Need help repairing mic preamp with faulty phantom power - Page 1
Forum section: electronics / beginners

author: JHAnd
author_rank: Contributor
timestamp: March 28, 2021, 08:11:30 am
content: Hi!Didn't know if I should post this on begginers or repair forum, I'm new here.I've done electronic repairs before, but I'm not great at understanding circuits, but often you can see whet component needs to be replaced.In this case I have a burned resistor on one of the mic preamps in a digimax d8, and phanom power is not working.The other 7 mic preamps on the unit are working fine and have 48v between pin 1 and 2.I was tempted to just switch out the burned resistor, but then I don't know what caused the resistor to burn in the first place.So I'd like to know what to look for that could have caused the resistor to burn?I discovered two faulty mic cables that I had been using with the mic preamp in question;Cable #1 1,2 and 3 shorted to each otherCable #2 Pin 3 looked a bit corrugated in colour, whitish, and had been broken off at the solder point.I have used these faulty mic cables on the mic preamp while testing the cables and mic preamp.Could a faulty mic cable short phantom power and cause the resistor to burn?Or is it more likely to be a circuit problem?This is what it looks like (burned resistor in rightmost circle, healthy to the left);Thanks for help!

author: mvs
author_rank: Frequent Contributor
timestamp: March 28, 2021, 11:19:04 am
content: Quote from: JHAnd on March 28, 2021, 08:11:30 amYes, short circuit in a cable or in a connector is the most probable cause. It is a bit pitty, that this preamp can not withstand such abuse.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 28, 2021, 12:01:59 pm
content: Yes, a shorted input might cause such damage, however, I don't think that is the case here. Phantom power is often delivered by a pair of matched resistors of the order of 6-7k. These would absorb the most power in the event of a short - but only 0.4W at worst. The smoked resistor is clearly unique to each channel and not one of a pair. I would look a little deepr for the possible cause of the fault.It's hard to tell from the photos but I would guess that the smoked resistor forms an RC filter to supply a common +48V rail to the middle of the balanced 6k8, phantom pair of resistors. The resistor is toast - cut it out and check the resistacne to ground at the input side - I would suspect that the "C" of the RC filter is shorted.You can obtain the value of resistance from the adjacent channels, although, I doubt that its value is critical. Sketch-out the cicruit of the input and post it.

author: drvtech
author_rank: Regular Contributor
timestamp: March 28, 2021, 01:21:32 pm
content: Looking at the schematic, that resistor could be burned out by a shorted 100uF 63V capacitor which will be very close by. One end of the resistor is the 48V supply, the other end feeds the 6k81 resistors and also has a 100uF cap to ground so that cap going short would definitely toast the resistor. The actual value of the resistor should be 169 ohms.Edit to add: Can't read the values from this angle but it is almost certainly the right hand electrolytic in your picture

author: JHAnd
author_rank: Contributor
timestamp: March 28, 2021, 08:26:45 pm
content: This is a picture of two channels, the channel on the right is the faulty one.In the middle of the two black plastic input sockets, there is a button that engages phantom power for both channels, (phantom power gets engaged in pairs across the unit)Colored rectangles top to bottom;Red - Pair of resistors marked 6811 Ohms, 6760 Ohms measured in circuitGreen - Marked 1003 Ohms, cannot measure resistance. changes like an electrolyticYellow - Marked 169 Ohms 0.25 W (The one on the right channel is the one that burned)White - 100uF 63V (the larger capacitor in the middle) This one is connected to ground on one side and to the burnt resistor on one side.Turquoise - 47uF 63VPurple - Marked 10.6 Ohms, Measures 10.6 Ohms (all 8 channels except the rightmost one in the picture measured at 10.5 ohms, probably not significant but note taken)Pink - 100uF 25VWhen measuring resistance (in circuit) over the three capacitors, they all start to fill upp and resistance is increased gradually. Including the capacitors in the broken channel.I've read somewhere that this does not necessarily mean that they are functional as they might be parallel to another capacitor that is functioning.@Andy Watson I'm not really sure how to go about sketching out the circuit, but maybe @drytech could post the schematic to the Digimax D8 if you have it available? Or I could post more photos of the circuit.I've checked resistance to ground on both sides of the 169 Ohm resistor (yellow rectangle) on all channels. There seems to be no difference either side of the resistor to ground; On channels 1-2, 5-6,7-8 resistance increases with 1k at a steady phase perhaps 1k per 0.5 sec.On channels 3-4 resistance increases noticeably faster, skipping numbers, so maybe 2-4k per 0.5 sec. I get the same result when measuring over the capacitors directly on channels 3-4 compared to the rest. Capacitors on channels 3-4 charge faster then the other six channels on the unit.i don't know if this is significant in any way.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 28, 2021, 09:47:23 pm
content: Try this link for the schematic:https://www.eserviceinfo.com/downloadsm/57032/PRESONUS_Digimax%2048.htmlIt seems to concur with the input section.

author: JHAnd
author_rank: Contributor
timestamp: March 29, 2021, 11:54:19 am
content: Thanks for the schematic, I can see the RC with the 100uF cap.I bought a cheap BSIDE Transistor Tester, and tested the 100uF cap.It measures as a resistor at 0,4 ohmsThe smaller pair of 47 uF caps tested OK.One was 45.75uF, ESR 0.40 Ohms the other was 45.16 uF, ESR 0.49 OhmsSo you guys where right, its the 100uF cap that is the culprit.Couple of questions...Would you just change this one cap or would you change all the 100uF caps for all 8 channels?Or would you change all electrolytics in the whole unit, to be sure?I bought the device 7 years ago.

author: drvtech
author_rank: Regular Contributor
timestamp: March 31, 2021, 08:01:21 am
content: Glad the prediction turned out to be correct. Regarding changing caps, I would just replace the faulty one, unless it takes half a day to disassemble the unit to get to it, in which case replace all of the corresponding 100uF caps. Reasoning - electrolytics which fail because they're old usually go high ESR or low capacitance or both, but not usually short. Therefore this one was likely just a random failure. The specification is reasonable in that it's a 63V part with 48V across it. If you can find a reputable brand replacement cap with a 100V rating, which will fit, then that wouldn't do any harm. I certainly wouldn't start doing a blanket replacement of all electrolytics. You'd almost certainly introduce more problems than you potentially avoided.Good Luck!Dave

author: JHAnd
author_rank: Contributor
timestamp: April 01, 2021, 10:33:53 am
content: Hi Dave,thats good advice, I've replaced the faulty one and a couple of others on the same channel, just to be sure.But I wont replace all the caps then, good to know how someone with more experience would think about this.Thanks to all of you, for your help!/Jonas