Workon the 2400 begins
Wednesday, October 10, 2012 at 10:01AM Back in 1962 one of the coolest and rarest NuTone Intercom models was the 2400 Stereo Music Intercom System. The 2400 consisted of 4 main components, a stereo amplifier, stereo AM/FM tuner, stereo turntable and either the optional stereo reel-to-reel tape player or a the less expensive LP record storage unit.
The stereo amplifier and tuner are vacuum tube designs, which makes them exceptionally rare and cool today. What most people don't realize is that back in 1962 FM radio was still a fairly new idea as was stereo music. NuTone's decision to design and build a stereo music intercom system would be equal to Apple's decision to design and build the first iPod. The model 2402 stereo AM/FM tuner places NuTone in the same company as Fisher, Magnavox, Sherwood and Scott for designing stereo tuners.
This 2401 amplifier and 2402 tuner arrived at the shop in basically all original condition. Neither unit works and they definitely are in need of rebuilding. Working on tube equipment is different than modern solid-state models. The 2401 and 2402 are hand assembled models with point-to-point wiring and they use electronic components that are unique and difficult to replace with today's modern parts.
The first order of business was to clean both units, they were covered with 50 years of dirt and dust. A simple and gentle method of cleaning these units is to use a soft bristle brush and a vacuum to sweep away the dirt.
After the cleaning it's time to test the tubes. The 2401 amplifier has 10 vacuum tubes, 6 of the tubes tested bad. One interesting discovery was that the 4 pre-amp tubes have been replaced with higher quality tubes. NuTone typically used Mullard brand tubes in their intercom sets, original tubes will be stamped with the NuTone logo. In the 2401 I found that the 4 12AX7 preamp tubes were vintage Telefunken brand German tubes.
Telefunken tubes are highly sought after by audiophiles for their high quality and smooth sound. Finding this high level of tube in the 2401 must mean that a previous owner cared a lot about have their system sound as good as it could.
Testing on the 2402 tuner was next. The 2402 has 11 vacuum tubes, 7 of the tubes tested bad. This was not to surprising because all the tubes in the tuner were original NuTone labeled tubes.
Along with the replacement tubes other various parts are planed to be replaced. These models use various capacitors which after 50 years need replacement. In the 2401 there is a Selenium Rectifier that must be replaced with a modern Silicon diode.
Tubes and parts have been ordered and should be delivered next week, more to come...
See all of the 2400 project pictures here.
Cindy & Christopher
Work is progressing on the NuTone 2402 AM FM Tuner. After the new replacement tubes were installed, the tuner was given a short test. The FM reception and sound was fairly good, however there wasn't any AM reception at all. This problem was traced back to a loose connection on one of the tube sockets. Once the loose connection was repaired the AM worked well.
I moved on to replacing the original electrolytic capacitors throughout the chassis.
Mounted on the top of the chassis is a multi-tap can capacitor which contains 2 – electrolytic capacitors. Each capacitor has a value of 40 mfd @ 300 volts.
Finding exact replacement can capacitors can be tricky, no only the values must be right, but the physical size must match also.
I found a CE brand can capacitor which physically fit the space in the chassis, and contained 4 capacitors with the values of 20 mfd @ 475 volts. This is a good choice for a replacement part because it fits into the chassis correctly and the values can be matched to the original capacitor by wiring the new capacitors in pairs, in parallel.
I took many pictures of the original connections as I disassembled the wiring, to make sure the new component was reinstalled correctly.
The new capacitor fit perfectly and wiring it up was straight forward. The blue arrows point towards the ground (negative) tabs (which are common with the can itself) these are twisted slightly to lock the can on to the insulating plate. The red arrows point towards the positive terminals of each of the 4 capacitors inside the can.
I found that the original can capacitor had leaked at some point, this is why it's necessary to replace these 50 year old parts.
The red arrows point to where jumpers have been installed between pairs of capacitors to increase the value of each capacitor set from 20 mfd to 40 mfd.
With new capacitors installed throughout the tuner, the over all sound quality is much improved. In fact this tube tuner puts many modern solid-state tuner to shame. It has a nice spatial sound quality that often is only heard on tube tuners.
The final phase of the tuner repair will be the FM Stereo Multiplex module.
See all of the Nutone 2402 Tuner Pictures