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Hitachi TRK 8190 - no stereo reception in FM

Hitachi 8190

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#1 beamrider

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Posted 09 June 2019 - 12:58 PM

Hi all, this is my first post. Thanks for adding me to the forum.

 

I've been trying to get my 8190 fully working after changing the belts. 

 

I've opened it up a few times and on the most recent reassembly noticed that I had no stereo in FM mode even though the signal was fairly strong. It's not just the LED as the music is also in mono.  The cassette tape plays correctly in stereo.

 

I didn't notice this before, so maybe something I've done accidentally. Does anyone have any suggestions as to how I might go about repairing this?

 

Also, there seems to be a white and yellow wire coming from the antenna, but only a pole connector for the white wire? Anyone else noticed this?

 

Thanks in advance!

 



#2 goodman

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Posted 10 June 2019 - 10:10 PM

Hello and :hi: to forum.

Is the any difference if you switch knob "mono - stereo" on top?



#3 hopey

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Posted 10 June 2019 - 11:24 PM

You can lose a couple of db though dirty connections. There is a crimp lug screwed onto the aerial base accessed from inside.

There should be two posts on the tuner PCb. One for white,  one for yellow. Try swap these ones.



#4 beamrider

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Posted 11 June 2019 - 08:29 AM

Thanks for the replies.

 

@goodman - no the mono/stereo switch has no effect on the radio

 

@hopey - I can only see only one connector post (white), the yellow antenna connector seems to be just an unused hole on the PCB.

 

The signal strength indicator is showing 5/6 LEDs which I thought would be sufficient to decode stereo?



#5 toshik

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Posted 11 June 2019 - 08:31 AM

Was the pilot tone frequency measured?



#6 beamrider

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Posted 11 June 2019 - 08:55 AM

Was the pilot tone frequency measured?

 

no, I don't know how to do that. It sounds like it's beyond my level of expertise/test equipment.



#7 toshik

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Posted 11 June 2019 - 10:45 AM

OK



#8 beamrider

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Posted 11 June 2019 - 12:30 PM

I had a look in the service manual. Would this be step 6 FM MPX adjustment?

 

I tried turning RT302 throughout its full range whilst monitoring the FM stereo light and also tried some deoxit and tweaking the other adjustable pots (everything reverted to pre-marked positions).

 

Guess I would just need a cheapish frequency counter for this? I only currently have a multimeter.



#9 Fatdog

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Posted 11 June 2019 - 12:53 PM

@hopey - I can only see only one connector post (white), the yellow antenna connector seems to be just an unused hole on the PCB.


That could be your problem. Is there any wire at all running from an antenna and connecting to a board?

#10 beamrider

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Posted 11 June 2019 - 01:09 PM

There are two wires from the antennas (yellow & white). Only one antenna post on the PCB (labelled white).

 

The signal strength looks healthy at +3db (only one unlit LED) with a piece of wire into the white connector.



#11 Fatdog

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Posted 11 June 2019 - 04:17 PM

Usually, in boomboxes with dual antennas, one will be for AM/MW and the other for FM.  I don't have my 8190 handy at the moment to check my wiring to the board.  I'll try to do that later this evening.



#12 hopey

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Posted 11 June 2019 - 08:34 PM

The yellow and white wires are for built in antenna and external antenna. As you seem to have enough signal strength it points towards the Mono Stereo Switch. 



#13 caution

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Posted 11 June 2019 - 09:01 PM

RT301 is the FM separation adjustment. That's where I'd start. Measure the resistance across the ends of the pot and see if you read 1K ohms.



#14 Reli

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Posted 12 June 2019 - 12:22 AM

I own that model. When removing the rear panel, there are 4 wires you need to pull. If it's less than 4, you have a problem.

 

When there's a stereo problem on any boombox, it's usually because the stereo pilot adjuster is out of adjustment, probably because the radio was  knocked around during shipping.  It's usually marked VR01 or 19KHZ on the tuner circuit board, while VR02 is usually the stereo separation pot (which I rarely ever touch).  Adjust VR01 until the stereo light comes on.  Simple.



#15 beamrider

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Posted 12 June 2019 - 02:55 AM

Thanks for replies again. 

 

 

The yellow and white wires are for built in antenna and external antenna. As you seem to have enough signal strength it points towards the Mono Stereo Switch. 

 

It could be that, but it does work correctly for tape operation.

 

RT301 is the FM separation adjustment. That's where I'd start. Measure the resistance across the ends of the pot and see if you read 1K ohms.

 

I'll give that a try tonight....

 

I own that model. When removing the rear panel, there are 4 wires you need to pull. If it's less than 4, you have a problem.

 

When there's a stereo problem on any boombox, it's usually because the stereo pilot adjuster is out of adjustment, probably because the radio was  knocked around during shipping.  It's usually marked VR01 or 19KHZ on the tuner circuit board, while VR02 is usually the stereo separation pot (which I rarely ever touch).  Adjust VR01 until the stereo light comes on.  Simple.

 

It does have 4 wires in total, 3 Power + 1 Antenna.  

 

Adjustments available are

  • RT302 FM Multplex adjustment - it should read 19khz +/- 100Hz according to the service manual.
  • RT301 Stereo Separation 

I've tried turning both through their full range of movement but the stereo light never comes on. (I reset to original position afterwards).

 

I don't have a frequency counter, but ordered a cheap one from EBay, so will try and check RT302 when it arrives.

 

Once again, thanks for the helpful replies.



#16 Reli

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Posted 12 June 2019 - 09:47 AM

The reason your tape deck plays stereo is because it's just playing whatever is on the tape (which is stereo music).  It doesn't need to decode the signals.  They're already in stereo.  Same with the AUX jacks.   But the tuner is different, it needs to decode the broadcast into stereo.  That's why there's a decoder chip, and pots that need to be adjusted. 

 

The correct pot is usually labeled VR1 or VR01.  If your service manual says it's RT302, that's odd, and pretty dumb on their part to use a proprietary code nobody else uses.  In any case, tune to one of the strongest FM stations and then adjust that pot.  If the stereo light still doesn't come on, maybe that pot needs cleaning, as it often gets corroded.  In fact, some people have needed to replace it.

 

Another possibility is dirty switches on top of the boombox.  Particularly the Function switch, the Band switch, and maybe the Stereo/Mono switch (although that's never given me as many problems as those other 2 switches).



#17 beamrider

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Posted 13 June 2019 - 01:28 AM

The band switch is a bit iffy, so that could be a suspect. I've tried squirting some de-oxit inside it but hasn't made much difference, in-fact may have even made it worse...

 

It's a slide type inside a metal housing, guess I'll need to de-solder it and strip it down..



#18 Reli

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Posted 13 June 2019 - 09:27 AM

Sometimes you need to wiggle the knob in between the bands to get the best result.



#19 beamrider

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Posted 14 June 2019 - 08:47 AM

I've cleaned the band switch and function selector switch but it hasn't made any difference.

 

The resistance across RT301 was ~1200 ohms. I could adjust it down to 1097 but the stereo light never came on so I've reverted it for now.  

 

+ the stereo/mono switch works for the tape, so it can't be the switch itself.



#20 fededesalta

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Posted 11 July 2019 - 05:26 PM

Soy nuevo en el foro y tengo el mismo modelo de Hitachi, se pudo arreglar?.Tengo el mismo problema.Gracias.

#21 Fatdog

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Posted 11 July 2019 - 05:57 PM

Soy nuevo en el foro y tengo el mismo modelo de Hitachi, se pudo arreglar?.Tengo el mismo problema.Gracias.

¡Bienvenido a bordo! Es posible. ¿Qué has tratado de reparar?



#22 fededesalta

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Posted 16 July 2019 - 02:40 PM

Fm no sintoniza claro ni se enciende la luz de fm estereo

Tambien tene los controles de graves y agudos con troblemas.

#23 Fatdog

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Posted 19 July 2019 - 12:11 PM

Fm no sintoniza claro ni se enciende la luz de fm estereo
Tambien tene los controles de graves y agudos con troblemas.

Los controles de bajos y agudos probablemente podrían repararse con una limpieza a fondo.  Sugiero descargar el manual de servicio para la luz de FM estéreo.  :yes:

 

http://www.mediafire...Manual.pdf/file



#24 fededesalta

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Posted 04 August 2019 - 04:14 PM

Gracias por la respuesta. Ya solucione  lo de el potenciometro.Desarme uno que no uso y le adapte los contactos de adentro. quedo muy bien.Con Fm me paso que cuando lo compre estaba muy muy sucio, decido pasarle aire a precion y despues ya no sintonizaba en estereo. La primera vez que le pruebo la sintomia de fm si recibia en estereo. Alguna idea que me puede haber pasado?



#25 Fatdog

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Posted 04 August 2019 - 08:41 PM

Hmm...  ¿Quizás el aire desconectó un cable?  :hmmm: :huh:



#26 fededesalta

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Posted 08 August 2019 - 11:32 AM

He limpiado el interruptor de banda y el interruptor selector de funciones, pero no ha hecho ninguna diferencia.

 

La resistencia a través de RT301 fue ~ 1200 ohmios. Podría ajustarlo a 1097, pero la luz estéreo nunca se encendió, así que la he revertido por ahora.  

 

+ el interruptor estéreo / mono funciona para la cinta, por lo que no puede ser el interruptor en sí.

Hola , pudiste reparar la recepcion de fm estereo?



#27 BoomboxLover48

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Posted 15 August 2019 - 10:13 AM

Was the pilot tone frequency measured?

Great suggestion for an expert here...... but can you make things simple by directing what to do...  :yes:

Please help with things to look at and follow up.  :yes:

Otherwise the comments you make are of no help here....

No offence though... just a suggestion!



#28 beamrider

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Posted 31 August 2019 - 05:14 AM

I tried to follow these steps from the service manual using my soundcard scope). But I couldn't obtain a 19khz signal anywere, although the instructions aren't that clear.  I got either 50hz or 100hz depending on how I made the connections.. Some further help would be appreceated.

 

 

MPXAdjust.png



#29 Superduper

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Posted 31 August 2019 - 09:08 PM

Ok, first, you need to eliminate either RT301 or RT302 as possible problem sources.  RT301 is a 1k Ω variable resistor and RT302 is a 4.7k Ω variable resistor.  Both should be adjustable from 0 to the max reading.  You might not be able to get max resistance if there is a parallel path, but something is wrong if you can't get as low as zero.  If the readings aren't within specs or not what you are expecting, take them out of circuit and check again.  No sense proceeding if either of these are failed or out of spec.  On post #19, you mentioned something about getting a high resistance reading from RT301.  Although that's not the adjustment for pilot signal, the readings that you are sharing does not sound right for a properly working 1k Ω pot.

 

Then once you've confirmed this, if any pot is/are bad, replace and recheck.  If problem still persists, you should consider that it's entirely possible that your MPX decoder chip is bad.  Happens a lot.

 

As for your 19khz adjustment, you should understand that a "true" frequency counter is the best tool.  A cheap one, a makeshift one, a computer one, arduino counter, or even a counter on a good DMM won't necessarily pick up the part of signal that is important to you.  The reason is that every good dedicated frequency counter will have adjustments not found on cheap ones such as low pass filter, trigger sensitivity, etc.  In a perfect world, a frequency from an oscillator would be clean and stable.  But in the real world, they often are not.  If the signal is noisy, it signal might be unusable for a frequency counter because it is unable to differentiate from the actual part of the signal you want to isolate and capture.  You did not set a location in your "profile" so nobody knows where you are located, and I'm not going to waste my time guessing, but since more than 1/2 the countries in the world use 50hz mains power, I'm going to presume that is what your counter is picking up.  Of course if you were using a more sophisticated counter, you would be able to use a low-pass filter.  I have in the past, found that my fluke digital multimeter using the frequency mode was unable to capture the signal I wanted, but a high quality dedicated one had no problem by tweaking the trigger sensitivity.  But is your particular problem because you don't have a good clean signal, signal too weak for the counter to pick up, or is it because your MPX decoder isn't actually outputting a proper signal?  I don't know.  All I know is that what it's picking up isn't the pilot signal, it's likely 50hz from the mains.

 

Hopefully, you check the first part regarding ensuring that the pots are actually working properly first, then go from there.



#30 beamrider

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Posted 01 September 2019 - 10:27 AM

thanks for that - I think the 1K pot is a bit corroded, I'll replace it and see what happens..

 

[edit] - I only have a PC based oscilloscope so I'll have to make the best of it unfortunately... I can run the unit from a 12V battery to eliminate the 50hz signal and give it a helping hand to lock on to any 19Khz modulation