SCA-35 output transformer testing

[Raco]

Hi all,

I have an SCA-35 which I got about a year ago, but ever postoned the project of starting restoration until a couple of weeks ago.

At this point I don't even remember what was claimed to be working - I got this off of ebay - but I started assessing the state of various things in the order of importance or criticality in the functionning of the component.

After making sure the PS supply is in good shape (I'm using an SDS labs board), I took off all of the OTs connections, and measure their sections in DC, comparing from left to right for relevance.

They all match, except for one section - BLU/GRN measures about 125ohm DCR on one channel while the other is at about 280ohm...

I can think of two types of issues with transformers - open winding (DCR is infinite, so not this case), and shorted windings (and then DCR would differ like I'm observing, as they can short out across sections etc., though not likely).

What do you guys think? Do I have one bad output transformer? I am not sure what else would be recommended to rule out any problems on the OTs. I could use an impedance meter I have (C use), just to jump up testing one step from DCR to a more real world test.

I'd be very upset to discover that an OT is bad - these are the cloth early type. Not sure of Q differences, but I prefer them. But heck - gotta figure out what's going on.

Many thanks for all thoughts and suggestions!
aR

[dcgillespie]

Raco -- Two thoughts:

1. Are both transformers of the same manufacturer, or (for example) has one been replaced? I'd do a careful visual inspection. Do they appear to have ever been taken apart?

2. Check the turns ratio of both transformers to see if they match. Use a variac and your DVM to determine this.

The difference is significant enough that if both are from the same manufacturer, something could likely be wrong with one. If they are of different manufacture however, it may be they are both fine.

Dave

[Raco]

Dave - I was actually hoping you'd pitch in, as I know you have a lot of experience with these Dynaco's...
Some answers below.

Raco -- Two thoughts:

1. Are both transformers of the same manufacturer, or (for example) has one been replaced? I'd do a careful visual inspection. Do they appear to have ever been taken apart?

Not really, they have the same imprints ("Z565"), seemingly Dynaco original manufacture, even the loss in color of the cloth seems to match ('blu' and 'grn' are just as 'identical' left to right... I had to label them as I cannot tell them apart).

2. Check the turns ratio of both transformers to see if they match. Use a variac and your DVM to determine this.

I might endeavor in hooking up my variac on the windings and measure at the other end, though I'm not at all sure of the methodology I'm expected to follow. Can you elaborate on this?

The difference is significant enough that if both are from the same manufacturer, something could likely be wrong with one.

Pfff... yeah. The one thing I find extremely confusing is that ALL other sections measure identically L/R. I think this thing alone rules out the different manufacture hypothesis (unfortunately...). Assuming that another manufacturer used a different gauge wire for the windings (for instance), the DCR should mirror across sections, at least over symmetries of the circuit.

If they are of different manufacture however, it may be they are both fine.

I'm losing hope...

It has been suggested to me (AA inmate and friend) that I make sure the drives are fine (usual suspects cleared, like coupling caps impedance-measured and replaced, resistors checked for drift, etc.), then variac-power it up with a known-good fuse in place, and then check for the stage to operate correctly.

BUT: to me, this DCR discrepancy is pretty disheartening. They SHOULD match - isn't that a fact? I am looking for an audit of my thinking process here...

If one would be bad - what have other people done? I'd hate having different OTs L to R. Triode Electronics - the one source I know for replacements, luckily located in the same town as I am (Chicago) - sells them at an arm and a leg pricing. I'd rather buy two, though, and sell the original Dynaco (known good) I have. I could also try to acquire another one like the good one I have (cloth out). Anyone has one for sale?...

Thanks much, Dave, for your input.

Best regards,
Radu.

Dave

[Raco]

"drives" = "drivers"

[Ty_Bower]

I'm left wondering why you are measuring from the plate to the screen tap. I would recommend you measure from the B+ lead across each of the primary sections. I think the B+ is the faded red wire.

It sounds to me like you missed a stripe somewhere, and are actually measuring from the plate connection on one side over to the screen tap on the other side. There is some data describing the expected typical DC resistances for the Z565. You'll find it in this thread:

http://www.diytube.com/phpBB2/viewtopic.php?t=4169

[Raco]

I'm left wondering why you are measuring from the plate to the screen tap. I would recommend you measure from the B+ lead across each of the primary sections. I think the B+ is the faded red wire.

I will repost measurements for B+ (red wire) to other sections, but that doesn't delete the issue of why a certain section ("BLU" to "GRN," or "plate" to "grid2") measures asymetrically. I actually detached the OT from the circuit, so when I say "plate," for instance, that is just for reference, and in fact I mean "BLU" wire.

It sounds to me like you missed a stripe somewhere, and are actually measuring from the plate connection on one side over to the screen tap on the other side.

I am certain I measure on one single transformer across either primary or secondary winding on the same section. I hope I'm not missing anything major. In fact - I hope I am! It would be cheaper than replacing the transformer!...

There is some data describing the expected typical DC resistances for the Z565. You'll find it in this thread:

http://www.diytube.com/phpBB2/viewtopic.php?t=4169

Thank you very much! I will read this.
Thanks much for your input,
Radu.

[dcgillespie]

As Ty mentioned -- make sure you are measuring between the same wires between the two transformers!

If -- on each transformer -- you leave one meter lead connected to a known plate lead, and then measure to the screen tap for that plate lead, then to the B+ lead, then to the other screen tap and finally to the other plate lead, your readings should increase with each step. They may not be identical between each transformer, but a given reading should easily be within say 5 ohms of the same measurement taken on the other transformer.

If you are still getting readings well off from each other between the two transformers, and you are sure of what the leads represent that you are measuring, then we can go on to checking to winding ratios, which is easy to do. Let us know!

Dave

[Raco]

(as a physicist by background) I am very methodical - I only measured either between strictly same color-coded taps (GRN-BLU left to GRN-BLU right, for instance), or across one "end" of a transformer and similarly coded subsequent taps, hoop by hoop. I am sorry, but am not sure what the criticism or warning is...

If we assume that I was well organized and measuring throughout consistently between equivalent taps, DCR is a pretty consistent factor in deciding that a trafo is defective, if sections present discrepancy - right?

I think I'm trying to emotionally salvage an original Dynaco cloth transformer, and you guys are helping... A transformer which does not stand reasonable analysis.

I think you guys are wonderful... But be blunt.
Thank you very much,
Radu.

[dcgillespie]

Radu -- No criticism of your method was intended. It's just that in trying to help solve an issue by written communication with someone who you have no idea of their skill or diligence level means that often, the very basics of methodology and accuracy must be emphasized so we can be assured that the facts we are dealing with are real. That was the sole intention here.

Taking the facts then as they are shown to be, the transformer measuring 280 ohms between the blue and green leads is the unit of concern. Because the concern is for a reading that is too high rather than too low, that is the reason I asked if we are dealing with transformers of the same manufacture. Given that we are, and accepting that the transformer in question was manufactured correctly to begin with, then a reading that is at least double the expected reading clearly implies that a portion of the winding between these two leads has burned open, and the area carbonized around the fault to still allow a level of continuity to be had with your ohm meter, albeit at an elevated level. Given the nature of the implied fault however, it is likely that in actual service, the transformer would deliver very low if any actual plate voltage to the plate of the tube the blue lead is connected to, which would be very damaging to that tube since it would allow screen voltage to be present without the appropriate plate voltage. Under those conditions a tube can be destroyed in a matter of moments. Therefore, if you do attempt to try this transformer in an operating circuit, do so first with tubes of little value due to the potential damage that can occur.

As a double check of my own thoughts here, I just went and measured two authentic Dynaco Z-565 transformers between the blue and green leads for you and found them both to measure between 124 and 125 ohms, backing up my concern that it is the transformer reading 280 ohms between these two leads that is at issue.

At this point, it would be good to remove the end bells of the suspect transformer and see if any visual damage can be seen, or if it has any burnt odor about it.

Given the facts, this is likely the scenario at hand. I wish it were otherwise as very likely, a replacement transformer will need to be obtained. Good luck with your hunt, and your project!

Dave

[Raco]

Dave & Ty - I believe criticism is a good thing!... I was only (unskillfully!) conveying that I was confused due to not being able to identify an error in my work. I am here to receive clear straight-forward feedback, which is exactly what I'm receiving. Many many thanks for that. Never even came close to take offense during this conversation - on the contrary!... I hope you guys share this with me.

You are confirming my suspicion. You are also confirming the DCR value given by an AA inmate. I will take the transformer out and do a physical assessment of it.

I will keep you guys posted.
Thanks much!
Radu.