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How can I remove the heatsink from a TO-5 style transistor?
How can I remove the heatsink from a TO-5 style transistor?
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Old 2nd October 2017, 05:40 PM   #1
TankAudio is offline TankAudio  United States
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Default How can I remove the heatsink from a TO-5 style transistor?

1969 Kustom Guitar Amp with NTE129/RCA39737 driver transistors. I found a bad 3055 output transistor after a blown 1ohm 5W resistor tipped me off. I replaced the 3055's as a set (p-p output section) and the (single) blown 1ohm. Amp works great.

Now, to ensure reliability, I believe I should replace the driver transistors in a set as well. The NTE129/RCA39737 transistors have a heatsink attached. I do not see a clip. How should I proceed?* Thanks.


*I think I should test the replacement drivers in the circuit at low power first, since I do not have a Beta tester. If the amp still looks good, then drill/hammer/acetone the old drivers out to salvage the heatsink. I would use a thermally conductive epoxy to glue the transistors into place.
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Old 2nd October 2017, 05:54 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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If they have worked since 1969 then I would leave well alone.

If you have replaced the 2n3055 with a new modern one I would check for oscillation on the output. I changed one on a 1980's Maplin amp and the new one oscillated as it had higher gain. I had to increase VAS capacitor a little to stop oscillation.
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Old 2nd October 2017, 06:25 PM   #3
TankAudio is offline TankAudio  United States
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Thanks Nigel.

I put the driver xs back in and took some screenshots on the scope, 8 ohm load. Does this look like normal noise or osc.? It has a small value of 20 or 30mVpp. Volume is at 0, tone controls at 10.
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Old 2nd October 2017, 06:29 PM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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If its oscillating it usually just one frequency, that looks like general noise.
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Old 2nd October 2017, 06:45 PM   #5
TankAudio is offline TankAudio  United States
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Great News. Thanks for the swift reply!!
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Old 2nd October 2017, 09:48 PM   #6
Johno is offline Johno  Australia
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How can I remove the heatsink from a TO-5 style transistor?
"Top Hat" heatsinks on TO5 transistors usually screw together....
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Old 4th October 2017, 05:29 PM   #7
TankAudio is offline TankAudio  United States
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Should I measure bias across the 1ohm resistor? Attached is a schematic. On the top and bottom, you will see Q1-Q4. Q2 and Q4 were replaced as a set, when Q4 was found to be faulty. When testing for bias, Q1 and Q4 show 22mA when measured across the 1ohm resistors, where Q2 and Q3 measure 35mA.

Nearly all resistors in here are CC 10% tolerance. I would like to know what the cause is for this difference in bias current. When cold, all (4) 5wt resistors measure the same. Before the replacement, the original Q1,2,3 transistors developed the same bias currents across the (3) good 5wt resistors.

What are your thoughts on this amount of difference in bias for a guitar amp? Is it the driver transistors that have determined the amount of bias current?

Thanks
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Old 4th October 2017, 05:45 PM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by TankAudio View Post

What are your thoughts on this amount of difference in bias for a guitar amp? Is it the driver transistors that have determined the amount of bias current?

Thanks
If there is an output offset voltage then currents will be different.
Get the offset voltage as close to zero as you can before testing bias currents.
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Old 4th October 2017, 05:53 PM   #9
TankAudio is offline TankAudio  United States
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That's interesting.

The GND lugs of the speaker terminals are tied to the CT of the dual power supply caps, and I found 24mV between the tip and shield connections on the output.

What is this voltage?
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Old 4th October 2017, 07:54 PM   #10
Nico Ras is offline Nico Ras  South Africa
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Quote:
Originally Posted by TankAudio View Post
Thanks Nigel.

I put the driver xs back in and took some screenshots on the scope, 8 ohm load. Does this look like normal noise or osc.? It has a small value of 20 or 30mVpp. Volume is at 0, tone controls at 10.
Usually if you cannot get your scope to trigger on the displayed signal, then the signal is random (noise like) not an oscillation.

You might find if you turn the time-base down a mains signal could be the cause of the amplitude deviation or noisy looking display, but 20 mV is hardly something to worry about.
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