I've been helping a local church install a new pipe organ, and the Allen 301 MOS1 organ went silent in the service two weeks ago. The pipe organ is about 10 weeks from done, the 301 has been sold. The PO voicer and I measured the two Allen S-100 amps, at maximum volume got about 3 VAC (analog VOM) out into the two 5.5 Ohm DC resistance loads. So I volunteered to replace the main PS caps (14000 uf 40 v Sangamos) and some other e-caps inside. Mainly the other cap I replaced was the 10 uf power up silence cap. I missed the NP designation on the 15 uf NP stage coupler cap, and the 60 uf NP power limiter cap (2n3904-3906 sense output, collectors couple to the middle of the spreader bias diodes through the 60 uf np cap) , so I didn't order any.
The new rail caps were CDE 50v 15 mf 3000 hour service life. With those and new PUR caps, both amps were putting out 10 VAC into a DVM AC scale yesterday. The flute channel amp was sounding good on piano FM radio source, but the main channel amp was sounding transistor-radio ish on piano source. Volume was very good, much louder than before. The sound was probably good enough for another ten weeks, since organ signals have such low high frequency content, and no attack volume variation to speak of.
Inside on the amp PC board were two 22 bullet sized silver cylinders with red ends, one end obviously going to the ground plane. Marking was 1310 and round the other side, 110. Anybody think these might be 10 uf tantalum caps? Tantalums are very high on my list of usual suspects, I might cut them out, one end anyway. They don't appear on the schematic I can find. I really don't want to drop a $9 freight bill on another order for this, but might spend 20 minutes cutting loose the tantalums if that was what they were. The puky sounding amp had 2 VDC coming out the speaker terminals with no speaker attached. The good sounding one had 0.15 Vdc coming out the speaker terminal. There is no DC adjustment on the PC, or I've had done that already.
This organ model was first sold in 1973 and church lore holds that this was a remainder bought at discount in the early eighties. If so, probably the speaker midrange surrounds are cardboard instead of the short lived foam surround ones Allen was so infamous for in the mid 80's. No easy way to get to the speaker boxes anyway, they are high up in crawlspaces.
And BTW, I realize the "official" way to fix an old Allen is to get a couple of $1000 bills ready, call the Allen authorized rep, who will replace the entire amps with incompatible new ones that require an adapter kit to even plug up. As if RCA jacks in and spade terminals under screws out suddenly became obsolete as buggy whips.
No, I can't publish the schematics, Allen threatens people who violate their copyrights. The output stage is a push pull NPN-PNP Moto TO3 pair, the rail voltages are 36, the input op amp is a 381, the input cap a 4.7 uf, the bias spreader is a double diode not on the heat sink, there is no idle bias adjustment pot. The heat sink is massive, as befits an amp required to run for hours near the 100 w 4 ohm limit. So a little excess OT idle current is probably not a problem.
Thanks for listening.
The new rail caps were CDE 50v 15 mf 3000 hour service life. With those and new PUR caps, both amps were putting out 10 VAC into a DVM AC scale yesterday. The flute channel amp was sounding good on piano FM radio source, but the main channel amp was sounding transistor-radio ish on piano source. Volume was very good, much louder than before. The sound was probably good enough for another ten weeks, since organ signals have such low high frequency content, and no attack volume variation to speak of.
Inside on the amp PC board were two 22 bullet sized silver cylinders with red ends, one end obviously going to the ground plane. Marking was 1310 and round the other side, 110. Anybody think these might be 10 uf tantalum caps? Tantalums are very high on my list of usual suspects, I might cut them out, one end anyway. They don't appear on the schematic I can find. I really don't want to drop a $9 freight bill on another order for this, but might spend 20 minutes cutting loose the tantalums if that was what they were. The puky sounding amp had 2 VDC coming out the speaker terminals with no speaker attached. The good sounding one had 0.15 Vdc coming out the speaker terminal. There is no DC adjustment on the PC, or I've had done that already.
This organ model was first sold in 1973 and church lore holds that this was a remainder bought at discount in the early eighties. If so, probably the speaker midrange surrounds are cardboard instead of the short lived foam surround ones Allen was so infamous for in the mid 80's. No easy way to get to the speaker boxes anyway, they are high up in crawlspaces.
And BTW, I realize the "official" way to fix an old Allen is to get a couple of $1000 bills ready, call the Allen authorized rep, who will replace the entire amps with incompatible new ones that require an adapter kit to even plug up. As if RCA jacks in and spade terminals under screws out suddenly became obsolete as buggy whips.
No, I can't publish the schematics, Allen threatens people who violate their copyrights. The output stage is a push pull NPN-PNP Moto TO3 pair, the rail voltages are 36, the input op amp is a 381, the input cap a 4.7 uf, the bias spreader is a double diode not on the heat sink, there is no idle bias adjustment pot. The heat sink is massive, as befits an amp required to run for hours near the 100 w 4 ohm limit. So a little excess OT idle current is probably not a problem.
Thanks for listening.
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