I would think that without the secondary ground reference the feedback signal would be some relatively low-voltage AC.pdf64 wrote: ↑Sun Apr 23, 2023 2:49 pm Without a 0V common reference for the OT secondary, the potential divider of the NFB network may not have had the expected effect. Too much NFB can result in the feedback actually being positive, sending the power amp into oscillation, possibly ultrasonic.
That could overheat the output valves, screen grid resistors, and the OT.
Dead output transformer
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martin manning
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Re: Dead output transformer
Re: Dead output transformer
What I gather is that Mojo doesn't want it back. If you get to keep it, you've got at least 3 options:
1) Try to fix it. If the problem is near the outer winding, you may find you can get to the bad spot, maybe unwind a few turns and attach a new external lead. I've done this successfully. Not real difficult.
2) Split off the lams and unwind it to uncover the wire sizes and number of turns. Number of turns will be far to tedious, but you can probably estimate by wire gauge, size of the window, and DCR of a fixed length of wire. Compute turns ratio the from the impedance ratios. This should give you a comfort level for validation. It will take you a whole afternoon, maybe more, but I think everyone should do this one time. It involves using a razor blade to cut through the exterior varnish and then maybe a hammer and sharp screwdriver. Stack the lams in order for the next person to reuse. Sell on ebay.
3) Use it as a doorstop.
You seem like a smart guy. I'd go for it in the order listed.
1) Try to fix it. If the problem is near the outer winding, you may find you can get to the bad spot, maybe unwind a few turns and attach a new external lead. I've done this successfully. Not real difficult.
2) Split off the lams and unwind it to uncover the wire sizes and number of turns. Number of turns will be far to tedious, but you can probably estimate by wire gauge, size of the window, and DCR of a fixed length of wire. Compute turns ratio the from the impedance ratios. This should give you a comfort level for validation. It will take you a whole afternoon, maybe more, but I think everyone should do this one time. It involves using a razor blade to cut through the exterior varnish and then maybe a hammer and sharp screwdriver. Stack the lams in order for the next person to reuse. Sell on ebay.
3) Use it as a doorstop.
You seem like a smart guy. I'd go for it in the order listed.
Re: Dead output transformer
I really appreciate all the help. This has been so much easier to troubleshoot than it could have been. Pdf64, your comment jogged my memory that when I first started troubleshooting the blown fuse issue, I tried putting it on the lightbulb current limiter again and noticed that when the standby switch was closed the limiter would begin to strobe on about 1 second cycles with the tubes plugged in. That would seem consistent with oscillation, though it seems like you're describing a higher frequency oscillation than what I observed.
Martin, I thought the same -- I understood the OT secondary taps to put out pretty low voltage, high current AC, so it seems like very little could pass the initial 10k resistor in the NFB network. There's a part of me that hopes it's as simple as the missing ground reference, though. Between me not being able to think of anything else, though, and wanting to redo my sloppy layout, I'm going to redo my power board (making sure to include the ground reference). I'll post my updated layouts soon (probably in a different thread). I saw you came up more than a few times when I searched for Hiwatt OT information on the thread, so I'd love your feedback on the layouts if you're willing.
Phil, you're right. They're saying my non-inclusion of the ground reference for the NFB circuit was equivalent to running the OT without a load, i.e., broke due to user error. They're offering me a discount on a replacement, and so between finding roasted EL34s and wanting to redo my sloppy layout, I'm taking the opportunity to avoid paying shipping on the parts I wanted anyway by just getting a new one. I was considering making another Hiwatt (probably more like a "vanilla" DR504), though, so rehabilitating this OT would definitely help. I noticed the short appears to be a contact point connecting the bell to the joint where the brown primary lead connects to the primary winding. The paper wrapping had burnt through at that spot, and there was a dark spot on the inner face of the bell end. It seems like insulating that spot again might resolve the issue, but I'm concerned that my disassembly job would make the OT unusable. I think I could redo the paper wrapping, but I wanted to get a sense if the detective work I did wouldn't make that a fool's errand.
Martin, I thought the same -- I understood the OT secondary taps to put out pretty low voltage, high current AC, so it seems like very little could pass the initial 10k resistor in the NFB network. There's a part of me that hopes it's as simple as the missing ground reference, though. Between me not being able to think of anything else, though, and wanting to redo my sloppy layout, I'm going to redo my power board (making sure to include the ground reference). I'll post my updated layouts soon (probably in a different thread). I saw you came up more than a few times when I searched for Hiwatt OT information on the thread, so I'd love your feedback on the layouts if you're willing.
Phil, you're right. They're saying my non-inclusion of the ground reference for the NFB circuit was equivalent to running the OT without a load, i.e., broke due to user error. They're offering me a discount on a replacement, and so between finding roasted EL34s and wanting to redo my sloppy layout, I'm taking the opportunity to avoid paying shipping on the parts I wanted anyway by just getting a new one. I was considering making another Hiwatt (probably more like a "vanilla" DR504), though, so rehabilitating this OT would definitely help. I noticed the short appears to be a contact point connecting the bell to the joint where the brown primary lead connects to the primary winding. The paper wrapping had burnt through at that spot, and there was a dark spot on the inner face of the bell end. It seems like insulating that spot again might resolve the issue, but I'm concerned that my disassembly job would make the OT unusable. I think I could redo the paper wrapping, but I wanted to get a sense if the detective work I did wouldn't make that a fool's errand.
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Stevem
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Re: Dead output transformer
If you drive a OT without a load, or without the secondary shorted then the OT acts like a car engine spark coil, but that double the B+ voltage swing has no where to go but to short out across the windings.
The key is driving it.
You can have the amp fully power up ( at idle ) with no load and no harm will come to the OT until you start to play thru the amp.
The key is driving it.
You can have the amp fully power up ( at idle ) with no load and no harm will come to the OT until you start to play thru the amp.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Dead output transformer
Update, if it helps anyone reading down the road: It seems like the floating speaker ground did indeed kill the output transformer. I finally rewired it and today I got it working. I ran it for about an hour with no issues, and it played great! Since the combination of negative feedback plus floating ground killed the output transformer within minutes, I'm hoping this means I'm out of the woods. I'll keep updating if something goes wrong, but hopefully this helps someone in the future with a similar issue (or anyone curious what happens if you have both NFB and floating speaker grounds).
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Stevem
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Re: Dead output transformer
Yes, it sounds like you will be fine now!
Having no ground reference is a sure fire way to kill a OT every time if you drive it.
Having no ground reference is a sure fire way to kill a OT every time if you drive it.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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dorrisant
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Re: Dead output transformer
Just curious... Why would you not apply a few patches of electrical tape (Scotch 33) between the shorted tap and the bell? Even applying the tape to the inside of the bell would work great. It is already marked at the exact spot that needs insulation. If the fuse blew and there was no real stress to the OT before that, then I don't see why this OT wouldn't be useable.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
Re: Dead output transformer
Thanks again everyone for helping me find the problem. Dorrisant, I think you're likely right that the OT would still be usable. During the disassembly and teardown process, I ended up cutting the plastic/paper insulating layers over the primary taps, so it'd involve a more extensive repair than taping over the shorted lead. I wanted to make absolutely sure that what I was seeing was that short and to get a peek inside the transformer (just a curiosity thing). I'm sure rebuilding the OT could be done (not sure the cost to have it done "properly" with the paper covering), but I decided that's a different project down the road since I don't have experience with that kind of remanufacturing work. Had I approached teardown differently and kept those insulating layers intact, I think re-insulating the tap would likely have worked.