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Old 6th Feb 2023, 12:57 pm   #1
cooperman
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Default Armstrong 400 series - Z10 PSU PCB

I have an Armstrong 426 tuner/amplifier that is generally in good order. I've noticed that the 7 watt 270 ohm resistor on the Z10 PSU pcb gets quite hot (even when there is no music playing). I don't have a diagram of the Z10 (the generally available Armstrong diagrams only cover the later Z17 pcb). Does anyone have a diagram for the Z10 that they could share with me? Also if anyone has a 400 series example (with the Z10 pcb), does that resistor run hot on your set?
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Old 6th Feb 2023, 7:28 pm   #2
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Default Re: Armstrong 400 series - Z10 PSU PCB

I doubt a manufacturer would fit a 7W resistor if they didn't have to, so I would expect it to run rather warm at least.
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Old 8th Feb 2023, 1:26 pm   #3
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Default Re: Armstrong 400 series - Z10 PSU PCB

In the absence of a diagram for the Z10, I've compared the PCB with the more common Z17 PSU pcb. They seem to be almost identical apart from some additional capacitors on the diode bridge and the 270 ohm resistor having been changed for two 12K resistors in parallel. The Z17 diagram shows these as 293 and 225 (see attached). This seems a very big increase in value. The A17 resistors look to be about 5 watts.
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Old 8th Feb 2023, 7:50 pm   #4
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Default Re: Armstrong 400 series - Z10 PSU PCB

Presumably D8 is a Zener providing the reference voltage for the regulator circuit. What would make sense for the value of the zener series resistor would be the values of the resistors and pot in the divider chain, the zener voltage and the regulator output voltage.
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Old 9th Feb 2023, 12:03 pm   #5
cooperman
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Default Re: Armstrong 400 series - Z10 PSU PCB

The Zenner measures 12.1 volts across it on the Z10 pcb.
HT voltage (measures at F2) is 50 volts
VR8 is 1K
R294 is 1K
R296 is 3.9K
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Old 9th Feb 2023, 8:39 pm   #6
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Default Re: Armstrong 400 series - Z10 PSU PCB

W=V*V/R =5.4W, yes, it will get hot! ...but it is inside the 7W rating. That's 140mA so the Zener is dissipating 1.7W and its probably only rated for 2W.
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Old 9th Feb 2023, 8:43 pm   #7
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Default Re: Armstrong 400 series - Z10 PSU PCB

I'm still puzzled about the single 270 ohm (7W) on the Z10 pcb being replaced by two 12K parallel resistors (5W) in the later Z17 pcb - the rest of the circuit seems to be the same on both cards.

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