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Old 1st March 2005, 03:23 PM   #1
koumbar is offline koumbar  Canada
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Join Date: Feb 2005
Location: Toronto
Default Threshold 800A assistance

Hi guys,
I bought a non working 800A from ebay a while back and got it powering up and playing at lower levels. There were 8 transistors 2N5878/76 missing and a bunch of resistors from on side only. I put in the transistors. I replaced the resistors with identical ones from a surplus store, matching them and being guided by looking at the other half which is symmetric. I have bypassed the two "volume" pots.
Now, when I power it up and it plays clean and clear at low level but when I try to increase the volume it goes into stand by mode. I added a "little" (6X32000uf & 4X15000uf bypassed with 10uf MKP, snatched from an mri rf amp ) capacitance to the power supply which made it sound a lot more velvety but the problem is still there.
Do you think the rest of the transistors are the problem?
Should I just get rid of all the transistors and replace them with new ones, as I feel that the rest that are there are not doing what they are suppose to. If so, and since money will be spent, what about going to another transistor. From what I have looked at the MJ21195/96 might do the job with less sweat. However as I am not an expert I am open to suggestions.
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Old 1st March 2005, 07:47 PM   #2
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Join Date: Mar 2001
You can find a channel schematic at

Sorry I don't have a copy of the control circuits for the
protection - and that may be where the problem is.

Offhand I don't see a reason to replace all the transistors,
unless some measurement shows them to be faulty.
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Old 1st March 2005, 08:48 PM   #3
koumbar is offline koumbar  Canada
diyAudio Member
Join Date: Feb 2005
Location: Toronto
Hello Mr. Pass,
thank you for talking the time to look at my problem. Can I check the transistors on the boards and what should they mesure? if I can.
Another thing is that the sound gets a bit harder just before the amp moves to standby. This is why I thought it might be the transistors.
Maybe I will redo the little board on the bottom of the amp ( I guess that would be the one with the protection? ) with new parts and perhaps this will cure the problem.
However even at the low level it is playing at it still trashes everything I have heard so far. I am in shock to think that people in 1977 were listening to such awesome equipment!!
Thanks again,
I wish I knew what you forgot in the last year!!!!!!
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