I've a conrad johnson pre amp model pv 10 which's
stop working. no sound from the preamp output, just a slight hum. (i do plugged it to working power amp, i've checked with other preamp)
If i plugged it into the ac outlet,
although I didn't turned on the switch, the front panel led start glowing but not too bright. I assume that was a protection led warning.
I can hear the pre amp o/p relay click, the relay working fine
I never work at a tubed unit, so I really like to have some input what to do.
If there is someone who is willing to share his experience or schematics..I greatly appreciate.
Thank you all for reading.
Look for problems in the power supply. It might be a blown rectifier or cold soldered joints. These have been the main problems I have found when working on this unit.
P.S. I would also check with Grey (GRollins) as he might be familar with this unit.
[Edited by jam on 07-06-2001 at 09:02 PM]
I've got a number of the CJ schematics on hand (and have owned several pieces, for that matter), but don't have the PV-10. I may be able to lay hands on it, though.
In the meantime, I agree with Jam, the problem is almost certainly in the power supply or a cold solder joint (I once worked on an MV-75 with a cold solder joint that took forever to track down).
I'm assuming that there's no sound out of either channel, correct?
The output relay is going to be a 12V unit run off the same supply as the tube filaments. Check to make sure that all tube filaments are glowing. The front panel LED also runs off of the 12V supply. There is no fault indication, per se. If the LED is half-lit, it's a side effect of whatever else is wrong.
Okay, on to the power supply. Check for DC on the first bulk supply cap (probably a 400-500V electrolytic bypassed with film caps) after the rectifier (probably a discrete full wave rectifier comprised of four 1N4007 diodes). There should be something on the order of 350-400V there. Trace from there to the regulator. CJ used pretty much the same regulator circuit in all their equipment. You're looking for a string of Zener diodes (probably 1N5257B). They shouldn't be hard to find, as there will be 10 or 12 of them all together. One end of the Zener stack will be at ground potential. The other should be at something like 300 to 330VDC. Count the number of diodes and multiply by the Zener voltage for that part. The 1N5257 is a 33V Zener, so if there are 10 of them, you're looking for 330V at the top of the stack. There will likely be three transistors nearby. See if you can identify an MPSA92, MPSA42, and an MJE340. The MJE340 is the pass transistor. It's the only one with a metal tab on top. Trace from it to the main power supply caps (several yellow or white film caps that say CJ-D on them). One side of the caps will be at ground potential. The other should be at about 330V (or whatever voltage you determined by counting the Zeners).
If there's no voltage on those CJ-D caps, you've dropped a power supply. As long as it's not the transformer, it won't cost more than a few dollars to fix.
Check that stuff and get back to us. We'll determine where to go from there.
Thanks for the information Sir Grey and Sir Jam
there's no sound out of either channel
All tube filaments are glowing,
I've try changing the tube..still no sound
there are two 20mF SCR/400Vac caps and another yellow CJ caps, plus another big SCR
two electrolytic caps in the psu area (4700mF 35Vplus small cap )
after the caps
8 zener and 4 1n4148, a couple of 1n4007
3 transistor... MPS u60, MJE 340-akwardly put, and the one with the big flag HS I assume it's mje340 (couldn't see i)t
no voltage on all the big caps end (i pick the one with high voltage rating)
I also checked the x-fer ac voltage... none on the hv side,
the filament measured 18V ac.
I checked continuity on the x-fer.... none on the hv side
It seems the x-fer has blown which's confused me ..why ... no lightning or power surge
is cj x-fer really that fragile?
should I rewound, buy another brand x-fer or buy the original one?
Should I look into other psu parts?
Once again thanks a million for Both of you
Just to make sure it's the transformer, unhook the high voltage leads completely from the board and test for AC voltage with the transformer unloaded.
CJ transformers are no more fragile than any others out there. Does the transformer have a silver sticker saying that it was made by Northlake Engineering? (I think I'm remembering that company name correctly.) If so, you'll have two options, one is to contact CJ for a replacement part, the other is to contact Northlake directly. I've never dealt with Northlake, but the CJ folks are pretty decent.
Rewinding a transformer can be done, but it's extremely tedious. Most people don't even attempt it. If you can find another transformer that has all the right voltages and plently of current, there's no reason not to substitute. Just keep in mind that you'll need to physically mount the new transformer somehow.
It wouldn't hurt to check the various parts in the power supply, as it's possible that a short could have been the cause of the transformer winding failing. Don't bet on it, though, as the line fuse would probably have blown (not to mention the shorted part).
Thank You for the reply.
I pulled out the x-fer connection from the pcb and plug it into the ac outlet, still no hv.
it seems the hv winding is officialy death.
I also look around the pcb and found out that 3 resistors have been changed into regular metal film. The pre amp was bought used and I didn't check it thoroughly.
Would you be kind enough to tell me the resistor brand and seller for those blue resistor?
Thank you for your kind attention.
The blue/gray resistors are CGW--Corning Glass Works.
Note that CJ didn't always use CGW resistors all the way into the power supply. They frequently used (what we would call ordinary now) 2% metal film, which were still considered pretty exotic in those days (the CGW were considered even more exotic). If you wish to use something better, by all means do (Caddock might be a good bet), but it's quite possible that those are the original resistors as long as they're just in the power supply.
thank you and another help needed
thank you Sir Grey and SIr Jam for the reply.
my cj preamp is now working.
I also has some problem for sonic frontirs tube power amplifier SFS80.
The bias run wild, it won't stay put..goes up to the max and sometimes down.
I've check the tube (kt99)and all the tubes are still in 80% range.
i measured the tube with kt88 setting as the manual don't list any kt99.
I would like to ask your kind help... pls inform what I should look at for the problem...
Thank you for your kind attention.
Look for leaky coupling capacitors. These are hooked up to the grids of the output tubes.
Also check the bias voltage to see that it is stable and does not bounce around, as you might have bad components in the bias circuit.
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