Electronics > Repair

Troubleshooting Noise in Cassette Player Technics RS-M17

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Zoli:

--- Quote from: floobydust on December 23, 2024, 02:52:29 am ---...
For Q401, 2SC1846S regulator transistor, it has a built-in insulator TO-126 (SOT-32) package which is quite hard to find, I've never seen that and Panasonic has them as obsolete. "TO-126B package which requires no insulation plate for installation to the heat sink".
...

--- End quote ---
BD13916STU from onsemi is full epoxy body(no exposed heat sink on the back), doesn't require insulating plate.

dankinzelman:
Q401 has a heatsink, here is a photo. The transistors you are suggesting look like they won't be compatible with the heatsink I have, so I presume I will need to purchase an appropriate heatsink as well? Suggestions?

Also, some more info:

R401 measures almost exactly 2k (in circuit) and I measure 18.6V on the diode side and 15.4V on the downstream side. So I guess the regulator transistor is not getting enough voltage to operate? Is this a reasonable current draw?

Also I presume there is some uncertainty measuring resistors in circuit due to interaction with other components. In this case it's parallel with the collector and base. Do I need to lift a leg, pay attention to DMM polarity, or is there something I can do (discharge caps, etc) to improve accuracy? Or is this close enough for our needs?

I have been unable to locate R7 or R8 anywhere on the board and forgot to measure resistance to ground from the play head. As it turns out, on closer inspection, the 100R resistor I changed (and thought was R7) was actually R9. Both R9 and R10 are 100R (as seen in the schematic). I have taken some photos, the board printing has R7 and R9 at the same spot, and on the other side of the rec-play switch, I see R2 and R8 printed very close to each other, but I see no 100k resistors nearby.

Zoli:

--- Quote from: dankinzelman on December 23, 2024, 09:18:24 pm ---Q401 has a heatsink, here is a photo. The transistors you are suggesting look like they won't be compatible with the heatsink I have, so I presume I will need to purchase an appropriate heatsink as well? Suggestions?
...

--- End quote ---
Sorry for the late reply, here's my take: re-use the original heatsink, attached with a screw(M2.5-M3) and heat transfer compound to the new transistor; don't forget about transistors different pinouts.

floobydust:
The Q401 you have looks different than the transistor in the RS-M17 posted schematic. A little cheaper.
It has a clip-on heatsink that slides up and off, it's put on a common transistor in a plastic TO-92 package. Easy to trash the leads pulling on it. No need to yet...

At R401 you measure 18.6V and 15.4V which tells me that is fine - the zener is alive just over 15V.
So what is coming out of the Q401 regulator? Measure at R402 (both sides) or C404. R402 looks 100Ω not 33Ω on the schematic.

Resistors rarely fail (unless in a power circuit) and lifting a leg to test them can do more damage to a board. I suspect most resistors last. You can troubleshoot quite a bit without doing that.
Low value parts you can check in-circuit like R402 33Ω or 100Ω - know you should never read higher than that resistor's value unless there is stray voltage on a capacitor somewhere.

R2 is the 10Ω part coming off the tape head. I see BRN-BLK-BLK-GOLD which is 10Ω not 100Ω. Check your colour codes you might be mixing them up.

dankinzelman:
Thanks, I'm traveling for work now but will check when I get back.

I can't read resistor color codes (without a cheat sheet), I usually just measure them. R2 is indeed 10R, the resistor I was speaking of as 100R is R9, which is between C1 and pin 2 (the schematic is missing the R9 label). I am attaching another version of the schematic here, this one is from the service manual.

As I mentioned, I have not managed to find several resistors on the board, the photos were intended to show where the 'missing' resistor names are printed on the board, but are not actually present. R402 is among the resistors I have yet to find. Do any of you have one of these units to look at?. When I get back home I will see if I can at least measure 100k between C1 and ground, and 33R between collector and C404's positive terminal. What does (1V2) mean next to 33 on the original schematic I posted? 1/2 W?

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