Something unusual I noticed on both my L and R ch. PC-18 boards. The SCA-80 manual (manual links below)states R3=27k

Now look at this photo of both my SCA-80 PC-18 boards. They (R3) are both 330 ohm. I did measure to confirm.
(R3 is between R9=2.2k/1watt and R6=1k 0.5watt; all three are diagonally mounted)

Above: L and R ch PC-18 boars, SCA-80.
https://i.postimg.cc/wMbDP91Z/IMG-1938.jpg

I looked into the R3 anomaly issue a bit. This site features an ST-80 (same as amp section on SCA-80; (manual links below) ...
https://jimkerkhoff.com/2019/03/01/a-dynaco-stereo-80-upgrade/

And the board has the same 330 ohm R3 (the St-80 manual also states R3=27k)

(R3 is between R9=2.2k/1watt and R6=1k 0.5watt; all three are diagonally mounted)
https://i0.wp.com/jimkerkhoff.com/wp-content/uploads/2019/03/Driver-board-with-old-caps.jpg

I investigated more ... maybe the SCA-80Q might resolve the mystery. But no! SCA-80Q schematic only slightly up the value: R3=30k

Note the somewhat similar ST-120 also has R3=30k, per orig .manual schematic (manual links below).

So what's going on?
Note that my SCA-80 was factory wired.

REFS:







Above images are from SCA-80 32-page manual. They pertain to the issue directly. If you want to see the entire SMs, links below.
PDF manuals/schematics:

SCA-80: https://www.updatemydynaco.com/documents/SCA80.pdf

SCA-80Q: https://www.updatemydynaco.com/documents/SCA80Q.pdf

ST-120: https://audiocircuit.dk/downloads/dynaco/Dynaco-S120-pwr-sm.pdf

 

That middle resistor is a 30 kΩ resistor:



Orange-Black-Orange-Gold =30000 Ω, 5% tolerance.

If you are measuring about 330 Ω by putting your probes on that resistor without removing at least one leg from the board, then you are measuring the combined effect of every other component that is reachable in the circuit in addition to that resistor.

 

That middle resistor is a 30 kΩ resistor:

View attachment 368686

Orange-Black-Orange-Gold =30000 Ω, 5% tolerance.

If you are measuring about 330 Ω by putting your probes on that resistor without removing at least one leg from the board, then you are measuring the combined effect of every other component that is reachable in the circuit in addition to that resistor.

You are correct! My bad!
Many issue with other troubleshooting on this unit. Including a another 330 ohm R on the board . Confusing is my duh!

Actually, the value has drifted UP to 32K, after 60 years of moderate use.

BTW:
Do carbon comp R's drift, over time, even if never used? Some YouTuber unboxed a brand new SCA80 kit and had to replace many of the "new"-old CC resistors because of drift.

 

Do carbon comp R's drift, over time, even if never used?

Perhaps a small amount, but as a practical matter, NO. Electrolytic capacitors are orders of magnitude more prone to age.

Today's film resistors can cheaply meet higher tolerances and produce less electrical noise, so it might make sense to replace certain key components.

My SCA-80Q was built by me 50 years ago! Still works.

 

that doe exceed 5% of 30k (it is almost 10%) so it should be replaced. also schematic says this was supposed to be 27k so reading 32k means this is 20% out. but as stated before, capacitors are usually much bigger problem.

 

My SCA-80Q was built by me 50 years ago! Still works.

Dynaco sold a LOT of kits and assembled units. So I'm guessing the orig design topology was pretty good.
Also, the metal chassis and other parts are very robust. I have several other Dyna units that are going strong, e.g. almost no rust.
Even the 60-year-old pots, switches are clean, and track and ENGAGE well.

I tested all the orig. transistors, and all still holding with v. respectable parameters like hfe.

Of course, storage conds, abuse and personal care are big factors in the longevity and reliability of any device. I bought this SCA-80 two years ago, and the interior was quite filthy and a quite greasy. Was it used in an auto repair or machine shop for a while, perhaps??? The main PSU filter cap (C9, 5000 uF) and the two output DC block caps (both 5000 uF) are all orig and test v. well on my Eico leakage tester, and PEAK esr tester. So I'm guessing low total hours on this 60 years-old SCA80. That said, multi-section C11 cap was a bit out, so I replaced with three new Nichicons. And 500 uF/16v C5 on PC19 (left ch, of course) was a bit heat damaged and out of spec.

Dynaco units still make for fun projects -- that is: upgrades, restorations and mods.

I'm having some decent success with this mod project:
https://gammaelectronics.xyz/audiox_08-2005_Dynaco-SCA80.html

that doe exceed 5% of 30k (it is almost 10%) so it should be replaced. also schematic says this was supposed to be 27k so reading 32k means this is 20% out. but as stated before, capacitors are usually much bigger problem.

The left ch. PC-18, component side sits close to that rectifier board (PC-19) that emits quite a bit of heat from its 7w 400 ohm dropping resistors. So it gets cooked a bit. And its components are unsurprisingly more out of tolerance as result.
Idling the SCA-80 still consumes only about 22 W.

I do want to eventually replace PC-19 with a modern PSU/regulator solution. But the drop from 74Vdc to 36Vdc is a bit unusual. I haven't looked into issue extensively but some simple 3-pin voltage reg. design, with much less "wattage consumed" than those 7w 400 ohm R's, but still able to deliver the goods (spec'd current to pwr amp board, PC-18), is ideal. Let me know!