Replacing a Solid State Rectifier

[Perkinsman]

Assuming you don't have the means of measuring AC Mains current, we're just looking to see if the transformer by itself has an internal short (drawing loads of current). if you have a Light Bulb Limiter, use it. With the secondary wires disconnected, so there's NO LOAD on the xfmr, it should draw very little current. The LIght Bulb Limiter wouldn't glow. I'm guessing the two GRN wires are one winding, the two RED wires a separate winging, and perhaps the BRN wire a C/T between the RED wires, though we'll find out. With your DMM, set it to OHMS, lowest range. If it has the ability to NULL the lead wires, short them and NULL, so you get zero ohms. If not, no big deal. Measure between the two RED Wires. If you get a resistance reading, it will be low, typically. And,if you do, see if there's resistance reading between either RED wire and the BRN wire. If no, then measure between the two GRN wires. If you get resistance, it will also be low, typically. See if there's any resistance between them and the RED wires. Then see if the BRN wire has resistance between it and the GRN wires.

If the transformer works, there will be AC voltage between the two RED wires, half that voltage if the BRN wire is a C/T to the RED wires. There should be AC voltage between the two GRN wires, and if BRN is a C/T to the GRN wires, you'd see half that voltage between the GRN and BRN wire. Position the wires so they don't short to each other or the chassis. Of course, it could be that the windings are RED & GRN instead. DMM will point the way.

For measuring the AC Voltage on the Secondary windings, start with the highest range, as if there is a tube circuit, you'd have much higher AC voltage than if it's all Solid State.

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With power on, I measured:

Primary measures 119.5vac.

red to red-0vac
red to green-0vac
red to brown- 0vac

I shut power off and detached all 5 secondary wires. I then measured resistance of my two meter probes- .2ohms

red to red: .6ohm
either red to brown: OL
either red to greens: OL
green to green: .3ohms
either green to brown: 1.8ohms

[nevetslab]

So, from the resistance measurements, the RED to RED winding should produce a higher VAC than the GRN to GRN winding. Odd that you're not getting ANY AC Voltage on either RED-RED or GRN-GRN windings. Not sure what to make of the GRN to BRN wire. If it was a Center Tap for the GRN-GRN winding, it would be half the resistance.

I suspect there is an open thermal switch in the primary of the power xfmr. With the power cord disconnected from the wall, the power switch turned on, do you get any resistance reading across the power cord's connector terminals? It should be a low resistance, like 10 ohms or less. If it's open circuit, then there may be an open circuit past the AC Mains fuse that you were replacing. Many products also contain a Open-On-Rise thermal switch inside the power xfmr. The cheaper ones use a part that doesn't reset, if the transformer heated up from a fault current. Only way to tell now is to see where the power cord/primary wiring goes to with regards to the power xfmr primary leads.

[g1]

It appears there is a protection circuit for the primary. Schematic attached.

[nevetslab]

I think he hasn't determined primary & secondary wires correctly. Single primary winding, fused, fed thru the relay-controlled circuit, while the secondary had three sets of CT windings. Good find with the schematic! 4 fuses on the secondary side leading into the regulators, and the one Mains fuse, no doubt rear panel part. We'll have to wait to learn if he's able to read a schematic and relate it to the physical unit.

[Perkinsman]

It appears there is a protection circuit for the primary. Schematic attached.

I measured the resistance between the prongs with the switch "on" and "off", both read OL. Would the Power Relay Unit be bad? Can it be tested?

[The Dude]

Even if the relay is not working, there is a .5 ohm 5W resistor in parallel with the relay, so you should still read some low resistance- not OL. Still, the switch could be bad, there could be connection issues, fuse could be bad, resistor could be open, etc. It would be best to check the primary resistance right at the transformer wires to verify whether it is open or not.

[Perkinsman]

Even if the relay is not working, there is a .5 ohm 5W resistor in parallel with the relay, so you should still read some low resistance- not OL. Still, the switch could be bad, there could be connection issues, fuse could be bad, resistor could be open, etc. It would be best to check the primary resistance right at the transformer wires to verify whether it is open or not.

I measured the resistance of the 2 white primary wires at .9ohms. These wires were attached at the main fuse & switchplug in the back of the unit with power off(see pic) The .5/5w resistor measured OL (see pic). I don’t know how to test the power switch on the front(see pic) of the unit.

[Perkinsman]

[Perkinsman]

Update: I replaced the detached secondary wiring on the rectifier and replaced the .5ohm/5w resistor on the Power Relay board that measured OL & it powered up!!!! it was on the dim bulb 150w but the bulb wouldn't dim...I shut it down and rechecked the 2 wires to the rectifier I had just reattached and found that I had one on the positive terminal so I reattached it to the correct terminal and powered it up on the variac. At 90v, the relay clicked and the unit came alive, and at 120v, it drew a stabile current of 1.6A...for a minute, then a crackle and smoke, quite a little bit too! I assume it was a cap on the power supply board but looked around and couldn't find a burned one.

So this is somewhat good news that it fired up but caused another issue. Any recommendations as to next steps?

[nevetslab]

As I see at least one edge connector in your first photo, and see on the schematics wiring connecting to the various circuit assemblies by way of a numbered terminal, if those are all terminals on the edge connectors, which allow assemblies to be unplugged, then you now need to find WHAT is drawing fault current.

I would begin with unplugging all of the PCB assemblies, leaving you with just the regulated power supplies, found in schematic #4. With all of the circuit assemblies removed, or, remove one at a time, and closely scrutinize each for signs of overheating / burning, since you experienced smoke. There WILL be evidence of where that came from, which is a plus in troubleshooting. If you DON'T find what smoked, then proceed ahead and check for shorted power transistors, diodes, signal transistors in the power amp circuit boards, as well as the other boards.

Those regulated supplies each have fuses following the transformer secondaries. So, you can also remove the fuses, and start with one power supply at a time, being the only load on the transformer. As you verify each supply to be either good (or not), lets see if we can first verify all power supplies are operational. If the supplies are good, and if you didn't find failed semiconductors, open emitter resistors and such on the power amp assemblies, then I'd try installing one power amp assembly at a time, to see if the fault current is in there. It will be a step-wise procedure until you find where the fault is.

If you have a power analyzer or current meter associated with your variac, that's a major plus....it can aid in finding the circuit board that's causing fault current. I gather it was major current, since your initial task was replacing a 16A Bridge Rectifier.

[g1]

Did you check that the new resistor is still good?

[Perkinsman]

I checked the new resistor & it’s still good but I’m the power supply board again, I found what I believe was the bad parts, a 1000uf cap & the F2 fuse. I replaced them, then tested the transistors & diodes, all good.... powered up, held for 30 seconds but then it sounded like the relay clicked & then another pop & smoke bomb!

[Perkinsman]

Btw, the new 1000uf cap was very hot

[Enzo]

Perhaps it is in backwards?

[Perkinsman]

No, it's in correctly but it's the replacement cap of the one that blew on the power supply so if it's getting hot too, wouldn't that indicate a resistor or short on the power supply board?