RC-M70JW with permanent low volume issues

[lberstein]

Hello everyone. First of all, english is not my native language so please forgive my mistakes or weird choice of words.
I have read some wonderful threads on restoring M70s, which I found because I seemed to have some similar issues. This is my parents' boombox, which I have dissasembled a few times to fix different simple issues. I come to you experts now in hope you can put me on the right track in finding the fix for my current issue. I'll try to be as detailed as possible so forgive me in advance for what might be a long post.

How it started:
Latest repair I did was cassette player not working properly: tapes would slip, and it would randomly autostop after a few seconds. Slipping was due to bad rubbers, which I changed and that was fixed. Random stopping I suspected a faulty blank detection chip (BA335, which is IC602). I disconnected the solenoid while sourcing a replacement. Tape player worked fine meanwhile. Once I got a replacement BA335 I took out the old one and soldered the new one in, and reconnected the autostop solenoid. Everything worked fine.

Current situation:
A few weeks later, the device has very low volume even when volume sliders are at full max. Sliders don't seem to be faulty, as volume is consistent, and wiggling the sliders doesn't produce any change. Sliding them down does still lower the volume even further. This is the same for both radio and tape mode. Also both channels give exact same volume (which furthers the theory that sliders are working fine), and level indicator needles are consistent with (low) volume level. When switching the power on it gives a louder bump (it always has), but wiggling the power switch doesn't produce any changes or faulty contact either.

I read a very detailed thread of someone having the same problem, in that case it was due to a LED in the MMS, so I assumed in my case it was the MMS system as well, but disconnecting its multiconnector doesn't change anything. Same for disconnecting the tape recorder module: volume is still low on radio. I assumed it still had something to do with the IC602, since it's the only thing I replaced, and its job is exactly to mute the system while in programming mode, so I changed the chip once again (I had the precaution of buying 3). No changes, still low volume in exactly the same way.

If I turn the MMS on, the first LED lights normally. If I then push the "program" button, the volume goes even lower as long as I keep pressing it, and goes back the "normal" (low) when I release it, which makes me think the muting function is working properly (but the whole volume levels are offset down).

MMS system does behave weirdly, but since it has never been used I can't really tell if this is a new issue or not: if I push program, second led flashes quickly, but that's it. I cannot get any other (numbered) LEDs to stay on besides the first (that indicates programming mode). In other words, programming is not working. Still, disconnecting the whole MMS module doesn't change the volume issue, so I believe these to be 2 separate issues.

Can you give me any advice on what to try or test? Any suggestion would be deeply appreciated. Also let me know if I forgot to mention something vital.

 

[lazol]

I don't recall if the earphone plug is driven by the amp or preamp, but did you try it out?
Are the VU meters acting normally?

 

[Radio raheem]

I WOULD GUESS IT HAS SOMETHING TO DO WITH THE MUSIC SEARCH MUTING SYSTEM OR THE RECORD BAR, good luck, sorry about the caps

 

[lberstein]

I don't recall if the earphone plug is driven by the amp or preamp, but did you try it out?
Are the VU meters acting normally?

Thank you for your suggestion!
VU meters are working "ok", meaning they show low volume, which is correct. They act according to the true sound volume that the device is outputting (low).
I didn't try out the earphone plug, it seems a pretty logical thing to check. I'll need to get hold of some earphones with a 1/4" plug, or an adapter. All my earphones use the 1/8" jack.

 

[lberstein]

I WOULD GUESS IT HAS SOMETHING TO DO WITH THE MUSIC SEARCH MUTING SYSTEM OR THE RECORD BAR, good luck, sorry about the caps

Thank you!
What do you mean by the record bar?
I also suspected the MMS muting system first, but I replaced the IC responsible for that twice and nothing changed. I also noticed that the lower goes even lower when in programming mode, and returns to "normal" (low) when not in programming mode, which makes me think the muting system is working as intended. Also disconnecting the whole MMS and tape recorder assembly doesn't fix the issue.

 

[Radio raheem]

im trying to help on both forums....can you show me seb968 repair posts...i need to read them, thanks

in the meantime check all music search led's are working with a mustimeter, i think it has something to do with a blown led....i know this is really strange but sebs fault was something like this

 

[lazol]

I would exclude the music search module causing the issue, because the tuner gives you the same problem.
Did you clean all the switches? Especially the 'function' switch, to reach it you have to open the back because it's far from the actual switch.
Try deoxite all the switches and work them up and down several times, also the record bar, as suggested (the record bar is easy to spot, it's the one activated by the tape deck).
When the problem is equal in both channels you need to focus on the elements on the signal path involving both channels together, and they're not a lot: usually it's a switch issue.

 

[lberstein]

im trying to help on both forums....can you show me seb968 repair posts...i need to read them, thanks

in the meantime check all music search led's are working with a mustimeter, i think it has something to do with a blown led....i know this is really strange but sebs fault was something like this

This is the post: http://www.stereo2go.com/topic/index.php?content_oid=361866108350028016&board_oid=193392314111653319

I did try to check the LEDs using a multimeter and the service manual, following seb's post, but didn't find anything. All LEDs seem to work (despite the MMS not working correctly), but I still don't think the volume issue is related to that.

 

[lberstein]

I would exclude the music search module causing the issue, because the tuner gives you the same problem.
Did you clean all the switches? Especially the 'function' switch, to reach it you have to open the back because it's far from the actual switch.
Try deoxite all the switches and work them up and down several times, also the record bar, as suggested (the record bar is easy to spot, it's the one activated by the tape deck).
When the problem is equal in both channels you need to focus on the elements on the signal path involving both channels together, and they're not a lot: usually it's a switch issue.

Thank you! I haven't try deoxite, I've read about it in several posts but I'm not sure what the equivalent is here. I'll have to go to an electronics supply store and see what I can find out. I did clean most of the device with IPA. It is currently open on my dinner table, back cover removed, and I know what swtich you mean, it has a long plastic L shaped bracket that translates the movement from the visible "switch" to the actual switch on the board. I'll try to clean that in particular and see how it goes. I still find it odd that there's no noise at all, no ripple, no crackling nor anything when moving the switches or sliders around.

 

[lazol]

Thank you! I haven't try deoxite, I've read about it in several posts but I'm not sure what the equivalent is here. I'll have to go to an electronics supply store and see what I can find out. I did clean most of the device with IPA. It is currently open on my dinner table, back cover removed, and I know what swtich you mean, it has a long plastic L shaped bracket that translates the movement from the visible "switch" to the actual switch on the board. I'll try to clean that in particular and see how it goes. I still find it odd that there's no noise at all, no ripple, no crackling nor anything when moving the switches or sliders around.

Cleaning the switches won't hurt anyway.
Don't forget to clean the headphone jack and the external speakers/line in/out jacks.
By the way, did you try it with the line-in? Same issue?

 

[docs]

Eliminate the record bar of the cassette deck first. You can do this by operating the unit and seeing if engaging the switch changes the volume. If it does then that is likely your issue, if it doesn't move on.
Check voltages at the power amp IC's 302 and 402, there should be 15v at pin 1. Pins 2 and 17 are outputs and should read 7.5v. Inputs I think are pin 7 and 12, should be 7.5v.
Eliminate these to ensure there isn't a voltage issue.

 

[lberstein]

Cleaning the switches won't hurt anyway.
Don't forget to clean the headphone jack and the external speakers/line in/out jacks.
By the way, did you try it with the line-in? Same issue?

I did try the Line-in, but it produces no sound whatsoever. I might be doing something wrong, but the connection is pretty straightforward, I used RCA cables and the output from another (working) tuner. Trying the other way around (line out from the M70 to line in on the other tuner) also produces complete silence.
I'm yet to try the heaphone jack as I haven't been able to get hold of !/4" jack yet.

 

[lberstein]

Eliminate the record bar of the cassette deck first. You can do this by operating the unit and seeing if engaging the switch changes the volume. If it does then that is likely your issue, if it doesn't move on.
Check voltages at the power amp IC's 302 and 402, there should be 15v at pin 1. Pins 2 and 17 are outputs and should read 7.5v. Inputs I think are pin 7 and 12, should be 7.5v.
Eliminate these to ensure there isn't a voltage issue.

Al voltages on the ICs are correct. Slightly over what their are supposed to be, but consistent. I get 16v on the 15v pins, and 8.5v on all the 7.5v pins.
I'm not entirely sure what you mean by the record bar. Is it the recording head that comes down when pressing record on the tape player? In that case, it doesn't change anything. Also, disconnecting the whole cassette player assembly still gives low volume.

 

[lazol]

Al voltages on the ICs are correct. Slightly over what their are supposed to be, but consistent. I get 16v on the 15v pins, and 8.5v on all the 7.5v pins.
I'm not entirely sure what you mean by the record bar. Is it the recording head that comes down when pressing record on the tape player? In that case, it doesn't change anything. Also, disconnecting the whole cassette player assembly still gives low volume.

This is a record bar (generic), push the plastic extremity up and down several times, possibly after spraying it with contact cleaner.

 

[lberstein]

This is a record bar (generic), push the plastic extremity up and down several times, possibly after spraying it with contact cleaner.
View attachment 52507

Thank you!
I located the record bar on my device. Pushing the plastic end doesn't produce any changes at all. I sprayed it with cleanr just in case, but still no change

 

[lazol]

When you get your big headphone jack, first thing clean the headphone output with it, working it on and off and turning it, possibly using contact cleaner. Do the same with the 'external speakers' jacks: I saw the schematic, and the signal pass from there before reaching the speakers (so no need to try with the headphones, you'll get the same result).
Nothing is impossible (like multiple components failing at same time) but having the exact same problem in the tuner/tape deck and left/right channel at the same time, and having the amp chip tested working, lead me to think that the issue is in the function switch.
It could be the switch itself or something external, like a cold solder joint.

 

[docs]

Did the audio work before you rebelted the deck? You original post seems to indicate it did.

The audio path goes across X305/405 and into the board where the bass/treble/main volume are, through the bass and treble then to the loudness board through the loudness switch and back out through the main volume and to the power amp, then from there to the headphone socket, the external speaker socket then the speakers.
If you think about it, assuming the power amps are ok by the voltage readings but then getting no audio in the headphone jack doesn't make any sense unless there is a faulty set of wires.

It would be easier using a audio signal tracer at this point to be able to detect where you can hear audio if at all. Also a signal injector may help if you have any of these tools?

Make sure you test voltages using DC power.
If voltages are still accurate at power amps we move on.
All functions on this unit lead into the line amp, look at X305 transistor and check voltages, again ensuring you use DC. At base 0.9v, at collector 6v and at emitter 0.3v.
Also check variable resistor/sliders bass, treble, main volume and the loudness switch. You can check continuity changes between pins too which would at least check the brushes are intact without opening them. If any of these are faulty, ie: the brushes are loose, this could lead to audio issues.

If all the above checks out, as lazol says, goto the function switch and test it best you can, it might need removing and taking apart.

 

[Hajidub]

After deoxiting all the knobs, buttons, and switches (from the inside) if it's still acting up, try using canned air on the output transistors while it's playing, if it crackles or breaks up they need replacing.

 

[lberstein]

Thank to everyone for the messages!

Yes, audio worked fine before rebelting. It still worked fine for a while both immediately after rebelting (while the solenoid was disconnected), and after replacing the IC602 blank-detection chip (and reconnecting the solenoid). The issue appeared some time after that.

I finally managed to get an old pair of headphones with the 1/4" jack. Audio volume is still low when playing through them, so the issue must be originated somewhere earlier. Cleaning the jack and plugging/unplugging the headphones several times made no difference.

I have been using the unit with an AC main, I'll need to find an apporpiate power supply for the 12V DC in with the appropiate socket. I'll do that next before testing more components, as suggested above. I didn't think the power source would be critical, I assume all power supply transformation came way before the audio electronics.

Thank you docs for the detailed tracing for the signal. Unfortunately I don't have specific tools for this, only my trusty multimeter.

The only other thing I've noticed is that the cables that come from the connector at the upper right (looking from behind) corner that connect all the sliders and switches board to the main amplifier board, produce some noise and loss of radio tunning if I touch them (over the insulation!), but I assume this has something to do with grounding by induction.

Also, just to cover all the bases, VU meters do reflect correct volume level, as they do give a spike up that coincides with a short crackling when quickly turning the unit on and off.

 

[docs]

Only reason to test on DC is because the service manual voltages are tested and produced using DC supply.
Don’t forget you can rule out cables or connecting traces using your multimeter on continuity test at both ends of the route.