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Feeback on one channel - 8080


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#1 Atomtan

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Posted 11 January 2019 - 10:29 AM

After a what I thought was a successful restoration of a poorly 8080 it functioned perfectly for a few hours of tape playback and then threw a wobbly I cannot diagnose.

 

When you press play, you get extremely loud feedback from the left channel (the VU needle kicks wildly over to show this), the feedback subsides to a low noise leaving that channel at a lower output level than that coming from the right (and with some background feedback on the left channel).  I have rebelted it (twice as one was misshapen), cleaned the mechanism, cleaned and lubed the record bar and switches, checked and rechecked the seating of the tape within the door and push on wire connections throughout the box. 

 

The weird thing is, if I swap the playback selector to mono the feedback goes away and the channels return to what I would say is normal balanced playback.

 

Any suggestions gratefully received as I cant move on to the next box, until this one is done and its taken toooooo long.



#2 jimmyjimmy19702010

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Posted 11 January 2019 - 04:33 PM

I had my GF-9090 do the same thing recently - the record bar was the cause. It’s the long switch that sits on the main board behind the deck. It is prone to bad contacts due to a lack of movement of this switch over the years due to the fact that it is only activated when you hit the record button.

It carries the power supply to many circuits and must be cleaned with contact cleaner. The switch must then be manually switched up to 100 times to ensure each contact is cleaned.

#3 Atomtan

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Posted 17 February 2019 - 11:17 AM

So I have concluded record bar cleaning is far more involved than I thought.

 

As I had cleaned them twice before, this time I decided to create an 'almost' encasement of each bar using thin plastic strip and bluetak.  I drowned each bar, moved the machine around, pressed the record many times, and repeated.  After that I purchased some of the dioxit fader lube and drowned each bar in that as well.  After that,  I cleaned off the surplus solvent, allowed some drying time and with little hope tested it.

 

Finally, that cracked the issue.  Feedback gone, and both channels restored to even sound.  Not the best sounding box in the world by any means, but finally it is revived.

 

Thanks for your help. 



#4 Fatdog

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Posted 17 February 2019 - 08:15 PM

Glad you got it sorted!  :thumbsup:



#5 BoomboxLover48

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Posted 18 February 2019 - 08:44 AM

Hello Atomtan,

 

    My 8080 still won't play. All other functions work. Do you remember the square belt size. I cannot understand how that belt size will fix the play function because I mover that wheel on the belt back and forth and nothing happens. I have no clue where the catch is to the take up reel axle. 



#6 Fatdog

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Posted 18 February 2019 - 07:51 PM

Hey Royce, this is from the Hitachi belts list.  I would imagine it's one of these:

TRK-8080H/HC	Belt, Flywheel		FRW10.1
		Belt			SCX10.5
		Belt, Counter		SCY7.0

Also, I just uploaded a service manual to the Boomboxery Library:
 

http://www.mediafire...80E_SM.pdf/file



#7 Atomtan

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Posted 19 February 2019 - 09:03 AM

Not sure I can any more guidance on the take up reel.  I used my knowledge from my two 8200 rebuilds on the 8080, but they are very different machines in the way they are assembled.  I wondered about buying the service manual as I worked through the 8080, but as its a box I like the look of, but didn't really hold in a lot of reverence, I plowed on until I either fixed it, or became too old trying.  Fatdog has now added the manual, so I can only suggest you go there and wish you the best of luck.

 

As a post script, I have noticed that the re-wind function is slow on my 8080, which I wonder remains belt size sensitive, but I have taken the view now to leave it be.



#8 BoomboxLover48

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Posted 19 February 2019 - 10:31 AM

Hey Royce, this is from the Hitachi belts list.  I would imagine it's one of these:

TRK-8080H/HC	Belt, Flywheel		FRW10.1
		Belt			SCX10.5
		Belt, Counter		SCY7.0

Also, I just uploaded a service manual to the Boomboxery Library:
 

http://www.mediafire...80E_SM.pdf/file

Thanks so much Bobby!  :bow:  :bow:  :bow:



#9 baddboybill

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Posted 20 February 2019 - 06:06 AM

I always has issue with 8080 FF or Rew. Can’t remember which but close to end of tape it would slow down or even stop. I believe issue was the counter belt which put drag on it.

#10 Nickeccles

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Posted 29 September 2019 - 04:07 PM

These suffer from take up tyre failure over time & getting to it to replace the tyre is extremely difficult (For me anyway!) You have to strip the deck right down - When I went to remove the layer of gears & the take up reel idler & tyre, there was a quick ping & to my dismay springs gears & levers shot everywhere!  :bang: :bang: :bang:
 
That was my 8080 archived for many months when I bought a scrapper from the bay that had a great deck once re belted!! So I built one out of the two & it's now working like a dream producing that warm late 1970's sound..................
 
Attached File  11.jpg   128.19K   2 downloads

#11 Atomtan

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Posted 03 February 2020 - 11:11 AM

Back to the same problem that started the thread on my 8080.  After a brief period of working okay, it has returned to intense feedback on the cassette when you press play, which subsides over the next 20 seconds, but then plays fine on right channel, and nothing through left.  The radio is perfect through both (interestingly line in is poor, but I have read this is common on an 8080).

 

I want this to be last time I tear this box down, and I don't think the record bar will benefit from any more of the usual techniques, but I maybe wrong.  I wondered if Azimuth could be my issue?  I was hoping for some advice on what order to go through it again rather than just trying 'the next thing' on my list.

 

Thank you.



#12 caution

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Posted 04 February 2020 - 01:03 AM

You're not hearing a head misalignment, that's a muffling sound. It's probably still the same switch if it sounds the same as before. Contact cleaner only works to a point. I had a loudness switch go bad again and again after spraying it well, it would slowly start to add noises into the audio channel from losing good connection. I'd switch it a few times and it's good for a bit, but would always return. Eventually it goes to be every couple minutes. I took it apart and all the terminals had a black coating of oxidation or carbon or dust or something... Problem is, normally the contacs only touch the terminals along a thin line, the vast majority remains untouched, coated with gunk ready to seep back on the active area of contact. Most brackets for these have little pockets that the contacts side up inside, so no spray is going to do a very good job pulling crud out when it wants to pool in the pockets.



#13 Atomtan

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Posted 04 February 2020 - 10:30 AM

Thank you for that, I did wonder if like some sliders on other boxes, the record bar could reach a point where only disassembly will do.  Never seen one of those apart before, time to start looking through the forum.  Everything tells me to leave the box be and set it aside, I just can't as it bugs me being so near to fully operational....



#14 caution

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Posted 04 February 2020 - 10:44 AM

Here's a few posts with pics of switches open for cleaning

http://boomboxery.co...-dead/?p=341984

http://boomboxery.co...resto/?p=313844

http://boomboxery.co...resto/?p=360513



#15 Atomtan

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Posted 07 February 2020 - 06:08 AM

Thank you so much, those threads have given me the confidence to open this up one more time.  

 

Your photographs are stunning by the way.



#16 Superduper

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Posted 07 February 2020 - 11:53 AM

Before you open one of those, you may want to consider soaking them in deoxit first. Spraying has limitations on penetration but soaking is more effective. Those leafs are very fragile and putting the switch back together isn’t as easy as it may seem. If you end up mangling one of the leafs, you will be up the creek. Just remember that there’s a certain amount of risk with the procedure if you choose to disassemble those.

#17 BoomboxLover48

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Posted 11 February 2020 - 10:45 AM

These suffer from take up tyre failure over time & getting to it to replace the tyre is extremely difficult (For me anyway!) You have to strip the deck right down - When I went to remove the layer of gears & the take up reel idler & tyre, there was a quick ping & to my dismay springs gears & levers shot everywhere!  :bang: :bang: :bang:
 
That was my 8080 archived for many months when I bought a scrapper from the bay that had a great deck once re belted!! So I built one out of the two & it's now working like a dream producing that warm late 1970's sound..................
 
attachicon.gif11.jpg

I still have no luck with the rebelting of it. I even replaced the whole deck assembly. I heard there is something tricky with the belt tension. 

It will rewind, fast forward, but play won't engage. I pulled lots of hair...... I still cannot understand why the contact wheel won't engage. Sorry.... I am going in another tangent .... not related to the main post here.



#18 Nickeccles

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Posted 15 February 2020 - 06:33 PM

As I say, it was the take up tyre on mine that had failed - I have found up until I bought mine that the deck is extremely reliable & rarely fails, I guess the passing of time results in more failures!!

 

Keep looking for a scrapper, chances are the deck will work after rebelting! Mine did  :yes:

 

As for the OP's 8080 it does sound like record bar failure of some sort although it could be a duff capacitor or two as it takes time to manifest on playback!

 

I have a working mainboard for an 8080 I'm sure - PM me & I'll find it & post to you! 



#19 Atomtan

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Posted 18 February 2020 - 10:12 AM

Nick, thank you for the kind offer.  When I get some time, I will have one final crack at the record bar, if that fails then I will be in touch via PM.