Amplifier with broken 4 pin pot, can it be replaced with a regular 3 pin one? - Page 1
Forum section: electronics / repair

author: d3vCr0w
author_rank: Contributor
timestamp: September 05, 2020, 01:05:40 am
content: Hi people, I received this week a guitar amplifier (Marshall Valvestate VS100 R) and unfortunately the volume control of the 'Overdrive 2' channel is not working, I noticed that the knob was loose, so I decided to take a look inside and found that it was replaced with another type of potentiometer, one not attached to the chasis by a nut, but insted held in place with hot glue directly to the PCB, so maybe because of that weak attachment it just broke, here is a picture of ithttps://imgur.com/a/6vTxnX9None of the electronics shops near me have the replacement for it (I live in Colombia), and importing it from the USAhttps://www.ebay.com/itm/150471584887would cost me more than 20 bucks (yes, it only doesn't ship here, so I'd have to ship it to the USA and have a friend send it to me from there, so importing it would cost what the pot costs) which is kind of a lot for a single potentiometer, so I'll have to go with a regular 3 pin potentiometer (one of this typehttps://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Fimages.campyent.com%2Fsites%2Fdefault%2Ffiles%2Fuc_products%2Fr-v100kl-m-16.png&f=1&nofb=1) which I can easily get here and is pretty cheap.The problem is that these original PCB potentiometers have 4 pins and the one I would use has 3 pins, does anyone know how would I wire it to the PCB? Here is the schematichttps://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-VS100R-100W-Schematic.pdfand the pot is "VR9".Thanks by the way.

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2020, 01:15:17 am
content: The schematic only shows three pins used so it *should* be possible. A closeup photo of the underside of the PCB round VR9 would help determine how to procede.

author: d3vCr0w
author_rank: Contributor
timestamp: September 05, 2020, 08:04:59 am
content: Hi, here are some pics of the broken Volume potentiometer (yes, the solder points are loose but I need to change it anyway for something stiffer)https://imgur.com/a/bXD2nv4and some pics of the pcb belowhttps://imgur.com/a/NZdh3dn

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2020, 10:30:17 am
content: You can clearly see, comparing the underside with the top side that the pad for the fourth pin doesn't have a track. The remaining pads must be the two track ends and the wiper. I suggest measuring resistance with power off between pins of a similar volume control pot, while turning its shaft to figure out which one's the wiper and then which of the remaining two are the clockwise and counterclockwise ends of the track.You'll need to make a bracket for the new pot to mount it to the PCB. I suggest cutting and bending tin plate (tinned steel sheet, cut from the bottom of a large biscuit tin or similar) to form it. Edges should be folded back and soldered to stiffen it , and tab legs left or thick wire legs soldered on to go through the mounting holes in the PCB. As there are only two holes for mounting tabs , you'll have to design the bracket to hook round the edge of the PCB, insulated from it with Kapton tape or similar stuck over the track that runs there, to get enough mechanical support. Prototype the bracket by cutting and folding thin card and trying for fit, then unfolding it as a template to mark the tin plate for cutting. Don't forget to add on any allowance for bent over edges, mounting tabs etc. that weren't part of the cardboard mockup!Fit the pot, tags up, to the bracket with a nut, (You did check the clearance to the panel so the nut will be hard up against it when designing the bracket?), solder the bracket in place, and run wires from the tags to the PCB pads, taking care to get each track end and the wiper to the correct PCB pad even though that will probably mean the wires cross.I think you've lost the pad from one of the mounting holes, so for that one you'll need to cut a very small rectangle of tin plate, trim the corners and make a slit in it to fit tightly over the leg protruding through the board. Push it over the leg and right down to the board so its in firm contact with the bracket also tight against the board, solder it, then at the end after you've cleaned up the flux residue from the PCB, and thoroughly dried the board, run a little superglue under it to firm up that leg. I'd do that leg first so you don't strain the remaining pad for the other leg.These videos may help you start learning the craft of tinsmithing, i.e. the techniques required to work with tinplate successfully:http://tinplategirl.com/category/videos/