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In Reply to: Vintage Pilot260 posted by Edmund on April 10, 2007 at 20:43:14:
That does seem low but your probably measuring DC resistance not AC impedance. What you can do is wire a 100 watt light bulb in series with the line cord for testing purposes. This will limit current flow.
Follow Ups:
Thanks for your advice. Yes, I meansured the DC resistance that shown one ohm. I compared the DC resistance of other power transfomer as well. Therefore, it is simply a reference. Is it possible to measure the AC resistance ? An how? Thanks.
No, not with a basic multi-meter (read the meter instructions or it's dial for capabilities). By measuring at least some DC resistance, you have shown there is not an open winding & no direct short (even if some of the winding is shorted, which seems to be your concern). If there is a pilot lamp in parallel with the primary winding, it will lower the *expected* resistance measurement. If you are loath to power up via the light bulb method, then disconnect the power transformer secondary windings from the circut & measure AC voltage on those windings when plugged in. In this way you can evaluate the transformer without endangering the circut. As Aretha Franklin sang, THINK! (because you are working with lethal voltages & currents).
Ron, Thanks for your advice.
The AC input is fused. I suppose in case of short circuit of primary. The fuse shall burn. Actually, my Pilot 260 work perfectly before I dismantle if to spary with paint. That is it, the transformer should not be get worse without connecting to AC power. I wish some pilot 260 users could kindly measure the resistance and share the informtaion with me. Thanks
I sent you diagrams and photos. Please notify me that you received them.
I get a DCR of 2.4 ohms.Did you bypass the on off switch properly? I don't remember if yours had the factory jumper plug for use without the preamp.
Al
Thanks AL. I'd much appreciate your detailed mail concerning Pilot 260. Please read my mail too. Edmund
Finally, I confirmed the AC input can be as low as 0.8 ohm. I connected it to 1117V AC main eventually. No fused burnt, no explosion of any kind. I just put one 5R4 instead of two in place. But the H+ is too high. My fluke meter read 504V DC but circuit shows that should be 430 V . The one should read 295V read 436V in mine Pilot 260. The 240V read 365 V in my Pilot 260. All H+ voltages are over, some even 50% higher. I awared there were two 400 Ohm cement resistors im parallel were added in series with 33 ohm 10 W to the can capacitor. I suspect that was the case the owner make the voltage drop to a reasonable level. In that case , I may have to put that two 400 ohm resistors back to the circuit. That will discrepant with original circuit. Hi friends! Any comments on that issues. Thanks .
Go by the diagram.It looks like the 7 to 8 jumper takes the 295 volt supply through 2 15K resistors in series to ground. I have not worked on it in a while.
The 295 volt supply being loaded down drops all the B+ voltages then. 800 ohm instead of 3000 ohm is too much I think.
Use thr diagram to go back to the factory configuration.
I installed 2 jumpers installed on the preamp socket. One jumper bypases the on off switch. The other puts resistors in paralell with the power supply to compensate for the increased voltages caused by not powering the preamp. This was not clearly shown on the Sams Photofact diagram as I recall.
AI,
You are absoultely right in this issue, I omitted that part. I will put that two 15K 2W resistors back and measured the voltage again. Whether or not to put that two 375 ohm 10W parallel connected resistors back will depend.
All H+ voltages are normal after I connected two 15K 10 W resistors back to the circuit and with all tubes in place.
Thats a good thing. Next try this amp with a passive preamp.
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