Hitachi AXM67 Stereo Repair/Small Modification

[Benjo]

Hi everyone, some help/discussion on this would be appreciated very much

(TL;DR at bottom)

I have this pretty old (at least 10 years) Hitachi stereo that wasn't in use (I have better equipment as far as stereos/amps are concerned) but recently I've found myself in need of an inline volume control able to be adjusted remotely. I did a lot of shopping but it's a real pain to find anything that is good value and that won't take weeks to arrive (I'm in the UK and literally every single motorised pot version comes from Hong Kong/China).

So I dug it out to use as an inline volume control since it actually has a remote and receiver etc.

It actually does the job ok, besides a few moderate snags.

One being that it has a woofer output. I don't need that since I'm using it as an inline volume control and I don't want it messing with the audio in any way (besides volume lol) yet I'm pretty sure that when using the stereo the resulting output audio sounds less bassy (which I'm assuming is because the bass frequencies are being isolated to the woofer output - but I'm only using the red and black cable clips [not pictured]).

On top of that the woofer output jack (small red square) doesn't even work. It's dead. I can't tell where it might be broken and it's a lot of effort to go through just to fix a feature I don't need.

Here are some pics of the main circuit:

I think that the coil at the top (long red square) is to isolate the bass frequencies, much like a crossover circuit does. If I remove it and join the residual connections on the circuit together, would that solve my problem? Or is that silly thinking haha, it does sound like it.

The other issue is the LCD display's backlight is dying/dead. It connects at the point circled in green (I think - either that or the two pin port at the bottom right which says +4.5v -4.5v - one connects to the battery compartment). I heard that the capacitors are likely the problem. Anyone know which ones it may be? There is some brown sticky-looking solder joins (I don't think they are burn marks) - that where they have leaked?

Also (because learning), does anyone know what the (presumably) adjustment screws circled in blue might be for? Just curious.

Any tips or info/discussion anyone can share about this kind of work would be greatly appreciated, thanks

Sorry for the lengthy post and, thanks for reading

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TL;DR: Stereo appears to be not outputting low-end frequencies (due to "woofer out" - how do I have all the sound output from only the L/R jacks?) and the LCD backlight has died. Picture is the mainboard, how do I fix?

 

[73's de Edd]

Sir Benjo . . . . .

Initially . . . as per . . .

I think that the coil at the top (long red square) is to isolate the bass frequencies, much like a crossover circuit does. If I remove it and join the residual connections on the circuit together, would that solve my problem? Or is that silly thinking haha, it does sound like it.

Whereas . . . .

I KNOW for a fact . . . that instead . . .it is being a ferrite antenna that used for the reception of your AM radio stations portion of the receiver.

Your designated BLUE circles markups are the RF tuning adjustments for the AM portion if the receiver with the other FM portion of the receiver using fixed ceramic filters.
The central I.C. chip is being a <radio in a chip> along with a few peripheral transistors, that you see surrounding it.
DO NOT touch those adjustments, as they are combinational and critical tuning adjustments.
If a layman touches all of those adjustments . . .with the mentality of . . . . lets turn this one to see if things work better . . . . then leave that adjustment and move on to trying each of the others
he is ScROOOOOOD.
Akin to me taking a virgin Rubik's cube and doing 5 twists on it and handing it to him and then watch him spending an initial 5 minutes . . extended to 5 hrs extended to 5 days extended to 5weeks . . 5.months . .5years . .5 lifetimes.
RF alignment instrumentation and techno expertise is needed to correct and re optimize.

To be precise on the other questions, the model number of /and the brand is needed.
I do see the main power supply in the left foreground near the main fuse and the rear heat sink and large blue capacitor.
The deposits are probably surplus rosin stains or adhesives used to mechanically stabilize larger components
The two other heat sinks could be for sub power supplies, or power audio outputs stages.
Or the audio could be the small black IC at the very right front corner .
No more info . . . . . without a model number.

Afterthought . . .the display MIGHT be a Panaplex fluorescent type which has thermochemical elements that have output decline with time,
A vewy closeup of the lit display could confirm its type .

ADDENDA:
JUST now noticed the model in he header instead of the content of the post . . .will research, but you still photo bomb the display.

73s de Edd

 

[Benjo]

Very detailed response, thank you very much.

Haha, that's fine about it being an antenna anyway, like I said, it's only going to be used as an inline remote volume control, so even if I messed with it, it's no big deal: I have already removed some components and functionality (one of them being a fault cd laser assembly), and I'm considering attaching the whole thing to a new enclosure, since it's going to be considerably smaller haha.

I can't get the screen to light up, but I uploaded a pic anyway. When it does light up, it's blue, if that helps. And sorry I probably should've specified the make/model in the post as well

Yes you can clearly see the input power supply. It doesn't specifiy what voltage the main input is (it comes from mains, and then there's a big unmarked magnet/transformer) on the outside or on the mainboard.

And since you can verify that the coil is not involved in the audio then Yeh my next guess would be the chip.

It's bizarre because the RCA out for the Woofer is attached right next to the inputs, yet there is an entire other output circuit for the other outputs (speaker cable clip type). I would've thought that the Woofer out would also be on the output board.

I'll take the screen bit out and take a look, see what I can find. Good news is despite the things I've taken out, it still works. I'm at a loss as to what do with the bass though. I wonder if bridging the connection to one of the right/left outputs would work?

I think I'll move the whole circuitry into a new enclosure anyway, maybe a small one knocked up out of MDF or something.

Here are some more pictures that show the underneath of the mainboard (with what looks like a small processing chip), the speaker output circuit, and the front of the stereo with the unlit but powered up display.

 

[Benjo]

OK I'm looking at the screen component right now and I'll test the backlight LEDs just to make sure it's not them.

Edit: Can confirm the LEDs do work as desired but they will not light up at all when the whole circuit is connected. It connects to the internal power supply previously mentioned (at the bottom left first image).

 

[73's de Edd]

I did confirm that the unit did not use the other Panaplex type as mentioned.

It uses blue backlighting, behind a transmissive LCD screen, as evaluated from the photo of another unit being seen lit up.

Pass me all of the larger IC chips I.D. s and I can evaluate functions and root out the circuitry to
locate an audio take off point that would be encompassing FULL range sound capability.

A time lag may be present . . .as I am currently assisting on 4 other posts.

73s de Edd

 

[Benjo]

Wow man that's amazing! Thank you! Sorry for the late post I've had a strange couple of weeks, I'll try to keep the project in mind and get the info you need - by all means take your time with helping though, I'm in no rush.

Can you tell which caps need replacing to fix the LED backlight flicker for the display (or should I just replace most of them?)

There is a through-hole chip on the audio input part of the circuit (the same place where the sub-out is). It's a TOSHIBA TA21498N. In around the same location but on the underside of the board is also a small 6 pin surface-mounted chip if that helps , but it's blank.

There are also two more surface mounted chips (plus a small processor looking one) on the underside, but I suspect they are not relevant to the audio itself. I can get their names but they have been somewhat eroded.

There are an additional three through-hole chips on the output circuit, but again the names were faded. They'll take a bit of deciphering and I'll post back with their names.

Apologies again for the late reply and thanks for your continued help with this, it's a lot of fun having some guidance with unfamiliar circuits