EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: uslrs on April 23, 2024, 10:33:58 am
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Hi All, I'm restoring this LEM Baby (built in the 1970's) and there has been a major issue with the power amp and repairs carried out at some point in the past. I've been unable to find a schematic, only one for a later version which has an IC in the first stage and has a complementary output pair, which mine hasn't. I've produced a schematic (attached) from the pcb but most of the transistors are unmarked and I'm not sure whether they are original or replacements. I'm struggling to make any sense of this design so would much appreciate any help or information which might enable me to get this power amp working. I've attached a notated photo of the pcb components to identify the transistors in the schematic. Hoping someone recognizes this and can provide some information. Many thanks.
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From what I can see LEM took the amp schematic from this old RCA databook (RCA Transistor Manual SC12 1966).
.... with some small variations, but it's very similar.
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Many thanks Picburner - that seems to be very much like the LEM circuit. It will certainly be a help with the repair process.
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So it looks like an amplifier, but C5=100uF, is that there to delay the signal to the top output transistors and make the echo effect ?
I play guitar, but I haven't tried any effects circuits yet.
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That capacitor seems to be for filtering.
Effects of all kinds would be done at earlier stages and lower signal levels, and the power amp just makes it bigger.