A little background: A friend of mine has a Harmony H306 and he wanted me to get it working. I've built 2 hand-wired champs and an 18W pcb amp (all three worked the first time), so my soldering skills are good, but I'm trying to develop my troubleshooting skills.
I open the amp up and it looks original, but there are a few capacitors and resistors that have come loose. I clipped a few caps out to test on my ESR and they were shot, so I plan on doing at least a cap job. There are carbon comp resistors in it as well, and would only take another $20 to replace them.
My question is I'd like to test both trannies on my meter to see if they are working properly, but not quite sure how. If they are not working properly, I'll probably scrap the rebuild because of cost. Just want to test them before any money is spent on the rest of the amp. Thanks!
Kevin
How to test a PT and OT?
Moderators: pompeiisneaks, Colossal
Re: How to test a PT and OT?
I've got an H306 that I converted to have a 6G3 pre + 5E3 PI&Power stage. It's one of my favorite amps. Still has the 6SH7 trem that sounds really good. The guts on it don't look that great, but it works. 
First, set up a 3-prong plug and ditch the death cap if it still has it. Hot to fuse to switch.
I have more pictures somewhere showing the PT and OT lead colors, but I can't find them at the moment. For the OT, you should be able to check the secondary and make sure that it isn't shorted to the primary with a meter. Make sure that there's continuity between the two secondary leads. Make sure there is continuity between all 3 primary leads. If they all test OK with continuity where there should be and none where there shouldn't, the OT is probably good. The resistance of the two halves of the primary winding should be similar, but not necessarily identical. There's a method for checking for internal shorts using a neon bulb and a battery that's pretty handy. I'll find a link when I can and post it.
For the PT, lots of the same stuff. Make sure that the windings aren't shorted to each other. You may need to disconnect the 6.3V from the power-tube cathode and the HT center tap to be able to test them. Maybe just one of them.
Once that's done, follow the paul ruby startup guide. When I say "follow", follow it religiously. It's caught several problems on amps I've either built or had given to me to fix:
http://www.paulrubyamps.com/info.html#FirstPowerUp
If I find the other pictures, I'll post them. Don't give up easily on the amp, it's got good iron and is a good donor if nothing else.
Matt
First, set up a 3-prong plug and ditch the death cap if it still has it. Hot to fuse to switch.
I have more pictures somewhere showing the PT and OT lead colors, but I can't find them at the moment. For the OT, you should be able to check the secondary and make sure that it isn't shorted to the primary with a meter. Make sure that there's continuity between the two secondary leads. Make sure there is continuity between all 3 primary leads. If they all test OK with continuity where there should be and none where there shouldn't, the OT is probably good. The resistance of the two halves of the primary winding should be similar, but not necessarily identical. There's a method for checking for internal shorts using a neon bulb and a battery that's pretty handy. I'll find a link when I can and post it.
For the PT, lots of the same stuff. Make sure that the windings aren't shorted to each other. You may need to disconnect the 6.3V from the power-tube cathode and the HT center tap to be able to test them. Maybe just one of them.
Once that's done, follow the paul ruby startup guide. When I say "follow", follow it religiously. It's caught several problems on amps I've either built or had given to me to fix:
http://www.paulrubyamps.com/info.html#FirstPowerUp
If I find the other pictures, I'll post them. Don't give up easily on the amp, it's got good iron and is a good donor if nothing else.
Matt
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Re: How to test a PT and OT?
After checking each of the PT windings for appropriate continuity with your R-meter, you can test a PT by hooking the primary up to the mains (with an appropriate fuse in line with the Phase/Active mains lead, and appropriate mains earth grounding of the case) and measuring the VAC on the secondaries. It helps to load up the secondaries with wire-wound resistors across each secondary to emulate a load.
You can test an OT by hooking the secondary(s) up to a low VAC source and measuring the VAC on the primary. If you are really cheap you can use the 5V or 6.3V secondary on your PT as the VAC source to test your OT.
You can test an OT by hooking the secondary(s) up to a low VAC source and measuring the VAC on the primary. If you are really cheap you can use the 5V or 6.3V secondary on your PT as the VAC source to test your OT.
He who dies with the most tubes... wins
Re: How to test a PT and OT?
Cool little amp!
I'd suspect the transformers are okay.
pull the rectifier tube, measure secondary voltage from power transformer and make sure the voltages are resonable.
pull the power tubes and use you ohmmeter to make sure the output transformer windings aren't open.
that would be enough for me before recapping it. also, measure the resistors and replace any that are more than 20% drifted.
I'd suspect the transformers are okay.
pull the rectifier tube, measure secondary voltage from power transformer and make sure the voltages are resonable.
pull the power tubes and use you ohmmeter to make sure the output transformer windings aren't open.
that would be enough for me before recapping it. also, measure the resistors and replace any that are more than 20% drifted.
Congress Park Amps