It's a shame the B+ wasn't coming from an external Hammond tone cabinet or Leslie speaker as both of these have current limiting resistors which would have prevented the transformer insulation from even getting warm, mind alone warm enough to even cause damage. However, the B+ power supply was made by a very competent Hammond technician who may very well have added in these current limiting resistors, I'll have to check this out. If these were in place (and if the thought that's gone into the rest of the organ is anything to go by, it's more than likely they are), then the transformer is almost guaranteed to be safe.
Speaking with a Hammond expert, it's unlikely the transformer insulation either between the primary and frame or primary and secondly has been compromised. The only difference with the 56 pulled is there's no current flowing through the primary. This means it's not 100%, but still a fairly high likelihood, that the transformer is fine.
The same can't be said for the coupling cap (C7) or anode or screen resistors. Apparently, the next test would be to remove the C7 cap and then see if the noise stops. If it does, the problem would be in the first stage, if not then it's in the second stage. Unfortunately, due to not having a soldering iron, I can't do this.
I know the organ was a work in progress a bit when I got it and it's probable the preamp hasn't had new caps or resistors (I haven't checked yet). After nearly 80 years, it's fair to say these should all be replaced anyway. So regardless of their age, I'll get them all replaced. Only then can anything else really be troubleshooted. It's highly probable that these parts were already failing, but this voltage spike was the final straw that they could take, causing them to ultimately fail noticeably.
Speaking with a Hammond expert, it's unlikely the transformer insulation either between the primary and frame or primary and secondly has been compromised. The only difference with the 56 pulled is there's no current flowing through the primary. This means it's not 100%, but still a fairly high likelihood, that the transformer is fine.
The same can't be said for the coupling cap (C7) or anode or screen resistors. Apparently, the next test would be to remove the C7 cap and then see if the noise stops. If it does, the problem would be in the first stage, if not then it's in the second stage. Unfortunately, due to not having a soldering iron, I can't do this.
I know the organ was a work in progress a bit when I got it and it's probable the preamp hasn't had new caps or resistors (I haven't checked yet). After nearly 80 years, it's fair to say these should all be replaced anyway. So regardless of their age, I'll get them all replaced. Only then can anything else really be troubleshooted. It's highly probable that these parts were already failing, but this voltage spike was the final straw that they could take, causing them to ultimately fail noticeably.
I hope its not the tranny of course... but I guess its kind of 50:50 at the moment. Only by replacing those parts around the valve will you know for sure. When talking of insulation you have to rmember that transformers work just as well in reverse. So one with (say) a 10 to 1 turns ratio, where a voltage of 100 volts on the primary would generate 10 volts on the secondary, would equally generate 100 volts on the primary if you applied just 10 to the secondary. Apply a few hundred volts to the secondary would generate a massive voltage spike on the primary, and its that extreme high voltage transient taht could "puncture" through any insulation. So to be sure you need to rebuild that stage and replace all the components around.
Keep us updated on how it goes
Keep us updated on how it goes
In my experience, crackling noise seems to be coming from bad plate resistors. Check R58 (350KΩ) and/or R51 (1MΩ). You might have to replace one or both of them. R58 more likely since it's typically soldered directly to the socket pin and you said "giving the 57 a bit of a knock stops the noise." Worth investigating...
An externally hosted image should be here but it was not working when we last tested it.
Thanks for the reply! I guess it may not hurt to just take a look under the preamp at some point to check for any obvious signs like burnt resistors. I could also do the few basic transformer tests while I'm at it. I expect it to pass those with flying colours, but of course if it doesn't, then that's an easy way of instantly diagnosing a problem!
Thanks for all your help so far. Will definitely get all caps and resistors replaced anyway and will keep you all posted.
Thanks for all your help so far. Will definitely get all caps and resistors replaced anyway and will keep you all posted.
The organ has been fixed! No more crackling any more!! I've had it 2 months and 3 days and now it's fixed, I still can't play it! Haven't got the time right now with GCSE exams, typical, as luck would have it... I can't complain though, it's fixed and I have 3 months to play it as much as I want this time next month!
The problem was just 3 resistors! I wasn't here when it was fixed, so I don't know which ones they were, but I guess it was most likely plate/grid resistors as mentioned. Most of the resistors and small capacitors have been replaced I've been told, meaning some will be coming up for 80 years old! But as they're working and an engineer has said they're safe and within limits, they may as well stay. Some of the capacitors/resistors in there are pretty Hammond-specific values which shape the sound a bit so the longer they last, may as well keep them.
This B+ voltage must have, for that brief second, passed through the output (or matching? Hammond calls it an output) transformer. I was told these were very robust transformers and that has definitely proved that point! I'll be making absolutely sure that the transformer is very well looked after in future, I'm sure not even a transformer can cheat death twice!
Thanks for all the help, you were completely right. The help and knowledge on this forum is brilliant. While I'm incredibly busy with exams for the next month or so, as soon as I upload a video to YouTube I'll link it here in case anyone wants to hear what an amazing sound this gives out!
The problem was just 3 resistors! I wasn't here when it was fixed, so I don't know which ones they were, but I guess it was most likely plate/grid resistors as mentioned. Most of the resistors and small capacitors have been replaced I've been told, meaning some will be coming up for 80 years old! But as they're working and an engineer has said they're safe and within limits, they may as well stay. Some of the capacitors/resistors in there are pretty Hammond-specific values which shape the sound a bit so the longer they last, may as well keep them.
This B+ voltage must have, for that brief second, passed through the output (or matching? Hammond calls it an output) transformer. I was told these were very robust transformers and that has definitely proved that point! I'll be making absolutely sure that the transformer is very well looked after in future, I'm sure not even a transformer can cheat death twice!
Thanks for all the help, you were completely right. The help and knowledge on this forum is brilliant. While I'm incredibly busy with exams for the next month or so, as soon as I upload a video to YouTube I'll link it here in case anyone wants to hear what an amazing sound this gives out!
Sorry for digging up this thread again, just another update for those who were following this thread.
It was so silent, and all so well.... Now it's back!!! I had an Oscilloscope say the Output Transformer was absolutely fine, so completely in doubt, I had the preamp rebuilt again.
Actually, my B+ power supply was outputting 435VDC! That's far too much, it should only be 200-250VDC! It must've been doing this the whole time, basic Ohm's Law showed all the resistors in the B+ power supply were wrong. Now it's outputting a nice 253VDC.
Anyway, a rebuild and the power supply fixed, it's still slightly crackling! The Output Transformer primary insulation has now decided to break down! Oh yes, and did I mention these transformers are nearly impossible to find as spares?
My tech has a few OT's that should be pretty close to spec, but none that are exact. I can't find an exact one anywhere at all. I'll try a ring-around of Hammond folks in the UK, but I doubt it'll get any luck. My tech has it at the moment and is going to obtain the necessary information of the OT for me, to see if I can find a spare. All I know so far is it's made by Haddon.
I'd love to keep things all original, even the original transformer if it's still at all possible. This really truly is a museum piece. One of now 7 known in the world.
If I sent it to a company who rewinds transformers, could they fix the problem for me?
It was so silent, and all so well.... Now it's back!!! I had an Oscilloscope say the Output Transformer was absolutely fine, so completely in doubt, I had the preamp rebuilt again.
Actually, my B+ power supply was outputting 435VDC! That's far too much, it should only be 200-250VDC! It must've been doing this the whole time, basic Ohm's Law showed all the resistors in the B+ power supply were wrong. Now it's outputting a nice 253VDC.
Anyway, a rebuild and the power supply fixed, it's still slightly crackling! The Output Transformer primary insulation has now decided to break down! Oh yes, and did I mention these transformers are nearly impossible to find as spares?
My tech has a few OT's that should be pretty close to spec, but none that are exact. I can't find an exact one anywhere at all. I'll try a ring-around of Hammond folks in the UK, but I doubt it'll get any luck. My tech has it at the moment and is going to obtain the necessary information of the OT for me, to see if I can find a spare. All I know so far is it's made by Haddon.
I'd love to keep things all original, even the original transformer if it's still at all possible. This really truly is a museum piece. One of now 7 known in the world.
If I sent it to a company who rewinds transformers, could they fix the problem for me?
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