B+ voltage

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 3:08 am

OK, so I think I fried my OT ( can be found in a previous post "blown fuse with a THD hotplate") anyway, thought I was checking the B+ correctly but want to make sure. First what exactly is the B+ voltage? I think it can be tested from the power tubes, V4 and V5 on pin 3 to ground? Is this right?

I have been trying to test it but my 1/2 amp fuse keeps popping before I can get an accurate reading. I had clipped my meter to V4 and V5 pin 3 to ground and was reading 274 VDC. That doesn't seem right, I think it should be around 403 VDC or somewhere around there.

I was told I could test to see if I fried the OT by checking the B+ while playing through the amp and seeing if the there is a significant voltage drop as a chord is struck and sustained.

I am ready to purchase a new transformer from George but want to make sure that really is the cause.

Also is there a way to test the OT and the PT to make sure they are in working order?

thanks guys

Tony

p.s. I have JCM 800 2204 for backup, but it just doesn't compete with my metro JTM45 build for tone, and I want to get this thing back up and running!

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 3:10 am

Here is the link to the previous post

http://forum.metroamp.com/viewtopic.php?t=13054

thanks again everyone!

the_haunted · Post by **the_haunted** » Mon Sep 17, 2007 7:28 am

the B+ goes to the center tap of the output transformer.
The center tap probably goes to one of the caps. straight out of the rectifier.

swap tubes from the JCM to the JTM, just to be sure you don't have a funny tube problem, but it sure sounds like an arcing OT to me.

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 1:23 pm

Thanks Haunted, but how does one actually go about testing this B+ voltage? And I did notice something strange about the tubes. Where the glass of the tube meets the base (reddish colored plastic) there is what looks to be corrosion all the way around, almost like you would see on a car battery. Strange! Still though, the I smell a 'burnt' odor from the transformer.

Just wondering how to actually test the B+ voltage, and also if there is a way to lift the transformer wires and test the transformer, if thats possible?

Thanks again
Tony

ScottW · Post by **ScottW** » Mon Sep 17, 2007 2:29 pm

The B+ rail starts from the rectifier and goes to the plates, screens, PI and all the way to the 1st triode on V1. It's at its highest voltage right at the the PT center tap/first filter cap. You can measure anywhere along the rail, from any point to ground. The center tap/first filter cap will be the highest, the first triode on V1 will be the lowest. Most people will say that B+ = plate voltage at pin 3 of the power tubes. So, yeah, pin 3 to ground.

How to test your PT is spelled out here:
http://www.geofex.com/ampdbug/pwrtrans.htm

Good luck!

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 2:36 pm

Excellent! thanks Scott, thats the kind of info I was looking for. Thank you!!!

ScottW · Post by **ScottW** » Mon Sep 17, 2007 2:45 pm

tfrost33 wrote:Excellent! thanks Scott, thats the kind of info I was looking for. Thank you!!!

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 5:59 pm

OK, so... Using the instructions Scott posted in the link above this is what I have found so far.

Set meter to Ohms 2K setting:

-Orange common is open to ground & transformer chassis

( According to the instructions from Scott's link this should be more than 1M, same with the red primary. Both are open to ground!)

-Red 120V Primary is also open to ground.

-From Primary's red 120V to Orange Common I get a reading of .004K

-High voltage windings Red (from pin 4 of rectifier) open to ground, but reads .024K to the Red/Yellow center tap (of the 1/2 amp fuse holder) and I get the same exact reading for the other Red high voltage wire on pin 6 of the rectifier. Red wire from pin 4 of rectifier to Red wire from pin 6 of rectifier = .052K

Rectifier heater (yellow/yellow) pins 2 and 8 of rectifier. Each to ground = 117K ohms exactly. Yellow to Yellow = 0.1 ohms

Green heater filament taps on V5 pins 2 and 7 I unsoldered these and get continuity to ground on both of these. (Does this mean its shorted to ground?)

Can anyone tell me if this means the Power Transformer is bad? I assume that it is according to these instructions. Also is there a way to test the Output Transformer? I am worried about both since it seems like they both stink. But its hard to tell for sure just by smell alone.

thanks

Tony

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 7:10 pm

Scratch that about the green heater wires here is an excerpt from the testing instructions, mainly the last line!!! OOPS

***"Measure the heater (filament) winding(s) (usually green - grern or green - green/yellow - green) as you did the primary, with the same conclusions EXCEPT that the heaters are usually tied to the chassis through the centertap (green/yellow) or through a hum balancing pot or fixed resistors of 100 - 500 ohms, so that having it conduct to the chassis is OK."***

So far, it seems the only reading that mey be off is the Orange common and the red primary being open to ground. Unless i am interpretting it wrong? Here is what it says

***Identify the terminals of the power transformer.
Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.

Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
If it is now less than 1M ohm, the transformer is failing, and should be replaced.
If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.***

Since this reading to the chassis (ground) is open can I assume the transformer is bad?

Thanks again fella's

ScottW · Post by **ScottW** » Mon Sep 17, 2007 7:29 pm

I'm confused (not unusual

), but what do you mean by "open" as in the connection to ground is open? Do you mean there is no continuity, infinite ohm reading? Or do you mean that it's shorted, meaning zero ohms to ground?

Think of a circuit and a switch. If you turn the switch off, you're opening the circuit, so there's no current, the switch is open, and the lugs show infinite resistance. If you turn the switch on, you're closing the circuit, allowing current to flow, and the switch lugs show 0 ohms - you've shorted the lugs together, closing the circuit.

And, exactly which leads are you referring to?

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 7:40 pm

Yes, infinite or "like the switch is off"

This occurs on both the Orange common and Red primary. Each of them to ground is open, no continuity, infinite...

The instructions say it should read more the 1M.

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 7:45 pm

Yes, infinite or "like the switch is off"

This occurs on both the Orange common and Red primary. Each of them to ground is open, no continuity, infinite...

The instructions say it should read more the 1M.

ScottW · Post by **ScottW** » Mon Sep 17, 2007 7:57 pm

Well, infinite is more than 1M so I'm not seeing an obvious fault. I'd proceed with the tests just down the page and see what you find. Maybe someone else has a better idea??

tfrost33 · Post by **tfrost33** » Mon Sep 17, 2007 8:00 pm

10-4 thanks Scott! Will proceed... You happen to have any testing procedures like this for an Output Transformer?

Thanks again

ScottW · Post by **ScottW** » Mon Sep 17, 2007 8:05 pm

tfrost33 wrote:10-4 thanks Scott! Will proceed... You happen to have any testing procedures like this for an Output Transformer?

Thanks again

Ha! Of course!

http://www.geofex.com/ampdbug/outtrans.htm