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My old portable radio volume knob does not go to the lowest point

Discussion in 'Troubleshooting and Repair' started by Nokianas, Feb 28, 2021.

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  1. Nokianas

    Nokianas

    2
    0
    Feb 28, 2021
    Hello and thank you for reding this topic.

    I recently bought a Hitachi TRK - 5351L from a flea market here in Lithuania.
    [​IMG]
    It is in quite nice condition, despite it's age. However, when plugged in, the volume is quite loud. Turning the volume knob to the minimum makes no difference. But it works well when i try to increase the volume.

    So i decided to take it apart and have a look: all the potentiometer solder joint seem to be okay. I found that turning the pink potentiometer to the end (pointed with a screwdriver ) on the board reduced the minimum threshold of the volume knob ever so slightly, but it was still quite loud.

    [​IMG] [​IMG] [​IMG]

    Is there any way to fix this issue? Would be really helpful with any information as i really like the looks of this radio and would like to bring it back to life.
     
  2. PETERDECO

    PETERDECO

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    Dec 19, 2019
    Welcome to the forum. Try spraying the pot with contact cleaner.
     
    Nokianas likes this.
  3. Tha fios agaibh

    Tha fios agaibh

    2,176
    729
    Aug 11, 2014
    That pink pot is a FR choke not a pot. Don't fool with them as they set the stages of the radio.
    I would put its adjustment back where it was.

    The volume pot likely has a bad spot changing its resistance on the low end.
     
    Nokianas likes this.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,796
    2,749
    Nov 17, 2011
    To start from the back: This pink element is not a potentiometer. It is a tunable coil. I hope you remembered the original position. Turn it back to where it was. The change in volume was due to a misaligned tuning circuit, not the volume control.
    I suspect the volume control potentiometer. Despiet the connections looking good, check the, especially the left end
    upload_2021-2-28_12-34-40.png
    Check also the connection where the metal tab is bonded to the potentiometer, where the hole is, tap against that point. Does the volume change?
     
    Nokianas likes this.
  5. 73's de Edd

    73's de Edd

    3,215
    1,347
    Aug 21, 2015
    That PINK slug ISN'T asssociated with a potentiometer. its a transformers critical tuned circuit alignment adjustment..
    AND don't further do the typical idiot procedure of trying all of them to see if it makes your radio work better or solves your problem. Or, forbid. TIGHTEN down all of the loose ones .
    On your 3rd photo down the left pot is being the volume pot.
    Turn its knob max CCW to minimum volume.
    Then, if there is still any sound present, a bare copper wire jumper from the 1st hole in the pot, shorting over to the second hole in the pot should reduce the sounds volume to ZERO.
    If that proves to be true , there is some possibility that the spraying of an electrical contact cleaner into the access stop / opening at the top of the control.
    Then you make mult i i i i i i i i i i i i rotations back and forth, might clean the pot enought to work again.
    Maybe for a limited time, until a new replacement pot is required.
     
    Nokianas likes this.
  6. Nokianas

    Nokianas

    2
    0
    Feb 28, 2021
    Thank you all for such a quick reply!
    As many of you indicated, i've returned the pink tunable coil to its original position. Im new to this hobby, therefore I am more than happy to face my mistakes and learn from them. Thank you for correcting me!
    As for the potentiometer. A very interesting thing happened. I've cleaned volume potentiometer with contact cleaner. The slight friction feeling when turning the pot has gone. However, I noticed that the sound quality increased quite noticeably. Moreover, the volume, when potentiometer is in "0" position, increased too!

    The sound is very good now, but the volume issue still remains.

    P.S. Tapping on the contacts made no difference to the sound and shorting 1st and 2nd potentiometer tabs did not mute the sound.
     
  7. Technomaniac

    Technomaniac

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    12
    Oct 31, 2020
    If shorting the tabs made no difference then the potentiometer is probably OK. What is happening is that the end terminal is not getting its path to the internal common or ground of the circuit. With the set switched off, and battery removed, you need to find where the earth to the pott is open circuited. Chances are that it is right handy to the pott, so have a good look at the copper tracks connected to that end of the pott. You will do better with an ohmmeter. The connection right at the terminal might look OK but it is the most likely place. Magnifying glasses will help. look at the join between the solder and the copper track and wiggle the pott backwards and forwards to see if you can witness any movement across the solder joint, between the blob and the copper track. Since it is most likely a mechanical fault, it should be possible for a newby to find. It is often found in manufactured items that the copper tracks are tarnished when the solder process takes place. This can mean that a good joint has never been made to the track. If you need to resolder, make sure that you shine up what you are soldering to get a good joint. This looks like it might be a double sided board - copper tracks both sides. If so, note that they sometimes need to connect a track on one side to a track on the other side. This might be done with something like that earth pin of the pott, or a rivet. Sometimes such links may be under a component and not readily accessible. If you can't find the problem, you can add a link from the end terminal to another point that is known to be ground, but if you make a mistake, the radio cassette will be history.
     
  8. shrtrnd

    shrtrnd

    3,827
    520
    Jan 15, 2010
    Save yourself a lot of time and trouble and replace the bad volume pot.
     
  9. 73's de Edd

    73's de Edd

    3,215
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    Aug 21, 2015
    Sir Nokianas . . . . . .


    With the forthgoing supplemental text of our Right Honorable and Esteemed colleage, Technomniac's info, you should have this situation all nailed down.
    I initially declined reply, until a further in depth circuit design analysis confimed that your volume control was not feeding a variable DC voltage into a variable electronic gain block.
    This provided schematic / wiring diagrams " relevant snippet " now confirms that not being the case, its just using an " old school " conventional variable voltage divider attenuator . . . . . ergo . . . no " fly by wire " techno design aspects are being incorporated.

    THE UNITS AUDIO SIGNAL FLOW PATH . . . .


    Hold on tight and lets go thru an audio circuit run.
    AM or FM audio comes in at the top right corners RED arrows and flows across and down to the Radio input of section 2 of Switch 601 and out its rotor and then down *** to the A terminal of the volume control and its rotor / wiper portion passes the signal on to coupling capacitor C712 and on thru it to the base of the Q702 transistor which amplifies the signal and then passes it on to the audio power amp section that finally feeds the units frontal speaker.
    (*** Coincidentally, that same audio signal also feeds across to the Radio terminal of S601 section 4 to connect into the series audio tone control circuitry composed of C721 and RV702 tone ontrol.. )
    Now refer to the right sides YELLOW inserts of three volume control setting variants.

    Max Vol has very little series resistance between A and B and thefore gets max signal thru and there is a high resistance path between the B wiper and C terminals ground

    Mid Vol has moderate series resistance between A and B and thefore gets further reduced signal thru and there is a now lower resistance path between the B wiper and C terminals ground, thereby weakening the signal even further.

    Min Vol now has considerable series resistance between A and B and thefore gets greatly reduced signal thru and there is a now a very low resistance path between the B wiper and C terminals ground, thereby weakening the signal even further.


    You now have told us that at max volume that the unit is outstandingly / excessively loud, and at mid volume is still way too loud, and even at lowest volume, that it is still being too loud.
    Now with you being able to differentiate and establish different volume shifts within that total range, that the volume pots A-B resistance changes are being good, BUT subtle, it just seems to have a problem in the B-C loop that grounds out a portion of the audiio.

    The grounding of RV701 C terminal . . . . or . . . .Where do I go ?

    Well ? . . . . . . in consulting the map and with GPS back-up you can go down to the first BLACK dots junction and can go to the left to
    find that the S601 section 1 is having no termination on its Radio position.
    So, now lets go back to the right and then find that path will run into terminal 3 of RV702 tone control. HEY ! . . .you know whuts . .it looks like the tone control is needing to SHARE the same path to ground as the volume control uses.
    Perchance, did you check to see if the tone control had any effect on your audio ?

    Tone control works thusly . . . .

    You have my earlier referenced alternate audio path into S601 section 4 at the Radio terminal and then out the wiper into C721 0.066 ufd coupling capacitor and then down into terminal 2 of RV702 tone control, thru the resistance path to terminal 3 and it finally grounds.
    With the tone being at max resistance of 1-2 position and the resultant series audio path thru C721, will have the least tonal coloration effect upon the audio. Then, as you start rotationg terminal 2 towards terminal 3, there are more of the highest audio frequencies being filtered off to ground. Now you . . .ole fewl that you are . . .think that you are a gittin' mo' BASS . . . . . while you are being fooled by the decrease of the treble frequencies presence .

    Lets now follow that last possibility left and go straight down with C terminal and initially encounter P terminal of S701 section 1#.
    Thats P as in Playback and not its alternative Record option.
    Now I / we know that in order to preclude accidental / unwanted record-over, that this electrical switch is mechanically interlocked such that BOTH hands, two thumbs , three fingers and two vile curse words are equired to successfully get it latched into its actual record mode.
    Now since I also know that you are merely wanting to be listening to the radio, I am seeing that they have the BLACK switch contact bridging the switch into its record mode. I am now correcting and inserting the actual RED dotted in switch contact position, in place of the BLACK position used when recording. . . .and that reverts it to normal Playback mode.
    BEHOLD . . . now use the GREEN arrow grounding path to travel down into the Play terminal, thru the RED slide contact, and up and out the center switch contact and finally into the large wide BLACK ground buss.

    Now . . . . . HOWDOYOUDOITTOIT ?

    This will be back and forth between the two reference types, initially using the RV701 pot photo to see that terminal B has the worst looking solder blob surround . . . . BUT . . . . A and B seem to minimally change the volume.
    YOU . . .yes, you, can see the green resist covered foil at the C terminal and also see which directions it branches out to and eventually connects to.
    I tend to think that the 4 ORANGE pot support brackets might be grounded, and valid grounds but only you can see the foil paths to confirm.
    I can see that my VIOLET rectangle marked up C711 BLACK electrolytic is NOT showing a silver ground band on its viewable side so I can correctly assign that wire terminals solder blob a RED circle positive polaity marking.
    SO . . . . that makes its next inwardly located wire and its solder blob . . . BLACK X marked . . . . terminal as being NEGATIVE polarity, and it is definitely a ground connection on the schematic as its now marked in BLACK X.
    Now you use that same jumper wire used before and place one end on this now confirmed ground and the other wire end at the
    C terminals hole. With the volume control B wiper rotated near C terminal end the volume should drop upon the jumpering action.
    If it does, it is then just a further tracking down of the few other points on the GREEN arrow path to ground.
    If no volume drop, we need to look into the left pages side of the inset of the YELLOW outlined contact portion of C of the pot.
    Thaaaaaaaaaaassit . . . . . .

    I now await for the findings of your situation(s) . . . . .

    [​IMG]



    VISUAL REFERENCING DATA . . . w / its marked up . . . . connotations


    [​IMG]



    73's de Edd . . . . . .

    upload_2021-3-2_11-24-22.png


    .
     
    Last edited: Mar 2, 2021
  10. Technomaniac

    Technomaniac

    94
    12
    Oct 31, 2020
    Its dirty contacts on the record-play slide switch.
     
  11. 73's de Edd

    73's de Edd

    3,215
    1,347
    Aug 21, 2015
    Nokianas . . .? where you g o o o o o o o o o o ? ? ?


    CAVEAT . . . if you do attempt to put on your circuit cleaners mini poly extender tube and shoot cleaner into . . .first . . .
    the central HOSIDEN brand switch and flood it and then work its contacts back and forth 67 times, be sure to have the
    right R410-R506-IC501-R501-C509-C516 components as being the units gravity down / bottom position , such that no excess cleaner, in its draining out, can flow across and into the two CT153-CT154 small round trimmer capacitors and their big main tuning capacitor to the left.
    Then you cleanse the REAL target in its being the other shorter switch and a 83 time back and forth workout.
     
    Last edited: Mar 4, 2021
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