How do I check transformers.

[ejfud]

I will be getting my 2 SCA35s toay from gogzhad. One of them doesn't work.

How do I check the transformers to see if they are functioning properly?

I would like to keep the working on for my work shop system.

Gary

[EWBrown]

First, remove all the tubes from their sockets, and then proceed thusly:

(Corrections made Friday evening)

On the Z-565 the color code is as follows: Blue: Plate 1; Green: SG 1, REd is B+ Center Tap, Green/White is SG 2; and Blue/White is Plate #2.

THe secondary is Black: 0 ohm (Common), Orange 8 ohms, and yellow 16 ohms. There is no 4 ohm tap.

The easiest way to pre-check the OPTs is to check the resistance from plate to plate (should be about 362 ohms DC) this will make sure the primaries aren't open. Actual DC resistance measurements, just made on a vintage Z-565: Blue to Red = 167 ohms, Blue/white to Red = 194 Ohms. From Green to red (SG to B+) = 42 ohms, and Green-white to Red (other SG) is 49 ohms. The secondary is low resistance, perhaps one ohm from black (0) to yellow (16 ohms) and about half an ohm from black to orange (8 ohms), The DC resistance is always lower than the impedance value - the Z565s are typically 7100 ohms CT primary.
If the OPTs are bad - which isn't very likely, the primaries will read either open or a short - or very low resistance.

As for the power transformer, first remove all the tubes, check the fuse to make sure it isn't blown, then measure the DC resistance between the prongs on the plug, it will be a fairly low DC resistance, but should not be a short. For this step, it is a good idea to replace the fuse with a 1 amp fuse, so it will blow quickly is there is any problem. Connect your meter across the secondary red leads, this will be about 660 VAC, so connect them with clip leads BEFORE plugging it in, set the meter to the highest AC voltage scale, and make sure that there are no short circuits.

If you have a variac, so much the better, then it can be powered up gradually. An alternative is to connect a light bulb socket in series with the AC power, which will limit the input current - a 50 to 100 watt bulb is good for this. Power it up and the voltage should read about 660 VAC, and the two filament windings will read 6.6 to 7 VAC each.
If the power tranny primary is shorted, or the rectifiers or filter cap are bad, then the fuse will blow, or the lightbulb will come on near full brilliance.

Alternatively, connect the negative lead of the meter to ground, and the positive lead to the end of the 50 ohm power resistor that is connected directly to the two silicon rectifier diodes. Set the meter for DC volts, and the 600 or 1000 VDC scale. Full DC voltage, without any current loading will be around 460 to 480 VDC (330 V * 1.4).

If all is OK, and you are using the lightbulb in series, it will briefly light up brightly, then fade out as the caps charge.

This isn't the most scientific and technically advanced testing method, but it can be done with a minimum of tools and test equipment.

Make sure that the capacitors are fully discharged before touching any of teh circuitry, the electrolytic caps can hold a hefty charge for quite a long time, and deliver a nasty surprise if one isn't careful.

If you have any doubts, don't be afraid to ask questions here, before going ahead.

/ed brown (edited to enter correct Z-565 DCR measurements)

[erichayes]

Hi All,

Ed, the light bulb might not be high tech, but it was required equipment for authorized Sony Service stations back in the '70s and '80s. Kept the turkey-techs from destroying a drawer full of replacement output transistors because they forgot to check the bias or differential pair.

[EWBrown]

Years ago, I made up a simple but vital piece of test equipment, forever to be known as the "chicken stick". Basically a 6 foot long 2X4 piece of wood, with an outlet box on one end, and a regular wall switch box on the other, and a ceramic light bulb socket next to the outlet box, and, of course, a line cord. The device under test was plugged on, a lightbulb (or a properly rated screw base fuse) was inserted into the lamp socket, and then, the device io be tested (or sometimes detonated) was turned on with the switch on the far (safe) end of the stick. This ensured an extra margin of safety, by having to be at least six feet away when the switch was flipped.

(Yes, I had one too many catastrophic capacitor explosions in my past).

Actually I remember seeing a similar arrangement on a test and repair bench when I was doing techie work in the army way back long ago (1969-1972). Sometimes the term "fire it up" was just a bit too literal...
Later on (1993-1996) when I was doing product safety testing, which involved intentionally creating malfunctions in power supplies, and making sure that they failed in a safe manner (no flames, gushing hot fluids or shock hazard) , this same technique was used, as sometimes the test resulted in a rather spectacular explosion, showers of sparks and hot electrolytes, and shooting flames.

/ed brown

[Shannon Parks]

....(Yes, I had one too many catastrophic capacitor explosions in my past).
.../ed brown

One reason I don't like oil capacitors. What a mess they can make!

Shannon

[erichayes]

Hi All,

Ed, I had a high school buddy who took the "Chicken Stick" concept one step further. He took an 8 oz fishing weight, some string, a couple of small pulleys, a paper towel core and a pushbutton switch, and rigged the whole mess up so the stuff he built was at one end of his garage and he was at the other (door) end.

My childhood projects had a much more straightforward final test: "Ma, I'm finished. Come turn it on for me."