Power supply trouble shooting, input needed

[valley001]

This is branching from my Dana voltmeter repair thread.   I figure a separate thread may garner more attention. 

The transformer is heating up which has lead me to look into the PSU and loads.  The entire meter draws roughly 50 watts or so, which is actually 10 watts LESS than a similar unit (Dana 5500). 

I believe I have a problem with the 300v rail in the schematic below.  I have tested C7, it reads 60uf .7 esr or so. CR3-4 and 13-14 test OK in circuit.  C5 is at least not shorted, but I have not tested it out of circuit.  R1 reads 210K or so. 



Here is the unloaded wave form of the 300v rail, looks ok to me. 





Here is the 300v rail wave form under load  I think it is drawing in the neighborhood of 20 watts, perhaps a bit less.



My understanding is this drastic change in the wave for can be either a filter cap issue or a load issue.  I am wondering if anyone here can tell by these wave forms which of the two it might be? 

[PKTKS]

I would guess a load problem ...

and put a amp meter int that rail to check

2V of ripple on 300V line is a considerable power
there should be a current drain somewhere...

Paul

[Renate]

Mmm, I presume that's a 2V/division that I'm looking at, maybe 5.5 V P-P on the ripple of the 300V?

Let's do the math:
With a pure resistive load (which it isn't):
V = Vo * e^-(t/RC)
Let's presume that the diode is conducting for maybe 0.5mS? 8.3mS-0.5mS
7.8mS*-ln((300-5.5)/300)/50uF = 8.5kohm

That's all presuming that the 50uF is actually that.
If the cap has aged and gotten less capacitance the load is actually less.

300V^2/8.5kohm = 10.5W

Of course we can take the simpler route and just presume that it's constant current:
V = It/C
5.5V*50uf/7.8mS = 35mA
300V*35mA = 10.5W

[valley001]

Thank you for this reply.  I have much to study re the math.  This is a hobby and not my profession, but the math fascinates me and is something I will work on. 

So based on the wave forms this rail is supplying 10.5 watts.  Is this a reasonable load for this type of power supply?  I see evidence of resistors over heating in circuits associated with the 300v rail , so I thought maybe this could be due to the 5-6v ripple.  Should I replace the filter cap and hope for improvement on the ripple or is this misguided?  I scoured the circuits associated with the 300v rail for evidence of a short or bad component causing the current draw and have come up empty so far. 

It does not help that I do not have a schematic for this model, the 5500 is close but there are some major differences in the 300v circuits. 

Mmm, I presume that's a 2V/division that I'm looking at, maybe 5.5 V P-P on the ripple of the 300V?

Let's do the math:
With a pure resistive load (which it isn't):
V = Vo * e^-(t/RC)
Let's presume that the diode is conducting for maybe 0.5mS? 8.3mS-0.5mS
7.8mS*-ln((300-5.5)/300)/50uF = 8.5kohm

That's all presuming that the 50uF is actually that.
If the cap has aged and gotten less capacitance the load is actually less.

300V^2/8.5kohm = 10.5W

Of course we can take the simpler route and just presume that it's constant current:
V = It/C
5.5V*50uf/7.8mS = 35mA
300V*35mA = 10.5W

[valley001]

OK, im stumped.  The transformer is heating up with the 300v rail load removed entirely.  It heats up more slowly but it still heats up, I have not left it to run once it gets past 40 degrees Celsius.   Without the 300v rail the total power on the primary side is 23 watts. I dont know why it would heat up under this small load.   I have another similar unit with the same transformer running at 60 watts with no noticeable change in transformer temperature. 

I went back over all of my work checking and installing this transformer:

-It is a dual primary and I have it wired in parallel.  Each side of the primary measures 24 ohms and in parallel it measures 12 ohms. 
-I wired line power directly to the transformer and measured watts from both sides of the primary, they are the same. 
-I double checked all of the secondaries, none are shorted.  The winding for the unused 40v rail is not shorted and is unconnected in the PSU.


I am starting to think this replacement transformer is bad, but I dont know why. 

[Renate]

I was wondering if the primaries are correctly in phase, but I can't imagine anything working at all if that were the case.
And, if they are white/black like the schematic says it wouldn't be easy to confuse them.
That is, unless you mixed up some white/black of the 40V stuff???

It's a head scratcher.
I'd disconnect (and tape) all the secondaries and see how it goes.

If it's still doing bad stuff, disconnect one wire of the primary and measure the AC voltage between the loose lead and where it was connected.
(That is, you're running the transformer with no load on only one of the primary windings.)
That should be showing a couple of volts maybe, but certainly not 2 x line voltage = 240!
That would be showing that the phase was wrong.

[valley001]

When I checked the transformer before installing I ohm'd all 4 primary wires, 2 white and 2 black.  They read open white to white and black to black.  White to black I get 24 ohms for each set, white-white and black-black I get 12 ohms.  This, I assume, should be correct for 120 line voltage. 

There is also shielding around the leads for the primary wires that is connected to ground.  I checked between the primary winding's to ground and they read open. 

With all secondaries disconnected I get 1.1 watt draw measured from the line plug. 

With the PSU connected alone I measure 5 watts draw.

I was wondering if the primaries are correctly in phase, but I can't imagine anything working at all if that were the case.
And, if they are white/black like the schematic says it wouldn't be easy to confuse them.
That is, unless you mixed up some white/black of the 40V stuff???

It's a head scratcher.
I'd disconnect (and tape) all the secondaries and see how it goes.

If it's still doing bad stuff, disconnect one wire of the primary and measure the AC voltage between the loose lead and where it was connected.
(That is, you're running the transformer with no load on only one of the primary windings.)
That should be showing a couple of volts maybe, but certainly not 2 x line voltage = 240!
That would be showing that the phase was wrong.

[valley001]

Thinking back to my checking of the primaries I decided to re check the primary winding resistance, this time with my 3456A in 4 wire mode rather than the hand held.   

These are the results I get.

Primary 1: 21.418 ohms
Primary 2: 21.680 ohms

Is this enough difference to cause problems? 

[Gyro]

That small difference will be due to the slightly different winding diameters. It's the number of turns matching rather than the absolute DC resistance matching that is important.

One thing that may not have shown up in your testing is eddy current losses due to accidental shorts on the transformer laminations. Check things like physical damage to the lamination stack and hot mounting bolts.

[valley001]

Ill advised or not I have connected one primary and am slowly increasing load with a temp sensor on the primary leads.  So far I see a 20% reduction in power draw at the line plug..

No change, still heats up.  Under full load it was drawling less power though which is odd.  43ish watts rather than 50.

[valley001]

Here is the internals of the transformer.  It is in very good physical condition, no dents etc.  The core is assembled as shown, with alternating pieces stacked on top of one another. 

I could eliminate eddy currents by powering up the transformer and observing whether its the winding's or core that heats up correct? 

[valley001]

Hmm..

So the other similar meter that uses the same power transformer but draws 60 watts also has a cooling fan.  The one I am working on does not.  I decided to try placing a small fan blowing down across the power supply and transformer.  Now it seems like temperature is stabilizing.   

[Renate]

Well, the question is: how warm is too warm?
Is everything working normally?

When we were talking about the 300V, I missed the point that there are no tubes in this only, Nixies.
300V seems a bit overkill. It's enough voltage to wake the dead, let alone fire a Nixie.
How much current were you thinking of running these Nixies on?
You might want to change the value of the anode resistors to get some longer life out of the Nixies.
Those resistors are probably dissipating over 5W now.

They might have undersized the 300V winding.
What's the resistance?
What's (35mA)^2*R?
i.e. 800 ohms = 1W

[valley001]

I hopefully have the correct set of schematics coming, what I assume are the current limiting resistors are on a board that is not covered in the data I have.  These resistors are 1 or 2 watt carbon 130K resistors.  When the thing is running those heat up to 65-70C, maybe more.  I can feel the warmth rising off of them and the PCB is burned underneath. 

Speaking of nixie voltages, it may be noteworthy that the original nixies are all worn badly with a lot of sputtering deposit on the glass.  My understanding is this can happen with too high running voltage.  I have not poked around there yet with it running since I lack an appropriate schematic.  I suppose I could draw one.    I was thinking this 300v rail is the reason the transformer heats up, but it heats up even with the 300v rail disconnected. 

Well, the question is: how warm is too warm?
Is everything working normally?

When we were talking about the 300V, I missed the point that there are no tubes in this only, Nixies.
300V seems a bit overkill. It's enough voltage to wake the dead, let alone fire a Nixie.
How much current were you thinking of running these Nixies on?
You might want to change the value of the anode resistors to get some longer life out of the Nixies.
Those resistors are probably dissipating over 5W now.

They might have undersized the 300V winding.
What's the resistance?
What's (35mA)^2*R?
i.e. 800 ohms = 1W

[Renate]

Well, something is screwy here.
You've got 5 Nixies fed by 130k resistors.
300V/130k*5 = 11 mA (IF the Nixies were a dead short).
You're drawing maybe 35 mA? Did you double check that current to see if we're crazy?
Moreover, you're looking more like (300V-150V)/130k*5 = 5.5mA (SINCE the Nixies are running ~150V steady state).
Are you sure about that 130k? 0.5 mA per Nixie is very little.
Moreover, the power if it were 130k would be much less than would cook everything around it.
I'm thinking around 30k is the right value for 5 mA per Nixie.

[valley001]

OK, this may make more sense.  The resistors heating up to 65-70C are the decimal/function indicator neon current limiters and do read 130K. The current limiting resistors for the nixies are actually on the decade boards.  They are 20K, also get pretty hot but I have not measured their temp rise.   When I have a schematic I was going to measure voltage drops etc.

These are zm1032 nixies, should draw about 4ma at 170 volts or so. 

Edit- 

I took some voltage measurements.  The voltage at the 130K  current limiting resistors is 290v.  When a neon is engaged the measured voltage on the other side is 70 volts.  When the neon is not engaged the voltage is 1.6 volts.  All of the resistors get very hot, I guess I should lift the resistor and get some current readings? 

If the neons are drawing .0005 amp, that is .0005 * 220 is .11 watt power dissipation correct? Id think the resistors would not heat so badly only dissipating .11 watts yes/no? 

Well, something is screwy here.
You've got 5 Nixies fed by 130k resistors.
300V/130k*5 = 11 mA (IF the Nixies were a dead short).
You're drawing maybe 35 mA? Did you double check that current to see if we're crazy?
Moreover, you're looking more like (300V-150V)/130k*5 = 5.5mA (SINCE the Nixies are running ~150V steady state).
Are you sure about that 130k? 0.5 mA per Nixie is very little.
Moreover, the power if it were 130k would be much less than would cook everything around it.
I'm thinking around 30k is the right value for 5 mA per Nixie.

[valley001]

Here is the wave form of the winding for the 300v rail. 

[Renate]

The 130k should only be dissipating 285 milliwatt.
If those resistors seriously lost resistance that could account for some things.
You should be able to easily measure them in-circuit.
The same goes for the 20k on the Nixies.
In either case the Nixie/neon would be glowing more purple than a delicate red.

Could you measure the resistance of the 300 V secondary?
You should be able to easily measure that in-circuit.

[PKTKS]

Actually I would measure AMPERAGE in both points

before and after that filter... the waveform looks crappy

Paul

[valley001]

Focusing back to a load issue, here is my attempt at drawing the schematic for the decimal/function indicator board.  Unfortunately I do not have a proper factory schematic.  This board draws 10-12ish watts all by itself and never illuminates more than 3 neons at once.  Later today I should have a proper extender card and can take more precise current readings.  The 130K resistors get VERY hot, 75 Celcius at least. 

This drawing is truncated, the circuits for the remaining neons seem the same as L4-7 in this drawing.

[shakalnokturn]

I admit to not understanding this schematic, are you sure this is drawn right?

[valley001]

I admit to not understanding this schematic, are you sure this is drawn right?

I am not sure.  When I have more time I will go back through everything and see where I may have gone wrong.  Like I said this is my first attempt. 

Below is a schematic from a similar unit, maybe it will root out some of my errors.  I have several original DANA service manuals for various multi meters from this era and NONE of them cover this type of display board.  I am unable to locate a manual for this particular unit. 

DANA 5400 display board schematic:



DANA 5403 display board:

[valley001]

The 130k should only be dissipating 285 milliwatt.
If those resistors seriously lost resistance that could account for some things.
You should be able to easily measure them in-circuit.
The same goes for the 20k on the Nixies.
In either case the Nixie/neon would be glowing more purple than a delicate red.

Could you measure the resistance of the 300 V secondary?
You should be able to easily measure that in-circuit.

The nixies and neons seem to have their normal orange glow. 

The 300v secondary measures 72 ohms or so.