2) The 100K ohm pull up resistor coming off the resistor bridge at the phase inverter input, and on the other side of the presence pot, is measuring only 4K ohms. Seems very low for a 100K resistor. The only reason it could be that low is I'm measuring it in circuit, and there could be a very low impedance circuit load in parallel with the 100k resistor. It's labeled 47x or 47k with (100K) in parenthesis just under the 47x. Its is in fact a 100K resistor that's installed there.
one end of the feedback resistor (which is what I think you are referring to) is seeing a very low impedance connection to ground since it's connected to a speaker output tap (output transformer secondary output tap). The other side sees ground ground through a 5k pot or 4.7k resistor, so this (100k in parallel with 4.7k or so = about 4.5k) is probably what you are reading when you see "4k ohms".
3) I can't read DC resistance through the two .022uf caps feeding the phase inverter output from V3 into the power amp stage input (V4 & V5). This could be fine & normal, because they are caps.
on a good cap you shouldn't be able to get a DCR reading
5) A side thing I noticed is the secondary side of the output xmfr, which connects to the speaker jacks via the impedance selector read nearly a short to each other and also to ground. Does this sound normal for the speaker side of the xfmr? Seems like awfully low resistance.
> Orange to Violet = .4 ohms
> Orange to Grey - .8 ohms
> Orange to Green = .6 ohms
> Orange to Yellow = .4 ohms
> Orange to Ground = 0.0 ohms
looks normal. Output transformer secondary from 0 ohms (orange) to yellow (4 ohms), green (8 ohms), then grey (16 ohms) and the DCR is going up incrementally, so seems ok.
Could it be the bias caps causing the current to keep increasing & the voltage to keep dropping from 410VDC all the way down to 320VDC?
the more the current draw (from the power transformer) the greater the voltage drops. Sort of like if you grabbed a branch on a tree and it bent downward. The more weight you put on the branch the more it bends downward. Put too much and it breaks. The more current drawn, the greater the voltage drop. Draw too much current, and something gives out (fuse, copper winding (insulated wire) inside a transformer that is only capable of passing current up to a certain point).
What else could be causing the current to keep increasing, besides the bias caps ?
a bad power tube, possibly other things.
How do I soecifically do this to assess the 50+50uf cans? ---> "it will probably be cheaper to use a series resistor, apply a voltage to a cap, and measure the voltage to figure out the leakage"
(If you google you should find a better explanation, but basically) take the loads off of the B+ path (high voltage supply). That is, the tubes (since they are sort of like resistors across the power supply drawing a few milliamps for the preamp tubes and tens of milliamps for the power tubes (but more when there is a signal). Install a resistor, say a 100k (make sure the fuse is still in the path), power up, monitor the DCV across the 100k resistor. The voltage seen here should drop over time (a good cap should slowly repair itself with a current limited applied voltage). Ohm's Law: V/R=I, (Voltage/Resistance = Current) will tell you the current total. If the caps are good, the current (leakage current) should be small. (If you look at aluminum capacitor datasheets, they will give a formula such as "0.02CV" (0.02 * capacitance value * voltage) for the amount of leakage a cap should have under specific conditions. So if you see something that looks abnormally high, then that should be a hint that one or more caps have too much leakage (and should be replaced).
On the 9VDC battery test, do I just jump (+) & (-) of the battery directly to the 50+50uf cap can (+)n & (-) cap inputs? How long do I apply the battery to the caps? If the caps are good, should I just see 9VDC (or closer to that) across the caps? How long should the caps hold that 9VDC charge before they dissipate back down to nothing? I drained all the can caps using a resistor on added to the jumper clip.
(Assuming the cap is discharged and not holding a high voltage first.) you need to "see" (monitor) the voltage, so the leads from the DVM need to be on the cap while you test (alligator clip-equipped leads or retractable hook type leads are useful for "hands free" connections). For the 9V battery, you can use a battery clip with alligator clip lead extensions or touch the battery contacts directly to the cap, but you MUST get the polarity correct (or you can damage the capacitor--IIRC polarized alu electros only withstand 1-2V of reverse voltage) . Hook up the battery to the cap. You should see 9V(or so if the battery is fresh). Disconnect one side (red or black--remember that as the term "electronic circuits" implies, circuits are made up of loops (the electricity goes somewhere and it has to come back through a path). (The loop will be battery plus -> cap plus -> cap minus back to battery minus.) If after interrupting the path (disconnecting), the voltage drops very quickly (immediately), the cap is likely bad. If the cap is good, the voltage should stay for a while. How long exactly? I don't know but probably at least tens of minutes or (possibly way) more (the less leaky the cap is the longer it should hold the charge).
Anyway if you really want to service the amp yourself it would be a good idea to find some good sources (tons on the web) and try to understand some of the basics at least. Youtube looks quite good as well for basic theory on tubes and electronics. (Like these: )
Electronics At Work (1943) Westinghouse
Vacuum Tube Primer
Vacuum Tube Amplifier Theory
(Lots of books here: )
http://www.tubebooks.org/
some theory, amp calculators:
http://www.ampbooks.com/home/tutorials/lesson-001/
etc.
I don't know very much but I still find the bit that I have learned useful for making simple repairs, troubleshooting etc. so I would say it was worth the time I put into learning things.