The JVC 4VR-5456X Protection board mystery!

[Kadosas]

Hello,

I have a question to those knowledgeable about this JVC receiver.

I have this beautifull tank of a receiver. I have recapped most of it and replaced all the infamous 458 transistors.

I turn it on and everything works. So far so good.

But. Everytime I turn it on there is a rather loud pop in my speakers on the exact moment the relay is activated, also with the volume down. This was both with the old and new relay I installed.

But I remembered the first time I opened the receiver and looked at the power supply/protectionboard I saw this weird ”constelation” of af diode, resistor and transistor tied together. See the picture. (two transistors in the middle). On the backside of the pcb there is also a resistor between the emitter and base.





This looked weird to me and I thought it probably wasnt factory.
But now I think it is, because I saw a picture of another JVC 5456 with the same ”constelation”.

So I changed it back to the way it was (thank God I took some pictures beforehand) before and expected that it would take care of this popping sound.

The good news is. The receiver powers up without a problem, the relay is activated- but still this popping sound.

So this is a longish story.I hope you still here;-)

My questions:

- Does anyone know about this problem or is it just my receiver?

- Most important. Does anyone have an updated version of the Powersupply/protectionboard part.

The service manual you can download from Hifi engine and the likes don't seem to correspond with what I have.


Thank you for your time!

 

[Kadosas]

there is probably a (small) dc voltage on the o\p

What is o\p? And how do I get rid of it?

 

[Kadosas]

measure the dc on the OUTPUT terminals, there is a preset is there not for each channel

There is no outputvoltage on the terminals, only around 5mV.

 

[Kadosas]

ah, maybe the relay is operating too soon then, what delay is there ?

Yes. Very good point. I also tought of that. It is about a second wich is not that much. When you look at the drawing of the protection circuit. Which capacitor (resistor) do I have to increase to get a longer RC constant? Is it C753 and R763?

 

[Kadosas]

i would replace the 2 100uf capacitors or just the left side one 752, juggle about with the values (increase) to get it long enough to get rid of the thump

I will definitely try that thanks. Weird though that it is necessary. Doesn't seem to make any sense. That's why I like to get hold of an updated version of the service manual, if there is one

 

[Kadosas]

I will definitely try that thanks. Weird though that it is necessary. Doesn't seem to make any sense. That's why I like to get hold of an updated version of the service manual, if there is one

I changed both 100uF´s to 470uF's just to see... I solved the problem! No pops anymore Thanks for your help.

 

[NAD80]

Leaky caps I'll bet. Great fix. Now for the rest of the caps, maybe.

 

[MegaHurts]

Older thread, but nice to see that the weirdness on the power board is not a random one off. I have the same thing on the receiver I'm looking at which is what brought me here. I was concerned about transistor x753 tied to the resistor as it's a 2SA628 where the schematic calls for 2SC711 (PNP vs NPN). Also 2the diode tied into R757.