Greetings. I joined to try and help as well as get some clues on Audio Research AR-1 speaker woofer amplifiers. Have a friend who owns several. I've attempted to repair several of them with little success. Worked in the business for years (Audio Labs and Stereo Sound Studios in Des Moines) before moving on to public television and eventually transmitters.
Keep my hands in things by re-doing Hafler power amps, building scratch pre-amps etc. and occasional repairs for friends, co-workers and other acquaintances.
Greetings from an Iowa native (Burlington), now living in St. Cloud, MN, where it's even colder. What are the tubed components in your pix?
Hi steelhanky. The blue box is a phono pre-amp built on a Chinese board that doesn't seem to be available. When I first put it together I used a 240VAC transformer that I had laying around. It had horrible low frequency oscillation and I set it aside for a year or two. Since the board requested 300 Volts I suspected that might be the trouble, even though it did the same thing when powered from an external DC supply. Finally I found a suitable 300 volt unit.
After it arrived and before I tried it I thought more about it and compared what I could (didn't come with a full schematic) with the original Marantz 7 which it pretty closely resembles. The only thing I could really find different was the coupling and the load after the first stage was way different (.1 vs .01). Installed original values and put it all back with the 'original' 240 Volt transformer and the oscillation was gone. Went ahead and put the 300 Volt unit in, found some tubes that were not too noisy and am pretty satisfied with it now. The other unit with tubes is an Audio Research SP-6 (C?) that came to me years ago after another friend's repair attempt went on too long. I added a mute relay with on delay as these seem to blow up power amplifiers when the power dips.
So I guess it's good to trust but verify those Chinese boards? Stay warm, spring is on the way!
After it arrived and before I tried it I thought more about it and compared what I could (didn't come with a full schematic) with the original Marantz 7 which it pretty closely resembles. The only thing I could really find different was the coupling and the load after the first stage was way different (.1 vs .01). Installed original values and put it all back with the 'original' 240 Volt transformer and the oscillation was gone. Went ahead and put the 300 Volt unit in, found some tubes that were not too noisy and am pretty satisfied with it now. The other unit with tubes is an Audio Research SP-6 (C?) that came to me years ago after another friend's repair attempt went on too long. I added a mute relay with on delay as these seem to blow up power amplifiers when the power dips.
So I guess it's good to trust but verify those Chinese boards? Stay warm, spring is on the way!