Fisher X-100 trouble help

MickeySmile

Well-Known Member
I have recap all the filter power supply capacitors as well as in signal paths.
It ran good for few week then now the fuse blew. I take off the 5AR4 out and put in a new fuse, when I turn on the fuse blew again. Is it the PT short?
Please give me advice.
 
It could be, or something else shorting. I don't have the diagram but maybe there is a selenium rectifier in there and if that failed it could blow the fuse.

Removing the rectifier tube proves that the problem isn't in the high voltage section. Post the circuit and we can make some guesses.
 
It doesn't have selenium rectifer, it uses rectifier tube. I did remove rectifier tube (5AR4) and all other tubes, put in new fuse and blew again.
I don't have scanner, so here is a power supply part that I use with camera.
 

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Very simple power supply. Only 2 high voltage taps (with CT), a 5 volt and a 6.3 volt tap.

You can take volt/ohm measurements of the primary. If its not shorted, you should see 0 to 1 ohms on the primary. Same with secondary. Just to be proof positive, dis-connect all the secondary transformer windings. If fuse still blows, yep, its the power transformer!
 
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You can take volt/ohm measurements of the primary. If its not shorted, you should see 0 to 1 ohms on the primary. Same with secondary. Just to be proof positive, dis-connect all the secondary transformer windings. If fuse still blows, yep, its the power transformer!

I did ohm measurement of the primary and it was more than 3 ohms. I will dis-connect the secondary tomorrow to make sure that the PT short.

In case for replacement, can I use the PT of Scott LK-72?
 
If your LK-72 has 365 volt X2 with center tap, 5volt and 6.3volt, then yes, it will work. If you know the LK-72 transformer to be good, take ohm readings from it and compare to your fisher.
 
I don't think resistance readings are enough of an indication. In any case, if the replacement transformer is about the same weight as the old one, you can't be too far off power wise.

Sometimes you can repair transformers depending on the construction.
 
If your LK-72 has 365 volt X2 with center tap, 5volt and 6.3volt, then yes, it will work. If you know the LK-72 transformer to be good, take ohm readings from it and compare to your fisher.

I don't have one but I saw on ebay there is one running right now. I don't know it has enough power to run on fisher
 
I have it out and do the measurements. The primary is 3 ohms. On the secondary, the 5V and 6.3V taps are nearly 0 ohm, and the 360V tap is 11.2 ohms. Is it short?
 
You can't tell from that. Put the primary on the ac line (with a fuse of course) and if it blows, you have a short.

If you have a short, then you need to see which winding is shorted. Or if one winding is shorted to another; this last test you do with your Ohmmeter before you apply power.
 
You can tell from ohm readings if it is shorted. I just checked a known good television transformer with 5, 6.3 & 700 volt CT and it reads 0 on all windings so I would have to say that your high voltage tap is shorted.

Of course, power added with a fuse, will also tell you if the fuse blows!
 
There is also one qualitative test you can run, only if you have an analog Ohmmeter. If you short the leads to the analog Ohmmeter, the needle jumps to full scale immediately. If you measure the primary winding of the transformer, it will pause before it does that. To verify, reverse the leads.

Many digital meters will go unstable on inductive measurements. If yours is doing that, you probably have a good transformer.
 
Make sure there is not a short between the primary and any of the secondary windings. Your resistance measurements are about normal. But impedance, not resistance is how transformers are measured, yes, with power transformers too. You should also know that it is normal for the secondary high voltage windings to be unequal. It is the turns, not the resistance that matters. The outside winding of the HV secondary should read slightly more because it has to go farther for the same number of turns.

The 6.3VAC and 5VAC windings are made with heavy wire because they carry a lot of current. And, they are less turns than the HV secondary.

Does that make sense?
 
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I measured the center tap to one lead of HV got 33 ohms, and 33 ohms same as for another HV lead. I measured the two HV leads and got 12 ohms, so if the PT is normal I should have 66 ohms. Then it is shorted. Could some one confirm me on this? Thanks
 
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Your transformer is shorted. I have replaced many a power transformer on an X-100, so it does not surprise me that yours has shorted.

The Scott LK-72 has more than enough power to operate the fisher, as it is a more powerful amplifier than the X-100 is. If it will physically fit OK, it should be a very good replacement.

Dave
 
Your transformer is shorted. I have replaced many a power transformer on an X-100, so it does not surprise me that yours has shorted.

The Scott LK-72 has more than enough power to operate the fisher, as it is a more powerful amplifier than the X-100 is. If it will physically fit OK, it should be a very good replacement.

Dave

Thank you Dave for your quickly reply. You have a very good night!
 
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