Transformer Diagnostic.

MyLastWord

Active Member
Is this transformer went BAD, when the resistor value on the primary side (say 2.9ohm) lower or equal to the secondary sides (say 2.9 and 5.4ohm)? This is the transformer from a dead Pioneer SX-980.
 
Those resistance readings are suspicious but don't confirm a transformer defect by themselves. It's best to open-circuit the secondaries, energize the primary, and check secondary voltages. If it lights up a dimbulb tester or blows fuses with no load on the secondary, then it surely has an internal short.
 
Yes, I check using DMM with the transformer out of circuit (both Primary & Sencondary). I am going to check for secondary voltage next.
 
I will assume for a moment that you know what you are doing, but have you checked the fuse(s)?

A power transformers primary will usually read a couple of ohms or more. If its not blowing fuses or smoking then its probably not shorted. If the primary was open, then you would get no reading and it would definitely be bad.
 
Base on the schematic of the Pioneer SX-980, the fuse (6A) for the primary was blown. I check in circuit for continuity, resistance value of the the primary and secondary wires (at the posts before the fuses). I also try to check for the voltage reading to the lamp and if I remember correctly, it was reading something like 4.3V instead of ~8V. So I remove the TF out and took some resistance reading value of the the primary and secondary wires. That's where I am at. That's why I am puzzle that the primary is lower than the secondary. If no reading, I would have known it's dead. Should I or could I continue to read the secondary voltages? Also, is the 2.2M R1 resistor shown here, build-in to the TF?

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Went home and checked, the R1 2.2M is external. Took the some secondary voltage measurement of the stand alone TF, they are seem to be all in order. The TF should be OK.
I will get replacement fuse and put the TF back some time later. Thank you everyone for your help.
 
I have put the TF back and got all fuses in places. I am attaching the snips from schematic of Pioneer SX-980 service manual for reference. This is what I have done to this board:
- Replaced C11, C12, C15, C16 and C23.
- Replace Q1, Q5 2SD712 with KSC2073, Q4 2SB682 with KSA940.
- Replace D1, D2, D3, D4, D6 and D7 10E2 with 1N4402 (may need to replace with 1N4404)
- Replace D12, D13, D14 and D15 SR3AM-8 with 1N5404.
Power it up, immediately blew 1A fuses FU5 and FU6. According to the schematic, I should get about 24VDC after D6&D7 before the 10ohm R9.
-What kind of VAC reading should I get at post 12 and post 13, relative to the ground (metal frame)?
-Could I the same method to check for VAC reading at post 10, 11 and 5, 8 and 6?
-Are the replacements 1N4402 the cause for blowup the fuses?
I think this will help me to see if the TF is bad.
Thanks..PS Schematic.PNG PS Board.PNG
 
You need to learn to isolate circuits, suggest isolate the transformer secondary in order to determine if it has issues. Any component in the secondary circuit could cause the secondary fuses to blow.
the ckt connected to 12,13 is a voltage doubler, check/change C23 (2200uF/35V) and if that does not do it, check the rest of the parts in the 13VDC regulator.
 
I think you may have added to your issues.
Build a DBT, learn it's use. Stop blowing the expensive fuses. This process can damage even more of the circuits.
Recheck your 13 volt supply circuit for mistakes in component placement.
Remove all the fuses and using the DBT check and see if you still have a short.
It might be a blown output and damaged amp causing your original issue. You might remove the connections at the big filters to isolate the outputs. Can't remember if their screw types?
 
Yup! I have been put off building a DBT for so long. I think it's time to build one to do proper diagnostic. I will return to this and post the finding.
 
A DBT can be put together for less than the cost of a box of fuses, so it even makes economic sense.
 
You need to learn to isolate circuits, suggest isolate the transformer secondary in order to determine if it has issues. Any component in the secondary circuit could cause the secondary fuses to blow.
the ckt connected to 12,13 is a voltage doubler, check/change C23 (2200uF/35V) and if that does not do it, check the rest of the parts in the 13VDC regulator.
@rcs16, Forgive me for contradicting you, but the circuit connected to pin 12 and 13, comprising of D6, D7, C23 and C10 is a +24vdc full wave rectifier driven by the green leads and grounded brown center-tap transformer secondary. This is followed by the +13vdc regulator.
 
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So after I built the DBT, and connect the receiver (SX-980) to it to check some voltage reading. Upon power it up, the bulb went bright then dim. Recheck vac of pin12 and 13, each measure at about 26vac. Then measure the voltage at D6, D7 and it only show about 9.5vdc, not good..
 
VAC between pin12 and 13 is about 33VAC. VAC between Pin5&8 and Pin6&8 is about 32VAC, between pin10 & pin11 is about 88VAC.
 
So after I built the DBT, and connect the receiver (SX-980) to it to check some voltage reading. Upon power it up, the bulb went bright then dim. Recheck vac of pin12 and 13, each measure at about 26vac. Then measure the voltage at D6, D7 and it only show about 9.5vdc, not good..

Are both fuses FU5 and FU6 intact? Check with a meter, not visually.

Are both diodes intact? Test with power off.

Is C23 intact?
 
Continuity check for FU5 & FU6 are good (new fuses), Diode test for D6 and D7 (replaced with 1N4002s) are goods. C23 should be good also, as it was replace (2200uf/50V).
 
If that is all correct, there isn't much beyond that but the 10Ω resistor and the regulator transistor array. If that's all intact, start measuring the loads on the 13V and 7½V supplies (in mA).
 
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