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Display JVC receiver RX 450

The display of my JVC RX 450 receiver doesn't light up anymore. Using the service manual on this site (thank you!), I replaced the two 12 V 0.15A lights on the display PCB. When switching on the receiver, the lights still don't light up. Any thoughts of what the underlying issue might be would be most appreciated. Thanks!

Re: Display JVC receiver RX 450

I looked at the schematic and there are two zener diodes across the lamps; these could be shorted. There could be a problem with the power supply that delivers current to the lamp board, or a connection problem. I couldn't see the part of the schematic where the lamp board cable goes to; it may not have been scanned. I would follow the three wires back from the lamp board to the ENH-091-1 board to see if there is a problem there. Hope this helps.

Re: Display JVC receiver RX 450

Many thanks for your immediate and helpful reply. The display PCB (the one with the two diodes and two lamps) doesn't seem to be getting power from the main board. The amp itself works well. I am a bit hesitant to take out the main board altogether, as it is linked to coolers etc. Does it make sense to try and get power from elsewhere?

Re: Display JVC receiver RX 450

You may not need to remove the main board. I would follow the three wires back from the lamp board to the ENH-091-1 board and look for open resistors. Power has to come from this board. I looked for half an hour an I can't see where this connector goes to; it must not have been scanned.

Re: Display JVC receiver RX 450

Johnnysan, many thanks again! I assume that with an open resistor you mean a resistor that is not connected to the PCB? I don't see any of those. By the way, there are only two wires from the main board to the display board. The connector has three pins, but one of them is void. That sounds reasonable, two wire to power two lamps should be fine. There is definitely not a third loose wire. I will try to remove the bottom plate tonight, in order to inspect the other side of the main board.

Re: Display JVC receiver RX 450

Johnnysan and others, could I ask your help one last time please? As you suggested I checked the two Zener diodes. One was shorted, and I replaced it with a new one. If I connect a 9V battery to one of the lights (not connected to PCB), it lights up. Same for the other one, so both lights are OK.

If I connect the 9V battery to the PCB, none of the lights light up, suggesting there is still something wrong with the PBC. Both Zener diodes are well soldered to the PCB, that is not the issue.

I also measured the voltage at the connector to the display PCB (when the connector is not connected to the display PCB). Strangely enough, the voltage starts as just above 0, and then increases to 15 Volts or so, very slowly, maybe 0.1 Volt every 8 seconds.

Do you have any suggestions please?

Many thanks, Hans

Re: Display JVC receiver RX 450

On pages 35 & 39 of SM you can see (at left side) the "BC481" connection that goes to lamps PCB. This is the ENH 091-1 circuit (Power amp). Take a look to R 826, R 827 & C 819... Q 807 may be involved too (on ENH 091-8: lamp driver board). I hope this "lights you up"...

Re: Display JVC receiver RX 450

bronco billy, many thanks! I can't find R 826 and R 827 on the spare parts list, but according to the drawing (and my layman interpretation of the resistor color coding table I found on the web) both should be 22 Ohm, correct? The drawing says 22 UMF, I have no idea what UMF stands for. Question 1: Do you?

Using my multimeter, both resistors seem to be open, so that could well be the cause. My multimeter doesn't measure capacitors, so I guess I will start by replacing both resistors. According to the drawing (and to may layman interpretation of a color table I found on the web), both resistors are 22 Ohm. Question 2: 22 ohm, correct? What does UMF mean (that is what the drawing says)?

In the footnote on the drawing I see a triangle with exclamation mark inside, indicating that I should be sure to replace these resistors with exactly the same parts. Moreover I noticed an asterisk on the drawing next to both resistors. Question 3: anything special I should pay attention to when ordering these resistors?

Many thanks again!

Re: Display JVC receiver RX 450

Hi HansReus:

I think you are right about the value of resistors. They are listed on page 18 of S.M. and here you can see there are two different types ("UNF. carbon" and "Fusible"); for six different regional codes ( codes are on page 55 of S.M.)

I think your unit is "D" type (Europe & Australia model) and so you need the "Fusible" type. Everyway this must be marked on same place of the board themshelves and you must check it (ENH 091-1 D) If I am right...

I don't know anything about the meaning of "UNF. carbon" or " Fusible"; I only know ohms and watts, but for sure they are special types according to safety standards; and because of it I can't give you an useful advice about your order.

Good luck!

Re: Display JVC receiver RX 450

Issue solved! I replaced both fusible resistors, plus the zener diode and both lamps. Like new :) I was given this receiver by someone who passed away, so very glad it works again. Couldn't have done that without your help, so many thanks!

Re: Display JVC receiver RX 450

I would think that the problem has to be on the main board where the power comes from. I would check voltage with the display board lamp connector disconnected. Note: zener diodes will test like normal diodes; for the lamp circuit they are used to regulate voltage to the lamps (not a very practical circuit though). Also, when you apply DC (battery) to the lamps, you need to have polarity correct or the zeners will act like a short.

When the one zener shorted it may have caused a resistor to open on the main board. As I said before, it appears that part of the circuit has not been scanned, as I can't find it.