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Re: 250-4 [message #28669 is a reply to message #28618]

Thu, 14 July 2022 11:56

kb0rex
Messages: 33
Registered: June 2022

Member

so it looks like I take out the 3-wire jumper from 5066 to 5069 and jumper 5069 j1 pin1 to pin 2.. ok will report back

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Re: 250-4 [message #28670 is a reply to message #28618]

Thu, 14 July 2022 12:08

kb0rex
Messages: 33
Registered: June 2022

Member

ok i jumped the 5066 and its now loud good signal still has vibrato and tremolo

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Re: 250-4 [message #28671 is a reply to message #28618]

Thu, 14 July 2022 12:09

kb0rex
Messages: 33
Registered: June 2022

Member

for now the amp is not pulling down the power supply and is running cool to the touch. Looks like maybe the amp portion is ok and time to move upstream.

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Re: 250-4 [message #28672 is a reply to message #28667]

Thu, 14 July 2022 12:25

kb0rex
Messages: 33
Registered: June 2022

Member

unplug just the 3wire connector right then jump 1&2 on the 5066 board right?

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Re: 250-4 [message #28673 is a reply to message #28618]

Thu, 14 July 2022 13:19

kb0rex
Messages: 33
Registered: June 2022

Member

now it looks like noise is being introduced somewhere in the preamps, I have a parasitic ocillation on the wave form on all preamp and 5069 board, gets worse the higher the frequncey. So far been a buger to isolate. any good way to jumper past the 3 boards one at a time to find problem? I can put my scope on the input and when I turn it up just a little boom trash on top of sine wave.Hmmmm

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Re: 250-4 [message #28763 is a reply to message #28618]

Fri, 09 September 2022 16:46

kb0rex
Messages: 33
Registered: June 2022

Member

Finished repair a couple of week ago. Thought I would update final repair. Amplifier worked but had a odd 3mhz oscillation on the output. After a lot of head scratching a talk with Bill we added snubber caps on the drivers which were replaced. This did take care of the problem. The new transistors of today are of higher performance and higher bandwidth gain then the old transistor, so you have to degrade or reduce the bandwidth to not introduce the reproduction in this case of the power supply ripple. Nice to know now and makes sense. Amp is stable and really a nice product with its matching dual 15-inch cabinet in folded finish. The customer got this at a garage sale for 50$ and seems pretty darn happy now.

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Re: 250-4 [message #28764 is a reply to message #28618]

Fri, 09 September 2022 20:06

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

Good to hear and I am glad you had the patience to stick with it!
You customer being happy will be great word of mouth for you.
Here's a old story you will relate with I think.

I had SS Vox Beatle head that was a very late model made just before Thomas organ ditched them.

These had very long wires to the T03 output transistors on the far side of the chassis.

This guys amp would blow the outputs when others that I repaired would not.

Now in these late amps I found out that they changed the wiring layout.

At first I thought that it was because this guy used to play the amp dam loud and the amp would go into run away heat over load, so I put a big Ass fan blowing on the heat sinks but it blew yet again.

I finally look at the thing and found that it was oscillating way up there like yours.

I came to the conclusion that it was due to the long and uneven wire length feeding the outputs which was making for a uneven impeadence load on the outputs.

To counter that I added a 10 ohm 1/2 watt resistor in line with all of the four base leads right at the transistors.
The result, I never saw that amp again and in fact I know the next person who now owns it and where talking 18 years since the new owner has had it!

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