Post-mortem on a 80 year old 2A3 amp - diyAudio
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Old 25th July 2012, 11:11 PM   #1
cuibono is offline cuibono  United States
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Default Post-mortem on a 80 year old 2A3 amp

The amp in question comes from a late-30's RCA-designed, Story and Clark electric piano, the Storytone. It is a 15W output beast, single channel, with four 2A3 tubes in push/pull parallel. It has two power transformers, and three signal transformers. At idle it consumes about 200W! The piano itself is beautiful, very 30's art deco, and was possibly the first electric piano.

I was able to find a schematic luckily. The power supply was fried at some point - I attached a picture of the melted transformer next to a 2A3 tube for size comparison. Its a pretty big transformer, and was used to generate the plate voltages. I'm hoping to be able to replace it, figure out what went wrong, and get the instrument working again.

Heres the evidence so far:

1) a 30 amp fuse, where a 3 amp is called for - duh!!
2) plate power transformer melted and still gooey
3) 5Z3 rectifier tube internally melted
4) no other components melted. One of the 2A3 tubes has very low output (tested at 40% quality on a hickok tester), but doesn't look to me like the reason the amp fried

The schematic shows that the plate transformer, T4, is supposed to have 120V in and 340Vac out. Its also says the primary is 2.9ohms and the secondary is 70ohm total. I actually have a second power supply with a good transformer, so I can do measurements on it if need be.

So my questions are, could the 5Z3 tube short internally, melting the transformer? And where/how can I replace the transformer? I'll have one custom made if need be.
Attached Images
File Type: jpg 1transformer.JPG (95.0 KB, 820 views)
File Type: jpg 2_5z3.JPG (79.5 KB, 784 views)
File Type: jpg 3storytone schem.jpg (215.5 KB, 789 views)
File Type: jpg 0piano.JPG (108.9 KB, 782 views)
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Old 25th July 2012, 11:17 PM   #2
vaughn is offline vaughn  United States
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I have no answer to your question, just want to comment on what a beautiful object that piano is
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Old 25th July 2012, 11:28 PM   #3
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Are you sure the plate PT is bad? Have you checked it electrically?

I'd be certainly interested in also seeing a schem of the remainder of the piano - eg. the tone generation/sgnal processing.

Can you provide a photo of the underside circuitry?

Top stuff!

Ciao, Tim
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Old 25th July 2012, 11:45 PM   #4
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Quote:
Originally Posted by cuibono View Post
So my questions are, could the 5Z3 tube short internally, melting the transformer?
Technically possible, but unlikely. More likely is a short somewhere else after the 5Z3. But for sure that 30 amp fuse caused the meltdown when it didn't blow. I've seen this too many times in various equipment where some moron puts a "slug" in place of a fuse.

As far as transformer replacement goes, you'll have to search the various transformer manufactures listings for one that's close. I'd start with Edcor, Hammond, etc. And also search eBay for a good used or NOS Stancor, Thordarson, Triad and others. Having one made special would be my last resort because of cost.
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Old 26th July 2012, 12:32 AM   #5
kevinkr is offline kevinkr  United States
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Edcor can probably build something suitable for not obscene money, however it probably will not be physically identical.. There are transformer rewinders that could do a complete rewind on this transformer for what is probably a substantial cost - given the nature of the beast it might just be worth it.

Most likely one or more of the supply filter caps croaked. I have an Atwater Kent Radio from 1932, mostly original parts except for the electrolytics which I stuffed with modern caps. The originals were wet types and had totally dried out. (and shorted)
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Old 26th July 2012, 12:49 AM   #6
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The wax or pitch of old transformers would often melt out over time. For sure, excess expulsion indicates gross heating, but may not end in winding failure. A resistance test, followed by a low AC voltage excitation test, followed by a megger/insulation resistance test would be the first actions I suggest.
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Old 26th July 2012, 05:16 PM   #7
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As a compromise, if you can find a similar transformer of the same core size, the end bells can be swapped to keep it looking original. I would be amazed if the winding WASN'T shorted or open, and wouldn't trust it if it did measure OK... as mentioned above, shorted cap would cause rectifier and transformer to fail.
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Old 26th July 2012, 05:37 PM   #8
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Check the electrolytic cap's. I have some problem with old sets including tube rectifiers, in which electrolytic become shorted or very leaky. As the cap drains more current that rectifier can supply, it becomes red plated, and then gas is liberated inside the bulb. Then, more current, and so. The final is the filament melted, and touching the plate were arcing or directly shorted to an anode. In an old Paco oscilloscope S50, (from 70's ?) it happened with a 6CA3, when the final was the cathode shorted to heater. And more recently, with a restored Eico 368 signal generator of my own. Then, I rewind the transformer for 220V (The original was 110V primary), and made a separate heater wind to the 6X4 to prevent cathode-heater insulation damage. This set works with 380V cathode voltage, when RCA specifies about 200V absolute maximum between cathode-heater voltage.
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Old 26th July 2012, 06:41 PM   #9
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In any case electrolytics caps have to be replaced.
And I second Edcor as the source for transformer.
You live quite far from SF Bay Area, and I have more project than can afford, otherwise I would offer my help in restoration of this beauty.
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Old 26th July 2012, 10:45 PM   #10
7N7 is offline 7N7  United Kingdom
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Quote:
Originally Posted by vaughn View Post
I have no answer to your question, just want to comment on what a beautiful object that piano is
It is absolutely stunning; I adore Art Deco and this instrument is a fine example indeed.

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