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T-80 Board Layout

By Dan Banquer

Most FM tuners from this era suffer from two major problems. They have a high and reactive output impedance, and the grounding scheme inside the unit is at best dubious. This is a unit from the era when single sided boards where standard issue. The near total lack of ground plane and high impedance returns to the power supply are very much part of this unit. One wonders what the performance might have been had Yamaha decided to use a double sided board with a fair amount of ground plane on the component side of the board. The following will show an approach to tackle both of these problems.

The following schematic is of the T-80 output circuit and the buffer installed to lower the output impedance so I can drive a Jensen Audio Isolator, which is just a pair of Jensen line level transformers in a package.


Please note the feedback network from pin 2 to pin 6 of the op amp. The 2.55k ohm resistor is the approximate resistance match of T-80 output circuit and the two capacitors in parallel represent the capacitance match of the T-80 output circuit. Although it is not shown here, when the T-80 has the high cut filter added the capacitance will double. The high cut circuit is only activated when you find a very noisy station that is not coming in well in local mode. For my present antenna configuration I really don’t need this and the output circuit is “optimized” for local stereo mode. This circuit will reduce upper order distortion, especially when connected to a relatively low impedance input and lower the output impedance to 100 ohms to drive the Jensen Audio Isolator. For technical information on the Jensen Audio Isolator Go Here: http://www.jensentransformers.com/datashts/ci2rr.pdf

And for a good price on this unit try here:

Jenson CI-2RR RCA Audio Hum Eliminator

At time of writing the CI-2RR is available from www.markertek.com for 119.00

Although transformers certainly have some drawbacks, they are very effective in isolating the ground of one unit from another. Given the grounding scheme used in the T-80, the use of the transformers has been very effective in reducing noise. They are not cheap, but many of us find the performance well worth it.

When I received the tuner back from alignment at Stereo Surgeons in East Hartford CT. http://www.stereosurgeons.com/ I noted many performance changes. Sensitivity, stereo separation, and distortion had definitely improved, but with the increase in sensitivity noise had also increased. I went back to the schematics for further study, and also had a good long look at the board layout. I noted that all the 0.01uf and 0.022uf bypass caps in the RF and IF sections were ceramic disc caps, and after looking through some data sheets on class three ceramic disc caps I noted that aging characteristics on class three ceramic caps was somewhat similar to electrolytic caps. I decided to replace them with NPO ceramic caps. This may well be excessive for this application and in all probability the CKO5 series caps would do just fine. So I will plead “mea culpa” to being a certifiably excessive here. I also added 0.1uf bypass caps across the electrolytic capacitors in both the RF and IF sections.

As I previously noted in this article the board layout is single sided and ground plane in the unit has been sacrificed to accommodate a single sided board. The main ground to the chassis appears to be a chassis support bracket that goes down between the RF section & the rest of the unit. I added ground straps using 1/8 inch flat braid from the return of the power supply to the chassis. From the return of the power supply I added a ground strap that went to small ground plane near the audio output. From there a ground strap was connected to a small ground plane of the IF section. From that same ground plane in the audio output section I added another ground strap to ground plane next to the bracket. The very last thing was to install a 75 ohm F pin connector for the RG6 cable run from the antenna. When the Yamaha T-80 was produced Yamaha used their own custom 75 ohm connector and these parts are no longer available. Note from the photo the short lead length down to the P.C. Board.

The decrease in noise was very audible from the above “modifications”, and now needs to go back for measurement to verify this, but it is right here that I ran into a brick wall. After e mailing a number of people recommended at www.fmtunerinfo.comI found the following: The shops that still do FM tuner alignment have found it cost prohibitive to keep their equipment up to original factory specifications and also find that there is little demand for this from the public, especially when it comes to accurate measurements of noise in this application. While I certainly understand from a business standpoint I am still saddened by this state of affairs. I would like to apologize to the readers of Audioholics, but please keep in mind that Gene and I appear to have exhausted all possibilities. If this changes, we will gladly update this article.

Well; what’s the final verdict here? And, just how good is FM radio in the Boston area as viable music source? The first thing I need to tell you is that there is no shortage of variables here. FM pop music stations that add their own compression on top of the compression already on the CD will not be discussed further. My observation is that some stations appear to care about the quality of their transmission, and have the budget to do so. National Public Radio in particular and the local college stations are O.K., but they really don’t have the budget they would probably like to have. Playback can vary from records to CD’s, but I will tell you this: with this tuner and antenna set up, when NPR decides to spin a well recorded CD, the playback through this tuner comes very close to a top notch reel to reel tape player in near perfect working order. I’ve even surprised a few folks into thinking that they were listening to a CD., and the level of performance that is possible has raised my eyebrows from time to time. This maybe hard to believe folks but FM radio here in Boston is better than I ever expected and I have been well rewarded for the time and money spent. If you can find some money in your charity budget give to National Public Radio and your local college stations, and you have a decent tuner, your ears will be well rewarded.

 

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