Help finding switches and pots for my KA7100 amp project - Page 1
Forum section: electronics / projects

author: toli
author_rank: Frequent Contributor
timestamp: February 18, 2011, 08:53:33 pm
content: Hi Guys,I've decided its time for me to move on to the project I've always wanted to do, and I need some help. I'm going to give my KA7100 amplifier (Kenwood) an overhaul. I've decided I'll replace all the internal wiring (I have some cross-talk between different inputs so probably shielded wires), all of the electrolytic caps (should I replace the ceramic caps as well?), all of the jack, and switches/pots.First, and ideas as to what else I should replace?The schematics of the amplifier can be seen here:http://img638.imageshack.us/img638/6176/kenwoodtrioka7100.jpgI've opened it up today to measure caps (physical size), and see what else I can find inside, and the switches + pots on the front panel are of a type I have never seen before, does anyone have any idea if I can still get these, and if so, where?Here's a picture:http://img199.imageshack.us/img199/4523/img0247ej.jpg(The switches are 30mmX11mm)And the back side so you can see the position of the solder joints:http://img819.imageshack.us/img819/4821/img0249wx.jpgThank you.

author: Simon
author_rank: Unknown
timestamp: February 18, 2011, 09:04:38 pm
content: hm you might want to look current handling specs of your rectifier diodes or change them too is you see fit. The capacitors will not overload the transformer which will cope with the odd current surge but but may blow the diodes if on start up they draw too much current through the diodes to charge up from zero

author: toli
author_rank: Frequent Contributor
timestamp: February 18, 2011, 09:14:17 pm
content: The Diodes are labeled in the full schematic as: "GP25D or 53V10 or U05C",but I was thinking of replacing them anyway, so recommendations for these are welcome as well.

author: Simon
author_rank: Unknown
timestamp: February 18, 2011, 09:41:40 pm
content: well if your doubling your caps try and increase the diodes slightly, although the more important thing is the peak current capability. Also look out for the capacitor voltages. Old equipment seems famed for using electrolyctics that have a voltage equal to the supply, double check your transformer output and make sure the caps are twice that (the transformer has peacks os 1.4 X Vout to start with)

author: toli
author_rank: Frequent Contributor
timestamp: February 19, 2011, 05:02:01 am
content: The transformer is rated at 33V - its actually 66V with a center tap (I will measure it just to be sure), the caps in there are 50V, I'm planning on replacing them with 63V caps.BTW, the stock volume pot is 150K. I was planning on replacing it with an ALPS RK27 but could not find it in 150K, just 100K. Will it do or should I look for a 150K from a different company?The stock pot also has a fourth leg going to the loudness control, I'm not familiar with this, do modern volume pots have this as well?BTW, I went through the Mouser catalog last night (I have a printed version at home so it easier than online), but could not find any switch or pot of the type I need. I would highly appreciate it if someone could help me find replacement items for these.

author: Simon
author_rank: Unknown
timestamp: February 19, 2011, 08:20:17 am
content: yea to be safe go for 63V caps, the pot impedence should not be a huge issue but I've never heard of a 4th leg, maybe best leave it alone

author: toli
author_rank: Frequent Contributor
timestamp: February 19, 2011, 09:09:41 am
content: Hopefully someone else can help understand the fourth leg. If not I'll try removing the pot and measuring it out of circuit to see where exactly the fourth leg connects.At the moment the main issue is the switches (the pots I can connect using wires, but the switches are mounted to the PCB and that's what keeps them in place. The stock switches are problematic, because of their old age the don't always make contact and I need to play with the until a good contact is made on both channels.BTW, I've checked again, the diodes inside are GP25D. I could only find this:http://www.alldatasheet.net/datasheet-pdf/pdf/62149/GE/RGP25D.htmlThey are rated at 2.5A average, and 100A surge current. I was first thinking of some MUR820 with a 47-100pF cap in parallel to each of them but the MUR820 are TO220 and the holes in the PCB are simply to far apart (about 20mm).

author: Simon
author_rank: Unknown
timestamp: February 19, 2011, 09:56:17 am
content: just get some bog standard 10 amp diodes, not sure what size your current ones are but if they are the next size up from the 1N4007 diodes you can get 10A diodes in the same pakageWith the switches have you tried spraying some WD-40 into them ?

author: toli
author_rank: Frequent Contributor
timestamp: February 19, 2011, 10:03:34 am
content: I've used some contact cleaner more than once, but it doesn't solve the problem.

author: djsb
author_rank: Super Contributor
timestamp: February 19, 2011, 07:24:59 pm
content: It's a lot of effort to go to for an old amplifier. Why not build something from this page instead. May end up with a better sound and have the satisfaction of having built it yourself.http://sound.westhost.com/projects.htmorhttp://www.passdiy.com/projects.htm(a bit more advanced but very simple circuits).or are you wanting to gain confidence first by working on your amplifier?David

author: toli
author_rank: Frequent Contributor
timestamp: February 20, 2011, 06:46:07 am
content: a. I really love this amp.b. It sounds great.c. Its easier to replace caps and a few other parts than build a new amp.d. I've build a couple of projects already, and repaired a couple of stuff too, the KA7100 project is something I've been waiting for for a couple of years now.

author: timd
author_rank: Newbie
timestamp: April 22, 2011, 05:33:30 am
content: Quote from: toli on February 19, 2011, 05:02:01 amLooking at the symbol is it maybe dual-wiper, or just a second terminal to the wiper?

author: toli
author_rank: Frequent Contributor
timestamp: April 22, 2011, 06:29:04 am
content: I've actually managed to get the right pot for the volume with a loudness tap out. THLaudio carries this item from ALPS. They only have a 100K version and not 150K like the stock, and the loudness tap is about 20K from ground as opposed to 50K in the stock, which will probably have some influence on the sensitivity of the loudness control, but it should work fine any way.