Help with replacing a vintage tube reciever power transformer

nabeelsayegh

AK Subscriber
Subscriber
Hello AK...

I have a Pioneer SX-800 that has some very loud buzzing coming directly from the transformer. The unit has gone through a full recap and has had a number of other things done to it like replacement of power resistors and such. On my variac, the PT is fine until you get past say 105VAC. At 110 you can start to hear it buzz and at 115 or better, it pretty much annoying. Part number on the transformer itself is an AT-5296. Output transformers (which i don't think have anything to do with this) are AT-6133. Anyone know where i can get replacement PT's from?

(BTW....i already went down the path of checking grounding and tightening bolts on the transformer....none of that helps.)
 
You might not want to go into this much depth, but you could always disconnect and un mount the power transformer, pull off the end bells and see if the entire winding wrap moves or feels loose. You could probably use a wooden shim to tighten it up. I found a wooden shim inside my recently destroyed output transformer so its probably safe to use.
 
Hello AK...

I have a Pioneer SX-800 that has some very loud buzzing coming directly from the transformer. The unit has gone through a full recap and has had a number of other things done to it like replacement of power resistors and such. On my variac, the PT is fine until you get past say 105VAC. At 110 you can start to hear it buzz and at 115 or better, it pretty much annoying. Part number on the transformer itself is an AT-5296. Output transformers (which i don't think have anything to do with this) are AT-6133. Anyone know where i can get replacement PT's from?

(BTW....i already went down the path of checking grounding and tightening bolts on the transformer....none of that helps.)
Heyboer transformers. Is the place google them and contact Alden
 
For an exact replacement you would need to source a donor unit.

One alternative would be to have the current one rewound from someone like Transformer Rewinding Service, Heyboer etc. Transcendar does replacement AT-6133 OPTs so might be worth inquiring if they would do the PT or rewind for you.

Another alternative could be to look for a transformer with similar characteristics (e.g. a different receiver that runs 7868 tubes, such as Fisher 400, Sherwood 8000, might have similar specs but might need tweaks for different power supply design or tube set, or might need enhancing such as with separate filament transformer.

However, to expand on Cademan's point, have you taken electrical measurements and determined that the unit is working within design parameters & not overloading the PT when getting to full voltage. i.e. determining it is definitely an electrical issue with the PT rather than electrical downstream issue or a mechanical problem (even though you have tightened the end bells).
 
One thing I have pulled all of the tubes out of the system and disconnected the feeds going to the bridge rectifier and i still get a buzz. When i measure voltages at all test point locations on the secondary side of the power transformer they all measure higher than what is notated on the schematic. (My area i have high line voltage....around 125VAC so i would expect to get higher numbers. I got a variac and dialed it down to 115 and the numbers did measure closer to schematic spec. So i have to believe i there is nothing generating stress to cause this issue.) if you have any suggestions of what other specific test i should do to ensure it is not an internal issue...i am open to it.

By the way, on trancendars site....there is nothing i see there on how to contact them or order parts. Anyone have info there?

I will definitely look at heyboer and find out what they can do as well. My fear of getting a donar system is 1...it is going to cost a lot and 2, i do not know what i am going to get. Could be buying one with worse issues.
 
Transcendar used to sell on eBay and that was a way to contact them. From this recent thread it seems AKer dallen9494 recently emailed, so perhaps you could PM and ask for the contact info.

Transformer Rewinding Service is here. I have no affiliations with any of these.
 
As has been mentioned,you have to first determine that the noise is not being caused by an electrical overload.Are you experiencing any overheating or the associated smell,and are (all) the operating voltages correct? If everything is ok from an electrical standpoint,you are in luck,as there are several options available,depending on the type and severity of the mechanical noise.

As has been mentioned,a tapered wooden shim (I use the type intended for shimming during door and window installations) between a loose bobbin and the lamination stack is an old and good trick. I find that using several shims and coming in from all sides to be the most effective.

Another trick,when the noise is minimal but still intrusive,is to stuff the endbells with a foam-rubber material.I use a specialized 3M product,but anything used for automotive applications should be suitable. Hell,you can even use old sofa foam if you're hard up!

My last trick requires some ''messy'' workspace area and especially good ventilation. Basically,what you are going to do is try to encapsulate the loose windings by impregnating them with varnish. I use old paint cans for this,and a varathane floor varnish,which has been severely over-diluted with an appropriate thinner to get it really thin.
You immerse the transformer (obviously just the stack,without the endbells!) and let it sit overnight or longer in the sealed can. In the morning,open the can,and hang the transformer by it's leads to drip into the can for a while.If you like,you can also wipe down the stack to remove excess varnish.

Just to add,I have a very good vacuum pump that is used in my self-made ( Loricraft copy) record cleaning machine that I connect to a fitting in the can lid to help ''vacuum impregnate'' the windings.This step is not absolutely necessary,and I only do it because I have the pump on hand and feel that it adds to the quality of the job.I may be a little fussy.......:rolleyes:
 
I have one, AT-5253, from a SX-800A (used 7868s, so current rating is likely higher, though voltage is about the same). About EI150 size, 4.5" or so in long dimension.
 
Actually, I don't think the AT-5253 is the right one, as it is tagged "550VCT" and seems to have a 5V winding too. But the one next to it might just be the SX-800A - it isn't marked, but looks Japanese from color codes. I can measure it. Is a bit rusty...
 
couple of points.

if you can unmount the trafo, try different material compression "washers", and perhaps
even rubber strip underneath as you bolt it back.

since it appears to be a voltage issue, then add a bucking trafo to get it under the buzz-floor
and adjust the PS resistors to re-deliver the required voltages to the respective stages,

there may be a reason for the buzz, that the replacement will also symptom, assuming
its a perfectly working replacement.

I once pulled a working disk brake caliper, mounted a new one, it was perfectly
brand new but the piston popped out as I tested the brakes, scared the crap out
of me for about a year, and replaced it with a new one that did work.
 
Well, i guess i have nothing to lose more than time. One thing of concern to me is that this power transformer, like most tube receivers i have worked on....get pretty warm when operating for a long time. Wedging in wooden pieces seems like it would be a fire hazard to me. Not suggesting this is not a common practice, just trying to reality check if it is a good idea or not. Again...i am 99% sure the issue is with the transformer. I did consider a bucking transformer, but would really like to have this guy stand on it's own two feet; or in this case, on its own windings ;)

Wish me luck.
 
If this helps, according to the schematic, the power xfrmr is a dual primary 1.8115/0.9A 230V. Secondary's are: 6.3V - 3.1A, 6.3V - 5.2A, 5.5V - 1.2A, 165V - ?A, 26V - 0.3A
 
After reading through this thread, DC offset on the AC mains line came to mind. I believe light dimmers and flat screen TV's on the same circuit can introduce this "DC on the AC" supply voltage and cause transformers to buzz. I read some article a few years ago calling this DC offset the silent transformer killer. The DC simply heats up the windings and messes with the magnetic fields and causes buzzing.
Have you tried this unit on a different circuit in your house or looked for any DC riding on the AC with a scope?
I believe an isolation transformer before the mains is the solution.
 
Well, i guess i have nothing to lose more than time. One thing of concern to me is that this power transformer, like most tube receivers i have worked on....get pretty warm when operating for a long time. Wedging in wooden pieces seems like it would be a fire hazard to me. Not suggesting this is not a common practice, just trying to reality check if it is a good idea or not. Again...i am 99% sure the issue is with the transformer. I did consider a bucking transformer, but would really like to have this guy stand on it's own two feet; or in this case, on its own windings ;)

Wish me luck.

Considering that many many millions of transformers used cardboard bobbins and paper insulation between the windings,I don't believe there's much to be concerned about.
If your transformer is operating at anywhere near the ignition temperature of wood,you have far more serious issues to deal with,and the transformer would have gone up in flames long ago,regardless of insulating materials used.
 
After reading through this thread, DC offset on the AC mains line came to mind. I believe light dimmers and flat screen TV's on the same circuit can introduce this "DC on the AC" supply voltage and cause transformers to buzz. I read some article a few years ago calling this DC offset the silent transformer killer. The DC simply heats up the windings and messes with the magnetic fields and causes buzzing.
Have you tried this unit on a different circuit in your house or looked for any DC riding on the AC with a scope?
I believe an isolation transformer before the mains is the solution.

This was my first thought, as well. I have a client with a Rogue Audio Zeus amp, that's gone through this ever since he's owned it. He measured several volts DC on the line, and confirmed the buzzing was caused by this, when he plugged in an Audioquest Niagara 7000 power conditioner (funny enough, the buzzing then MOVED from the transformer in the Zeus, to the transformer inside the Niagara!).

He's actually planning to go to the extent of wiring a big isolation transformer in, at his breaker panel, for the circuit the stereo uses- so that the buzzing can happen THERE in the isolation transformer (on the other side of the house), instead of in the room, at the amplifier...

Regards,
Gordon.
 
The OP's variac should provide isolation.

If someone has DC or significant AC across the panel Ground/Neutral then they need to get their panel groundings and line neutral lugs in the panel and at the meter tightened, cruddy ground rod, water pipe clamps checked.

I'd also check the breaker lug for good tightness. I could see how a loose hot line could cause AC noise under load.
If his variac has a ground, it won't isolate completely.
He needs an ungrounded (2 tine plug) isolation transformer. No way DC can pass thru that.
I have one for testing purposes only. It's an inexpensive tool useful to have around.
 
I could see how a loose hot line could cause AC noise under load.
Not talking about AC noise. I'm talking about DC riding on the AC causing the transformer core to act like an electromagnet with a fixed north and south pole. The core's not supposed to do that. The primary side is supposed to create an alternating magnetic field that induces an alternating EMF into the secondary side. The magnetic pole flips at 50hz/60 hz to do that. A fixed DC voltage from the primary side wrapped around the core will turn the core into an electromagnet no different than wrapping a wire around a nail and connecting it to a battery. This causes heat which can damage the insulation of the transformer's windings.
This is why DC riding on AC is called a transformer killer...it causes unnecessary heating of the core and the insulated wires that make up the transformer winding's. Eventually, the insulation on the winding's will fail causing a transformer short circuit and death. The added stress can cause a transformer to hum.
The safety ground is not tied to the variac secondary.
Not necessarily true. It depends on the Variac.

This is bad.


Variac.jpg

The input and output ground sockets of the variac appear to be connected to something, possibly to the cabinet. Same for the RFI filter. The schematic says they're connected to the house ground.

At the service panel (fuse box) the Neutral buss is connected to the Ground buss.
Therefore every neutral wire of every wall outlet in the house is tied together as well as to ground at the service panel. The neutral on the variac's input and output are therefore directly connected via ground in the service panel. No isolation at all as DC can still travel from anywhere in the house along the neutral through the ground buss/Neutral Buss connection in the service panel. Every neutral in the house is connected regardless of whether or not the outlet or appliance has a ground plug.
In the diagram above the RFI filter only passes AC noise to ground via a capacitor. Since capacitors block DC, no DC on the primary side can pass to ground. Any DC on the primary side (can be neutral or hot wire) will therefore magnetize the core like a nail.
Grounding the transformer's core only means if a wire loses it's insulation and contacts the core and goes to ground, the fuse will blow. This is an example of transformer death. The insulation on the windings has failed.

This is still not good.
isolation-transformer-wiring.gif

Neutral is still connected to ground at the service panel. If the AC output in the above diagram has a ground, it's still connected to neutral. No isolation.

This is ideal.

main-qimg-3efc1a4b00a76c5995f306134abe03e0

A two wire isolation transformer. No pathway from primary to secondary for DC to travel. The Secondary is pure AC. If any DC is riding on the primary side, only the isolation transformer will be at risk, and possibly hum. No ground for DC to travel via the Neutral to Ground buss connection at the service panel. No way for DC to pass even if a light dimmer or flat screen TV puts DC on the Neutral/Ground or hot wire.

If a variac has 3 wires in and out...I don't see how it can serve as an isolation transformer if the output ground is connected.
 
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The OP's variac should provide isolation.


Not necessarily.

Many variacs are autotransformers- meaning the output is simply tapped off of the primary. If so, then at least some of the DC on the input, may wind up on the output, since there's a direct connection between input and output.

You need a true transformer- separate primary and secondary, not connected- to block DC.

Regards,
Gordon.
 
If a variac is an isolation transformer it still isolates the amp that's plugged into it even if the amp has a 3 -wire cord and the safety ground plugs into the variac.
Right. But I think we've established that not all variac's are isolation transformers, some are in fact autotransformers.
If any device in the home dumps DC onto the neutral for that circuit, then it will be grounded to 0v at he panel if the ground rod connections are good and won't go any further.
Right. I highlighted the operative phrase here:if the ground rod connections are good. Realistically, more than likely they are not. Corrosion between the rod/clamp/and ground wire is not uncommon. Additionally, soil moisture around the ground rod(s) plays a part as well. Old code in my area was one ground rod, new code is two ground rods.
Ever read about or get the impression a stereo sounds better after a good soaking rain? Ever wonder why?

What kind of device, if working properly, would ever rectify/filter a primary AC voltage input and mix that DC back onto the hot line to go back to everything on the circuit's phase buss,
I'm going to quote Nelson Pass for this one:
Quote from Nelson Pass:

"If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer."
"The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise."

In case you aren't sure who Nelson Pass is, I think you should google his name because you should be impressed. If he say's DC can ride on AC and cause a transformer to hum....who am I to challenge the above quoted statement?

The statement
"The hum comes usually from toroidal transformers, which
saturate easily with DC"
simply means toroidal transformers are more prone to saturate than conventional transformers. This propensity to saturate might be the reason why toroidal transformers have not overtaken conventional transformers in high end gear.

From another source with explanation:
DC off set is caused by the half wave bridge rectifiers used in all switching power supplies that draw current in uneven pulses .. this drives the sine wave off it's proper zero degree crossing point when the phase turns from positive to negative in the cycle.
The humming in the transformers is caused by the transformer drawing an extra cycle and the plates snapping back ... but the real issue is the loss of efficiency your equipment experiences.
Just about every appliance in your home including that flat screen TV ... cable box all your computer devices and some of your audio equipment uses switching power supplies and cause this problems with the harmonics they create
this drives the sine wave off it's proper zero degree crossing point when the phase turns from positive to negative in the cycle.
This means the height of the sine wave above the zero degree crossing point is different than the height below. In effect, there is a constant DC voltage on the AC because the point where the phase turns has moved.


In my opinion, if you really want to rule out the possibility than any DC is riding on the AC and causing the transformer in the device under test to hum, the most straight forward to understand, easiest and most cost effective method is to use a 2 wire isolation transformer. It isolates the device under test from the rest of the house's wiring.

To offer an alternative to the OP's original question "Help with replacing a vintage tube receiver power transformer"

I believe a proper Isolation Transformer is a much cheaper and easier way to determine whether or not replacing the original vintage power transformer is necessary or even appropriate. It's another diagnostic tool for your tool box that might just pay for itself more than once.

main-qimg-3efc1a4b00a76c5995f306134abe03e0
 
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