You are having bad juju with vintage electronics, at the moment.Tks. BTW, while I was cheking this w5m, fuse blew on Dynaco ST-70
I'm sending some good vibes out.
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Heathkit w5m blowing fuse.
- Thread starter Alex Kruz
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I'm sending some good vibes out.
Be sure to keep a good supply of the correct fuses. After so many fuses blow, it's so tempting to use a bigger amp-rated one. That's when the door to disaster gets propped open.
trobbins
Super Member
Post #17 measurements are suspect - or your identification of V3 and V4 pins is in error - or your meter is NQR.
What meter are you using for resistance, and what does it measure with probe tips shorted? 0V/Gnd to start of 330R should be circa 125R, and to end of 330R should be circa 455R, and to either pin 8 should be circa 485R. The 100uF e-caps may initially cause quirky resistance readings for the previous sentance, but if you changed them then unlikely they are at fault.
You indicate 150R from pin 3 to pin 6. The W5M manual shows pin 6 is used for screen resistor to UL node, so 150R appears to be the OPT winding DCR from anode to UL tap. Is that reading the same for V3 and V4? You indicate a shorted resistance from OPT CT to V4 pin 3 - can you repeat that measurement ? Do you also get a shorted resistance from OPT CT to pin 6 on V4 ?
What meter are you using for resistance, and what does it measure with probe tips shorted? 0V/Gnd to start of 330R should be circa 125R, and to end of 330R should be circa 455R, and to either pin 8 should be circa 485R. The 100uF e-caps may initially cause quirky resistance readings for the previous sentance, but if you changed them then unlikely they are at fault.
You indicate 150R from pin 3 to pin 6. The W5M manual shows pin 6 is used for screen resistor to UL node, so 150R appears to be the OPT winding DCR from anode to UL tap. Is that reading the same for V3 and V4? You indicate a shorted resistance from OPT CT to V4 pin 3 - can you repeat that measurement ? Do you also get a shorted resistance from OPT CT to pin 6 on V4 ?
No kidding, just came by mail from amazon 10pc 3a resistors) (Be sure to keep a good supply of the correct fuses. After so many fuses blow, it's so tempting to use a bigger amp-rated one. That's when the door to disaster gets propped open.
:~)You are having bad juju with vintage electronics, at the moment.
I'm sending some good vibes out.
YEs, you right I misspoke, center tap to pin 3 on V3 is OL not 0v and unfortunately that confirmed by 2 multimeters. :~(. On V4 p.3 to center tap is 318R, p.3 to p.6 is 150R on both V3 and V4.
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trobbins
Super Member
Given the rarity of the OPT, as a second opinion, you could also apply say a test 6Vac across the 16 ohm secondary, and measure Vac on the primary side sections (eg. circa 75Vac from CT to anode on the good side).
An open-circuit CT to screen tap would suggest a KT66 screen has shorted internally, and caused an overload current to take out that section of the winding. The 270R screen stopper appears not to have saved the OPT - even though it was 1/2W rating - which could suggest a lower wattage rated (eg. 0.25W) resistor may have a better chance of opening first, or the use of a PTC.
The 16458 OPT has 50% turns ratio screen taps. Apart from using that OPT as a door stop, you may find someone who wants it to then rewind, or to use the 50% of good turns to provide a 2k5 PP OPT for some diy amp.
An open-circuit CT to screen tap would suggest a KT66 screen has shorted internally, and caused an overload current to take out that section of the winding. The 270R screen stopper appears not to have saved the OPT - even though it was 1/2W rating - which could suggest a lower wattage rated (eg. 0.25W) resistor may have a better chance of opening first, or the use of a PTC.
The 16458 OPT has 50% turns ratio screen taps. Apart from using that OPT as a door stop, you may find someone who wants it to then rewind, or to use the 50% of good turns to provide a 2k5 PP OPT for some diy amp.
personally I'd be very leery of using a half-open transformer like that. How long until it goes further open, and what about the winding hanging off the end that hasn't gone open.
I've had somewhat better results using metal film vs any flavor of carbon resistor as a sacrificial device. Carbon will keep conducting way past its wattage rating for a fair amount of time.
I've had somewhat better results using metal film vs any flavor of carbon resistor as a sacrificial device. Carbon will keep conducting way past its wattage rating for a fair amount of time.
I'm not sure it is rwindable. This is another one from w5m (((( It is 16309 BTW.
But could you please explain it for a total newbie how to do this: "you could also apply say a test 6Vac across the 16 ohm secondary, and measure Vac on the primary side sections (eg. circa 75Vac from CT to anode on the good side)." Also, what it neans "circa 75Vac"?
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he's talking about back feeding the transformer from the secondary to see if you have voltage on both sides of the primary. If you have an open circuit though, any readings from that aren't going to be meaningful. It might spit out AC but its not going to carry any sort of current.
trobbins
Super Member
Some people may like to reuse the core for a rewind - not an uncommon situation for scarce OPT's.
The turns ratio of the complete P-CT-P primary is about 25:1 to the 16R secondary winding. Energising the 16 ohm winding with say 6Vac would generate about 6x25=150Vac PP on the primary side, if the primary was good. Each section of the primary (plate1 to screen1, screen1 to CT, CT to screen2, screen2 to plate 2) would generate about 38Vac, except for the bad CT to screen section, as the UL taps are presumably 50% turns ratio. It's just another way of confirming an open-circuit in a CT to screen section.
As an option if you were into diy audio, connecting the two screen taps and using that as a new CT terminal would make the OPT act as a 2.5k PP OPT (ie. no UL taps). Sure you don't get the same HF performance, and there is a substantial winding section (screen to screen taps) that is exhibiting an open circuit - which is a risk as the open-circuited region could leak or reconnect/arc with a high enough voltage across it and so cause a shorted winding path - but maybe not, and possibly testable with a megger - just saying .
The turns ratio of the complete P-CT-P primary is about 25:1 to the 16R secondary winding. Energising the 16 ohm winding with say 6Vac would generate about 6x25=150Vac PP on the primary side, if the primary was good. Each section of the primary (plate1 to screen1, screen1 to CT, CT to screen2, screen2 to plate 2) would generate about 38Vac, except for the bad CT to screen section, as the UL taps are presumably 50% turns ratio. It's just another way of confirming an open-circuit in a CT to screen section.
As an option if you were into diy audio, connecting the two screen taps and using that as a new CT terminal would make the OPT act as a 2.5k PP OPT (ie. no UL taps). Sure you don't get the same HF performance, and there is a substantial winding section (screen to screen taps) that is exhibiting an open circuit - which is a risk as the open-circuited region could leak or reconnect/arc with a high enough voltage across it and so cause a shorted winding path - but maybe not, and possibly testable with a megger - just saying
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This is a neat way to find the turn ratio (and the primary impedance) of an unknown output transformer.
1.) Using a signal generator, place a 0.5VAC signal at 1000Hz across, say, the 8-ohm tap of the secondary.
2.) Measure the voltage on the primary side — end-to-end — center tap not connected. Say you get 14VAC.
3.) The ratio of 14VAC/0.5VAC = 28. This is the turn ratio between the primary and secondary windings of that OPT. Twenty-eight turns of the primary to every turn of the secondary.
4.) The square of the turn ratio = (28) X (28) = 784
5.) Multiply 784 x 8-ohm secondary tap = 6272 ohm. This is the impedance of the primary side.
Average data for typical AB1 push-pull outputs:
The 6V6 and EL84 have a primary impedance range of 6600- 8000.
EL34 and 6L6GC in the 4200 - 6600 range.
6550 and KT88 in the 3400 - 5000 range.
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Sorry, I was away for the weekend. *Sounds like* one half of the primary is shorted. But please double check. With output tubes removed, check V4, pin 6 to center tap. Should be c. 150 ohms. Pin 3 to CT should be about 310 ohms. Is it 0 ohms (shorted, unlikely) or is it "no reading" or open? A "no reading" or open circuit would look like "0.L" on most meters.
Pin 5 to ground on both sockets should read c. 100K ohms. If it's 130 ohms then there a serious problem with the grid circuit. The 100K grid resistors are not properly connected.
Can you remind us about the original issues with this amp and what has been done to repair/restore it?
We know what the OPT is and we know what the readings should be. Everything else is irrelevant. Either the OPT is in working order or it isn't. Either the grid circuit (pin 5 to ground for c. 100K ohms) is properly connected or it isn't.
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