JVC QL-F6 power transformer replacement

[olato]

Hi Experts

My JVC QL-F6 platter have stopped spinning. I opened it up and measured the voltage across the
Power Transformer Secondary winding. Zero. Voltage on primary is 220VAC:
Per the service manual at vinyl engine that is supposed to
be 32VAC. I removed the power transformer and ohm´ed the primary and secondary. The primary is
around 110-120ohm, while the secondary is around 3.4ohm. That seems right, but on the other hand
no voltage, so it definitely point to the transformer being broken and need replacement.
And yes, Fuses are all good

Now, the service manual identifes this to be a JVC E03032-33C part. Did an ebay search without luck,
so I need a replacement. As this is supposed to be a 120/220VAC to 32VAC transformer that should be
relatively straight-forward. However, when I look at the schematics of the rectifier I´m wondering whether
this is actually a true 32VAC, but rather a VCT 16-0-16, and that the output from the transformer fed into the
rectifier is actually 16VAC? As the output VDC voltage is supposed to be 22-24VDC, my
recollection from school are that Vac == 0.71*Vdc after rectification (diode bridge).
So, that should correspond with 16VAC and NOT 32VAC as the service manual states.
This makes me wonder, should I get a replacement power transformer as either 32VCT (and only
use the middle output) or 16VAC?
For convenience I´ve attached the schematics.
Service manual: http://www.vinylengine.com/ve_downloads/index.php?jvc/jvc_qlf6.pdf

Anyone that can help shed some light onto this?


============
Maths:
Vac = Vdc x 0.71
Iac = Idc x 1.61
Pac = Pdc x 1.14
Rectifier circuit:

 

[whoaru99]

0V and 3.4 ohms does not compute in my book, but...

I would get what the schematic shows. I'm not a solid state expert, but perhaps the circuits immediately following the rectifier bridge are for voltage regulation. For voltage regulation, generally, the input voltage has to be maintained some amount higher than the output voltage (including troughs of ripple voltage and load sag) otherwise it falls out of regulation.

 

[olato]

Let me clarify: 0V was measured at the secondary (with 220VAC connected).
3.4ohm is resitance (through the transformer secondary winding) when power is not applied and transformer is removed
from the turntable/other circuitry (Don´t measure resistance when power is applied!!)
The ratio between secondary and primary voltage /windings in a transformer is: primary voltage/secondary voltage.
The ratio between winding/impedance for the primary/secondary windings are:
SqRoot(Primary Impedans)/Primary winding = SqRoot(Secondary Impedans)/Secondary winding
Fill-in we have sqrroot (120) / 7 = SqRoor(3.4)/1 -> which is 1.56 ~1.84
This seems relatively reasonable.

 

[whoaru99]

I'm not clear what you're saying is reasonable. My point is I would not expect to have 3.4 ohms and 0 volts. 0 volts on the secondary implies open circuit, not 3.4 ohms, does it not?

 

[olato]

Yes - I concur. One would have expected to not measure any resistance at all on the secondary if that is broken,
However, it can be that the issue is on the primary, and maybe there is a short circuit there

 

[ralphfcooke]

Did you measure the secondary winding voltage with it disconnected from the rest of the circuit? If not you may have a problem with the rectifiers/smoothing capacitors. Try testing the transformer in isolation and see what you get.
If it is the transformer then I wouldn't be too worried about an exact match, though the specification calls for a 32V transformer a 30V of similar size would do fine.

Good Luck

 

[olato]

I just tested the transformer separately, and it indeed measures ~36VAC on the secondary.

 

[ralphfcooke]

Now we're getting somewhere, the next things to check are the rectifier diodes and the smoothing capacitors; depending on your skill level and available tools, substitution might be the easiest. then check the transistor marked as X14 on the schematic.
The problem beyond this point is to work out which component has failed, often the failure is quite obscure; it took me two days to work out what had failed on my JVC QL-Y3F, and I've been doing this for around a half century

 

[olato]

A brief update - I mounted the transformer back in to the turntable, and now - platter rotates!! Little bit of a mystery to me,
Now, the platter does not stop rotating after pressing reject. I´m wondering whether that have to do that the Arm is not mounted
yet. We´ll see.
Also, is it normal that the rotating speed fluctuates a when platter is not mounted?