Guitar amp repair (modifications) - Page 1
Forum section: electronics / projects

author: rhdf
author_rank: Contributor
timestamp: June 27, 2016, 10:53:30 pm
content: A couple days ago I tore apart my old "trusty" Dean Markley K-50. It have been standing in a corner at my parents place for 10 years and when i got it back I realised that it sounded.. bad. Scratchy pots, annoying hum, crackling inputs and a pot not working as it should. (hey its over 25 years old or so)I managed to find the schematics (http://www.deanmarkley.com/Info/LegacyAmps/Schematics/D1518.pdf), but I start to realize that there is some differences in the actual circuit and the schematics.Some values differs, some components is missing (as in: not even a place for them on the pcb) I suspect that there are 2 different revisions. Mine is built on one single PCB, mounted parallel with the front panel, with all wiring directly soldered to "random" spots. Some other revision seems to be on separate PCB:s (http://guitar-dreamer.blogspot.se/2012/11/diy-repair-your-solidstate-combo.html)In that revision the power supply has a diode-bridge (as in the schema) instead of separate diodes (as mine have), and the pcb seems to be a lot nicer laid out with connectors for all wires (on the other hand do the speaker-wires look a bit whimpy even for a 35W amp)So:Solving the hum seems to be as simple as replacing those 2 big caps, Anodia brand... Never heard ofCrackling inputs Can be just oxide, haven't checked yet. same thing with the scratchy pots.The non working pot was probably due to a bad solderjoint ,there where a few of them... BUT, there is one other with feels like its a crack in the track.2 problems with eventually replacing the pots..1) The values. PCB mounted 250k Audio pots are rare.2) 250k pots with loooong legs are even more rareDue to the looks of the pcb and the not so nice layout (components scattered all over instead of nice grouping) I started to think of a complete rebuildFirst I might focus on the powersupply. The mains wiring looks bad with exposed wires and only a SP mains switch. The DC-part seems to be able to do some improvements on.Next maybe separate the power-amp to a separate PCB and build a proper headphone-ampAny traps and real strangeness in the shema linked above??

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: June 28, 2016, 03:23:20 am
content: http://amprepairparts.com/pots.htm#peaveyspiderPart #P-VS250KA looks to be suitable if you remove extra pin

author: rhdf
author_rank: Contributor
timestamp: June 28, 2016, 08:56:40 am
content: That extra pin should have been there from start. Seems that they could give some extra stability and "un-flimsiness" when trying to fit 7 pots and 6 jacks into their panel hole at the same time...At the moment I trying to sort out the powersupply.. The mains wiring seems to be a bit.. unsafe or at least not up to the standard I like it to be.See image below.No shrink-tube on the terminals.Single pole power-switchearth-lead soldered to a wiring-post then bolted to chassisAnd then this cap between one mains-lead and earth. Depending on what direction you put the plug in the wall this can be Neutral or LiveSo basically I have a old ceramic disc-cap between earth and mains (read somewhere that these are referred to as "death-caps" by amp-repair guys)Its markedE 103M IKV* LK*Yes its a I not a "1" since it differs from the "1" in "103" But i think it means 1KV (seems logic)first step for my own safety (and mental health) seems to be to rewire the mains-part.I probably will go for a IEC-connector with integrated fuseholder and then replace the main-switch with a 2-pole

author: rhdf
author_rank: Contributor
timestamp: July 05, 2016, 06:50:02 pm
content: Update:The cap between Earth and Neutral/Live (depending on wich way you turn the plug) mentioned above:It turns out that its not even soldered in place. The leads are just semi-loose wrapped around the terminals.I've start analysing the circuit just by curiosity and to get some understanding of how it works (and if there is anything that can be modified)First of, the switching between the "clean" and overdrive channel is done by 2 FET:s One is connecting the input to the opamp for the od-channel and the other one "shorts" the feedback-loop on the opamp for the clean channel (giving it gain~0)Both channels connects to a third Op-amp via a 22k each, as it would be a mixer. somehow it seems that one of them can be left out (and connect the od-channel to the same 22k as the clean) since both channels never are active at the same time.Second:the caps, most of them are ceramics, either disc or "blobs" There are some filmcaps (I guess) But not where I expected to see them (except one) The FET:s mentioned above has one 10n between Gate and Ground probably by a good reason, just that I cant figure it out.I realize that the amp is built as cheap as possible and that's probably why I see some strange component choices made. For example. All resistors are 5% carbon and god knows what specs this "random" selection of caps has. At least I know the amp is anything than noise-less...But on the other hand, it probably give it some kind of character in the sound

author: calexanian
author_rank: Super Contributor
timestamp: July 05, 2016, 08:06:31 pm
content: Looks pretty standard for a guitar amp. I got my start repairing them. I would first reflow and reflux the board and deoxit the pots and jacks. That takes care of the great majority of problems. Occasionally you will get actual breaks in the traces on the PCB because guitar amp manufacturers are notorious for using the lowest cost board houses. That design is pretty standard.

author: rhdf
author_rank: Contributor
timestamp: July 05, 2016, 09:26:16 pm
content: Well, The PCB looks "decent" Seen worse. But the fabrication looks.. well.. cheap. Notice the caps in one of my pictures in the tread-startAlmost every single one is mounted ~.5 inch above the pcb, and its not because the holes are to far apart to manage to get them sit properly (in most cases). Then we have this "beautiful" method to connect wires to the board. A big blob of solder.One thing that really made me thinking:There is a "mod-wire" going from the ground at the input to a small ground "island"The thing is that these 2 points are already connected with jumpers (see red markings on attached image)why? Did this point of the circuit need extra good connection to ground? That "island" connects to C13, VR2 and pin5 of U2 (see linked schema)If so, why not connect the wire to the central grounding-point (blue ellipse) witch is closer?The main reason for taking it apart was to solve the hum (wich scared the sh**t out of my 2 year old son when he switched it on) and the scratchy pots. But to solve that I had to tear it down in a somewhat destructive way.That's why I looking at simply build a clone, with some potential mods (new config of the tone-stack witch I never liked the sound of anyway)

author: calexanian
author_rank: Super Contributor
timestamp: July 06, 2016, 10:36:12 pm
content: Single sided boards are notorious for stress cracking the solder joints. Typically you can just re flow the whole board and it fixes them.

author: rhdf
author_rank: Contributor
timestamp: July 30, 2016, 12:16:33 am
content: One step forward X steps back...Mains part doneInstalled a IEC-connector with built-in fuseholder and replaced the powerswitch with a 2-pole-switch and added shrink tube to all connectionsReplaced almost all electrolytic caps, 2 2200u, 2 100u and a bunch 4,7u. didn't have a 15u so i left that one.Reassembled everything to to a test..switched it on and... HUUUUUMMMMMM.....This time with a bit of smell... oupsie!!!Did I forget something? Anything connected backwards? (for a moment I feared I burned the reverb-tank)Started to measure stuff and realize I have 35V DC on the speaker-out.... Baaad. Ok i planned to replace that speaker sometime in the future but not now. The speaker might be toast, measures ~6 ohms now, i think it's supposed to be 8ohm since there is a external speaker output rated for 8 ohmSo.. either something was broken from the beginning (same loud hum) or I managed to break something in the process of tearing this piece of "built in the USA" crap down to even be able to fix some bad solderjoints.I tested the line out and everything works, no loud hum. I left it on for half an hour and tormented my dog with some "sweet" guitar playing with line out connected to my "test-amp" (dumpster-found home-stereo)The only thing I noticed when sniffing/feeling around is that one of the main filter-caps got a bit hot, not much but clearly noticeable.So obviously there is something broken in the output-stage that creates hum AND 35V DC on the output.The question is what. Sure I can replace everything it's just some trannies, resistors and random caps...To make things interessting the headphone-jack is fed from the power-amp, not via a dedicated headphone-amp I'm glad that I didn't plug in my headphones in this..

author: rhdf
author_rank: Contributor
timestamp: August 01, 2016, 11:09:39 pm
content: Update:Dis some (more) googling and found that most solutions to hum and DC on output is to reflow solder joints especially around filtercaps and input-jack. Well, I gave it a try, rechecked the board for ugly looking joints. Obviously the jacks and all the wires where hand soldered since they had a lots of sticky flux-residue around them. So I sucked all the old solder off and re-soldered them.Reconnected everything and did a quick test. Voila! No loud hum, No smell. Still got some hum left, especially when i turn the reverb up,Guess its the transformer (which hums by itself) that generates it.Now I at least got a decent working amp. Next question is if I shall build a new powersupply for it or simply build a rack-mount "clone" without power-amp. The latter seems to be more fun since I then can incorporate some "crazy" ideas and get rid of some stupidness of the originalAs an example, the clean channel and the overdrive are 2 separate things, so far so good. But the overdrive has 2 controls "Drive" and "Vol. 2"With Drive at 0 you get.. 0 sound, not clean sound with a very very small amount of overdrive (might be a fault in my amp, but its how it worked as far as I remember). I want 1 volume and one drive, thats it.Another crazy idea is to put everything under digital control. The amp has this nice "OVC" "overdrive voicing control". Simply a array with 8 caps and a dip-switch so you can control the character of the sound. Why not make this programmable? When I was using this amp (half a life ago) I could spend hours fiddling whith all the combinations of settings.Ok, I could simply walk down to my local music-store and buy something like a Line-6 POD, but where's the fun in thatNext level of crazyness is to build it as euro-rack modules.

author: CJay
author_rank: Super Contributor
timestamp: August 02, 2016, 11:00:14 am
content: You might not actually have a fault causing the hum, it looks like it's got a springline reverb which are dreadful for causing hum, I think it's 'just how they are, try grounding the wiper of VR7, reverb level.

author: rhdf
author_rank: Contributor
timestamp: August 02, 2016, 01:42:00 pm
content: Yes, The reverb is probably the source here, or at least a big part of it.If I disconnect the tank and turn up the reverb-level I get more humOne thing with my amp is that there is 2 resistors missing (as mentioned earlier, there is not even a place for them on the pcb)These are R60 and R61. both seems to connect the reverb in/out to ground.I noticed when testing the amp outside of the chassie that if I touched some of the pots I got some hum.I don't know Why, but obviously there is some kind of electric connection between the shaft/bushing and the circuit. Back in the chassie that hum was gone. So connect them to "Mains Earth" or at least connect them together solved it.

author: CJay
author_rank: Super Contributor
timestamp: August 02, 2016, 03:05:38 pm
content: It's a pretty awful design, DC coupled and no bias adjustment, no bias tracking, very simple and dreadful distortion figures for possibly 20-25W output.Probably bomb proof at the cost of that distortionThe hum disappearing when you link the bodies of the controls is also pretty common on unboxed amplifiers (I've seen is cause catastrophic oscillation on one particularly nasty bastard of a HiFi amplifier, Aiwa I think), it's not great practice to earth via the chassis, specially with a PA amp as controls get abused and the fixing nuts work loose, I'd maybe sand off some of the surface coating and solder a length of tinned copper wire to link the metal bodies of the pots to ground (if ground is connected to 0V)...As for the effect/preamp, it's a rare thing to find a springline these days, they do make a nice sound (ignoring the hum)Having said that, it looks like it'd be fun to tinker with (quite fancy having a go at building a clone and tinkering to improve it), how are your DSP skills?

author: rhdf
author_rank: Contributor
timestamp: August 02, 2016, 04:48:17 pm
content: Quote from: CJay on August 02, 2016, 03:05:38 pmThe chassis are connected to mains-earth (protective earth) And then all jacks (except the headphones) are screwed with a locking-washer to it, so there is where circuit-ground connects to mains earth.. (the later revision seems to have plastic connectors = no connection that way)QuoteWell its an "old" amp (late 80:s/early90:sObviously they have tried to get rid of some noise in that part since that op-amp has 2 100u caps for bypass, The only one having some sort of bypass for the +/-15V.QuoteWell, it started out as a repair, but after seeing the build-quality (or lack of in some places) I started to think that this could be fun to build a slightly improved clone from. Yet to decide if I will build it in the same box (and then have to use the same amount of pots) or simply build it in some other form-factor (rack or desktop) without the reverb and poweramp (or put a modern digital reverbtank in it)My DSP-skills sucks at the moment.