Kustom 250-4 [message #27628] |
Thu, 09 January 2020 12:23 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
johnnysynth
Messages: 45 Registered: June 2018
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Member |
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I'm rebuilding a Kustom 250-4 amp. It works but is noisey and distorts in the high end frequencies
with my Wurlitzer Electric Piano. Right now rebuilding the (2) preamp sections starting with
the 14pin dual op-amps XR4739/nte725 uA739
This part is not listed in Mouser or Digikey
Any clues on this or any other -hard to find- devices
in the pre-amp section?
I replaced the two 'big can' electrolytics already
in the pwr. supply and that brought the
idyl noise down considerable
That's as far as I've gotten
Jake Welsh
johnnysynth@yahoo.com
808 755-5886
Saint Augustine, FL
1/9/20
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Re: Kustom 250-4 [message #27631 is a reply to message #27628] |
Sat, 11 January 2020 06:27 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4767 Registered: June 2004 Location: NY
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Senior Member |
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Yes, NTE has discontinued the 725 chip, but I do not think that's where your high frequency distortion is stemming from as channel one and two get mixed together in that chip and then mergered they get sent to the driver board, and more then likely your channel one would be having that same issue.
What I would do next is unplug, clean and then replug in all the interboard connectors a few times to is if first that fixes it.
Be careful with the plug on the power amp driver board as I have had a few amps who's pins on that board where barely soldered in on the bottom side.
Check to see if you can rock them at all.
If the connector removing and cleaning does not fix the issue then I would look at replacing all the small Tantalum electrolytic caps on that board as one of them is likely partitaly shiorted and dropping down the needed +12 or -12 volts that powers that board.
I would therefore replace C6 and C7 on that preamp board ( 100uf 15 volt) with non polarized filter caps with a 25 volt or better rating, in fact if it where me then while I was in there I would replace all of these type caps on all the boards since they are all the same age and don't last forever!
Just be sure to work on one board at a time so that if you make a mistake you will have not screwed yourself as bad!
Also be darn sure to not turn the amp on without the connector on the driver board plugged in as that will damage the output stage.
Let us know how things go.
[Updated on: Sat, 11 January 2020 06:37] Report message to a moderator
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Re: Kustom 250-4 [message #27639 is a reply to message #27628] |
Thu, 16 January 2020 09:05 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
stevem
Messages: 4767 Registered: June 2004 Location: NY
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Senior Member |
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4 start electronics shows 1 ECG725 in stock which is that 14 pin stock.
You need to call for a price quote.
877 240 8595.
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