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Old 9th Sep 2021, 10:34 am   #1
DennisCA
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Default JVC 9401LF Radio (1970s)

I bought this radio recently and it has got this behaviour where the volume drops, but if you flip it from FM to MW or LW and back then it goes back to normal. My suspicion is a capacitor somewhere is giving up?

I have not yet gotten the radio, seller informed me of the issue and I bought it for cheap because of that and an issue with the cassette deck too.
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Old 9th Sep 2021, 1:23 pm   #2
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Default Re: JVC 9401LF Radio (1970s)

It’s quite likely that the waveband switches have tarnished/intermittent contacts and need a squirt of suitable (aerosol) contact cleaner and lubricant, such as Servisol Super 10.

This would require the case (maybe just the rear part) to be removed in order to gain access to the actual switch contacts. Otherwise repeated operation of the switches may clean them sufficiently.
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Old 10th Sep 2021, 5:59 am   #3
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Default Re: JVC 9401LF Radio (1970s)

I see, but would that really cause the volume to change over time? As I understood the problem (I hope to get the package today so I can try myself) the radio switches fine between the wavebands, just the act of doing it resets the volume.

Well it'd be a simpler fix so worth a try for sure.
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Old 25th Oct 2021, 7:14 am   #4
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Default Re: JVC 9401LF Radio (1970s)

I've had this radio working for a while, however I wanted to run it on batteries but that did not work. I opened it up and looked and found the terminal to the negative pole of the battery was loose (1st picture, next to yellow markings).

I turned the board over and soldered it and got an electrical connection back.

It did not work on battery despite that. I looked at the unknown component to the right (inside the yellow square) and when I put the 6V into the center pin the radio came alive.

So I am wondering if this component is broke, and I also wonder what it is? It's called CRB1 on the board and the schematic shows a component of two capacitors and a resistor. I am not quite sure what this is, some googling leads me to believe maybe it is a ceramic filter?

The lettering on it says
54
F2037
471
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Old 25th Oct 2021, 9:42 am   #5
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Default Re: JVC 9401LF Radio (1970s)

Hi Dennis

I suspect that the negative side of the battery should connect to ground, but via a switch. Sometimes this switch is part of the AC mains input socket, so that if you push in a mains cable, the switch is activated by an insulated plunger and the battery is automatically disconnected. This switch may need its contacts cleaning with switch cleaner.

That brown component looks like a ceramic IF filter, but I think it is a resistor-capacitor smoothing filter for one of the DC supply lines to the nearby integrated circuit. The centre pin most likely connects to ground. By connecting the battery negative to here, you are effectively by-passing the switch mentioned above. It is very unlikely that the smoothing filter is faulty.
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Old 25th Oct 2021, 10:36 am   #6
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Default Re: JVC 9401LF Radio (1970s)

I'm having trouble seeing where the switch could be in that case. The negative lead does not connect to anything but the battery directly. So a switch would be on the positive side I figure.

The positive side of the battery lead is near the AC mains input socket, if there was a switch I figure it would have been there somewhere but I cannot remember seeing anything like that, will have to double check. Power does seem to flow from the positive side as indicated by me being able to bypass it. Perhaps the switch is on the board itself?

I have a copy of the schematic here, but I am not that good at reading schematics, but it does look like the CRB-1 component provides power for the IC (pin 1), I see several things that look like switches but can't determine how they would affect battery power. The symbol for the battery itself does not make sense to me.

https://i.**********/O9FlSBe.png
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Old 25th Oct 2021, 11:03 am   #7
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Default Re: JVC 9401LF Radio (1970s)

The imgur circuit is very low definition (maybe 'cos I don't subscribe?) but it does show the positive supply switched between battery and mains by contacts associated with the mains input connector. +ve supply then goes via contacts on another "battery"? jack to the set.

Battery -ve appears to go to the set via yet another switch, possibly the record / playback switch?
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Old 25th Oct 2021, 11:55 am   #8
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Default Re: JVC 9401LF Radio (1970s)

Try this.
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Old 25th Oct 2021, 12:03 pm   #9
DennisCA
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Default Re: JVC 9401LF Radio (1970s)

The battery symbol confuses me, I've never seen it before and have no real idea of how to interpret it? How does one even know which is + and - ?
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Old 25th Oct 2021, 12:08 pm   #10
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Default Re: JVC 9401LF Radio (1970s)

https://commons.wikimedia.org/wiki/F...ery_symbol.svg

Lawrence.
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Old 25th Oct 2021, 12:12 pm   #11
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Default Re: JVC 9401LF Radio (1970s)

Yes that's what I'd expect as a battery symbol, the one in the schematic looks nothing like that. Maybe some kind of 3 pole switch, with some strange symbols.

It is called Ext Battery J4 in the schematic.

EDIT: ooh wait, I've been insisting on thinking of J4 as the battery, because it says battery. But perhaps the part marked J4 is the 6v outlet on the side? And the other part is the 6V battery, I had figured they where the other way around because the one was called battery and the other one was not.
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Old 25th Oct 2021, 12:17 pm   #12
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Default Re: JVC 9401LF Radio (1970s)

That's the external battery socket that has an inbuilt switch contact, the outer (the vertical rectangle) is the -ve input, the inner (the line with a "v" on the end) is the +ve input, when a plug from an external battery is plugged into it the switch contact opens which isolates the external battery supply from any internal supplies, eg internal battery or mains powered PSU.

The battery symbol for the internal battery in the schematic is a typical representation, long plate = +ve, short plate = -ve.

Lawrence.

Last edited by ms660; 25th Oct 2021 at 12:27 pm. Reason: extra info
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Old 25th Oct 2021, 12:29 pm   #13
DennisCA
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Default Re: JVC 9401LF Radio (1970s)

I tried powering the radio via the 6V input switch by the way, did not work either.
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Old 25th Oct 2021, 12:54 pm   #14
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Default Re: JVC 9401LF Radio (1970s)

Hello again

Looking at the full schematic, this radio appears to use a mixture of PNP and NPN transistors. The PNP stages are in the audio amplifier, and need negative supply to the collectors and positive supply to the emitters (inward arrows).

The designer has taken the mains transformer/rectifier and internal battery positive line via the mains/battery switch operated by the AC cord, then via the internal/external switch operated by the external battery jack and then to the "ground" of the output stages. The negative line goes via the tape/radio switch and then to the output stage collector supplies. All these switches would benefit from checking and cleaning.

It's hard to read the schematic, but the NPN input stages and the IC will need a positive supply and a negative "ground". It looks like there are ground symbols around these input stages, but these ARE NOT be the same as the output stage "ground". This is a recipe for confusion if ever there was one.

If you look just above the audio driver transistor (TR106 ?) you will see a reduced voltage negative supply taken off to the left to supply the input stage grounds. Similarly, the output stage ground is taken off to the left (in the schematic area nearby, just above the radiomuseum logo) to provide the input stage positive supplies.
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Old 25th Oct 2021, 1:25 pm   #15
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Default Re: JVC 9401LF Radio (1970s)

Sounds pretty complicated.... I dunno if it helps, but I believe the part of the schematic with TR06 transistor is for the tape player board and related parts. The left part of the schematic, which seems to be connected with only a single line to the other board houses the radio stuff.

I think the switch there is the one that switches from radio to tape mode. Of course the negative lead goes into radio board, on the schematic it looks like it ought to connect to the tape board. I guess schematics don't have to match the physical reality that well.
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Old 25th Oct 2021, 1:41 pm   #16
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Default Re: JVC 9401LF Radio (1970s)

Try using your meter to find out where the voltage is getting to, or rather not getting to.

If the set works on mains, then failing to work using an internal or external battery is going to be down to a switching fault.
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Old 25th Oct 2021, 1:55 pm   #17
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Default Re: JVC 9401LF Radio (1970s)

Hi

To help you understand the schematic:

TR1 and TR2 are the FM radio tuner
TR3 is for AM radio (Long and Medium Wave)
IC1 is for both FM and AM radio
TR100, TR101, ...., TR108 are for the tape player/recorder and audio amplification
S1, S2, ....., S8 are for radio band switching, with audio selected by S8
SA1, SA2 etc are for Record/Play switching
SB2, SB4 etc are for Tape/Radio switching

In general, the signal flow in schematics is from left to right

I suspect that the Tape section was an older existing circuit board (PNP technology), to which the radio part (NPN technology) was added on.

Apart from broken wires, the most basic check on this type of set is to clean tarnished switch contacts by applying a small amount of proprietary switch cleaner (not WD-40 etc) and working the switch back and forth. This dissolves the invisible film of non-conductive tarnish that builds up on contact surfaces, especially if they sit unused for a long time.
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Old 25th Oct 2021, 3:08 pm   #18
DennisCA
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Default Re: JVC 9401LF Radio (1970s)

Thanks for your help and also the education in reading the schematic. I finally got home to take another look at the radio and you know what I found, hidden under a bunch of cables on the board for the tape section, that I'd not noticed before? Another lead. I touched it with the negative terminal and the radio came alive.

This makes more sense to me, since the battery is shown on the tape section. I found it by accident when looking for broken wires.

The real mystery to me is the lead on the radio board, what purpose could it possibly serve? I bought this radio as broken and the wire in question was there when I took it apart, I am wondering if it was a previous failed repair attempt...
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