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Old 14th April 2006, 06:50 AM   #41
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The diodes are not zeners. I believe they are 1n914 or 1n4148s.

You need to find the reason that the negative voltage is not present on the 220k onm resistor. If there is no negative voltage, there is nothing for the optocoupler to send to the jfets. Pull the jfet and the electrolytic cap (or whatever cap is connected to the 220k ohm resistor). See if there is negative voltage on the 220k ohm resistor with those components removed (with the amp on on, of course). The black meter lead will go to the RCA shield ground.
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Old 14th April 2006, 12:33 PM   #42
Clipped is offline Clipped  Thailand
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not zeners? i thought clear orange were zeners, oh well learn something new everyday... i replaced them with the IN4148...clear orange.

well if i cant find the problem, think it would be ok to run the amp without the muting circuit? the only thing that seems to happen with the circuit inactive is that the volume doesnt roll up when turned on...it comes on suddenly...i can live with that

thanx
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Old 14th April 2006, 03:13 PM   #43
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It's OK to run it without the muting but... wouldn't it be much more satisfying if you repaired it properly?

The circuit is very simple. The diode passes negative pulses from one transformer winding to the resistor. The resistor charges the cap. The voltage on the cap determines the muting level. When you remove the cap and jfet, the voltage on the 'non-ground' side of the cap should instantly swing negatively when the amp is powered up. If this is true, reinstall the cap. The voltage should again swing negatively but more slowly as the cap charges. Normally, when you switch the amp off, the jfet would drain the cap but with it removed, the cap would remain charged (at least for a short period of time). If the cap charges properly, suspect the jfet. If the cap does not charge properly, suspect the cap (possibly leaking or improperly installed).

The color of a diode means very little. I've seen orange, black, blue and silver Zeners. I've seen black, orange and white rectifier/switching diodes.

When you remove any component, you must know what it is to replace it with the proper component. Even if it would have been a zener, you would have had to have known what it was to replace it. There are many different values and only one would have worked properly. For small fragile components (like the orange diodes), it's best to try to read the value before pulling it. Often, when they fail, they crack. When you apply pressure to it (when removing it), it will break up. Although you may still be able to read it, it won''t be easy.
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Old 14th April 2006, 04:50 PM   #44
Clipped is offline Clipped  Thailand
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this amp is sucking the life out of me...lol

ive learned alot in the process of fixing this(these) amps... even wound a new transformer 'exactly' like the original... i was amazed that it worked..lolx2

when i removed the zeners it was very hard to read the numbers even with a magnifying glass...when i bought them in a pack of 20 they said zener on them...ehhhh

i changed the jfet and the cap near the ps a while ago... but i will check them again.

thanx for the help

ps...the two big 3300 caps laying next to the transformer are charred at the terminals....i was wondering if it would be ok to parrallel three 1200 caps in place of the original, i cant find horizontal caps for the life of me... does the leg length of a cap hinder the esr rating?

just another interesting thing ive found along the way, i experimented with the two 1000 caps before the transformer
changing the values between 820-1500...with 820 the amp would drain alot more power...but with 1200 the ammeter on my power supply would literally follow the current draw with every beat...needle would dance back and forth extremely accurate in sync with the beat, as opposed to the 820 and 1000 values where it would linger.... what do these caps determine?

bass was tighter too, but maybe too tight...but for now i put the 1000's back in....ok i'll stop babbling now
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Old 14th April 2006, 07:23 PM   #45
AndrewT is online now AndrewT  Scotland
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Hi,
Quote:
two 1000 caps before the transformer
Do you mean on the mains side?
What are they?
Electrolytics then what voltage, ceramics, or film, X or Y rated?
What multiplier on the 1000? pF, nF, uF?
__________________
regards Andrew T.
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Old 14th April 2006, 11:48 PM   #46
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The two 1000uf caps are before the inductor on the B+ line. The 3300uf caps are on the transformer side of the inductor. Both sets of caps are connected to the chassis ground.

If the leads are burned open on the 3300uf, you can replace the lead with a very short piece of copper wire. It's best to replace the caps but if the copper wire is the same size or larger, it will be as good as or better than the original leads (which are likely made of tinned copper clad steel).

If the leads of the larger caps are burned open, I'm not surprised to hear/read that the amp's power supply didn't react properly to the current demand. There was likely a significant amount of ripple on the power supply line feeding the 594. The current draw from the power supply should precisely follow the current demand by the amplifier section.
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Old 15th April 2006, 04:24 AM   #47
Clipped is offline Clipped  Thailand
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would it be ok to parrallel three 1200uf caps (6 total) in place of the individual horizontal 3300uf caps?

i cant find the horizontal style caps around hear.

thanx
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