Japanese JVC M70 100V to 240V conversion advice needed

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jimmyjimmy19702010

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Hi Guys,

I'm helping out a local guy whose Japanese domestic 100V M70 that was fried when connected to 240V by a mate!!
View attachment 18449

He sourced an adjustable power board & transformer + main board for me. After checking the Japanese domestic 100V transformer and board, I found that the boards and wiring is different.

Anyone out there done this same conversion? As the unit now has no sound output, we were also hoping to swap an M70 main board into the 100V box but there also appear to be many differences between the two versions. Are there any compatibility issues I need to be aware of?

Thanks,

James.... :-)
 
alfie

alfie

Member (SA)
Hi James,

I have 2 victor M70s in storage which are both parts boxes both 100v. Im pretty sure you'd find what you need. Only problem is, I wont be able to access them for about 5 or 6 months....... If you're still struggling a bit further down the track let me know.....
 
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jimmyjimmy19702010

Member (SA)
alfie said:
Hi James,

I have 2 victor M70s in storage which are both parts boxes both 100v. Im pretty sure you'd find what you need. Only problem is, I wont be able to access them for about 5 or 6 months....... If you're still struggling a bit further down the track let me know.....
Click to expand...
P.M on the way.

Thanks,

James.... :-)
 
Superduper

Superduper

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The connectors are different but there should be no problem retrofitting the selectable voltage PS. You can either swap the harness over so it is plug/play or you can direct solder the wires to the mainboard where the PS connects at the bottom of the mainboard. However, there are additional differences with the mainboard including the interface connectors with the tuner -- I recommend that you just fix whatever ails your mainboard. These different models are hard to swap/interface together. Even the DIN and RCA boards of the north American and European models are sufficiently different to make these not a direct swap of the I/O boards, and likely not worth the trouble. The alternative is to swap the entire guts from a working unit into your better condition cabinets but unless it's another victor, there won't be the front pitch controls.

jimmyjimmy19702010 said:
I found that the boards and wiring is different.

Anyone out there done this same conversion? As the unit now has no sound output, we were also hoping to swap an M70 main board into the 100V box but there also appear to be many differences between the two versions. Are there any compatibility issues I need to be aware of?
Click to expand...
 
J

jimmyjimmy19702010

Member (SA)
Superduper said:
The connectors are different but there should be no problem retrofitting the selectable voltage PS. You can either swap the harness over so it is plug/play or you can direct solder the wires to the mainboard where the PS connects at the bottom of the mainboard. However, there are additional differences with the mainboard including the interface connectors with the tuner -- I recommend that you just fix whatever ails your mainboard. These different models are hard to swap/interface together. Even the DIN and RCA boards of the north American and European models are sufficiently different to make these not a direct swap of the I/O boards, and likely not worth the trouble. The alternative is to swap the entire guts from a working unit into your better condition cabinets but unless it's another victor, there won't be the front pitch controls.
Click to expand...
Ok, so assuming I have all the replacement parts I need from our parts board, where should I start? What parts would be automatically damaged with an over voltage situation? The sound is there but at a very low level so the pre-amp would be a good place to start?

The owner of this M70 is having a lot of trouble finding someone willing to fix the M70 so I know any help I can give him would be appreciated.

James... :-)
 
J

jimmyjimmy19702010

Member (SA)
Superduper said:
does the line out work at the right signal level or is it attenuated?
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Thanks for the quick response Norm.

Running the line out from the M70 to the line in of a Hitachi TRK-9300, the output level from the M70 sounds normal on both tape and tuner modes.

The owner of the M70 told me the sound level on all modes was very low but after testing it myself tonight on D cells, I can only get the odd crackling noise on all modes through the speakers.

James.... :-)
 
toshik

toshik

Member (SA)
I guess we should use plain common sense here. Run the main board off extranal DC power supply I'd guess it should say the correct voltage on the back (should be somewhere between 9-15 VDC) and see if anything is still alive.
 
Superduper

Superduper

Moderator
Staff member
Hey Jimmy, got to be a little patient since we are on opposite sides of the planet and our times are probably upside down. The fact that line-out is OK sounds to me like the (active) preamp portions are working fine. That signal then relays off to the passive preamp board (sound shaping) before getting passed off to the amplifiers. First thing you should check is the connector from main board to the slider board. If that connector is loose, it might cause that. Also, if the sliders have failed, this could also be the source of the issue. Lastly, the problem could be in the main amplifier. Extremely low voltage might contribute to that but it's doubtful that the voltage would drop low enough to cause low output and still have a proper working deck and strong tuner. It's simple enough to check of course by probing (live -- be careful) the backside of the mainboard during operation at the V+ pins of the amp chips (look at P.C.B. diagram in service manual for proper spot -- pin #1). If voltage is very low, then that may be the culprit.

If the amps are getting proper voltage, then the next thing to check is to see if the amps themselves are working properly. This involves injecting an audio signal into the amp (live) to see if it will amplify. Pin #7. Be careful since unless you have an attenuable audio signal generator to adjust and serve as a volume control, the output will be at full volume. If yes, then the slider board is suspect. If no, then the amps might be toast.

Lastly, this is a bit harder to troubleshoot for the novice but there are several muting related transistors. The best thing to do is perform VR (voltage/resistance) tests on those transistors and appurtenant circuit (live on transistors -- be very very careful with these as it's easy to toast a transistor with a slipped probe). Treat significant voltage discrepencies suspiciously.
 
J

jimmyjimmy19702010

Member (SA)
Superduper said:
Hey Jimmy, got to be a little patient since we are on opposite sides of the planet and our times are probably upside down. The fact that line-out is OK sounds to me like the (active) preamp portions are working fine. That signal then relays off to the passive preamp board (sound shaping) before getting passed off to the amplifiers. First thing you should check is the connector from main board to the slider board. If that connector is loose, it might cause that. Also, if the sliders have failed, this could also be the source of the issue. Lastly, the problem could be in the main amplifier. Extremely low voltage might contribute to that but it's doubtful that the voltage would drop low enough to cause low output and still have a proper working deck and strong tuner. It's simple enough to check of course by probing (live -- be careful) the backside of the mainboard during operation at the V+ pins of the amp chips (look at P.C.B. diagram in service manual for proper spot -- pin #1). If voltage is very low, then that may be the culprit.

If the amps are getting proper voltage, then the next thing to check is to see if the amps themselves are working properly. This involves injecting an audio signal into the amp (live) to see if it will amplify. Pin #7. Be careful since unless you have an attenuable audio signal generator to adjust and serve as a volume control, the output will be at full volume. If yes, then the slider board is suspect. If no, then the amps might be toast.

Lastly, this is a bit harder to troubleshoot for the novice but there are several muting related transistors. The best thing to do is perform VR (voltage/resistance) tests on those transistors and appurtenant circuit (live on transistors -- be very very careful with these as it's easy to toast a transistor with a slipped probe). Treat significant voltage discrepencies suspiciously.
Click to expand...
Thanks for your detailed response. The owner assures me the M70 worked 100% before the incident. So in a situation like this, the tone board could also have been damaged as it's the same circuit path as the main amp board. As a novice, I organised a complete main board for a simple plug and play fix but if the tone board could also suffer damage, the parts list required is starting to grow!

He has been quoted $110 per hour by a local tech! I'm thinking the cost of the repair is going to far exceed the value of the box.
I think I'll have to pass this one back to him as I don't want this M70 to be my first electronics causality.

Thanks again for your assistance Norm,

James... :-)
 
Superduper

Superduper

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Staff member
The slider board does not contain any "active" components. By active, I mean semiconductors and they are more prone to voltage over voltage than resistors. The caps could get hurt but that's highly unlikely since there are over 100 electrolytic caps in the rest of the boombox and you said yourself that line-out is good so I can't imagine the slider board caps getting special treatment. Therefore, when I say the slider board can be suspect, I am referring more to a connection issue or to bad sliders -- hardly unheard of with the M70. However if the owner claims it worked before, then the sliders should still be good but just keep in mind that I am highly distrustful of owners who say it worked 100% before. I have been snookered too many times to count and wasted lots of diagnostic time accepting "presumptions" that later turned out to be unreliable.

Truth be told, most of the circuits in the M70 are protected by voltage regulators. The amps themselves however are not and get full unregulated rail voltage. Doubling the voltage could likely have fried the amp chips themselves and in the back of my mind, I think there is a good chance that damaged amps might be the issue. Keep in mind that on AC, they already get like 18 volts. When you plug a 110 box into 220, that is going to result in the amps momentarily getting 36+ volts until the PS blows.
 
Superduper

Superduper

Moderator
Staff member
Oh and most boombox repairs are considered by shops to be "not economical to repair" or another way of saying the cost to repair exceeds the value of the repaired product, especially at $110/hour. Virtually every shop charges a minimum of 1-hour shop time and if they aren't familiar with the M70, disassembly and reassembly already could take up 30 minutes or more.
 
J

jimmyjimmy19702010

Member (SA)
Superduper said:
The slider board does not contain any "active" components. By active, I mean semiconductors and they are more prone to voltage over voltage than resistors. The caps could get hurt but that's highly unlikely since there are over 100 electrolytic caps in the rest of the boombox and you said yourself that line-out is good so I can't imagine the slider board caps getting special treatment. Therefore, when I say the slider board can be suspect, I am referring more to a connection issue or to bad sliders -- hardly unheard of with the M70. However if the owner claims it worked before, then the sliders should still be good but just keep in mind that I am highly distrustful of owners who say it worked 100% before. I have been snookered too many times to count and wasted lots of diagnostic time accepting "presumptions" that later turned out to be unreliable.

Truth be told, most of the circuits in the M70 are protected by voltage regulators. The amps themselves however are not and get full unregulated rail voltage. Doubling the voltage could likely have fried the amp chips themselves and in the back of my mind, I think there is a good chance that damaged amps might be the issue. Keep in mind that on AC, they already get like 18 volts. When you plug a 110 box into 220, that is going to result in the amps momentarily getting 36+ volts until the PS blows.
Click to expand...
Ok, so maybe transplanting the amp chips from our parts board is worth a try!?

If no luck, then I'll look into swapping the whole board (extra dramas with it coming from a different version M70). It would mean swapping the good amp ICs back again though.

James.... :-)
 
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