M90 Recording noise in left channel.

[Tinman]

Ok, I refurbished this M90 a couple of years ago and it's been sitting, nicely wrapped up, since.
I usually verify everything is working before calling a refurbishment done so I'm fairly sure the recording function was working before I stored it away.
I recently pulled it out and was checking a remote I have for it and tried the recording function and noticed the left VU meter was just holding steadily in the red/orange and it didn't matter if I was in manual (any volume) or automatic mode.
I just tried recording on a tape and sure enough, the left channel has a constant feedback noise and didn't capture my voice at all, the right side works fine.
It plays tapes fine.
I don't really have time to rip into this and troubleshoot it right now so I figured I'd see if anybody has any ideas.
I'm fairly sure I've had this problem before but can't remember which box it was or how I remedied it.

 

[floyd]

I would check the Record Head make sure there are no cold solder joints or disconnected wires. Could also mean the Record Bar is dirty.

 

[Radio raheem]

There is no record bar on the m90 lad, it's all electronic...apart from that i wouldn't have a clue
the hemiguy m90 doesn't record but i'm not bothered, aint recorded a tape in 20 years lol
try recording off the radio and see what happens then

 

[hopey]

Could be the Rec cable off the head try removing and spread the crotch and sweat back on. Channel maybe down to ground.

 

[Tinman]

There is no record bar on the m90 lad, it's all electronic...apart from that i wouldn't have a clue
the hemiguy m90 doesn't record but i'm not bothered, aint recorded a tape in 20 years lol
try recording off the radio and see what happens then

I'm definitely not going to be recording anything onto cassette any time soon, it just drives me crazy when something is malfunctioning.
I'm definitely a little anal., lol.

 

[Tinman]

Could be the Rec cable off the head try removing and spread the crotch and sweat back on. Channel maybe down to ground.

With me, it's never anything that easy but it's definitely something to check out.

 

[floyd]

There is no record bar on the m90 lad, it's all electronic...apart from that i wouldn't have a clue
the hemiguy m90 doesn't record but i'm not bothered, aint recorded a tape in 20 years lol
try recording off the radio and see what happens then

Out of the over 100 boomboxes I've owned the M90 is the one I worked on the least lol. The only thing I ever did was change the belts of the deck and nothing else.

 

[floyd]

Out of the over 100 boomboxes I've owned the M90 is the one I worked on the least lol. The only thing I ever did was change the belts of the deck and nothing else.

And I wouldn't even got that done without help from you know you guys back in the day too. Actually the M90 is the first Boombox I ever worked on and it sat for years taken apart until I can figure out how to get it all back together.

 

[BoomboxLover48]

Clean the external microphone jack with Deoxit. That jack should cut off the internal built in mic when you put an external microphone. Simply insert an external microphone male plug in and out and see if that helps.

 

[Tinman]

This was weighing on my mind so I decided to open it up on Saturday and take a look around.
One thing I noticed before taking it apart was that when I touched the Rec-auto/manual switch while in record mode, the meters would move slightly.
Even though I'm sure I thoroughly cleaned everything when I first worked on the box, I figured it couldn't hurt to give it another dousing of Deoxit.
I removed the back panel and then the chassis from the front panel.
Amazingly, this must've been sitting for over 2 years after seeing my date markings in the box.
The first thing I checked was the cassette head solder joints which looked perfect.
Then I figured I'd remove both main & pre amp boards from the chassis, deox all controls and look for cracked joints, loose wires, etc.
I went over both boards with my magnifying glass and ended up resoldering some suspicious joints on the pre amp board.
Oddly, one of the joints I found had a black mark next to it so I may have singled it out originally and forgot to resolder it.
I also added a jumper wire across a ground tracing that was cut (possibly on purpose when it was assembled?), it's right by the external mic jacks.
I believe I did the same thing on another M90.
It showed a slight resistance but was 0 after my jumper was in place.
I also sprayed Deoxit in the external mic jacks and worked them like BbL48 suggested.
After a few hours, I decided to reassemble everything and try it out.
Before sliding the chassis into the front panel, I verified that both mic wires weren't grounding out.
I also checked the little board they're soldered to which looked fine.
I really didn't have much hope since I didn't find any obvious problem.
Once back together, I hit record and both needles were sitting at -20 like they should be.
Switching from auto to manual and adjusting the level knobs showed the same.
FIXED!
How, I have no clue, lol.
If I had to guess, I'd say the Rec switch was fouled some how and deoxing it cleared it up.
Touching it now while in record mode doesn't cause the needles to react like it did.
It took most of my Saturday but it feels good to have it fully working

Here's a pic of what the needles looked like when in record mode.


Here it is working correctly in auto.


Here it is working correctly in manual with both rec level knobs at max.

 

[Radio raheem]

That's great news sorted

 

[BoomboxLover48]

That's Great News!

 

[hopey]

A good example of how Deoxit or Contact Cleaner is only a temporary fix. Need to remove and rebuild the Switches.

 

[BoomboxLover48]

A good example of how Deoxit or Contact Cleaner is only a temporary fix. Need to remove and rebuild the Switches.

Where was that example hopey?

 

[Radio raheem]

A good example of how Deoxit or Contact Cleaner is only a temporary fix. Need to remove and rebuild the Switches.

I agree with you Hopey, however rebuilding switches is beyond most on here imho

 

[Tinman]

A good example of how Deoxit or Contact Cleaner is only a temporary fix. Need to remove and rebuild the Switches.

I agree that taking apart and physically cleaning/sanding all contacts and applying grease (if that's what you mean by rebuilding) would be the best way but that's a whole lot more work.
There have been some switches that wouldn't work even after a cleaning with contact cleaner.
I remember removing the power button on my first SCR-8 after contact cleaner didn't work.
I assume the build up from the contacts arcing over the years of use is too much for contact or Deoxit to completely remove.

 

[Hajidub]

Maybe a better fix, but I've owned equipment that I've deoxit'd and has worked for over decades, including a Nak BX-300 with a supposed bad motor.

 

[caution]

I assume the build up from the contacts arcing over the years of use is too much for contact or Deoxit to completely remove.

Yeah, some switches aren't set up well for a flush, so grime just moves around inside, eventually making its way back onto the contact area and needing more spraying later. I'd think that the voltages aren't high enough for arcing, and that the grime is either coming from dust/dirt, tar from cigarette smoke, and maybe a little corrosion/patina.
That's what was happening to me on these switches, a full clean made my audio issues disappear