Help with B4, 24 resitance reading

dfed22 · 1289

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Offline dfed22

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on: January 05, 2019, 06:21:38 AM
A friend of mine sent me his amp which at one time was working fine.  It is version 2.1 and he replaced the output capacitors with Jupiters. 

In trying to get it working I did the resistance tests and got no reading for terminals B4 and 24.  Seeing that a 220 resistor connected to both I replaced the resistor, but I still get no reading for both terminals.  To me this suggests the problem is elsewhere but is revealing itself there.

Can someone point me in the right direction of where I might start to look?

Thanks!


Offline Paul Birkeland

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Reply #1 on: January 05, 2019, 08:06:19 AM
Quote from: dfed22 on January 05, 2019, 06:21:38 AM

Can someone point me in the right direction of where I might start to look?

Do you get a resistance reading at 14?  It might help to turn the volume pot down all the way. 

The 220 ohm resistor goes from that terminal to one of the grids internally in the tube.  It does not get its ground reference there.  On the other end of the resistor, it is wired to the volume pot, and the volume pot does provide a ground reference.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #2 on: January 05, 2019, 12:25:44 PM
Thanks for the reply.

I do get a reading at 14.


Offline Paul Birkeland

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Reply #3 on: January 05, 2019, 06:09:31 PM
What actual reading do you get at 24?  Is it OL, or something else?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #4 on: January 06, 2019, 12:11:36 PM
Yes the actual reading at 24 and B4 is OL.


Offline Paul Birkeland

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Reply #5 on: January 07, 2019, 08:00:37 AM
What reading do you get at each center pin of each RCA jack?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #6 on: January 07, 2019, 05:20:38 PM
Black is OL, red is variable it seems based on volume


Offline Paul Birkeland

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Reply #7 on: January 07, 2019, 05:31:14 PM
Follow the shielded twisted pair of wires from the black jack up to the volume pot.  The black wire will land on one of the outer pot lugs.  What is the resistance between this lug and ground?

The black wire going to 22 from the volume pot is probably not connected on one end, or is snapped off.  You can compare it to the black wire that goes from the pot to terminal 12.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #8 on: January 13, 2019, 01:06:42 PM
I appears my friend added a stepped attenuator. I took a reading where that black wire from twisted pair connects to the attenuator and it is 0 (not OL).  I don't know if the attenuator changes things for diagnosis or not.  I followed all the wires connected to it and they all look attached to me.


Offline Paul Birkeland

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Reply #9 on: January 13, 2019, 02:16:03 PM
All bets are off when aftermarket parts are installed.

If you want to post some photos of the area, we might be able to spot something.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #10 on: January 13, 2019, 05:22:13 PM
I think I'd like to restore the stock pot.  What are the specs of what I should buy?


Offline Paul Birkeland

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Reply #11 on: January 14, 2019, 04:28:16 AM
Quote from: dfed22 on January 13, 2019, 05:22:13 PM
I think I'd like to restore the stock pot.  What are the specs of what I should buy?

100K dual logarithmic pot.  The Alpha RV24 1/2W pot fits nicely.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline dfed22

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Reply #12 on: April 29, 2019, 05:58:15 PM
Ok I replaced the stock pot and so I know the black wire from the black jack that goes to the pot lug is connected, and the reading there is OL.

Is it possible the issue could be the wire itself connecting the pot to 24?


Offline Paul Birkeland

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Reply #13 on: April 29, 2019, 06:17:12 PM
That is something you can check with the continuity tester on your meter.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

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