ampex 351 psu

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bluesbaz

bluesbaz

Well-known member
Joined
Jan 9, 2005
Messages
201
Location
San Francisco CA
Ive been using my converted ampex for a while but lately I keep hearing crackling. I've recapped checked wiring heated the traces and still crackling. Im now in the process of rebuilding the psu. The trouble is every time i try to connect the B+ power the transformer sounds like something is about to start boiling inside. Any why im turning my xformer into a baked ziti?

Screen%20shot%202010-12-07%20at%204.52.24%20AM.png
 
Hi,

Is it not blowing the fuse?

A Hand on the transformer will tell you real quick.  I just recently came close to cooking a PT - same crackling sound and the potting compound was literally starting to boil.

Sound like you have a short or near short somewhere.  You may have enough time to measure the voltage drop across R55 or R58 and get a current draw figure before something starts smoking. Those resistors will likely be getting very hot.


1) Pull the 6X4 and check DCR between 1/6 & 7.  They can short internally.

2) Check diodes in the filament supplies.

Ideally a variac with ammeter & voltmeter would be better than throwing full line voltage.  Measure drop on R301 & R60 to see if shorting is in the filament circuits.




 
First of all, thank you!!! for the quick response.
I have not included complete info regarding my intentions.
I would like to remove the tube rectifier and replace it with some 1n4007s
The First state of B+ is supposed to be 250v at the output.
That lead at junction R55/58 is a flyback to the output transformer.

Im looking around at replacing the transformer with a hammond I 'm stuck and could use some wise words.
I've got two 12ax7s and a 12au7 to feed.
I believe they will draw no more than 6ma the data sheet says 1.5ma at 250v.
The heaters look to need 0.9a so I should be good with 2a.
Im planning on leaving out the phantom section and lighting the bulbs from the heater supply
269EX 380V C.T.@[email protected]
http://www.hammondmfg.com/263.htm
http://www.mouser.com/ProductDetail/Hammond/269EX/?qs=%252bHhoWzUJg4JFeqMvbuL8ng%3d%3d

Hows that stream of consciousness sound?



 
Something is shorted or miswired.

>Ideally a variac with ammeter & voltmeter would be better than throwing full line voltage.  Measure drop on R301 & R60 to see if shorting is in the filament circuits.

I find a lamp limiter much easier to use on tube stuff, especially since there's a tube rectifier involved.
 
Butterylicious said:
Something is shorted or miswired.

>Ideally a variac with ammeter & voltmeter would be better than throwing full line voltage.  Measure drop on R301 & R60 to see if shorting is in the filament circuits.

I find a lamp limiter much easier to use on tube stuff, especially since there's a tube rectifier involved.
Click to expand...

VERY interesting.  I opt for a SS plug-in rectifier when pulling these up: takes tube rectifier warm-up out of the equation.  And don't take your eye off the ammeter.
 
Thanks all for the generous help.
This short is confounding because ive beeped everything and tested on a verobard mockup.
Butterylicious said:
Something is shorted or miswired.

>Ideally a variac with ammeter & voltmeter would be better than throwing full line voltage.  Measure drop on R301 & R60 to see if shorting is in the filament circuits.

I find a lamp limiter much easier to use on tube stuff, especially since there's a tube rectifier involved.
Click to expand...

Im gonna build one of these lamp limiters, Ive got most of the parts lying around, and I plan to purchase a Variac.

As for the short I've disconnected the transformer and hooked it up to the circuit below the xformer still heats up mighty fast with or without the ct hooked in to ground.
The only difference is I get 240 with the ct and 570 without it, both show a 10 volt drop across a 33Ω resistor strapped output to ground.
Theres also 400Ω between each leg and the ct dont know if that means anything.
Is all of this normal?
Screen%20shot%202010-12-07%20at%2011.00.20%20PM.png



 
bluesbaz said:
As for the short I've disconnected the transformer and hooked it up to the circuit below the xformer still heats up mighty fast with or without the ct hooked in to ground.

Screen%20shot%202010-12-07%20at%2011.00.20%20PM.png
Click to expand...


Yikes! Where is the CT in this scenario?

Your diagram shows a full-wave bridge rectifier! If that winding has a CT, grounding it will short out the winding good and proper.

The trouble is every time i try to connect the B+ power the transformer sounds like something is about to start boiling inside.
Click to expand...

I suspect that, for whatever reason, it's already too late. A partial short in the HT winding seems more than likely.

The best initial check for your transformer is to disconnect all secondaries completely, power the transformer up - and wait ...

If it still cooks then, it's cooked.

If a normal load on any of the windings causes crackling, it's cooked.

 
Even sans bulb limiter and variac, there should be a fuse on this rig.  1/8A would've stopped what just happened.

Second,  check the DCRs on the PT and see if you're getting  decent looking numbers.  Maybe ~300r on sec.

10V on 33r is 300ma(!) & 3W.  Not sure if you still had circuit load connected but if not, then the 33r is virtually a short to ground - which will heat up the transformer really fast.  i.e you just created a short with that 33r resistor to ground

The drops should be measured with load connected - values taken from the R1 & R2 series string resistors

The short or whatever was approximating it must be reliably identified.  Remove that 33r and run it with no load and keep hand on the PT.  If the PT itself is still ok - it shouldn't heat up under these conditions.

PTs can be very durable - I had one that was literally boiling up the potting compound and it has survived and still works fine.

 
Once again thanks a million for sticking with me on this. The help is invaluable

lassoharp said:
Even sans bulb limiter and variac, there should be a fuse on this rig.  1/8A would've stopped what just happened.

Second,  check the DCRs on the PT and see if you're getting  decent looking numbers.  Maybe ~300r on sec.

10V on 33r is 300ma(!) & 3W.  Not sure if you still had circuit load connected but if not, then the 33r is virtually a short to ground - which will heat up the transformer really fast.  i.e you just created a short with that 33r resistor to ground

The drops should be measured with load connected - values taken from the R1 & R2 series string resistors

The short or whatever was approximating it must be reliably identified.  Remove that 33r and run it with no load and keep hand on the PT.  If the PT itself is still ok - it shouldn't heat up under these conditions.

PTs can be very durable - I had one that was literally boiling up the potting compound and it has survived and still works fine.
Click to expand...

Im not showing a fuse but I do always use one, and I do appreciate the good advice and concern.
I went back to the supply board and found a bad cap which was indeed shorting out and replaced it.
I hooked everything back up and im getting 380vdc out of the psu with a .8v drop across R58 the 20k resistor or almost no current
I have the heater supply temporarily unconnected.
but the xformer is still hot as hell in 3 min
as for dcr im getting 800Ω between legs
ive checked for shorts across the entire system.
Anything else I can do to get this thing to calm down.


 
Don't forget your early attempts may have caused this transformer to fail. Disconnect everything from the secondary side and turn it on. If it still heats, it has shorted turns, and its junk time! And buy that Variac quick!
 
I have the heater supply temporarily unconnected.
Click to expand...

This scenario with any shunt load /bleeder resistor to ground will still cause heating


Is this with the new SS rectifier and all new filter caps?  If the 6X4 is still in, check it for shorts.

When you say you found a bad cap on power supply board - was this an older can electrolytic?

If so, it's likely other sections are going bad.  Sometimes they will not show the short condition until the HT DC is applied.  This can be easily overlooked because a DCR reading on the cap will look normal when the power is off.

Other than that,  with power off, try measuring DCR to ground  from each leg of PT  ( 7 & 8 ) and see if anything unusual shows up.  Should read infinite or very high with the SS setup and good caps.

 
Thanks again
The 6x4 i have is blown so pluging it in would be of no use. The blown 6x4 was the impetus for deciding to convert the 351 to an ss rectifier. The resistance of the b+ lines to the ct/ground is 530Ω and 550Ω different for each leg 1.3k to each other without the ct. the 12ax7 heater legs are both 3Ω to the ct and 6Ω to each other. The rectifier heater is @ 7mΩ and the bulb legs are @ 2Ω. these readings are the all the same connected or disconnected from the circuit.
How about those numbers?
I've also changed all electrolytic caps.






 
And the thanks keep coming.
radardoug said:
Don't forget your early attempts may have caused this transformer to fail. Disconnect everything from the secondary side and turn it on. If it still heats, it has shorted turns, and its junk time! And buy that Variac quick!
Click to expand...
Oh and the transformer only heats up when the b+ legs are used. I even tried using the heater legs of the transformer on the b+ section of the power supply which spits out 13v regulated from a cool transformer.


 
Actually, it just takes longer for the transformer to heat up... but connected to nothing the thing is 150f in 5min
looks like I need to find a new power transformer.
I need to understand some key concepts
If I understand thing correctly, I need 310 after rectification so i divide that number by 1.4.
Is that correct?
 
Try powering it up again with no load.  Should give the full DC 380V and PT should stay cool.  Verify that condition before connecting the load. If there are anymore old electrolytic PS decoupling caps downstream I'd be weary of those.  There could have been one there that shorted first and helped take out the main C1, C2 caps, rectifier etc.      

Make sure as Magneto Sound said, that the original transformer CT is not connected to anything.

Since this supply uses a PC board, take a magnifying glass and check closely for any stray bits of wire or solder bridging unwanted connections between traces.

If it still overheats with no load it may very well be the PT itself causing it.  If you have another tube PT laying around you could drop it in under no load condition and see how it behaves.


 
f I understand thing correctly, I need 310 after rectification so i divide that number by 1.4.
Is that correct?
Click to expand...


http://www.clarisonus.com/Archives/Ampex/Ampex351.pdf

Looking at this version 351 schematic they're showing 330V rectified DC.  Whatever PT that's closest to either should work fine.  The Hammond you originally mentioned would be too high though. More like a 117-0-117 or 230ish no CT iron.


 
lassoharp said:
f I understand thing correctly, I need 310 after rectification so i divide that number by 1.4.
Is that correct?
Click to expand...


http://www.clarisonus.com/Archives/Ampex/Ampex351.pdf

Looking at this version 351 schematic they're showing 330V rectified DC.  Whatever PT that's closest to either should work fine.  The Hammond you originally mentioned would be too high though. More like a 117-0-117 or 230ish no CT iron.

I tested the Transformer at 630v but not sure if that's high as it's unloaded.
I would like to be sure and go to 310.
How does the whole heater supply thing go? Everything I'm finding has 6.3v with or without taps does everyone just use a voltage doubler?
Click to expand...
 

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