Pre73 crapped out

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Carnesd

Well-known member
Joined
Apr 17, 2010
Messages
73
Well, got everything set up to records some vocals today and hit the power button on the pre73 and light comes on then off and now won't power on.  I checked the fuse and its good, getting 26 vac to both sides of the fuse.

I'm not an electronics genius like alot of you guys, but I'm not totally a noob either.  If anyone could give me a couple checks I could make or any thoughts on what to check would be great.  Thanks!

I knew I shoulda saved for an N72 kit.
 
Yea sorry its just a cheap 1073 clone with crappy parts and crappy quality control. 

 
Ok, So I did some voltage checking and i think I've got a bad resistor but not sure, I followed the 24v AC signal path and it turned into 73v DC on one side of a diode and then too a 5w resistor I have 73v on one side and .4v on the other.  Pulled the resistor out and ohmd it out, it says 5w 270R J on the front but I'm getting 240k ohms. 
Am I correct in thinking this is bad?
 
Carnesd said:
Ok, So I did some voltage checking and i think I've got a bad resistor but not sure, I followed the 24v AC signal path and it turned into 73v DC on one side of a diode and then too a 5w resistor I have 73v on one side and .4v on the other.  Pulled the resistor out and ohmd it out, it says 5w 270R J on the front but I'm getting 240k ohms. 
Am I correct in thinking this is bad?
Get a fresh battery for your multimetermeter and measure again. ;)
 
don't have another battery, but i checked a couple resistors i had laying around and they are reading correctly.  So you don't think its a bad resistor?
 
You said the preamp was not working, but you didn't mention that you knew it was the PSU. Did you check your DC at the various PSU outputs? 
 
Oh, sorry  :-[  Yea,  the pre73 has an external 24v ac power supply,  Then it has a psu board which splits to the 24v for the preamp and 48v for the phantom power.  I have 48mv at the psu board output, but do not have the 24v to the output on the psu board.  I removed the board from the chassis and plugged it in and checked voltage from the fuse down to the connector, 

It turned from 24v ac to 73v dc after going through a diode, and then stayed 73v down until I went through the 5w resistor then it went to .4v and thats what I have on the 24v output connector on the psu board.

It seems like only one path from the fuse down to the 24v output connector, just some caps going to ground along the way. 

Sorry I don't know the technical terms for any of this,  hope its at least can follow it.

Thanks again.
 
I assume there are other diodes (like 3 more) near by? Your post is a quite confusing:
I have 48mv at the psu board output, but do not have the 24v to the output on the psu board. 
Anyway, was anything plugged in when you turned it on and it died? I'm assuming the 73VDC is after a voltage doubler (basically a bunch of diodes and caps, check google for schematics and see if you see something like that on the board) for your phantom. It would make sense to have a large watt resistor before the regulator assuming it's an LM317, basically to drop some of the voltage the regulator would otherwise have to do by itself.

Try to take a nice clear (macro if you can) picture of the PSU board because I don't think there is a schematic around for this thing.

Check some PSU schematics, specifically JLMaudio, since they use separate AC supplies, maybe you'll find something similar. 

Also, if you have a working 24VDC PSU, you could plug it into your circuit and see if it works.

good luck.  I like my cheap-o chinese 1084 from the groupbuy. It's actually a pretty nice preamp.
 
Yea, there are two connectors coming off of the psu board, one that says 24v(which I guess is the main power to the preamp) and one for 48mv(which I'm guessing is phantom power), (with the connectors unplugged and checking the pins on the board)  I'm getting 48mv to the connector labeled 48v.  But not getting 24v to the other connector. (which is labeled 24v). 

I did have a mic plugged in when it cut off. 
I will see if I can't take a good picture, thanks

 
DSCN1405.jpg


Did a quick diagram, maybe this will help,  I left out the phantom power side because it didn't seem like it was integrated at all.
 
I guess I'm just gonna order a resistor and solder it in and see what happens,    Do you think it would be alright to use a 100ohm resistor instead of the 270 ohm?    I can get a 10w 100ohm from radio shack.
 
Carnesd said:
I guess I'm just gonna order a resistor and solder it in and see what happens,    Do you think it would be alright to use a 100ohm resistor instead of the 270 ohm?     I can get a 10w 100ohm from radio shack.

get 3 and put them in series.
see if they've got 5 watters.
 
That's a very strange looking PSU. If the first diode had the cathode facing downwards, it would be a standard half wave doubler (are you sure it's facing that way?).  The 2 ICs are regulators? I assume you drew them viewed as if the metal tab were towards the back?  The first regulator is variable (LM317?) and the other is a 7824? Is the input of the second actually going to ground or is that a mistake?  Out of curiosity, what are the resistors around the first reg? (sorry for the questions, just for clarification).

Does the phantom tap off the 73VDC also? Just trying to understand why they did that...

Someone please correct me if I'm wrong, but, if my calculations are correct, that 5W resistor was perilously close to dying the first time the preamp was powered on:
Original Neve circuits are spec'ed to run on +24V @ about 120-130mA.  On paper, the doubler rectifier takes your 24VAC up to close to 68VDC, if your mains voltage is high or the trafo is not exactly 24VAC you could easily get the 73VDC you measured. That 5W resistor is there to drop voltage (as heat) before the regulator, and since we know what the circuit uses, we can figure out how much:  V=IR = .130A x 270R = 35.1V  (That's assuming the LEDs are running off another rail? I don't know.) So, if the circuit is plugged in, you should have about 35V less on the other side of the resistor.

OK, now for the power dissipated by the resistor under above circumstances:
P=IV = 35.1V x .13A = 4.563W !!! (If LEDs are running off this rail then I don't know how it didn't fry earlier).
Keep in mind that this is also not accounting for the couple seconds when you turn it on that Neve preamps suck double or more current (until the capacitors in the circuit fully charge). It's pretty quick, but still, way more than that resistor is made to handle. I wonder how many others might have the same nasty surprise coming....

They should have put a 10W in there. Better yet, they should have used a JLM  :p

As far as the resistors go, do as geoff says, but get the 10W for sure, especially if you use 3 in series. It'll drop about 39 volts, and put out 5.07W of heat.  Conversely, you could use 2 in series, drop 26V and dissipate 3.38W of heat, which should be OK for 5W resistors, but may or may not be OK for the first regulator, depending on what they have it regulating to.

As always, double check everything I wrote, I tendency to make mistakes is above average!  ;D

Once you get it working check the ACMP thread for some mods to these chinese circuits. I would assume yours has some of the same issues, like lytics instead of tant caps, wrong zobels on the trafos, RC filters on the input connectors... it's a long read but some things may make your preamp a bit better.
 
Well I bought it second hand and got no paperwork or anything with it so I doubt its in warranty. 

Good news is I got three 10w 100ohm resistors from RS and soldered them in and boom!  Works great  ;D

So I ordered a 10w 270 ohm resister from digikey and will solder it in there permanent cause theres no way three of these bad boys will go in the case.

Thanks alot for all of you guys help!  I really love this site!

 

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