Revamping a Vintage Yamaha T-80 Tuner
Recently I acquired a Yamaha T-80 tuner. This tuner was manufactured in the mid 1980’s, probably in 1985 from what I can tell. I proceeded to put the unit in my system and connected it to a standard FM dipole antenna. My initial impression was that this had some potential, but was going to need a lot of work. This appeared not to be your usual consumer toy tuner.
I did a bit of web searching on FM tuners and found one particular web site that had a fair amount of information: www.fmtunerinfo.com The general consensus seemed that in the mid 1980’s a number of Japanese and American manufacturers made some very good consumer issue FM tuners. The general consensus also to be that after the 1980’s most of these companies decided not to pursue this much further. This may well be due to the decline in this section of the market.
I then proceeded to contact the Yamaha service department for the service manual. Yamaha did not have extra manuals for sale but they did send a Xerox copy of the one they had in the service department at no charge. Thank you Yamaha!!! It’s not often that one can find a service department this helpful and cooperative.
After looking over the schematics I was pretty impressed by some of the techniques that they were using in the RF section and made the decision to bring this unit back to original condition and better. I took some initial data on the +/- 12volt supply lines right at pin 4 and pin 8 of the output op amp. The data was not exactly what I would call acceptable.
One of the biggest problems that aging units have are the electrolytic capacitors. A generally accepted life span for an electrolytic capacitor is approximately 15 years for consumer applications. There is plenty of variation here as heat will accelerate the aging process. One of the more pronounced effects of electrolytic capacitor aging is that they start to behave more like resistors rather than capacitors, and when they do finally fail they generally take a bunch of circuitry with them! Considering that this is a twenty year old unit with hard to find, if not impossible to find parts, the first step will be to replace ALL of the electrolytic capacitors. This may appear to be excessive for some folks, but good quality, high temp (Sprauge 517D series:105 C) electrolytic capacitors are not exactly expensive. My replacement cost was about forty dollars, including shipping, for the entire unit.
Replacing all of the electrolytic capacitors is not exactly what I call exciting or glamorous; in fact it’s downright boring. I do have some general desoldering tips for the DIY folks. When you are desoldering a component from the PC board, use a drop or two of flux on the area that needs to be desoldered. The flux will help the solder flow quickly and easily, speeding up your work, which will give you less chance of destroying a pad or etch. Cleaning up can easily be done with an acid brush and 91% Isopropyl Alcohol available from your local drugstore.
After all the electrolytic capacitors were replaced I did add some 0.1uf film bypass caps in the power supply and added some 0.01uf ceramic bypass caps in the RF section. To do this; simply solder the bypass caps directly across the electrolytic capacitors. Most consumer units of this era did NOT have these in their units to start with. Once this was completed I went back to my original supply pins on the output op amp and found a 9 db reduction in ripple and approximately 18 db reduction in broadband noise.
A few words of caution to the DIY folks on electrolytic capacitor replacement: The first instinct is to increase the capacitance for better filtering. The problem with this is that surrounding design may NOT be able to handle the surge currents of larger capacitors and cause premature failure of rectifier diodes, pass transistors, and force a transformer into saturation. I would also like to note that on the + 12 volt rail I found the ripple was down to 1 mV p-p and the –12 volt rail was less than that after doing the electrolytic capacitor replacement.
My next step is to replace a good portion of the carbon film resistors in the unit. The carbon film resistor is typically the dominant source of thermal noise in audio circuits. Metal film resistors have much lower thermal noise. I proceeded to replace all of the carbon film resistors in the audio output circuit, multiplex circuit, IF filter section, and the bias resistors for the pass transistors in the power supply. To ensure accuracy I measured each resistor just to be sure I was reading the color code correctly and replaced each resistor with the equivalent value metal film resistor. The reduction of thermal noise is very audible to most folks and as the spec’s will show a real difference in the S/N ratio. In addition I replaced the JRC 2041 dual op amp with an OP275. An OPA2134 will work just fine in this application too.
Now that I have completed the basic restoration and noise reduction of the T-80 tuner it’s time to start on setting up a decent antenna. An outdoor rotating antenna would be the optimum choice here, but my present living circumstances are not going to allow that. I am not a DX guy as my major goal is to get decent reception from the many Boston area radio stations. A friend of mine who had come to visit recommended using the attic, which has enough space to put in a 4 element Yagi. I presently live about 15 miles up the coast from Boston so mounting a small Yagi from Winegard (Pro 6000), http://www.winegard.com/offair/pdf/pr-6000.pdf seemed like an acceptable alternative to the standard FM dipole. I spent a few evenings on antenna placement and found the following. Just about anywhere in the attic with antenna pointed toward Boston gave me about the same results. Even with an extra 37 feet of RG 6 added I found no real difference monitoring the signal quality meter and the multipath meter. I got the RG6 I am using at Home Depot. I also did a search through the Belden Master Catalog and found that RG6 typically has an insertion loss of 2.5db for every 100 ft. at 100 MHz. I am presently using about 32 ft. so that gives me just under 1db of loss from the antenna. The only spot where I had problems was near the chimney. The antenna itself was very inexpensive. The cost from Lashen Electronics in New Jersey was about 47.00, which included the mast and Balun to convert the 300 ohm antenna to 75 ohms. The antenna is extremely light so body building is not required for placement. The shipping cost came to about 35.00 due to UPS claiming it was an oversize package. UPS also managed to nearly destroy the cardboard packaging but the antenna itself was not damaged.
The final result confirms the age old RF truism: “There ain’t nothing like a good directional antenna to boost signal quality and reduce multipath.” Multipath distortion has that real grinding quality to it that all of us could do without. Lowering multipath should be of primary concern when placing an antenna. Fortunately the Yamaha T-80 has a multipath meter to monitor this when you placing the antenna.
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