wrote:

Yout voltage on the screens is higher than the plates.  That's not good.  Check those resistor values.

R40/R45/R46/R39 (Measure AT pin 3)
R41/R42/R47/R48 (Measure AT pin 4)
Again, R55 should be replaced (why not?) with a 10W 470ohm.  Has that resistor been replaced?  Do you know if it is currently measuring to spec?

Same v on pins vs on board. 470 10w measures .465k

Any other ideas?

D.

Offline

 

#30 2011-12-09 20:26:58

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

I just said on the pins because there are two sides to each resistor.  smile

The voltage at point B should be lower, which follows the problem with your screen voltage being higher than you're plate voltage. And your board looks pretty bad where the screen resistors are.  Investigate traces, etc....


Matthew

Offline

 

#31 2011-12-10 19:19:40

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

No luck. Am i correct in assuming the problem is somewhere between the 1k resistors and just after pt A?  My 100k resistors on the 2 capcans are good. I replaced the c19 cap with 2 22uf 500v in parallel. Im getting about 400v on the negative side of this cap. I get 573v on the pos side going to pt B. i relocated 1ks to the pin 4 on power tubes and wired off of them to the leg of the 470 10w and tied the wire from c19s pos there as well, so i could get them off the board and discount bad traces.

Im not sure where to look next.

Help!!!

Daryl

Offline

 

#32 2011-12-11 14:16:49

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

those 2 22uF 500v in parallel are not equal or equivalent to what the schematic calls for.   

The schematic has 2x 40uF 450v caps in SERIES (edit, sorry). That equals a 20uF 900v cap.

What you have equals one 44uf 500v cap with 571 volts on it. Or do you have C19 in parallel with part of a cap can (C10) as per the schematic?  If so, why not try using two resistors across the caps to even out the voltage across them and see if that more evently splits the voltage

Last edited by Liquids (2011-12-11 14:40:04)


Matthew

Offline

 

#33 2011-12-11 16:01:19

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Correct. My recap kit came with a 47uf 500v cap. I swapped out with 2 22uf in parallel to see if the cap was the problem. It is in series with one of the can caps.

Not quitesure what you mean with resistors?  Could you explain?

Daryl

Last edited by Drabidoux (2011-12-11 16:02:09)

Offline

 

#34 2011-12-12 13:22:36

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Like with the caps in series out of the rectifier - to balance /dvide the voltage more evently, you can strap resistors across them.  Se R56 and R57 for an example.


Matthew

Offline

 

#35 2011-12-16 01:52:10

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Will this help with the voltage problem?

Daryl

Offline

 

#36 2011-12-16 14:21:22

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Good question.  I'd guess not, but getting 400v at the junction of your filter caps is not good in it's own right...suggests that there may been some value drift...and this may help it even out if nothing else.


Matthew

Offline

 

#37 2011-12-27 15:30:28

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Did you give up and throw it out the window?


Matthew

Offline

 

#38 2011-12-27 20:11:31

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

Ha!!  Pretty much at that point.

I cant find any bridges,  i wired up 2 100k resistors.  No change. What resistor/caps cause the voltage drop? 

Im sure i wired things as they were with the new caps, but it seems like the problem is in that 470 10w and the 2 40uf caps.

Any where else i should look?

D

Offline

 

#39 2011-12-27 20:20:30

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

I feel reluctant after how many things I've said check that haven't turned up the solution, though at least the problem (screen voltage higher than plate voltage) is clearer.

You're pictures don't really show the cap can wiring entirely.  It is also a birds nest which is not a good thing, but that may be completely unrelated.

So at the junction of the 2 100k resistors that are now in parallel with the capacitors that are filtering the screen supply, are you getting more like 1/2 the voltage at the junction of 470R 10W and the first cap?

The 10W 470R resistor should be doing the voltage drop if all was working correctly.  What voltage drop are you getting across that resistor anyway?

Anyhow, just be sure you wired all your caps cans properly...if you can offer an additional picture of that area, please do, wouldn't hurt.

When I've reach this stage of troubleshooting myself, it's always come back to an incorrect assumption for me.  "Beginner mind" when looking it over, though frustrating, is important when you keep turning up with nothing.

Don't let this amp beat you, it's not untameable.


Matthew

Offline

 

#40 2011-12-27 22:11:33

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

I am getting no voltage drop from either side of the resistor.

I havent rewired since i got it. Not sure what was done after the factory, but should be stock wiring. ie: dont blame me wink

Ill get some better pics and post in the next week.

D

Offline

 

#41 2011-12-28 10:49:59

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

So, no measurable voltage drop when you put your DMM leads on opposite sides of the R55/10W 470R?  But when you set your DMM to 'continuity' aka 'beep' mode and put the leads on opposite sides of R55. it doesn't read continuity, and it reads as close to 470R of resistance?

Also, previously you wrote "when the amp is off and drained, Im getting about 400v on the negative side of this cap [C19]. I get 573v on the pos side going to pt B."  Now that you have a 100k resistor from point B to the negative side of C19, and then another 100k resistor from there across whatever cap you are using for the 'triangle' portion of  C18, what voltage are you getting at the junction?  And, if you clip your black lead to the chassis somewhere else and read the voltage  at the NEGATIVE side of the cap that is subbing for the triangle portion of C18, do you get a voltage?


Matthew

Offline

 

#42 2011-12-28 15:47:55

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

No voltage drop on either side of r55.  I am measuring all voltages to ground.  You aren't suggesting i should be measuring red on one side and black on the other of r55, are you? 

I have replaced r55 with another resistor.  Both of the resistors measure .463k on my dmm.

I do not get continuity on the resistor.  So no beep when measuring.

I'll try things from the second paragraph when I get back to the amp on the 1st of the new year.

Thanks Liquids.

d.

Offline

 

#43 2011-12-29 15:38:23

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

Measuring red on one side and black on the other side of the resistor will give you voltage drop.  Of note, that can (via some math) give you current draw as well.  That said, if you are getting the same voltage on both sides, that says something..we know that you're not getting even 1V of drop...measuring voltage from one side of R55 to the other will, yes, give you the reading for exactly how much voltage is dropping, but right now we know its small enough to be insignificant or zero.  Which is certain A or THE cause/result of the mysterious problem that you are having...

I'm stumped.  It's hard to troubleshoot amps 'blind.'  If you have a friend with a fresh pair of eyes, or willing to spend the money to have a tech look it over with the clear issue that the screen voltage is too high and not dropping over R55, I would do it.  We all reach our limit of skill/knowledge/experience at some point in the process, no shame in that.


Matthew

Offline

 

#44 2012-12-18 16:10:20

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

What ever happened with this amp problem?


Matthew

Offline

 

#45 2012-12-18 17:26:33

Drabidoux
Member
Registered: 2011-11-19
Posts: 21

Re: V4 Bias woes....

It's still dead in my basement. Don't want to sink anymore $ into it. If anyone wants it, pm me.

Daryl

Offline

 

#46 2012-12-18 21:15:37

Liquids
Member
From: CT
Registered: 2010-08-01
Posts: 491

Re: V4 Bias woes....

I'm probably not interested right now, but maybe; also, it depends on where you are located (shipping vs pickup) and how much you're asking...PM me...worth a shot...


Matthew

Offline

 

Board footer

Powered by PunBB
© Copyright 2002–2005 Rickard Andersson