I own and enjoy an Olson AM-240. In my efforts to make it play, as it arrived DOA from EBay, I had to do some extensive parts upgrades. After weeks of trying, I finally got my AM-240 to sound. That was many years ago, like back in 2002-2003. Now Fast forward, I just pulled the AM-240 out of storage last week and the first thing that I did was to power it up using my Variac. One of the Power Supply Diodes was shorted, blew the fuse, and this happened the last time I used the AM-240 in my Garage in 2013, it also blew a fuse. So I replaced both Diodes with GP15M/1431 that are 1000PIV, 1,5A and the amp came alive again. Next, I connected the AM-240 amp to my 100W Stereo Dummy Load and looked at 1KHz on my Dual Trace Oscilloscope.... oh. As usual, for older amps, one channel had less power than the other, and the top end of the louder channel was clipping. I adjusted the balance Control to compensate and it sounded good over my speakers when I put the AM-240 on duty, the distorting channel now behaved. But I was curious about the Channel imbalance, as this is a problem I have seen with many older Tube Preamps and Integrated Amps. On the AM-240, the Stereo Volume Control is not measuring well. One wafer was 1.23 Meg, the other was .882 Meg. This also happened on my Dynaco PAS3x where I was lucky to find a NOS Volume Control (out of production). I ordered a NOS 1 Meg Stereo Volume Pot with a Loudness Tap. The reason I post here on my AM-240 is that I had to sacrifice some of the internal stuff like the Headphone Circuit and also the Output Phase Switch to get it to play. I bypassed both of these, so that my Output Transformers are now directly connected to the 4-8-16-C terminals, getting rid of the extra stuff that I really do not need or want. Hey if anyone needs a DC voltage reading chart, I can provide it. The final thing, I get no sound from the Phono section, it was and still is totally DOA, I plan to eventually draw out the schematic on paper and also draw out the rest of the amplifier to share. Reverse Engineering will take a few month's, as AM-240 schematics are non-existent. How does the AM-240 sound? I like it very much. The sound is very full and has depth, good strong Bass, Midrange is very sweet and extended with great Treble. The Loudness button only works at low volumes, it is too brash and not really needed. The tubes that I use, are the Russian 7189 types as this Amp will roast EL-84's. I haven't tried the EL-84M yet but I suspect that they will not like the high plate voltage the AM-240 and 7189 amps of that period need, it is in the higher 390V to lower 420V range, and that is clearly out of EL-84 Plate Dissipation territory. One could possibly change the AM-240 to Cathode Bias and put some large Power Resistors in the PSU Voltage Divider, but this is not good, I have tried this brute force approach in the past with 7189 amps and the results were not good. If your AM-240 got totally roasted, a Hammond Power Transformer could rescue the amp, but I would go Cathode Bias to keep the effort simple. The tubes needed for the AM-240 are Russian 6p14pEV (a.k.a. 6p14pEB) and also the 6p14pER. These Tubes sound great and can take up to 450V on the Plates.
If I ever find a AM-240 schematic I will share it immediately, in the meantime I am available to help get youre up an running.