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Ah, ok that's simple enough. Just as an aside, the other day immediately after powering the amp down, I checked several caps (electrolytic) for residual voltage and none seemed particularly high (no more tham 12 vdc) and they seemed to dissipate rather quickly. Granted these weren't the caps located in the "cap can"... I'm just wondering because I've read quite a bit about caps holding on to voltages for quite awhile in many cases.
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As long as you only turn off the ac power switch and not the standby switch, the amp's circuitry will continue to draw power from the power supply and will rather quickly drain off the charge on the filter caps.
Your amp does not contain the "bleeder" resistors that are normally found in Fender amps, so allow it few minutes for the caps to drain.
A simple way to test the cap, is with an ohmmeter. With the amp off and unplugged, ground the black lead and attach the red lead to TP30. Depending upon how much residual charge is left on the cap, you should get a low reading that will start to rise as the meter voltage charges up the cap. It may or may not go to infinity, but all that matters is that it should read some high resistance value and not a low resistance reading.Comment
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Ok, here's what I got...R64 (hot side) 454vdc... C37 (test for continuity) -
(that's nothing). I tested a few other capacitors on the board (electrolytics-albeit smaller ones) and got various readings (between 1200 and 1800 [ohms?]). Am I right to assume that C37 is bad?Comment
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"C37 (test for continuity) - (that's nothing)." Sorry, does "nothing" mean zero ohms to ground & that you have continuity from hot side of C37 to ground, or, that you do not have continuity to ground? (My meters read "000" for continuity).
What does R64 read in ohms?Comment
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0 ohms to ground... didn't check ohms on R64. 454v on the hot side 0 on the other...Comment
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If there is no resistance from the hot (waisted end assuming it is an axial cap?) of C37 to ground then the cap is shorted & needs replacing. It would make sense to check the value of R64 whilst you are in there.Comment
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Ok, I'll check that tonight. C37 is an axial 350v 47mf cap. Will also check value of R64. Thanks for all your help.Comment
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Try this first. Make sure the amp is off and unplugged. Using a wire, short across the terminals of C37 to remove any residual charge on it. Then remove the wire and test it with your ohmmeter. It's possible that you can get a false zero reading if there is a small charge left on the cap. If it still reads zero or low ohms to ground, then replace it.
If R64 was working, it would mean that it was dropping 454 vdc! If this was the case, it would be hot enough to smoke and burn. My guess is that R64 is wide open.Comment
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Ok. Chassis to ground side of C37 there is continuity. Ground to + side gives me nothing. R64 no reading. Just for fun I checked several other resistors on the board, and for some I got readings (I assume resistance in ohms), some I got nothing , and some I got continuity (ie: beep). Why the differences? Is this purely due to differing resistance? All these measurements were made with the amp off and unplugged.Comment
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"R64 no reading" What no reading at all? If it is as it should be you should have 68K, if it is open (burned out) you will get no reading (as opposed to "0" ohms). You should be measuring R64 end to end in ohms.
No reading (continuity) from waisted (+) end of C37 suggests that this is not shorted.
Some resistors in the circuit are deliberately grounded (why you get a beep from one end), others are not (no beep when measuring continuity to ground).
Replace R64 with the biggest wattage 68K resistor that will comfortably fit on the board.Comment
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Yhea, R64 - No reading..."Replace R64 with the biggest wattage 68K resistor that will comfortably fit on the board I'll try hitting Radio Shack later on... they should have a 1 watt 68k resistor, right?
"No reading (continuity) from waisted (+) end of C37 suggests that this is not shorted That would be a good thing...I'm tired of paying more for shipping than the parts I'm ordering!
"Some resistors in the circuit are deliberately grounded (why you get a beep from one end), others are not (no beep when measuring continuity to ground). That's what I figured, thanks for the explanation.
One last thing; When testing C37, occasionally (usually the first time I touched the probes to the leg(s) of the cap) I would get a brief (maybe a second or two) reading which would quickly dissipate back to 0 or
.001 depending on scale. Is this just due to a residual charge in the cap?Comment
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Do you mean a voltage reading at C37? Yes, that would be normal. It's also normal to get a momentary beep when reading continuity to ground accross a functioning electrolytic cap.
I should rephrase this bit - "No reading (IN CONTINUITY MODE - e.g. you do NOT have continuity) from waisted (+) end of C37 suggests that this is not shorted"Comment
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"Do you mean a voltage reading at C37? Yes, that would be normal. It's also normal to get a momentary beep when reading continuity to ground accross a functioning electrolytic cap.
No not voltage (I was testing for continuity) I guess it would be resistance... very brief reading (starts around 1750)...Comment
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I believe what you are doing there, is charging the cap with your meter, that's why you get the brief meter reading.
What sort of meter do you have? Does it have an actual resistance setting? If it does, check across the leads of R64 not to ground.Comment
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Hi Bill,
It's a digital multi meter (don't remember the brand offhand, but it's not a Fluke...) I got it from a company in Arizona (internet based) and I also got a really nice variable voltage soldering iron very reasonably priced. I think I paid about 1/3rd what a similar Weller iron was priced at, so I though it was a good deal...They have both worked well over the past 2 or 3 years. Anyway, by an actual resistance setting do you mean Ohms? If so, yes, in addition to a sound emmiting continuity setting it's got Ohms settings of 2, 20, 200, 2000 and 20 or 200k (or maybe both...). I think you're probably onto something because if I use the straight resistance setting (not continuity) I get no reading on C37 or R64 (a 1 in the far left place holder) regardless of the scale used.Comment
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