NAD: Thunderstar Bass Amp

GAD

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I’ll measure the pots in detail in a bit. Is there an easy/temp way to bypass the tone stack? Can I just jumper around it or would that be bad?
 

SFIV1967

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Is it possible that 100k ohms resistor bare wire (on the side where your #2 is) is touching the bare wire of the .033 cap at the other side of the cap where #5 is? That would ground that whole thing!

jXPb6DI.jpg


Ralf
 
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GAD

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Is it possible that 100k ohms resistor bare wire (on the side where your #2 is) is touching the bare wire of the .033 cap at the other side of the cap where #5 is? That would ground that whole thing!

jXPb6DI.jpg


Ralf

Nah - that's just lack of depth perception in the photo. There's a good 2-3mm separating them.
 

GAD

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Well, I think my gut was right all along - the vol-1 pot is toast.

PotsResistance.jpg



The numbers on the Vol-1 pot are accurate. I checked multiple times. In fact, they've changed since yesterday because (and I think I even commented on it) yesterday it measured 500k on the outer two lugs. Today it's 10k. AND, it rises up to about 300k then back down to ~100k at around 5. In other words it has higher resistance in the middle of the sweep than at either end.
 

SFIV1967

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That's bad news... After all the trouble of desoldering the bunch of parts you can't even open that one as it looks since the bottom of the V2 pot is so close to it.
I have no idea how they are connected to each other.

V1 (I mean the log of front volume one) is that one with the red cross, or?

68NwONM.jpg
0019268000_merch_dtl_001_nr.jpg



How about to take it out (lots of work already) and wash it completely with Naphta inside before you apply fresh Deoxid? Maybe that saves it from disassembly?

Ralf
 
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GAD

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That's bad news... After all the trouble of desoldering the bunch of parts you can't even open that one as it looks since the bottom of the V2 pot is so close to it.
I have no idea how they are connected to each other.

V1 is that one with the red cross, or?

68NwONM.jpg
0019268000_merch_dtl_001_nr.jpg



How about to take it out (lots of work already) and wash it completely with Naphta inside before you apply fresh Deoxid? Maybe that saves it from disassembly?

Ralf


Nope - Vol-1 is the one closest to the panel.

The one you posted a pic of is a concentric pot. I've read of people taking those and soldering the two shafts together.
 

SFIV1967

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Nope - Vol-1 is the one closest to the panel.
That I don't understand. Isn't the bare thick wire at the 1 o'clock position in my picture above going from the middle of the log (front vol) pot (with the red cross) up to the 47pF and .01 capacitors on the PCB (the green and square red caps)? That bare wire comes from the middle of the lin. (back vol.) pot ??? The lin. pot only goes to a .01 cap in the schematic as far as I see.
Ralf
 
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GAD

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See those two ceramic disk caps going to the third turret from the left on the board? Those are the .001 caps between the bass and vol-1 pots (both audio). It's difficult to see in that pic because it gets dark down there and all the leads there are covered in black insulation. The bare wire you mentioned goes to the lug closest to the camera on the the linear pot (top in the pic) which is connected to a .01 (also connected to pin 2 on V2 which you can see in the pic) and the .022 cap that connects to the pre-amp input (the grey wire going up from the cap second from the left on the turret board).

Here's a possibly better pic:

Guild-Thunderstar-Bass-Pots.jpg



With the exception of the underside of the turret board, that area of the volume pots and the tone switch are the absolute worst places to work in the entire amp.
 
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GAD

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A "Stackpole Electronics, Inc" didn't exist in 1970 (jut to be correct...). The pots were most probably in the Electronics division of the Stackpole Carbon Company in St.Marys, PA or in 1970 already under the Stackpole Components Company in Farmville, VA and/or Raleigh, NC. Hard to say as I didn't find any vintage components catalogs from that time. The new SEI from the 90ies has nothing to do with those vintage parts anymore, unfortunatelly.
Ralf

Gah - serves me right for trusting an online tool. Thanks for the clarification.
 

GAD

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Here's my current thinking/plan for the dual pot.

Alpha makes a dual 1M audio (https://www.amplifiedparts.com/products/potentiometer-alpha-audio-dual-38-bushing)
Alpha makes a dual 1M linear (https://www.amplifiedparts.com/products/potentiometer-alpha-linear-dual)

I'm going to buy a couple of each and dismantle them to see if i can build a 1M audio/linear. The downside is the solid shaft. I don't know why, but Alpha does not seem to make a split-shaft dual 1M pot of either flavor. Also, they don't seem to have a linear in 3/8" bushing, but since that's the one I'll be salvaging parts from, that shouldn't matter.

I'll have to replace the knob and that sucks, I've got a lot of damn knobs so I'll figure something out.
 
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Nuuska

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Oscilloscope time! This is all just signal trace.

Question - is it normal for the voltage to drop so much after the tone controls?

ThunderstarBassVoltages.jpg


Hello

There seems to be voltage drop of roughly 16dB from #6 to #7

If that schematic is correct, then with knob turned to "10" there should be zero ohms between those two points - thus 0dB voltage loss.

I would try following tone control bypass :
- desolder one end of resistor that goes from #4 to ground
- desolder one end of condenser that goes from #5 to ground
- now with Treble @ 10 you should have zero loss unless that capacitor between points #2 & #6 is culprit.
 

Default

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You might be able to repair the pot. This is an interesting video and something I personally would try first.

 

SFIV1967

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See those two ceramic disk caps going to the third turret from the left on the board? Those are the .001 caps between the bass and vol-1 pots (both audio).
O.k., all clear now, I see and understand. I mixed that up obviously.
Ralf
 

GAD

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Today I tried an experiment and just put a jumper from the positions I have marked #6 and #8 (also tried 9). There is a noticeable increase in volume and drive - too much, in fact, as it doesn't sound good. Sounds farty and terrible, but it's as loud as I'd expect it to be. I couldn't easily remove the switch from the equation and I didn't do Nuuska's recommendation of lifting the ground from the tone pots because I didn't feel like soldering today. I may try adding a 1M pot into the jumper to try and add a bit of resistance to see if I can dial in a good tone, but at this point I think I need to walk away because I have a book deadline that this is distracting me from.

Steve, if you want to take a stab at it let me know and we can meet up somewhere. No rush and no obligation, of course. Just let me know if you'd like and I'll make it happen.
 

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You should be getting that replacement pot in the mail today or tomorrow. You'll be golden then.
 

GAD

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I replaced the pot with many MAY thanks to Default for supplying the part. It got better, but something else was wrong.

I finally got fed up trying to diagnose the horror-show that is the tone switch so I bypassed it and the amp just came alive. Well, it's a bit more complicated than that, but I'll cover that in my write-up, which I might actually get to complete now.

Thanks for the pot Steve!
 
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