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Superphon Revelation MK II PreAmp
Superphon Revelation MK II PreAmp
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Old 4th December 2007, 07:14 AM   #21
eeeehaw is offline eeeehaw  United States
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For the benefit of those replacing their Volume/Balance pots, here is a photo of my new pots installed (see Mouser part number above previous message). I had to drill a 1/8" keyhole thru the front panel in exactly the right locations (note that one is on top and the other is on bottom), in order to install the pots. The key tab on the pot are there to prevent the pot from spinning when the knob reaches the end of its travel.
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Old 4th December 2007, 08:25 AM   #22
beeger is offline beeger  United States
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relay replacement;

For anyone else needing a relay, I have a few on hand and perhaps we can establish whether your relay really is the culprit. It it is, then I can help you with a replacement. These only fit the Revelation II. Most muting difficulties are caused by aging of the first filter cap, but relays do occasionally fail.
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Old 4th December 2007, 08:51 AM   #23
beeger is offline beeger  United States
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question for eeeehaw;

Does the control you obtained for the balance have log/reverse log elements? That is, one element is log taper clockwise and the other is log counter clockwise? If not, that will most likely be the source of the channel mismatch. The original control had zero insertion loss elements when the control was at twelve o'clock. It only inserted resistance when moved to one side or the other. Just the first thought that came to mind when thinking about the problem.
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Old 4th December 2007, 03:44 PM   #24
eeeehaw is offline eeeehaw  United States
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beeger: Yep, I remembered (hard to do at my age) that Balance pots have to be log/reverse-log tapers, so the new one is correct. But this balance problem also occured with the old Balance pot, too, ever since the unit was new (both Line and Phono sources). I just lived with it all those years of use, as otherwise the sound from both channels is equivalently excellent. I get the opp to work on this thing in short spurts, but am planning next on double-checking the component match between Line stages to make sure they're the same values. Does the circuit contain a stage requiring a close match of xstr's?
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Old 4th December 2007, 04:35 PM   #25
SierraTech is offline SierraTech  United States
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beeger

The increase in resistance was a temporary measure until I can grab a new capacitor. But as seen on the scope the capacitor is shot to be sure. I have access to a Polypropylene cap which might be better suited for this.

Since this capacitor went leaky, I am considering changing the two 470uF 50V caps (filters for the preamps) while I am at it.

The 220uF is definitely the issue, and after seeing the ripple on the bridge, I should have considered it first. I will put the 12.1K resistor back in place so "Brown-out" conditions will trigger the mute circuit as designed.

Thanks for your input. I love my Superphon Revelation II (22 years), the sound is hard to beat.
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Old 4th December 2007, 08:16 PM   #26
beeger is offline beeger  United States
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eeeehaw;
If line input sources are affected, then the problem is surely in the line stage. Your o'scope is your friend here, just follow the signal from the jack to the switch to the pots and then to the board. If both channels deliver the same signal to the board then there is a gain mismatch in one of the channels. It is just as likely that one channel is high as it is to be low in gain. I would look for obvious differences in resistors between the two channels and then, if all are the same, check the resistors with an Ohm meter with the power off. In circuit tests will not be absolutely correct, but should be the same on both channels. Be sure to keep the probe polarity the same from one channel to the other when checking resistors. Reversing the probes could give a different reading by biasing a semiconductor junction. It could also be a tantalum capacitor in the gain divider. The gain is set by three resistors: One in the feedback, two in series to ground, with the one actually connected to ground bypassed by a capacitor so it is zero Ohms for AC signals but provides a necessary drop for proper DC bias. The input mosfets are matched, but shouldn't change the gain unless severely out of balance. You would need to replace both of these $40 parts if one is damaged. And, you would need to buy enough to get a reasonable match now you see why I warned you to be careful with meter probes when checking voltages with the power on.
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Old 4th December 2007, 08:26 PM   #27
beeger is offline beeger  United States
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Sierra Tech;
There are many good caps with low leakage and high ripple available these days. I can't get my head around the probable size of a 220 uFd film cap , but if you can get one in there it should last forever. You are right about the main supply caps for the audio section. They are just as old as the timer cap and should also be replaced with newer and better ones. I think we used Panasonic Z series caps and something of at least the same spec's would be appropriate. The newer Panasonic FC or FM caps come to mind. Or, maybe a lunatic might throw a couple of Black Gates in there. Let your wallet be your guide.
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Old 6th December 2007, 06:37 AM   #28
beeger is offline beeger  United States
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Sierra Tech;

One more note: The twelve Volt regulators used in the initial production did not suffer the over Voltage problem. As we used up the inventory and replaced with different brands we found some would go into shut down mode. A simple correction is to drop two fifteen Volt/ one Watt zener diodes in series to ground from the output side of the big resistor. This holds the output Voltage to no more than thirty Volts when the relay is not powered (mute mode). The current is small enough to pose no problem for the zener diodes. Remember the banded end goes to positive or you will suffer the consequences of a non-working relay circuit and some very hot diodes.
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Old 16th September 2008, 10:05 PM   #29
thistlehd is offline thistlehd  United States
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Default Superphon Revelation Basic

Beeger:

I am hoping that you could answer a question about the Revelation Basic for me. I recently aquired one that has an adjustable power supply, on the others I have the boards are set to about a 24v output, but this one could be adjusted. Is there an optimum rail voltage? This unit also came with a pair of higher output torridal transformers, (and a couple of blown tant. capacitors)

Thanks, Chris
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Old 17th September 2008, 03:25 AM   #30
beeger is offline beeger  United States
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Default regulator voltage

You are correct Mr. Thistle. The transistors are being pushed a little at 24 Volts. It will run fine at less than 24, but works best at 24 Volts. Bigger transformer(s) can sound better if you also replace the original 1.5 Amp diode bridges with 4-6 Amp units. I haven't been able to prove that high speed diodes actually sound better, but bigger ones do.

Good luck and have fun.
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