7T issues

JBL Rich

Member
Hi guys,

My dad pulled an old 7T out of the garage he had for parts. He thought it was working fine when he put it away 25 years ago. I brought it up on a variac slowly. DC offset seemed fine with no input.

Pulled her into the system, which has a 20B and a 250M. Friggin' thing SLAMMED, and I mean SLAMMED my 250 according to the meters. WTF? Any ideas from the peanut gallery. Fortunately the 250M survived the event.

Regards,
Rich
 
So, there was a little DC afterall.......

Do you have a scope?
Look at the signal with a tone generator / or a CD player with a test cd.

In 25 years caps can be totally rotten as do solder joints.
The 7 T is partially hardwired right?
 
Bad output caps....or input caps....

Recap....

The transistors are in sockets, clean the leads and spray the sockets with De-oxit...

BUT MAKE SURE YOU RETURN THE TRANSISTORS TO THE EXACT POSITIONS.

jk
 
Thanks guys. I'm on it. Ideas on brand/types of caps would be most appreciated.

Thanks,
Rich

I forgot what I used.... few years ago now...I usually try few back to back to find what I like in a given unit. There are some really small tantalum caps in there too... 22 uF at 4V if I remember correctly.

Besides the larger electrolytics, find those small tantalums and replace them with something better.


Also the headphone jack has switches in it to cutout the power amp when you use head phones... Bypass the switches, those 40 year old contacts can degrade the sound....


jk
 
I forgot what I used.... few years ago now...I usually try few back to back to find what I like in a given unit. There are some really small tantalum caps in there too... 22 uF at 4V if I remember correctly.

Besides the larger electrolytics, find those small tantalums and replace them with something better.


Also the headphone jack has switches in it to cutout the power amp when you use head phones... Bypass the switches, those 40 year old contacts can degrade the sound....


jk

Thanks. would something like orange drops be good? Or, something more common, or more esoteric? I've only been swapping out the 'lytics to date (nothing on this unit).

Rich
 
Thanks. would something like orange drops be good? Or, something more common, or more esoteric? I've only been swapping out the 'lytics to date (nothing on this unit).

Rich

Do not replace any of the Film caps.... THEY ARE GREAT QUALITY...Even at 40 years old.

Only replace the electrolytics and tantalums....

Besides the power supply caps ( which mostly need replacement ), there are only about 4 caps that need replacement for the signal path in each channel. Plus one more for the center channel.

And if you exclude the tape head amp there is only 3 caps per channel to replace ( including one in the Phone amp...) .



jk
 
I need to recap my 7t what kind of caps and where do I find the power supply caps the big one that splits into two?
 
My 7t sounds a little Noisey and hissy if I turn the volume past halfway mark what can I do to make it be quiet.
 
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