Fixing Someone Else's Mess PART 3 Biema Alpha Line Array Amplifier Repair NO BASS : No Schematics

A Customer brought this amplifier module from a Biema Alpha Line Array and says it is a very urgent repair. I've fixed the two original faults caused by a previous repair attempt and now I get audio output, but only higher frequencies, and it sounds a bit 'weird'. So I need to fix the audio problem but I DONT' LIKE what I find...
Part One of this repair is here : • Biema Alpha Line Array...
Part Two of this repair is here : • Fixing Someone Else's ...
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Thank you
Richard

[@LearnElectronicsRepair]

*Several viewers suggest the fault on this amplifier is caused by a large DC offset across the speaker. This is a good suggestion, and I checked this eventuality on part two but it is easy to miss that as this repair runs over several episodes. You can see me check for this here and there is no DC offset* kzbin.info/www/bejne/g6SxhaGwZ5Vmidk at 00:39:53

[@robertwebb5586]

Richard at 17:54 just below C4 there is a burnt resistor check that

[@sivoltage]

Yep I just spotted that too. Blown in half 🔥

[@mechanoid5739]

The last time I had this symptom on an audio amp I found there was a large DC offset being applied to the speaker. This was caused by a shorted cap. You could still hear sound but with no base because the speaker cone could not move as it was held fully out by the leaking voltage. The speaker was obviously getting hot too.

[@LearnElectronicsRepair]

I checked for DC offset on the speaker in part two (linked in the video description) and I didn't see one. Of course I can check again

[@JonH-j8i]

Richard, on a previous video you wondered about what caused the 10R 2W resistor that is in series will the capacitor near the speaker relay to burn. I guess that this is the 'Zobel Network', that is designed to give the amplifier a more constant load impedance at high frequencies so as to offset the rising speaker impedance at these frequencies. To me a burnt 10R resistor here is an indication that the amplifier has been oscillating at high frequencies and at highish amplitude levels. Check the outputs with a scope.

[@LearnElectronicsRepair]

Thanks for the info. I really did wonder how that resistor could burn up and having checked on the back of the PCB it does not go anywhere else. I'll be doing some more video on this amplifier next week

[@ralphj4012]

It may (or may not be) worth a look at the Biema FW2400 schematic. The chances of an exact match is highly unlikely but it does include the zobel (which blew up), the limiter / protect IC and a VPK01 which looks like some sort of adjustable gain stage / limiter (guesswork).

[@Hellhound604]

Hi Richard, invest in a function generator that can sweep and build that dummy load. Then your can do frequency sweeps and view it on an oscilloscope through all stages of the amp withour having to rely on a recorded audio track and can get real objective data without guessing. If you measure frequency reponse at input vs frequency response at output, then you can meausre at midway point, like the volume control and determine where the rollooff occurs, and do like a binary sort pattern, every time reverse engineering the stage from where the fault shows up to the previous stage where the fault does not occur, but that is just the way I find faults. Of course, you can also go in the opposite direction, jaut monitor the output level, and injecting the sweeping waveform earlier and earlier in the amp, but of course, with Class D amps, it becomes a little bit more complicated, as that process doesnot function on the digital part of the amp (but that is normally just the output stage, and the normal SMPSU-fault finding procedures apply). I like your fault-finding process and drawing circuits to figure out what is going on, but I would use a function/audio generator and oscilloscope a lot more, together with a binary search-pattern. Every person has his/her own way of finding fault, so don’t take my writing as negative criticism, it is just my way of doing fault finding.

[@LearnElectronicsRepair]

Yepp as I often say there are many ways to go about diagnosing a fault. There is no wrong or right way, there are just some that will get you there in the end 🙂

[@andymouse]

@@LearnElectronicsRepair Philosophical nonsense ! you sound like Yoda, get the kit and show the correct way to understand what an amplifier is doing ! as well as using the force !🤪🤣

[@PaulaXism]

Hi Richard. You shouldn't actually need that signal processor to test the main amplifiers. Find where the main amp inputs come out of that thing and inject your audio in there through a cap of course. I think the jfet is just a "soft start" mute circuit to stop the big power on thump..

[@rp5029]

I'm probably way off base but my thinking is that 2 watt resistor you replaced has something to do with the problem. specifically, where it connects to the capacitor. maybe that pad you lost connected to another component. maybe you can check that on the good amplifier. thanks for the videos. I really enjoy following along. cheers.

[@lambert0259]

it looks like the track next to the input to the professional amplifier processor unit is an input from the TL084 pin7 output2, may be the TL084 is faulty ? ideal time for the op amp tester.

[@LearnElectronicsRepair]

It's worth checking. Thanks

[@twithheldmwithheld8938]

I have no idea what difference this would make or not make, but the power switch on the panel is on (12:53) before you swapped the pre-amp board, and off after the swap (19:20), although the blue LED is lit in both cases.

[@darrenbird2526]

I love your videos and descriptions, i really learn so much, thanks again for all the time you put in. Cheers

[@Lightrunner.]

Hi Richard 🤗 18:15, your board 4 has a faulty NASA resistor🚀🔥 Question, why didn't do use your DSO or a audio signal tracer there even helpfully is. You can't measure everything with your DMM . Comparative measurements during operation of the amplifier are very helpful

[@LearnElectronicsRepair]

Watching the video, Preamp board 4 came from amplifier 4 and that one is working regardless what a resistor looks like, the owner gave it to me for comparison. Preamp 4 gives the same problem of no bass on amplifier 3. I now have Preamp 3 board on amplifier 4 and it that is also still working. How does that explain the problem of no Bass on amplifier 3?

[@izzzzzz6]

Well spotted, i see the blackened resistor near the connection terminal

[@Lightrunner.]

​@@LearnElectronicsRepair,,hi Richards, 18:15 is an input pcb and no preamplifier pcb mix up. Question: Where are the connections of the module (IDC1 cannot be deciphered exactly) connected? If the connections go directly to the power amplifier transistors, connect the pins together with a 1 microfarad capacitor. to test whether the output stage three is working. It is difficult to give you tips, thoughts and ideas if certain things that I would have done were not checked in advance. This module can actually only be an AD to DA converter with frequency separating properties (sound processor) or (cheap variant) a RC low, high, bandpass circuit that is cast in. In the audio sector I always prefer to use a signal tracker to control the path while the device is working. if necessary, inject an audio signal. Please keep going, you're doing a great job.👌👍

[@viccurtis7448]

Nightmare problem! Reckon you're on the right track. The JFET could well be a "mute switch" intended to keep the amp muted on startup to avoid the switch-on thump. The fact that you are hearing a thump makes me suspect there may be a fault in that part of the circuit. Is it possible that owing to a fault the mute circuit is actually not fully muting but letting some reduced signal through? I would be tempted to investigate the circuit around that JFET to see how it is supposed to be "switched", the adjacent quad opamp may well be implicated. Good luck with this one, I'll continue to watch with interest.

[@chrishartley1210]

My thoughts exactly. 2SK163 may appear to be the wrong part from the datasheet but my search showed multiple pictures of a T092 package. When it is off the circuit takes the longer route with the capacitor absorbing the signal and muting the input, once it turns on the audio path is direct.

[@LearnElectronicsRepair]

Thanks for the suggestion 🙂

[@BigBoss-rh7zq]

Looks like it behave like a low pass filter. Make any sense to inject an high freq signal and track it until it drops ?

[@Amplified208]

I was thinking similar but a signal sweep of 40-500hz and the see where the signal drops

[@LittlePetieWheat]

biema have an english web site with contact details. Perhaps they sell the module?

[@Amplified208]

Swap the processor modules. Great video look forward to the next video

[@joelsciamma9322]

Richard, just listen in on the audio with a small amp as it progresses through the circuit and either side of that module - you’ll hear the difference. That or use a simple signal tracer.

[@tezza3733]

from memory the diode looks like a varicap diode.and the board says VD14 varicap diode is like a zenner diode only the charge barrier acts as a voltage variable capacitor. never seen them used in Audio, but they are OFTEN used in phase locked loops to tune over a small or large range in transmitters and Linear amplifiers. watched to 42:09 where you tested the jfet, if that IS a varicap diode to the gate of the fet. makes sense.

[@jamesmdeluca]

Use a frequency sweep to check for affected sounds.

[@Karthor.]

17:30 burnt resistor to the right?

[@Backcountry_Bodger]

Maybe try refiting the digital amplifier and iusing a signal tracer on the two amps to see where the signal is lost?

[@anthonytidey2005]

It looks to me that may be the professional module had been worked on before as it had a lot of flux just around this component all the others around dont have it. Why dont you inject a audio signal in and see where it gets distorted. Most of my working life I was faultfinding to component level. As you did inspect for anything thst does not look correct eg burnt resistor, leaky cap etc. As there are so meny of these amps is it possable to get circuit diagrams. Like you method of fault finding. Thanks for the video, I will wait for more.

[@LearnElectronicsRepair]

This could be the case. The problem with tracing the signal path is the lack of schematics, but in this video by swapping stuff around I am fairly sure I know all is OK up until the DSP module. This amplifier had at least one prior botched repair attempt. Based on a the previous repairs I did for the same customer, I had the Basic 400 amplifier with a faulty front end op-amp (glued to the board) and the Lucas Performer 900 subwoofer with a faulty resistor in the negative feedback network causing one of the bridged class D amps not to work. I have a suspicion that this guy is bringing me a lot of stuff that has been in previous repair shops who could not fix it because nothing I am getting from him is straight forward. Either that or I am just really unlucky. If you watched the previous videos, I already fixed two major faults on this amplifier (and at least I got it to a state where it basically 'works' now) only to face even more problems with it. Hmmmmmm.......

[@007rgb]

As you have duplicate amps and little chance of securing a replacement pro amp processor component, could you at least show if it performs equivalently by using the oscilloscope to compare audio in and out for both processors?

[@danieladam86]

Is that a bar code on that DSP chip?

[@davidhollfelder9940]

Back in the day, we’d say it sounds like a “tin can”. As mentioned by Hellhound, signal injection/signal tracing, stage by stage would, be the way to go on this one.

[@stevehurcombe]

at 23:42 that variable resistor looks burnt

[@mw1fly1]

Is that a burnt resistor on the mixer board you marked as 4?

[@LearnElectronicsRepair]

Yeah it looks like it, but mixer board 4 came from amp 4 which the customer tells me is a good working one (and it seems OK to me too). When I put mixer board 4 on amp 3 I have no bass. When i put mixer board 3 on amp 4 it works just the same as before

[@mobility-tech]

Ok so i have a question - The alpha-a unit is listed as having a 3" and 2x8" drivers and freq 75hz -18khz. The alpha-b unit has a 18" driver and is 35-250hz. Presumably they function together as tweeter/mid and bass system in the field. They seem to have a built in DSP presumably for crossover and response tuning but there are also mixers and DSP available from this maker. How exactly does the customer have his equipment set up? Is the missing bass here (and what he complained of) actually missing or provided by an alpha-b that you don't have? + modified by other equipment? If the crossover is quite high the expected bass will be lacking. If its up near 150 hz it will sound a bit 'bonky' or hollow I think especially played through small test speakers. I note that some of their DSP have software available. Perhaps those are programmable to some degree... has someone messed that up if one is being used in the bigger scheme? Just looking outside the box 🙃

[@LearnElectronicsRepair]

I did wonder if that is possibly the case, that the amplifier is meant to be used in a different type of speaker cab. He tells me they both come from the same type of cab, though if for examp-le he bought these used or something, who knows what is the truth of this matter? I wouldn't be surprised if you are right and he is wrong.

[@izzzzzz6]

€30k For a "The box" line array Lol I mentioned this last video. Could they have different DSP setups for treble mid bass in different cabinets of the line array? Failing that I may try to crack that box and take a look inside. Might be silicone, hopefully not some sort of epoxy. I tried cutting a similar box open recently and i ended up slicing through an inductor in the signal path. I've replaced it with a similar inductor. In the end the problem was coming from bipolar diodes (to ground from +&- XLR in) TVS near the xlr input. I'll be chucking it all back together once I have some time and energy. If this is supposed to have the same dsp as the other one then it could be a break in ground somewhere. I would first swap the modules to double check then with not much to loose if confirming one module is bad only then try to open it up.

[@Dutch-linux]

the thumb usually means dc on the speakers you have to check that

[@LearnElectronicsRepair]

There is no DC offset on the speaker, I checked that in part two 😉

[@TheSilmarillian]

Interesting indeed new sub here.

[@vinnieluther6589]

Is it possible a DC blocking capacitor has lost its capacity and is now acting as a high pass filter?

[@LearnElectronicsRepair]

That was one of my initial thoughts, though I have eliminated everything before the DSP module by swapping the mixer boards and testing any other caps in the audio path. it could still be a cap after that but bear in mind the fault is on both amplifier channels when tested individually. Really I was interested at this stage to see if anyone knows what that DSP module is and where to get them before I proceed further for example by swapping the module form the other known good amplifier, which would take a couple hours to do and if it turns out one module is faulty I would then be unable to proceed further and actually get paid for this job.

[@bouhlassamohamed1337]

❤ awesome

[@TheElectronicDilettante]

You say you checked all the voltages, but they don’t match what’s written on the transformer. Granted they might be higher or lower depending on the rail but they should be close to factors of the supply. Sorry last thing. If that fan is janky isnt it going to go into some kind of safe mode. I saw that happen to Dave J. On EEVBlog.

[@LearnElectronicsRepair]

The voltages marked on the transformer are AC, the ones powering the amplifier are DC so they are going to be higher. Yeah I guess there is an outside chance the fan is causing the problem but if it was so important to know it is working correctly surely they would have used a three wire fan rather than a two wire one?

[@TheElectronicDilettante]

The α-a you have is from the 1990-early 2000’s when Biema was still made in the USA. Have you confirmed the power outputs from the Toroidal Transformer passive and under load? Maybe get that light bulb out of there and put a current probe or current to voltage transformer and hook a scope up to it to get a visual of what’s going on . If that checks out, take a curve tracer and start testing each component (in circuit) from one side of the board to the other. If you don’t get it sorted , do a live stream and throw a bunch of real time global brain power.

[@LearnElectronicsRepair]

Thanks for the good ideas 🙂

[@Mark_C1]

Isn't there another of those modules in the other unit ?

[@LearnElectronicsRepair]

Yes there is and as I made a point of in this video, if I know I can get a replacement one if needed then I think it would be worth swapping the module from the working amplifier to see if I can prove the other one is faulty. Bear in mind it takes over an hour to strip down the amplifier and remove the module from amplifier 4 to fit it to amplifier 3 and rebuild it to test. As I work on a no fix no fee basis I was questioning myself if it was worth doing that if I know I can't replace it regardless, but given the interest this video has sparked I suspect I will end up doing so anyway just to satisfy everyone's curiosity even if it ends up I can't fix this 😉 I have a nasty feeling that is the problem but then I could get lucky...

[@Mark_C1]

@LearnElectronicsRepair yeah I totally get the no fix no fee and the amount of work involved. I know it would drive me mad knowing I could know by swapping them over if it's the issue by doing it but also the pitfall of not wanting to have, potentially 2 broken units you can't fix, that would be a nasty situation. How about injecting 100 odd hz and using the signal detector pen you made to find where the signal ends it's low frequency path ?

[@viccurtis7448]

@@Mark_C1 Hmm, not a bad idea, and in particular now we seem to have identified the signal input feed to the "processor" module it would be interesting to see if the bass disappears before the signal reaches that module.

[@g4z-kb7ct]

When I come across something that has multiple boards and I have the luxury of having a 2nd working unit I swap each board one by one and test to find the faulty board then work only on that. In other words, flailing around working on random boards without knowing if it is actually faulty is just asking for trouble. I see a LOT of that in the arcade scene where almost everyone bar a few are cowboys lol. If you think the potted module is bad just swap it from the other working unit. Even if you don't have a replacement and it is found to be faulty it will bring closure to the job and you can move onto the next job. But from what I saw you didn't swap each board and test it and you haven't located which board is actually faulty? Or maybe you did and I just missed it? Anyway, I would work on that small board that you have identified as bad first. not really sure why you didn't since it is clearly faulty and has a black component in the middle that appears to be burned? Would love it to be one of the opamps then you could use your tester :-)

[@LearnElectronicsRepair]

The mixer board (4) came from the good amplifier (4) and it works on there regardless of the burnt resistor which yes I should have looked for. When I put mixer board () on amplifier (3) I still have no bass. Whe nI put mixer board (3) on amplifier (4) it still works OK. And for some reason now both protect LEDs work the same too. I don't think I was 'flailing around' on random boards. What we have here is mixer boards, both confirmed working. A bridged amplifier board, both channels of which work the same when tested individually, but one amp has bass and the other doesn't. And then the PSU board, I didn't swap but the supply voltages are all correct on both amps. Yeah the next thing is to swap the module from amp 4 to amp 3 but as it takes at least a couple hours to do that, and this video was already 45 mins, I decided to ask if anyone knew where I could get those modules, or even a pinout for it, before doing that. I believe that is fairly reasonable, but don't worry there will be a follow up. Before continuing I'm also trying to figure out how I can actually fault find on this thing properly as once it is assembled you can't get to the rear of the PCB 🤔

[@g4z-kb7ct]

@@LearnElectronicsRepair Oh I worded that poorly. I was just saying flailing around is bad. I didn't mean you were doing it. From what I've seen the work you do is very good :-)

[@Nebbia_affaraccimiei]

42:55 44:44 I swear it looks like theres already a module inside the holes and you're holding the "bad" one so was it already swapped??

[@LearnElectronicsRepair]

No there isn't look at 39:17 it's just the shape of the solder pads

[@Nebbia_affaraccimiei]

@@LearnElectronicsRepair optical illusions are fun!

[@LearnElectronicsRepair]

@@Nebbia_affaraccimiei Yep 🙂

[@joegibson9589]

According to this post www.elektroda.pl/rtvforum/topic1359666.html The Reloop RPA-2900 has this part. You can request parts from Reloop here: www.reloop.com/spare-part-inquiry Hope this can help. Love your work.

[@gregwmanning]

can a phone read a barcode like a qr code?

[@fredfabris7187]

2sk163

[@michaelbrown2470]

I'm thinking that JFET is used as an inverter to derive the push-pull signals. Are the emitter and collector resistor values identical? Also, this may lead to more info; "www.aliexpress.com/item/4001114739848.html". I block all Chinese IP addresses so I can't follow the link.

[@LearnElectronicsRepair]

From what I see the only (mono) audio signal passes through that JFET. I think it is more likely some sort of mute circuit, to wait a few seconds on power up before applying the audio to the amplifiers

[@aircraftclocks]

Test

[@aircraftclocks]

Test