On testing again the set drew proper current. I also re-balanced the pots so the movable contact was at its electrical center. The now functional pot resulted in the proper bias on the tubes, about -22 to -23 volts which was correct for the applied levels of plate and screen voltages according to my tube manual. I made sure the other pot which was intact was also cleaned. Cleaning the inside and applying some more DeOxit directly saved the pot. The sliding contacts were being interfered with by a greasy substance that I assume was hardened lubricant. I pulled the offending pot and opened it. These are nominally 100K ohm linear pots. While measuring fine end to end, the tap was not making proper contact. The problem turned out to be the balancing pot on the channel I was testing. However, I had new respect for this "weak" pair of tubes. Not the best thing for 7591A tubes which, at the voltages in this set, are running near their margin. I quickly isolated the cause, zero grid bias. Time to check under the hood (with the tuner-amp on its back). It was working, but the ammeter was slowly inching towards about 1.8 amps, too much power draw with only one channel active! I shut off power. After the 5AR4 pair kicked in, I heard the pleasing sound of FM off-station rush noise. Then, watching the AC ammeter closely, I slowly powered the set. I connected an antenna clip lead and a test speaker to the one channel and set the controls to FM stereo. I decided to just test one channel of the amp section at a time, using the weaker tube pair. On checking the output tubes and rectifiers with my stingy TV-7 military tube tester, I found one pair of 7591A tubes to be weak, but the other pair and the rectifiers easily passed the test. I had checked resistance on those pots and had not found a problem. I did not immediately check the balance pots for the grid bias adjustments on the output tubes, a mistake on my part. However, the voltage from the bias supply with the silicon rectifier in place was spot-on. I replaced the little selenium bridge with a silicon bridge expecting to have to reduce its output to match the 48 volts. It wasn't producing even half that which was obvious because the four 12AX7 tubes did not show a proper filament glow. That means the supply must deliver about 48 volts output after the filter capacitors and resistors. The manual and schematic do not show the voltage output from that bias rectifier, but it supplies the DC filament voltage for four 12AX7 series-connected preamp tubes. The bias supply in the Scott 340, like similar sets, is by way of a small full wave selenium bridge rectifier. Proper negative grid bias voltage is vital in such sets. The cathodes for all the 7591A output tubes are directly grounded. That is an important step in this set and many other high power audio tube sets using a fixed bias supply as opposed to cathode bias. I gave it a thorough cleaning with waterless hand cleaner and old toothbrushes and rags.Īfter safety checks such as checking the fuse for proper size and for any line to chassis leakage which was not a problem in this set, I often do an initial power-up without the high voltage rectifier tubes to check the AC voltages from the power transformer and proper current draw for just the filaments. Dirt is typical for a set that has not been used in a long time. September 1962 ad in 'High Fidelity' magazineĮxcept for an accumulation of dust and dirt, the set was in good cosmetic condition including the wooden case. The set is now often identified as a Scott 340A to distinguish it from the very different 340B. The optional case was an additional cost. Advertised price for the 340 was $379.95. It has a total of 20 tubes including four 7591A output tubes and a pair of 5AR4/GZ-34 rectifiers. The set essentially combines the Scott 350 FM multiplex tuner with the Scott 299C amplifier. Introduced in April 1962, the HH Scott 340 "Stereomaster" tuner-amplifier is rated at 27 watts per channel for a total of 54 watts RMS (advertised at 60 watts output and later 70 watts). Scott 340 "Stereomaster" tuner-amplifier without its walnut case HH Scott 340 Stereomaster tuner-amplifier HH Scott 340 "Stereomaster" tuner-amplifier
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