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Old 1st April 2014, 05:48 AM   #1
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Location: Santa Fe, Argentina
Default Olson AM-240 schematic needed

Hi fellow forumers
I'm going to restore this amp for a friend of mine
this is the unit: AM-240 Ampl/Mixer Olson Radio Corporation; Akron, Ohio, buil
Obviously, the one I've in the bench is not in such a good shape, take a look:
Well, lacking the schematic the restoring job, while not impossible, becomes considerable more difficult. So If one good soul can share the circuit, it will help in an just and heroic cause.
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"It is less serious than if worst"
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Old 1st April 2014, 12:43 PM   #2
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first and dirt measurements confirm that this circuit output stage works with 415V on plates, 400V on G2, and -20V fixed bias.
First decision: revolve heaven and earth to find the original 7189 tubes (a very difficult and expensive task in my neighborhood), or derate the B+ to a more maneageable level, in order to fit the humble 6BQ5's (standard flavour)?.
Since this unit is being powered through a 110 to 220 step down transformer, a proper 220V PT is likely to be purchased, so, adjusting to fit different tubes will not be a problem, but...
"It is less serious than if worst"
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Old 1st April 2014, 01:24 PM   #3
infinia is offline infinia  United States
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it's probably a knock off of a HH Scott design, so get a couple of their schematics might get you to 85%.
in the end looks like loads of work with an unknown quality result?
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like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
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Old 1st April 2014, 01:25 PM   #4
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Jim McShane ships world wide and it's likely other reliable tube sellers can accommodate you too.

Fortunately, a good and affordable 7189 equivalent is in production. It's the Russian 6П14П-EB (6p14p-ev), AKA EL84M, is what you should buy.

Be careful to distinguish between the 7189 and the 7189A. 7189As are extinct and amps using it need socket rework and (perhaps) operating conditions adjustment to employ the 6П14П-EB.
Eli D.
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Old 1st April 2014, 02:02 PM   #5
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Thanks Infinia and Eli. Much appreciated.
This weekend I'm going to check the amp against the Scott schemo, it will be fun.
I'm mostly concerned about some rotary switches, cables were cut off, and functions disabled, a real mess, ad opera some shitty incompetent TV repairman , for sure.
For what I saw, the tubes fitted (one is missing) are standard 7189's, not the "A" version.
I guess I'll take the 6П14П-EB route... Now I'm looking for some tubes on Mercado Chifle, our local version of Steals Bay
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