Power Transformer Question Following geofex steps

[newperson]

Hi,
I am trying to problem solve a PT.  It is blowing the fuse with no tubes plugged in and the OT in or out of the circuit.
Step one states to Ohm meter the two black primary lines. 
I am getting .06 Ohm.  Is this open and bad or less than 1k and fine?

Next step is primary lead to chassis ground.  I get over 120M.  The only part connected between here is the "death cap."  Could this be shorting out the PT?  It does not read open on meter. 

I am guessing the next steps do not matter until I can understand if the first two are okay.

Thanks for any help,
Paul.






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

Unplug the amplifier.
Open the case up and identify where the Power Filter Capacitors are. Connect a cliplead to the chassis, then clip the free end of the clip lead to the plate lead of any preamp tube to drain any charge from the filter capacitors. The plates of a 12AX7 are on pins 1 and 6.

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.

Measure the high voltage winding (usually red - red/yellow - red if it is centertapped, red - red or red - red/yellow if it is not centertapped) resistance from end to end. It should be less than 1K ohms. If it is over that, the winding is open, and the transformer should be replaced. The centertap, if present, should be tied to the chassis, perhaps through a standby switch.
Measure the rectifier heater winding (usually ? - ?) resistance from end to end. It should be less than 10 ohms. If it is more than that, the transformer is bad. Measure the resistance from either end of the winding to chassis. If it is less than 100K ohms, unsolder the leads and measure again. If this reading is less than 100K ohms, the transformer is bad.
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.
If none of the previous tests indicate a bad transfomer, make sure all leads are soldered correctly back in their original positions if you unsoldered them.

[newperson]

Also does it matter if about having a slo blow fuse?  I have not used one of those yet.

[G. Hoffman]

Also does it matter if about having a slo blow fuse?  I have not used one of those yet.

YES!  If it is supposed to have a slow blow fuse, it MUST have a slow blow fuse.  You are going to have peaks from time to time that (momentarily) exceed the fuse ratting.  A slow blow will survive those brief peaks, but still protect you from shorts in your circuit.

Second, I don't care if it's shorted or not, the "death cap" is well named, serves no function what so ever (assuming you have your amp properly grounded, which you should, and even if it's not the death cap serves a extremely limited function), and should ALWAYS be removed as fast as possible.  Get rid of it.  The switch will be equally functional without it.

For the rest of your questions, I'll leave it to others who have more experience than I.


Gabriel

[R.G.]

I am trying to problem solve a PT.  It is blowing the fuse with no tubes plugged in and the OT in or out of the circuit.
Step one states to Ohm meter the two black primary lines. 
I am getting .06 Ohm.  Is this open and bad or less than 1k and fine?

0.06 ohm is less than 1K. This is therefore fine.
"Open" in its strictest sense means "not connected by a conductor". It is not open.

Next step is primary lead to chassis ground.  I get over 120M.

Over 120M is fine, see below.

The only part connected between here is the "death cap."  Could this be shorting out the PT?  It does not read open on meter. 

If the "death cap" were shorting the PT, it would read much less than 120M. It's YOUR job to determine the meaning of "open" in the circuit in a relative sense.

"Open" is a subjective term. In it's relative sense, it means "not conducting a significant amount." "Open" is in this sense not something a meter tells you or not. You must learn to distinguish what open means in a relative sense. A MOSFET gate has a resistance of maybe a giga-ohm to a tera-ohm. Air humidity may constitute a short to such a resistance. A NiChrome wire may have a resistance of, say, 8 ohms. That's a dead short to a MOSFET gate, but a working load to a hair dryer circuit or an audio amplifier. And a NiChrome wire may be shorted by an equal length of copper wire, which would have a much smaller resistance. It is important to understand the relative nature of the situation.

In your particular case, anything over 10M may be considered to be open. And for shorted, much the same reasoning applies. In your case, you're looking for something that will cause a fuse to blow. Blowing a fuse requires approximately 2x the current printed on the fuse. If this is, for example, 2A in a small amplifier, you need something like 4A minimum to cause an immediate fuse blow. A current just a little over the fuse rating will cause it to blow in something like tens of minutes to hours.

So something in your amp is causing more than 2A to 6A (guessing at a 1A to 3A fuse) to flow in the primary. The primary voltage is 120V, so the resistance that makes 2A flow is 60 ohms or less. You're looking for a resistance across the AC power inlet or power to chassis ground of 60 ohms or less.

Let's backtrack a bit. What path of choices did you go through to get to testing the power transformer? Start at the beginning of the debug page.

[newperson]

Thank you for the replies.  I am working with a black, black, yellow, yellow, red, red, green, green, red/yellow Stancor PM-8408 PT.

Quote-
"Let's backtrack a bit. What path of choices did you go through to get to testing the power transformer? Start at the beginning of the debug page."

I found a small amp, 5D2 princeton that was missing tubes, looks to have the PT changed and OT changed and needed to be recapped.  I dusted it, recapped it, and put in a set of tubes.  But with no tubes (one each - 5Y3, 6V6, 12ax7 set) and the OT/speaker disconnected it blows the fuse the second it is turned on.  I have just clicked it on then off, then check the fuse.  I have only tried fast fuses 2 & 3 amp so far.  I need to buy some slo blo today.  The schematic shows 2A fuses, but not if they should be fast or slow.  So I started trouble shooting with the PT because not many things are connected with the tubes out of the circuit.  

The only thing I think in the circuit is the switch, fuse, (the lightbulb is out), the .05 600V cap to ground from the fuse, and 1 8uf 450V to ground.  I can cut the Yellow to get rid of the last thing.  


quote
"something in your amp is causing more than 2A to 6A (guessing at a 1A to 3A fuse) to flow in the primary. The primary voltage is 120V, so the resistance that makes 2A flow is 60 ohms or less. You're looking for a resistance across the AC power inlet or power to chassis ground of 60 ohms or less."

I see that one red is 23Ohms to chassis ground.  Is this okay because red/yellow (high power center tap?) is to ground?  The other red does not show 23Ohms to chassis ground.


Thank you again.  I have learned a lot just from your post.
Paul.

Here is a link to the schematic and layout of the amp,
http://www.ampwares.com/schematics/princeton_5d2.pdf

[R.G.]

So I started trouble shooting with the PT because not many things are connected with the tubes out of the circuit.  

Actually, there's a lot to test before the PT is even a suspect.

The only thing I think in the circuit is the switch, fuse, (the lightbulb is out), the .05 600V cap to ground from the fuse, and 1 8uf 450V to ground.  I can cut the Yellow to get rid of the last thing.  

... like these. A new PT is probably the least likely thing to be bad.

I'll be frank with you. Your note, indicating as it did that you don't firmly know what short and open mean, scares me on your behalf. Working on the power section of a tube amp before you have electronics wiring and safety firmly learned is a good way to get yourself electrocuted. Please, please, please be honest with yourself - if you don't know what you're doing, get live, on the spot help before you kill yourself. I advise you NOT to proceed unless you get that help. If you do, it is against my advice. That being said, I know that no one will heed that advice, so I'll try to tell you what to do safely. Proceed only knowing that you could get yourself killed, and could create an electrocution or fire hazard that will strike you or someone else at an unknown time in the future.
1. Unplug the amp from the AC wall socket. Wait five minutes. Solder one end of an insulated wire to chassis ground. Touch the other end of the insulated wire to the + terminal of the first filter cap, just brushing it across the contact. If there are no arcs, hold it there.
2. Before you do anything else, solder a 470K 1/2W resistor across the first power filter cap from + to - terminals, and leave it there. This is a bleeder resistor which will bleed down the capacitors to a safe level and not let them "regrow" charge, which they will otherwise do. This resistor will be permanent in the amp once it's fixed.
3. Unsolder the PT from all AC wiring; be sure the PT is disconnected. Put in a new 2A *slow blow* fuse. Get all personal parts and meter wires, etc. out of the chassis. Plug the AC power cord into the wall. Flip the switch on, then immediately off. Unplug the AC power cord. Check the fuse. Is it blown? If so, you must repair the AC wiring before you can do anything else. With the PT disconnected, the AC power is blowing the fuse all by itself. The PT has nothing to do with it.
2. If the fuse is OK, leave the AC power cord unplugged and wait five minutes. Then solder only the PT primaries to the AC power wiring. What winding did you decide was the primary? It is usually, but not always black and black.

Be very, very sure that *all* the secondary leads are disconnected from everything else in the amp, preferably taped over to prevent accidental shorts. If they are already soldered in, unsolder them and tape them up individually to prevent any accidental contact to any secondary. Nothing is to be connected except the primary. Now check to be sure that they're disconnected again. Get all your personal parts, meter leads, etc. out of the chassis. Make sure the fuseholder contains a good fuse. Plug in the AC power cord and flip the power switch on and off again. Unplug and check the fuse. If it's blown, Check another three times that only the primary is connected to anything, anything at all, and that no secondary is shorted. Then try again with a fresh fuse. If it happens again, the PT is toast, since the fuse did not blow on the AC wiring alone, and now blows when the transformer primary only is connected. IF YOU HAVE A SECONDARY CONNECTED, IT MAY GIVE YOU A "FALSE DEAD" READING.

3. If the fuse did not blow, unplug the AC power at the wall and wait five minutes. Connect your meter leads to the red/red leads, one on each lead, taping up the meter to the transformer lead. The meter leads are taped in place so that you do not have to touch the meter or its leads while the test is being made.  Check again to be sure no other secondary is connected anywhere. Get your personal parts out of the chassis, plug in the AC plug, and blip the power switch on, and read your meter **not touching the meter or leads while you do it***, which you have conveniently set for high voltage *AC* reading. Note the voltage. It should be 680Vac. If your meter will not read that high, try this same test but with the red/yellow and one red lead. This should read 340Vac or a little higher. If it's not, there is a PT problem, but not one which would blow fuses.

Repeat this test verifying that the green/green secondary is about 6.-7Vac, and the yellow/yellow is about 5Vac. I cannot emphasize enough: do all the picky little steps of unplugging the AC power and letting it sit to settle before making any change, and making certain that all secondaries are disconnected before each test. If the PT provides good AC voltages on this test, it's almost certainly good.

4. Unplug from the AC wall and wait. Solder the green/green winding to the filament heater wiring. Plug in, blip the power switch on and off. Fuse pop? Yes = shorted heater wiring. No = heater wiring probably OK. Disconnect AC and wait.

5. Plug in tubes. Plug in AC and blip the power switch. Disconnect AC and wait. Fuse stlll good? Yes = heater wiring and tubes OK. No = bad tube or socket.

6. Solder the 5V heater winding to the rectifier tube socket in the correct lugs. Plug in AC, blip the power switch on/off. Disconnect AC and wait. Check - fuse still good? Yes = rectifier heater socket wiring not shorted. No = bad rectifier heater wiring.

7. Check that the output of the rectifier goes to the *** positive *** side of the first filter cap (and its bleeder resistor). Check that the *** negative *** side of the first filter cap goes to chassis ground. Unsolder the wire leading out of the first filter cap to the rest of the amp. Nothing should connect to the first filter cap except its bleeder resistor and the wire from the rectifier. Plug in the rectifier tube. Connect AC, turn on the power tube and see if the rectifier heater heats up. If it does, the rectifier tube is probably OK. If not, the rectifier tube may be bad. If the fuse blows, the rectifier tube is shorted or the filter cap is shorted. Disconnect AC plug and wait. If you had one of the shorts, go figure out which one is shorted. If not, connect your meter, set for high voltage DC (400-500Vdc) to the filter cap + and - terminals, taping the meter leads in place. Do not touch the meter or leads while doing this test, as before. Let the rectifier tube warm up, and read the DC voltage on the filter cap. IF this voltage comes up to between 300Vdc and 500Vdc, the transformer, rectifier, and filament wiring is good. If the fuse blows, the filter cap has a soft-failure under high voltage. Power off, disconnect AC plug and wait.

8. Resolder the rest of the high voltage network to the first filter cap. Repeat test 7. Still blowing fuses? Yes = it's HV power wiring related. Power off, disconnect AC and wait.

9 Plug in tubes. Repeat test 7. Still blowing fuses? It's tube related.

I see that one red is 23Ohms to chassis ground.  Is this okay because red/yellow (high power center tap?) is to ground?  The other red does not show 23Ohms to chassis ground

.
No, it's not OK. Find out what the connection is. Start by (as always, unplug the AC and wait, then) disconnecting both reds from the chassis, then measuring them to chassis. It could be a short to the core inside the transformer. But you're going to have to unsolder them to make sure you're not reading something accidental. Isolate. Divide and conquer.

And once again, please, please, please don't mess with this unless you already know how. Trying to do experiments where death is a possible side effect of any misstep is not a good way to learn.

[petemoore]

  Electronics...is it that hard ?
 Not really, if you already pretty much know what you're doing, if you'd asked me that when I'd spent days finding things that should not be, yes, absolutely frustrating.
 Did I fry stuff ? oh yes.
 Is it lethal ? Oh Yes !, but not at safe DC voltages like come out of Wall Warts, DC is what the amp uses.
 Understanding AC and DC, having built and debugged some circuits at non-lethal potentials, you stand a better chance pulling the amp out of the bag [or box, keeps the dust off] and fixing it safely, less expensively that using it as a learning tool, one lethal lesson is end of amp-game.
  Build then read some low voltage DC circuits and get:
  http://geofex.com/Article_Folders/Power-supplies/powersup.htm
  downpat.
  Does the amp have a grounded chassis ? [chassis connected to ground pin of power cable]/