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Onkyo T-4055 (1974,
photo)
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The T-4055 is an older tuner that has some fans. Our contributor Walt says, "I know the T-4055 very well. In '76 or '77, The Absolute Sound (probably Issue 5 or 6) had a tuner shootout which included such items as a Marantz 10B, the first Sequerra tuner, and the T-4055. The T-4055 was rated as #2 overall, and #1 in terms of bang for the buck and ease of listening. In retrospect it was somewhat overrated at the time, but considering I was able to buy them for $141 at the time, it was a stone bargain. I probably bought and sold at least two dozen over a 2-1/2 year period. The T-4055 has a switch for an audio output of the multipath. Other than that and a warmish sound, it's pretty basic. The bass is lacking dynamics, and the treble in the 6-7K range might also be slightly shelved down. Overall, it's a pleasant-sounding unit. It's no DX rig stock, and the AM section is miserable. It has a lot of room to work on the boards. There are quite a few cheap coupling caps on the boards which have probably started to leak in the past few years, and they should all be replaced."
Bill Ammons says, "The front panel has an output level control, muting, and noise filter control, plus the AM/FM/auto selector and tuning knob. There is a switch in the back to make the multipath audible, and a wideband detector output for hooking into SCA and quad adapters. I did pop the top - it is a 4-gang FM, 3-gang AM design with three green 280 kHz ceramic filters (single bandwidth), with the IF stages having a 10.7 MHz ceramic resonator from emitter to ground on each of the IF gain stages. The T-4055 has for its price range a very good RF front end. The RF amp and mixer are MOSFET and it has some well-spaced higher Q coils. It performs well in strong signal areas." Our contributor Alan has compared the T-4055 to tuners like a Magnum Dynalab FT-101A Etude and says the T-4055 is "one fine tuner, both from a DX perspective and especially sonically." Our contributor Greg F. says, "I have two Sherwood S-3000Vs and next to them the Onkyo sounds grainy, but it picks up many more stations. Also, it has performed flawlessly for 25 years." The back panel has fixed and variable outputs and jacks for an oscilloscope. Apparently early units, but not all T-4055s, had a 400 Hz calibration tone with an on/off switch on the back. The T-4055 usually sells for $45-75 on eBay, but almost anything is possible.
General DIY Tips
1.
Return ground path - On the Kenwood KT-7500, there is a way to help ensure the integrity of the return ground path. There are four screws that clamp the circuit board to the steel frame, which holds the power transformer and more. Loose screws can cause electrical problems in the KT-7500 and other tuners. First, remove the screws and inspect the board and frame for corrosion or dirt. Second, clean the interface with Caig DeoxIT or another commercial electrical contact cleaner. When reinstalling the screws, don't re-tighten them so much as to crack the circuit board - just good and firm.
2.
Power supply - Raise the transformer off the chassis with rubber grommets. Measure AC on chassis to outlet gnd with plug oriented both ways, mark plug for lowest noise. New "low noise" diodes, with snubber circuits, bigger better electrolytic caps with smaller bypass caps. Dedicated regulators for audio/mpx sections if you are really going all out. AC power noise filters will also help, either internal or external.
3.
Audio section signal routing - Bypass the external de-emphasis switches on the back panel. Direct wire them on the board. Bypass muting relays, switches, board traces, old internal wiring, etc. In the KT-7500 (described below), the audio signal went through two switches, a relay, long board traces, around the power transformer, and then to the RCA jacks. This was all bypassed, and instead CAT5 teflon twisted pair (one wire grounded at one end for shield) was used to go direct from the op-amp output, through 100-ohm resistor, to the fixed RCA jacks. This change alone was responsible for a huge increase in resolution in the mid and high frequencies. Note: you lose muting, but it's worth it.
4.
Op-amp mods - Typical stuff, replace stock op-amp with new high performance unit (i.e., BB OPA2604, OPA2132, etc.) Gold plated socket allows swapping. Add local decoupling caps to gnd on +/- rails (many tuner have no decoupling at all).
5.
Coupling cap mods - Many small electrolytic caps are in the audio signal path. Replace these with either Black Gate N or NX, NH non-polar electrolytics (Michael Percy) for large sizes (47 µF) and/or use polypropylene caps such as Hovland, Auricap, or Solen (less expensive) for 1-10 µF values. Note that you really need a schematic to find these easily. In many cases, you'll find a very small DC offset out of the op-amp. If your preamp can tolerate say 20 mV DC (mine easily can), then bypass (pull signal from in front of) the coupling cap after the op-amp, usually a 47 µF or larger. The best cap is always no cap! Lastly, you can add a 5K or thereabouts resistor from - rail to output which will bias the op-amp into class A operation.
If the tuner has both fixed and variable outputs, you can do the wiring mods in such a way as to be able to still use the variable output with muting, and use this for tuning stations, then use the direct output for listening. This will use an additional input on your preamp, though.
These mods can be done in a long evening or weekend if you take it slow, and make a HUGE difference in the sound of most '70s era vintage tuners.
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