I've been using an old Heathkit bench supply for prototyping which is fine for 100mA or less. I have had an ISCO 494 for a year or more and other than checking the voltage indicated on the built in meter vs. my DMM I haven't used it.
Yesterday I powered it up to test it with a PP KT88 design (more mA than the Heathkit can handle) and got horrible buzzing (from the amp, not the PS). The DC voltage was a dead on match for the builtin meter reading. Out of curiosity I switched the meter to AC and found nearly the same AC voltage as DC on the output!
Anyone have experience with trouble-shooting or rebuilding these? From reading the manual I suspect some bad caps (the manual talks about "excess ripple" which I guess describes the AC on the output.
Yesterday I powered it up to test it with a PP KT88 design (more mA than the Heathkit can handle) and got horrible buzzing (from the amp, not the PS). The DC voltage was a dead on match for the builtin meter reading. Out of curiosity I switched the meter to AC and found nearly the same AC voltage as DC on the output!
Anyone have experience with trouble-shooting or rebuilding these? From reading the manual I suspect some bad caps (the manual talks about "excess ripple" which I guess describes the AC on the output.
Quick update-
The meter I was using to measure the AC was faulty. With two different meters I found the AC to be much lower than I first thought but still too high for a good PS. With the voltages set for 350 I was getting 30VAC on the outputs.
I put together an external CRC filter and the hum on the amp disappeared. I also visually checked the caps inside and nothing was obviously exploded. Next step is to do a more thorough check of internal components.
The problem is I don't know how much AC ISCO allows on the outputs of these units. I didn't find it in the manual though I'll double check that as well.
The meter I was using to measure the AC was faulty. With two different meters I found the AC to be much lower than I first thought but still too high for a good PS. With the voltages set for 350 I was getting 30VAC on the outputs.
I put together an external CRC filter and the hum on the amp disappeared. I also visually checked the caps inside and nothing was obviously exploded.
Next step is to do a more thorough check of internal components.The problem is I don't know how much AC ISCO allows on the outputs of these units. I didn't find it in the manual though I'll double check that as well.
Your 494 should be rock solid. 30VAC is not anywhere near correct. The manual suggests cap problems, which will be very spendy if you need to replace the big cans. You could run through the maintenance and diagnostic procedures in the manual. Be careful, as you may find 2KV inside.
Or, just get another one. Working properly, it is a very nice unit.
Or, just get another one. Working properly, it is a very nice unit.
Thanks for the input. I believe I downloaded the manual from a link you provided about a year ago. In any case I have it in PDF format so I'll check the diagnostics. The cursory look I had yesterday did seem to point to cap issues.
As I stated above the output is now acceptable, though with the added filter the built-in voltmeter is no longer exactly what I'm getting out. If the diagnostics indicated a cap replacement is in order and the cost is too high (I only paid about $50 for the unit) I may simply build a nicer filter into a chassis and feed through that.
This has not been my week for test equipment. First my scope's second channel went south on me, then my function generator started acting up and now this.
On the other hand I now have "justification" for some new equipment!
As I stated above the output is now acceptable, though with the added filter the built-in voltmeter is no longer exactly what I'm getting out. If the diagnostics indicated a cap replacement is in order and the cost is too high (I only paid about $50 for the unit) I may simply build a nicer filter into a chassis and feed through that.
This has not been my week for test equipment. First my scope's second channel went south on me, then my function generator started acting up and now this.
On the other hand I now have "justification" for some new equipment! Wow! That's quite a difference to the 30VAC I get with it set at about 350V using a true RMS meter. Definitely something amiss in my unit. Maybe later this week I'll find the time to try and track down the issue.
The problem as you said is that the big caps get spendy and there are several of these on eBay now for about $50 each so it probably isn't worth the cost to properly repair.
The problem as you said is that the big caps get spendy and there are several of these on eBay now for about $50 each so it probably isn't worth the cost to properly repair.
I'm not as sure. It appears all the big electrophoresis supplies have gotten very pricey. The one 494 is listed over $500 on a 'buy it now' auction. The 494 is an almost ideal box for most audio tube work and I might just bite the bullet for another 20 years' service. By then the digi-amp stuff will possibly have caught up for my by-then septuagenarian ears.
rdf said:
I have to admit being pretty impressed by the few digital amps I've heard. To my ears they sound closer to tubes than most of the solid state amps I've heard. I've been considering putting together a decent kit to compare to my slightly modded SI T-Amp. (I'd rather not tackle SMT components though. I have a hard enough time with 1/4W resistors!
)
http://www.eisenhower.org/eric/isco/
These are the docs I got from Isco on the 493 and 494. I just sent an email to Isco support, and they were happy to send it. They were also amused that people re-purpose them for audio.
These are the docs I got from Isco on the 493 and 494. I just sent an email to Isco support, and they were happy to send it. They were also amused that people re-purpose them for audio.
Just downloaded the doc, thanks..
Now another question - what do you do to deal with the ground fault detection since this is a floating supply?
When doing design and test work obviously in most cases the negative output is going to be referenced to ground through other test equipment. I imagine the output ground fault detection feature can be defeated or even better be configured so that it doesn't detect common earth, but does detect differences in currents flowing through the supply leads.
Now another question - what do you do to deal with the ground fault detection since this is a floating supply?
When doing design and test work obviously in most cases the negative output is going to be referenced to ground through other test equipment. I imagine the output ground fault detection feature can be defeated or even better be configured so that it doesn't detect common earth, but does detect differences in currents flowing through the supply leads.
ISCO 494 mod for 1200V max @ 150 mA (not 90 mA)
Hello, I am trying to modify the ISCO 494 that has an output of 2000V @ 90mA if all controls are set to max. My target is to get 1200 Volts @ 150 mA (this is the same power). You understand the use of it, it can feed 2 SE channels with GM70, 845, 211 etc. Or 1 Push Pull channel with them.
So by checking the circuit I see that 2kV's come out by the following circuit: The 250 Vac secondary is multiplied twice, by 2 separate triplers. Those triplers that give 1 KV and -1 KV referenced to their common are added so the final output is 2 KV. If 2 electrolytics are removed (good as spares!) and a couple of jumpers bypass their respective diodes, this thing becomes a dual doubler/adder that finally gives approx 1300 Volts. I will come back to write the exact parts to be removed, using their numbering as seen on the pcb.
So far so good, the ISCO became a power supply that can't go over 1.3 KV. Have in mind that if something fails in the regulation circuitry (as mentioned in the manual) the output can become 2KV full power, no matter if during tests someone is using it at say 1 KV.
So this easy mod at the worst case will make the unit not to exceed 1.3 KV.
What needs to be done next is to make all feedback circuitry keep working as before, and get the double mA's.
It must become adjustable from 0-1200 V, with current limiting from some mA's up to 150mA (or slightly more, why not?) . This is the part that I am working now, still no positive results, as for the moment the controls fail to work.
I welcome any suggestions.
Hello, I am trying to modify the ISCO 494 that has an output of 2000V @ 90mA if all controls are set to max. My target is to get 1200 Volts @ 150 mA (this is the same power). You understand the use of it, it can feed 2 SE channels with GM70, 845, 211 etc. Or 1 Push Pull channel with them.
So by checking the circuit I see that 2kV's come out by the following circuit: The 250 Vac secondary is multiplied twice, by 2 separate triplers. Those triplers that give 1 KV and -1 KV referenced to their common are added so the final output is 2 KV. If 2 electrolytics are removed (good as spares!) and a couple of jumpers bypass their respective diodes, this thing becomes a dual doubler/adder that finally gives approx 1300 Volts. I will come back to write the exact parts to be removed, using their numbering as seen on the pcb.
So far so good, the ISCO became a power supply that can't go over 1.3 KV. Have in mind that if something fails in the regulation circuitry (as mentioned in the manual) the output can become 2KV full power, no matter if during tests someone is using it at say 1 KV.
So this easy mod at the worst case will make the unit not to exceed 1.3 KV.
What needs to be done next is to make all feedback circuitry keep working as before, and get the double mA's.
It must become adjustable from 0-1200 V, with current limiting from some mA's up to 150mA (or slightly more, why not?) . This is the part that I am working now, still no positive results, as for the moment the controls fail to work.
I welcome any suggestions.
Sherman, what was the C-R-C values for outside filtering you applied? (I have a hum problem, too).Quick update-
The meter I was using to measure the AC was faulty. With two different meters I found the AC to be much lower than I first thought but still too high for a good PS. With the voltages set for 350 I was getting 30VAC on the outputs.
I put together an external CRC filter and the hum on the amp disappeared. I also visually checked the caps inside and nothing was obviously exploded.
The problem is I don't know how much AC ISCO allows on the outputs of these units. I didn't find it in the manual though I'll double check that as well.
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