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Old 26th April 2009, 10:18 PM   #1
ZZZDave is offline ZZZDave  United States
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Default Theta Amp problem

I have 2 Theta tube amps, monoblocks, (on the covers) "Theta Power Amplifier, SCN OPT, made in usa", both very modified, uses a 6AN8 and 2- 6550's. One amp works & the other blows fuses. The schematic I was given is roughly similar to the configuration. Am getting close, to finding the short but does anyone have a schematic for these? or experience with them?

In a past posting on rebuilding a Theta w/KT88's someone wrote that they had schematics on some Theta components but since I'm new here I am not allowed to email members yet. Could someone help?
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Old 26th April 2009, 11:29 PM   #2
llwhtt is offline llwhtt  United States
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I have a schematic, what's your email?

Craig
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Old 27th April 2009, 02:12 PM   #3
ZZZDave is offline ZZZDave  United States
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Default Theta Amp

Craig,

That would be Great! It's outbackdave@verizon.net.

Thanks, Dave
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Old 27th April 2009, 10:09 PM   #4
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Schematics are always a good addition to any serviceman's library

geek <at> scorpiorising <dot> ca

Cheers!
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Old 27th April 2009, 10:11 PM   #5
llwhtt is offline llwhtt  United States
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Geek,

Is that a hint?

Craig
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Old 28th April 2009, 02:50 AM   #6
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Yes, thank you
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Old 28th April 2009, 04:02 PM   #7
ZZZDave is offline ZZZDave  United States
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Default Theta Amp - Update

Thanks to Craig, but I was getting close, it turned out to be a shorted rectifier diode. The rectifier was designed to have 2- 1N4006 diodes in series on each half of the transformer output and one 1N4006 on the bias output (reversed). I had one diode replacing two marked with an M (circle around it), RQ 170, 8336. Looks the same as a TO-41(?) basic diode.

Now, the "modifier" added photo-flash capacitors for the filter capacitors and with some basic testing they seem to be OK but 2 of them have slightly swollen ends & that always worries me. I'm of the opinion that it is best to put the P/S section back together as designed on the schematic, then I should have the correct voltages to work with. Does anyone care to offer any helpful comments, advice, jokes?
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Old 29th April 2009, 12:09 AM   #8
llwhtt is offline llwhtt  United States
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Dave,

You can probably find some snap-in type capacitors close to the original values, 600UF@360V. CDE, Nichicon, or Panasonic should have what you need.

Craig
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Old 29th April 2009, 03:03 PM   #9
ZZZDave is offline ZZZDave  United States
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Default Theta Amp

I've been looking for caps. in the online distributors I normally use but so far nothing close enough has been found. I even tried looking in Radial, Axial, and Screw mount styles!

I guess it's "back to the books" to remember what I've forgotten about Power Supply design. As far as I can remember it's the resistance of the filter, including ESR of components, that determines the voltages after the filters. Whatever it is, I'll figure it out & might even have a P-supply design program hidden in a folder on the PC somewhere!

Re: Rectifier diodes, the NTE catalog lists all the 1N4xxx diodes and recommends using the 1N4007 in most cases because it has a higher voltage capability so that was easy.

Thanks, Dave
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