| Secret Chimp |
I have a 1979 Technics SA-300 that's in good shape, save for a slightly scratchy bottom end on the volume control, and the fact that the FM tuner doesn't work at all.
I get AM fine, and the little signal-power meter moves like it's supposed to when tuning AM; but when I switch to FM, the meter just ticks to the right a little and stays there, all while I get no sound whatsoever. I don't get any static or anything, and all other positions for the switch (AM, Aux, Phono) work fine. Flipping the FM Muting switch up and down causes a little change in the slight background noise it always makes, but I never have any static or signal jump in.
A dirty switch (in reference to the FM/AM/Aux/Phono one) doesn't really make sense, as the meter moving off-center suggests that something is getting through the switch in that position, and it seems like a dirty switch would have one position work fine with the others hindered, instead of the other way around. (The switch is a slide-type that's moved by a couple of gears connected to the knob)
I've taken off the top, and I can't see any wires that have popped off. There are a couple of box things with what look like flathead screws painted red or yellow on their tops next to the actual tuner, but I have no clue what those things do, and I don't want to mess up some other aspect of the thing.
Any idea what could be wrong? I could take some pictures of the insides if that'd be helpful; I did that when I was trying to find some specific caps in my Sharp boombox and it worked out well. |
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| anatech |
Hi Secret Chimp,
Check pin 11 for supply of around 10.3V on IC101 (AN377). It is possible that the coil adjustments on T101 are out. Don't touch it yet!!! (S - curve and THD adjustment).
Try putting a small ferrous object into the coil without touching anything. If the meter moves, the coil may need adjusting. You need an fm stereo generator, 'scope, freq. counter and THD meter to set it up. If you don't have these, take it to someone who does.
The other possibility is a bad AN377. Don't replace it yet.
-Chris |
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| Secret Chimp |
Heh...unfortunately, I know next to nothing about audio tech, and so I have none of the equipment you mentioned (unless an analog multimeter works the same as a THD meter for this purpose). Though you are the first person who's answered beyond "dirty switch," so thank you.
What I'm going for here is a know-nothing freebie/junk parts homebrew repair, if at all possible. I spent 25 bucks on this thing, and it'd be easier for me to just spend another 25 bucks or so on a matching silver Technics slimline digital tuner (or just another SA-series receiver) than at least twice that on a professional repair. Essentially I'm just looking for a self-fix via measured fiddling, something that I end up being good at after I actually know what I need to be doing.
It should be noted that at this point, I'm asking questions just to gain a better overall understanding of what needs to be done, not because I'm planning on diving in to it prematurely.
Anyways, I've found the IC101 and, I think, the T101 coil. There's a silver box with both a red and green flathead screw on the top, and the only other things around the T101 label on the board are resistors and caps, so I figure that's it.
I've never done anything with chips before, so I have a couple of questions on that:
-Are pins counted straight across the top, then across the bottom (Top 12345, bottom 678910), or in a row by row fashion - i.e. the first pin on top is 1, the first pin on the bottom is 2, etc.
-How would I check its supply voltage with my multimeter? Would I just touch one probe to the pin and ground the other out?
On the coil: do I need to somehow remove that metal box around it to do the object check, or would bringing a ferrous object close to the box produce the same effect (if the coil is off)?
If I end up understanding the first couple of steps (and if my multimeter will check the voltage properly) and the coil seems fine, what would be the next step?
Alternately, if I'm able to test the coil and I get a voltage change, would it be possible to remedy this by buying a new (or somehow OEM) coil, or through an unscientific screw-n-listen method, or is my only option a professional repair/diagnosis? |
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| anatech |
Hi Secret Chimp,
Hopefully, you have a friend that can help here when the time comes.
IC's are numbers from the dot (pin 1) along the side and around the back towards the front again. This is a 16 pin chip so the dot or dimple is pin 1, directly opposite is pin 16.
Put the negative lead of your meter to the metal chassis. Touch the positive lead to the point you are trying to measure. I hope you have a digital meter. If it's analog and cheap, consider replacing it with digital and a little less cheap.
Do not touch the shield for T101. Do not adjust it either. This is a very sensitive adjustment. Touch a metallic screwdriver to the core and watch the meter, try each core. You may not get any movement, or you might. Let me know. In general, do not adjust anything unless you have the equipment to do so. You will cause more trouble than you solve.
Alignment is a pro service. I recently repaired a Marantz tuner that another good shop couldn't fix. Someone had "twiddled" the alignment. The other shop even had the manual. I will let you know what to do when you reply.
-Chris |
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| Secret Chimp |
I found and tested pin 11; I started off with the meter on 10 DC V, expecting it to pop to the other side of the scale, but instead it says that I'm getting no more than 4.6-4.7 volts off of that pin. I touched the cores several times with a couple of different screwdrivers, but the meter didn't move.
So, unless I did something wrong, the T101 adjustments are fine, but the supply voltage for that IC is still off? |
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| anatech |
Hi Secret Chimp,
Check C110 for being leaky and R114 (100 ohm) for over temperature marks. Measure this if you can. Also measure the DC voltage at each end of R114. If you get around 13VDC on the outside end, the AN377 IC is likely bad. Replace it in that case.
-Chris |
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| Secret Chimp |
On R114, I get 50 volts on the end with the gold band, and around 60-65 volts on the other end (there's what I think is a little negative sign with a circle around it printed on the board next to that end)
R114 doesn't have any overtemp marks (that I can find) on it either.
My current meter doesn't have any kind of capacitance measure on it, so I can't check C110 for leakage, unless there's some method of checking with only a volts/mA/ohms meter. I don't plan on sticking with this thing forever, I just can't get my hands on a better meter for at least a week or so.
Would the cores of the coil having red/green paint over them have had any effect on that test? There isn't any bare metal on either of the screw-top things, just red or green coloring.
Regarding meters: I've found a lot of these meters for cheap on eBay. Judging by the top under the display, it looks like they're Chinese or something. Should I spend 40-50 bucks on a "good" digital meter, or will these comparatively cheapo things work fine? |
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| anatech |
Hi Secret Chimp,
Okay, R114 feeds the chip where you measured 4.6 ish volts before. The supply from the regulator transistor (Q751) is around 13VDC and it's fed by around 20VDC. What kind of meter are you currently using??
If you have about 6 V coming from Q751, then the regulator circuit has failed. Good news. Take some measurements from around that area.
The meters you show may be okay or no better than what you have. If you can possibly afford to buy a Fluke it is well worth it. If not, buy the best you can afford.
-Chris |
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| Secret Chimp |
Wow, do I feel dumb; I was reading the wrong scale on my meter.
The proper readings for R114 are 10 volts on the gold-striped end and about 12.5 volts on the other end (has a kind of pink/flesh colored band on it).
I checked all 3 pins on Q751, and they're 13 volts, 20 volts, and 13 volts (also reading the 50 volt scale instead of the 250 volt scale).
The meter I have is a GB Instruments analog thing. |
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| anatech |
Hi Secret Chimp,
Alright. The supply voltage is proper. Depending on the cost you may want to try another chip. There may be a twiddled transformer or a bad chip. This is too hard to tell without the unit being on my bench. If you touch the adjustments, may as well just go out and buy a new tuner.
I was hoping it was a bad supply. This is repairable easily.
-Chris |
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