info: fmtunerinfo.com
Onkyo T-4500 (1989, $320, )
The T-4500, a black digital FM-AM tuner, is the little brother of the T-4700. The two tuners are quite different inside despite their similar exteriors. The T-4500 has the equivalent of 5 gangs and 4 ceramic filters, 2 of which are used in Wide mode and all 4 of which are used in Narrow mode. Our contributor Stephan notes that the T-4500 and the T-4650, a European model that appears to be identical to the T-4500 except for voltage and tuning increments, use the Sanyo LA3401 MPX chip, and the Sanyo LA1266 IF limiter/mute/metering IC for the quadrature detector function. Our panelist Jim did a Shootout and offers this general information about the T-4500: "You like buttons? We've got buttons! This tuner is a button pusher's dream. I counted 41 and that's without considering that the 20 presets become 40 with the shift button. One button I used a lot during my DX tests was the cable/mute button, which also changes the tuning steps from .5 to .25 MHz. Functions are defeatable but when you change stations, the mute can activate. There is an 'APR' (automatic precision reception) button that chooses the best reception parameters. It, too, can be defeated per the operator's needs. The T-4500 has wide and narrow bandwidths and a local/DX RF mode. Also included is a manually selectable high blend and manual/auto tuning switch. This model takes a slide-on (quick-connect) 75-ohm connection - a screw-on F-connector won't fit."
The T-4500's "Classified Memory Scan System" allows one to group preset stations into six "classes." Like many Onkyos, the T-4500 has excellent selectivity even in stock form, largely because two of its four ceramic filters have a narrow 150 kHz bandwidth. The wide filters, with a 230 kHz bandwidth, are first and last in the circuit (X101 and X104), while the two narrow filters, with a 150 kHz bandwidth, are in the middle (X102 and X103). The T-4500 would be a good tuner for a DXer or anyone in an urban area with strong signals. Our contributor Stephan compared his T-4500 to a Kenwood KT-880D: "For tougher (more crowded) receiving conditions, I'd prefer the T-4500, since it features one more gang, one more IF filter and a nifty channel separation correction circuit for narrow IF mode (simple but effective; look out for Q202)." Stephan also speculated, "On strong signals, the Kenwood with its nominally lower distortion LA1235 (instead of the LA1266 as used in the T-4500) may sound better, though that's probably more a matter of alignment."
Stephan points out a problem affecting some T-4500s: "A significant number of T-4650s (and thus also T-4500s) appear to be affected by more or less serious amnesia. This has been traced to the microprocessor not going to sleep quickly enough when the tuner is turned off. (That would be triggered by pulling the /HLD line low.) Instead it continues running for a while, discharging the goldcap. You may not notice this as long as the goldcap is still in good shape, but as it grows older and its resistance increases (aided by nice warm temps near the display), the voltage may drop too much for the µP which promptly forgets all of its memories. My own T-4650 shows semi-serious symptoms, it wants to be warmed up properly in order to save its memories, but then it'll keep them for weeks without problems. It appears the goldcap is being charged veeery slowly. Maybe more is involved here. The cause for the problem appears to be cost-cutting during the main PCB design. The more expensive T-4700/T-4670 (same µP board, but different main PCB) features an additional transistor for pulling /HLD low and seems to be much less frequently affected. The successor T-407/T-4850 adds a 100-ohm resistor in series with the goldcap and another 1k resistor to avoid shock discharging of a 1µ/50V electrolytic involved.
"What about a fix, you ask? For a naive and untested approach, connect a matching NPN transistor (Onkyo used DTC114YS on the T-4700 and TC144ES on the T-4850) with the base and emitter in parallel to R906, emitter to ground, while the base (and that is where untested comes into play) should go to the anode of either R813 or R814 (the Q806-side end of R825 would also work; these three spots are all connected). I heard that another approach would also work, but have yet to receive details. My T-4650 had two mods done on the µP board, one with a diode affecting the /DSP line (maybe from the factory) and another, more amateurish-looking one that essentially pulls up /HLD via 2.4 K ohm (and down when +5.6 V is without juice). Apparently the latter was an earlier attempt to fix the problem, with only moderate success." Typical sale prices for the T-4500 on eBay have dropped from the $200 area in 2004 and early 2005 to just $50-85, a great bargain, but up to $115 (in 6/09) is possible and still not a bad buy. The all-time low and high are $22 in 9/08 and $255 in 1/07.
