Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

[nzphil]

I have a Yamaha YST-C10 mini-stereo that was made around 1990. It has two cassette decks, a CD player and an AM/FM tuner. When it is turned on in tuner mode, the cassette deck capstans normally run for about one second and then the radio will begin to play. Currently, however, the capstans run continuously for about five minutes followed by running intermittently in two second bursts for another five minutes. The radio will play softly when the capstans stop during the intermittent phase. After about 10 minutes of this, the capstans stop completely and all the stereo functions work fine.

I've looked for loose connections and tried pushing the reset button on the back of the unit. I also contacted Yamaha to get a service manual but was told it's no longer available.

I'm new to this forum and any suggestions would be much appreciated.

Cheers,
Tom

[R_J]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

It does sound like weak capacitors in the unit, likely in the power supply. post some pictures of the boards etc. so we can see what we are dealing with.
Here is a service manual for a YST-C11 which I suspect might be similar.

Start by checking the voltages coming from the power supply board. Because it uses transistors as regulators, it could be one of the electrolytics in the base of the regulator transistor causing a low voltage problem, these caps will usually look ok and just be dried out.

[nzphil]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

Ok, thanks. Here are some pictures. The power cord can be seen exiting the unit on the left in the second picture.

[nzphil]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

I checked the voltages coming from the power transformer. I measured 20.8 volts with red/black wires (shown in picture); 42.2 red/red; 2.7 red/blue; 5.0 red/yellow; 7.3 red/blue. I'm not sure how to check the voltages coming from the power supply. I tried touching two pins on one of the connectors on the power supply board and got zero volts with it set to 200 AC (see photo). Is this the correct way to do it? I also attached photos of the power supply board.

[R_J]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

It should be obvious that the voltages from the transformer are ok because the unit is basically working. If there is a problem it is likely with the DC voltage that supplies the cassette deck section.
Page 60 shows a plug going to the deck pwb, check if the voltages on that plug are correct. -15vdc, +15vdc, S_5v, S+12v etc.

[petehall347]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

is there a driver transistor for the motor ? maybe start there and work back if so .

[nzphil]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

I found the 13-pin plug on the power supply board that goes to the deck pwb and put one multimeter probe on the brown wire (on the far right in the photo), and the second probe on each of the other wires (#1 is on the far left in the photo).

I got these values for dc voltage:

1. 13
2. 19
3. 19
4. 14
5. 14
6. 14
7. 18
8. 19
9. 19
10. 1
11. 1
12. 1

Not sure how to interpret these values.

[R_J]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

Pins 9 & 10 are ground (see the schematic), not the pin on the right, that is pin #1 and should be -15vdc. If you don't have the meter negative lead on ground, the voltages don't mean anything.
Pin#1 is the brown wire, so count over to pin 9 and use it as ground, then post the voltages
Pin1 =
pin2 =
etc.

[nzphil]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

Ok, thanks. Using Pin 9 as ground, here's what I measured:

Pin 1: -14.2
Pin 2: 14.2
Pin 3: 12.2
Pin 4: 11.4
Pin 5: 5.5
Pin 6: 5.5
Pin 7: 1.4 --> 3.85
Pin 8: 0.02
Pin 9: NA
Pin 10: 0
Pin 11: 5.3
Pin 12: 4.96
Pin 13: -1.4

The voltage on Pin 7 started as 1.4 when I first turned the unit on (and the capstans were spinning continuously) and reached about 3.85 volts after 10 minutes or so (and the capstans were not spinning continuously).

I looked “upstream” of Pin 7 in the schematic and found transistors (?) labeled Q601 and Q932 (see p. 34-35 of the attached pdf).

I tried to measure the voltages for the pins on these transistors. Not sure how these pins are numbered, so I'll label them based on the letters that appear above them on the transistor case (X, S, and A). (see photos)

For Q601:
X and S: -0.1
X and A: 3.8 (capstans not running) -> 3.9 (capstans running)
S and A: 3.8 -> 3.9

For Q932:
X and S: -3.8 -> -3.87
X and A: 1.4 -> 1.51
S and A: 5.2 -> 5.4

[R_J]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

The main voltages seem to be ok.
Pin7 is the MUTE line, it will change slowly after you first turn on the unit, this is normal. Since this is a problem that seems to correct itself after a few minutes, you might want to try using a heat gun (hairdryer) and try heating sections of the boards to see if you can speed up the time to normal operation. It might be hard to trace down any other way.

[budwich]

Re: Yamaha YST-C10 stereo capstans run for 10 min when unit is turned on

what status lights / displays / indicators are being shown during the whole "episode"? Are you able to isolate the buttons from the boards so that they can be disconnected just in case there is some "pseudo / ghost" button action happening?