Asking for assistance diagnosing a preamp - Page 1
Forum section: electronics / repair

author: adauphin
author_rank: Unknown
timestamp: March 01, 2018, 05:50:04 am
content: This will be my first real dive into troubleshooting an electronic component. I've messed around with caps, resistors, rebuilding crossovers, or wiring relays, but this is more in depth.I have a 7.1 channel preamp/processor that's about 17 years old and is starting to give me some hiss from the tweeters and a very faint buzz from the midrange.I want to speculate a possible PSU cap or maybe a bridge rectifier. Either way, at its peak, it was and still is a very high quality component....I would like to possibly breathe new life into it or at least try.It still sounds great, albeit the noise. I have a Fluke 89 IV, and a trusty 465B...not sure how to fully operate the scope, but would love to try it here.The unit in question is a B&K Reference 50.I can give you pics and a schematic tomorrow, any input appreciated. Not sure how to start the diagnosis but that's why I'm here...

author: danadak
author_rank: Super Contributor
timestamp: March 01, 2018, 10:42:44 am
content: Generally speaking (using scope)1) If scope DSO place it on infinite persistence and look at power supply railsto evaluate noise. If scope analog, but storage, run a very slow sweep lookingat rails with controls set to long persistence.2) Any parts in sockets lift the part at each end slightly and push back into socket.3) Dirt/dust/grime buildup clean PCB.4) Ground input jack for a channel and using scope proceed to look, starting atinput, and proceed along the signal path all the way to output. Repeat with asignal source, like simple tone. Here you can get free signal generator from yourPC/Laptop via sound card.The following for people w/o scopes, but the software also have sound generators -If you don't have a scope, but have a PC (limited, will not be able to see high speedtransients, but still somewhat useful) -You can start with a PC sound card based scope for free. Will give you basicallyaudio range scope, spectrum analyzer, and function generator all using yourPC sound card.https://www.zeitnitz.eu/scope_enhttp://www.zelscope.com/http://www.ledametrix.com/oscope/http://www.virtins.com/downloads.shtmlBut first build a simple circuit to protect sound card inputs so you do notruin from transients, overvoltage. Google "protect sound card input".For examplehttp://makezine.com/projects/sound-card-oscilloscope/Sound card impedance bridge -http://www.marucchi.it/ZRLC_web/ZRLC/Steber_An_LMS_Impedance_Bridge.pdfhttp://www.sillanumsoft.org/ZRLC.htmRegards, Dana.

author: rhb
author_rank: Super Contributor
timestamp: March 01, 2018, 03:50:43 pm
content: The OP has a classic Tektronix 100 MHz analog scope. However, the rest of @danadak's instructions are spot on. The only thing I would add is look for solder flux residue and if you find any clean it off with 91% or more isopropyl alcohol. Make sure to let dry thoroughly before applying power if it's not 99%. I usually use a hair dryer on large boards with lots of places for liquid to hide.Flux is hygroscopic and as it absorbs moisture from the air can created unwanted parasitic capacitance and conduction paths. I usually see this as an issue in logic circuits, but noise is noise.I have a 465 which I repaired. So I know the scope well, but I'm sure I'd find myself somewhat confused trying to use it after not touching it for a long time. I bought a Rigol DS1102E because I wanted access to a scope and just could not face all those socketed transistors and 40 year old solder joints. But I will once I have enough workspace to keep it running on my bench.One other tip as the OP said this was his first foray into troubleshooting. After checking the power rails, test the audio chain as near the middle as you can locate with the input shorted to ground. If you see noise, the fault is between there and the input. If you don't see noise it's between there and the output. Of course, this only helps if you can locate the middle easily. Sometimes you have to trace from one end or the other to find the middle.

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2018, 02:00:40 am
content: FYI, purpose of posting the PC scope info was they have function generatorsas part of the code base, to use as a signal generator for testing. They are alsostorage, unlike 465, can capture, albeit low freq content, transients for examinationand troubleshooting.I like the idea of binary search trouble shooting, quick way to finding problem area.Regards, Dana.

author: adauphin
author_rank: Unknown
timestamp: March 02, 2018, 05:06:34 am
content: Great..thanks for the links..I forgot to mention I have a Global Spec 4001 signal generator...not sure if it's ideal for this project but I attached a pic.I also have an occasional hum in my power amps toroidal, I want to say I might have some DC on my mains.....not sure.Here's a pic of the Ref 50's power supply schematic.

author: adauphin
author_rank: Unknown
timestamp: March 02, 2018, 03:44:28 pm
content: Mods, feel free to move this to the repair forum, my mistake.Here's some pics, the ground connections appear clean, as well as the PSU connectors....wouldn't hurt to hit them with some cleaner to be sure.As far as using the scope, I want to be sure the chassis ground is where the ground clip should be. The caps look OK, probably wouldn't hurt to upgrade the PSU, also not sure what measurements to take.I'm all in on learning, been an interest of mine for years but now starting to get around to it.....also a great time to dive into a new entry level scope....

author: adauphin
author_rank: Unknown
timestamp: March 17, 2018, 05:24:34 pm
content: I was able to get the voltages of the power supply recorded, was surprised to see 240V but I'm not going to question the design.The 4 power supply caps are:3- 3900uf 25V1- 1000uf 50VUnsure of brand or model. These are warm to the touch when sitting idle for long periods, I can record those temps later.I labeled the leads from top to bottom.White: Mains neutralYellow: 121.8 VACOrange: 220.6 VACRed: 240.4 VACPurple: 2.71 VAC.......... Blue: 24.65 VACBlack: 185.3 mVAC......Gray: 16.22 VACPurple: 2.73 VAC...........Gray: 16.22 VACBrown: 4.68 VAC...........Blue: 24.68 VACBrown: 4.68 VACI assume the two rectangular pieces adjacent to the caps are the bridge rectifiers?This will also be the first project for my newly acquired 1054Z....As my first real project, I would like to also verify or simply check the audio signal from input to output for signal purity or degradation.Looking forward to this, all input and criticism appreciated.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 17, 2018, 06:11:09 pm
content: You may start with checking all solder joints and noise on the power supplies.One common failure on oldish hardware is aging electrolytic capacitors, especially when they are constantly exposed to high temperatures, which is typical in power supply sections.From your pictures, the caps don't seem to have leaked yet but it's worth checking the noise on the power supplies and changing the caps.

author: adauphin
author_rank: Unknown
timestamp: March 18, 2018, 02:43:47 am
content: Quote from: SiliconWizard on March 17, 2018, 06:11:09 pmI will check all the solder joints, but even after watching the vids and some searching...I'm not 100% confident on properly checking the lower supply. Not trying to sound lazy, or not wanting to do any research....I'm just new to this and dont want to mess things up.Do I need to probe all the leads? Curious about the 240V leads, that's a bit of juice.

author: dashpuppy
author_rank: Regular Contributor
timestamp: March 18, 2018, 03:11:14 am
content: first off, i’d feed a 1khz signal into one channel and then put the scope on the output. see what you see..

author: adauphin
author_rank: Unknown
timestamp: April 01, 2018, 05:22:31 am
content: Sorry for the delayed response, I just returned from vacation and was finally able to power up the 1054Z for the first time.Bear with me, this is the first measurement I have made with this scope, please feel free to critique any part of it as it will be a learning experience for me and possibly others.I used a sine wave generator from my iPad, channel 1 (yellow) was the output of the preamp with the volume on -5db, channel 2( blue) was the signal input. This is a 1kHz sine wave.Left channel:Right channel:What I'm getting is a decent amount of hiss from the tweeters, and a very, very slight noise from the mids, could be a buzz...but you need to have your ear next to the cone to hear it from the mid, the tweeter hiss is audible from 2-3' away easily.Maybe this looks ok, or real bad...I'm not sure. The sound quality is very good, but the hiss likely shouldn't be there.

author: adauphin
author_rank: Unknown
timestamp: April 01, 2018, 07:12:37 pm
content: This is a comparison between the mains input and the 120V transformer tap after the transformer.Yellow is mains at the first connection after the IEC inlet, blue is the 120V tap after the transformer.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 01, 2018, 07:31:57 pm
content: The high-frequency noise in the output is very noticeable in your scope screenshot.As for the power supply, take a look at the rectified and regulated voltages.

author: adauphin
author_rank: Unknown
timestamp: April 01, 2018, 07:47:57 pm
content: Quote from: SiliconWizard on April 01, 2018, 07:31:57 pmI had to run out for a family gathering so I will get some more data this evening.I assume that's a fair amount of HF noise...any settings on the scope I should set to accurately measure the rest of the voltages? Thanks for the help...

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 01, 2018, 08:07:57 pm
content: For all DC voltages, you can start with measuring them with a multimeter. Then take a look with your scope with DC coupling, and then switch to AC coupling to evaluate noise, you can then use a much lower voltage range to see what kind of peak-to-peak noise there is on all the DC voltages.And be careful. Your device is connected to the mains.

author: StillTrying
author_rank: Super Contributor
timestamp: April 01, 2018, 08:47:39 pm
content: Quote from: adauphin on April 01, 2018, 07:47:57 pmIf you don't stop measuring the mains, the family gathering will be for you, but you won't be there.DON'T try to measure the mains with a scope, until you have years of experience of scopes and grounds, ...and even then.As others say, it's low voltage supplies you should be checking.You should be able to move the thread to repair yourself if you want, - very bottom left.https://www.hifiengine.com/manual_library/bk-components/reference-50.shtmlhttps://elektrotanya.com/bk_reference-50.pdf/download.html

author: adauphin
author_rank: Unknown
timestamp: April 01, 2018, 09:45:18 pm
content: I appreciate those words of wisdom @StillTrying , I don't like measuring the mains even with a DMM, but I exercise extreme caution every time. I check and double check all the connections for proper grip, proper ground and neutral. Even with a partial family of electricians safety is never a back seat.I understand a scope is different and it's easy to short to earth if you aren't careful. That being said, I only want to ensure my incoming signal wasn't full of artifacts since I have been chasing a torodial hum also.I will focus on the lower voltages and other components now that I have that out of the way. Don't get me wrong, I appreciate the warnings from the experienced and look to heed the advice of my peers.. redundancy is key.

author: adauphin
author_rank: Unknown
timestamp: April 02, 2018, 03:54:58 am
content: I was able to probe the low voltage side, this is right after the transformer as it enters the circuit board.Going by this pic, the two white connectors moving from left to right, starting with the blue on far left. My voltage varies at my house and these values are slightly different from the original pics in this thread.Blue 24.4 VACGrey 16.04 VACGrey 15.89 VACBlue 24.36 VACBrown 4.68 VAC - AC CoupledDC CoupledBrown2 4.75 VAC - AC CoupledDC CoupledPurple 2.73 VAC - DC CoupledAC Coupled...Not sure I recorded this properly.Black 183 mVPurple 2.67 VACThe 4-pin connector had pretty much identical traces with AC or DC coupling.The 5-pin connector with the center purple lead the trace appeared to run from right to left.I will try get to the rectified leads later this evening.

author: adauphin
author_rank: Unknown
timestamp: April 02, 2018, 01:06:59 pm
content: With it being cramped near the rectifier, I wasn't able to safely probe anything on top of the board, but I did manage to get the three low voltage wires in the two white connectors, CON 19 & CON 20. The schematic shows what I believe is different voltages for two of the wires (+12), however I had lower readings. Not the trace I was expecting, unless I made an error in recording the trace.This was the orange wire in CON 19, I had a steady 4.965 VDC. (2 Pics)This is CON 20, yellow wire, steady 9.75 VDC (2 pics)CON 20 Orange wire, steady 4.976 VDC (3 pics)

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 02, 2018, 03:04:34 pm
content: I would suggest a noisy 3.3V, 5V, whatever... supply to the DSP,

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 02, 2018, 03:20:26 pm
content: Lots of these scope pictures are clipped. And the 50mV/div are also just noise you picked up out of the thin air. I'm more worried at the 'digital' noise some of your traces show. If this is supposed to be a supply I would check if the filter caps are OK.Anyway can you check how the high frequency stuff at the output looks like in more detail? For example with a shorted input to be sure the preamp doesn't pick up on the open input.

author: adauphin
author_rank: Unknown
timestamp: April 02, 2018, 06:28:22 pm
content: Quote from: Twoflower on April 02, 2018, 03:20:26 pmI can try for sure.I started to look for any grounding issues, so I checked the resistance of each outer shield in relation to ground. All the shields measured low, around .2 ohms...except for the Zone A & B digital coax output.I don't know how they are tied into the circuit with relation to ground, but I have a short between the center pin and the outer shield with infinite ohms to ground on the outer shield. This is only on the two outputs on the far left...ZA and ZB.I thought maybe a relay would change that so I cycled through all the inputs with no change. Unless there's a menu setting that activates a relay, I still don't think the center pin should be shorted to the outer shield....not sure. All the other digital coax jacks measure 75 ohms center pin to outer shield.Far left in these 2 pics:Far right in this pic:

author: adauphin
author_rank: Unknown
timestamp: April 02, 2018, 08:27:07 pm
content: I guess we can rule the dig coax theory out, the center and shield are joined together....far left coax output. Top pic goes to left of bottom pic.

author: StillTrying
author_rank: Super Contributor
timestamp: April 02, 2018, 10:40:49 pm
content: You're going to have put at least some logic into your noise testing and fault-finding, I sometimes think this thread is just a wind-up!

author: adauphin
author_rank: Unknown
timestamp: April 02, 2018, 10:49:05 pm
content: I hear ya...hopefully it's just an old cap...

author: coromonadalix
author_rank: Super Contributor
timestamp: April 04, 2018, 01:22:59 am
content: your orange con19 wire is not good we see the center pin, correct this ...normally get an service manual, sometimes you can order them directly from the company1 - check all the psus voltages ac, dc, ac+dc2 - put the equalizers setups effects to none / off / or flat no correction at all if there is3 - i would inject an 0db 1 khz signal and follow it up to the preamp and amp section, always compairing the 2 audio sides( left right) with an isolated main ac voltage scope4 - try every input line level, not the phono input if you have one lollAnd yes you have to do some fault finding logic, do go here and there thinking this is bad or else, sometimes you can pull out pcbs to simplify your search and signal tracingI had in the past an Sony prologic 5.1 sound system, the bug was after 30min - 1 hrs of use, i had dc component at the output and heard distortion, tons of boards to check, had to follow the signal path with an scope, an signal tracer, and a signal generator to help, and to my surprise there was a missing thru hole resistor in the amp feedback, straight brand new form sony lolll

author: adauphin
author_rank: Unknown
timestamp: April 07, 2018, 11:25:22 pm
content: Quote from: coromonadalix on April 04, 2018, 01:22:59 amThe CON 19 is probably using the center wire in the Series 2 version, mine only has the single lead.I do have the preamp in Direct mode...no eq.I will try all you suggested.