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JVC RX-401 display has gone dark

pioneer07

pioneer07

Member
I got this old JVC RX-401 for $25 recently and everything was working perfectly until the other day when the entire LCD display went dark. I am looking for advice on troubleshooting and fixing this issue. Seems like it is probably something basic since the entire thing went dark. The sound is still perfect and features all still seem to work. Just no display.

I have experience repairing vintage stereos, but older ones without so many integrated circuits. I have re-capped lots of old receivers, mostly in old console stereos. So I have tools to do some repairs.

I also got a hard copy of the service manual for this off eBay but it's not making sense to me. I am missing something. The manual says there are two different ICs that control the LCD display. One for the tuner section and one for the rest of it. But ALL of the LCD screen is dark. There must be some common power or common link between them that failed.
 
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I don't think that's an LCD. Based on the apparent vintage (late 1980s) and multiple colors, it's probably either a bunch of LEDs or a VFD.

If it IS an LCD, is the backlight lighting up? If not, you should still be able to see the segments changing if you shine a light on it.

399590-c7a5620b-excellent_jvc_rx401_am_fm_computer_controlled_stereo_receiver.jpg
 
is it backlit?

i did a couple jvc tuners that had no display and it was only burned out backlight bulbs
 
swechsler said:
I don't think that's an LCD. Based on the apparent vintage (late 1980s) and multiple colors, it's probably either a bunch of LEDs or a VFD.

If it IS an LCD, is the backlight lighting up? If not, you should still be able to see the segments changing if you shine a light on it.
Click to expand...

tie goes to you.
 
Cool, so yes when I shine a light on it I can see the sections changing. Like just when playing with volume I can see the level go up and down. So how does one go about replacing the backlight? This is a new repair for me
 
I'd bet it is a back-lit LCD. I don't see a service manual anywhere. If it's like other JVCs that are f this vintage they do weird stuff for the back-light circuit. For example in on of my receivers, they put 3 bulbs in series but each bulb has a zener diode parallel to it. I guess the idea is that if a bulb burns out, the zener will complete the circuit so the other 2 remain lit. Unfortunately they undersized the zener and if a bulb burns out, the zener ends up shorting out and then burns the other bulbs and zeners then it burns a couple of resistors up the line. Open it up and see whats there!
 
This is what I'm Talking about...


Capture.PNG


You need to check those diodes and resistors. Hopefully they learned to use a diode that is capable of dissipating the power if a bulb burns out.

The fuse lamps are 12V 150mA.
 
painttoad said:
well that would leave out the led mod like i used in a jvc tuner recently.
Click to expand...

I'd find out what the actual power supply is; the one that feeds the zener nonsense. Then I'd stick in a suitable LED and series resistor. LEDs need much less current so even if you're dropping a significant voltage across the series R, it's not that much power.

Assuming the bulb is taking the rated 150mA, that's also flowing through each of R807/808. P = I^2R = 0.15^2 * 22 = 0.495W. So the 0.25W resistors are way under-rated. No wonder there is a ! next to them...

If you assume the supply is 30V, and Vf is 3V, and you need 10mA to drive the LED, that's only 27 * 0.01 W = 270mW. 0.5W resistor would do, one 2k7 R per LED. Or stick the LEDs in series, and reduce the power even further...
 
cpt_paranoia said:
I'd find out what the actual power supply is; the one that feeds the zener nonsense. Then I'd stick in a suitable LED and series resistor. LEDs need much less current so even if you're dropping a significant voltage across the series R, it's not that much power.

Assuming the bulb is taking the rated 150mA, that's also flowing through each of R807/808. P = I^2R = 0.15^2 * 22 = 0.495W. So the 0.25W resistors are way under-rated. No wonder there is a ! next to them...

If you assume the supply is 30V, and Vf is 3V, and you need 10mA to drive the LED, that's only 27 * 0.01 W = 270mW. 0.5W resistor would do, one 2k7 R per LED. Or stick the LEDs in series, and reduce the power even further...
Click to expand...
i only said that it leaves out the mod that i did because my tuner did not use fuse type bulbs. it only used a grain of wheat bulb and the led fit right in the bulb grommet.

but yes, i'd definately use fuse type leds for this if that's what it takes, it's just gonna cost more than the 25 cents in parts that it cost me.;)
 
cpt_paranoia said:
Assuming the bulb is taking the rated 150mA, that's also flowing through each of R807/808. P = I^2R = 0.15^2 * 22 = 0.495W. So the 0.25W resistors are way under-rated. No wonder there is a ! next to them…
Click to expand...

Big error here… In the above circuit (post #9) the lamps are in series but the resistors R807 and R808 are in parallel. So, the current in each resistor is only 75mA and the power dissipated by each resistor is only 124mW. No problem with the 0.25W power rating of the resistors.

I admit that LEDs draw much less current but light is less diffused than filament bulbs and sometimes filament bulbs provide better lightning.
 
The total voltage on the lamps is less than the sum of the Zener voltages, so the Zener don't conduct. But if one lamp burns out, the current will pass by one of the Zener diode and there will be approximately 12V on the second lamp. There should be approximately 24V ahead (on the "right side") of the resistors.
 
I have the service manual. The manual says these are the Rohm part numbers of the LEDs in this unit:
SLH-34VC3F qty 1
SLR-34DC3F qty 5

I can't find these exact part numbers but I can find close. Would it be easier to just find an equivalent to these? Mouser has one that is one extra digit but otherwise the same part number to one of them
 
Where are they? Do they have a special shape?

The only reason to choose a specific LED would be if it had some special moulded case/lens, rather than the classic 3mm or 5mm domed top. IIRC, Rohm did specialise in this sort of thing. Brightness and colour you can deal with by choosing correct device and current.
 
I don't think those are for the back light. Those are for the remote received and whatever other things are lit with LEDs.

Did you check the fuse lamps?
 
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