Motorola SK32W Getting it going again

[Mperron]

Hello all! I have talked about this in another thread but it suggested I start a new one. I have a Motorola SK32W that I inherited when I purchased my house. It sat in the garage here in New England probably for 6 years or so until I moved it back in the house this summer. Not being up to speed yet on old audio equipment I plugged it in and it worked. Even the record player worked. I listened to it for a couple days and then it suddenly stopped. I started to investigate the problem and that is when I found this site. I located schematics and found the 3 amp slo blow fuse. jumped in a new one and of course it blew. I learned that all capacitors should be changed. So I did so. I replaced the can cap with the required elecrolytic caps and all the other "gum drop" ones. Jumped in a new fuse and it still blows as soon as I turn it on. I was told maybe the rectifier could be the culprit, so I removed it and it still blew the fuse. So now I guess I am looking for some more advice on what could be the issue. I don't have fancy test equipment but I do have a ohm meter. I have a basic understanding of schematics and my soldering is getting better!
I am going to post some pics of what I have so far.
Thanks in advance!

 

[cademan]

Before you go to deep into it, do a simple a process of elimination to help narrow things down.

First double check your capacitor orientation and make sure none are backwards.
Remove all the tubes and power up. One of the other tubes could be shorted. It's rare but does happen.
Power up the amp by itself with no tuner. You will have to know which wires to jump in the connector to get the amp to power up on its own.

A meter will tell you if a winding in the transformer is shorted but if you don't understand what the readings mean, you can simply disconnect the power transformer (Make a note of all connections), and set it up all by itself on your workbench and power it up with a fuse. If the fuse blows, transformer is bad.

 

[JamesR]

I don't know I can help but I have a question. I am getting ready to do the same job you are working on. Can I ask where you mounted the 4th capacitor from the can capacitor? I have been really worried about this part of my project since i ma new to this. The schematic says its supposed to have 30/40/40 400V; 100 25V? I see three on top but did you mount the 4th underneath? Maybe the black colored one underneath?

 

[RWood]

Hi James,

That fourth cap in the can is a cathode bypass capacitor for the pair of power tubes in the bass channel. If you look at pin 3 of the bass channel 6BQ5s there is a large cathode resistor the runs to ground. There is likely a wire connecting it to the other tube's pin3. When I replace caps like this I run it from the 'other' tube's pin 3 to ground, ideally the same ground point as the cathode resistor. Wiring it to the other tube's cathode gives it some physical distance from the cathode resistor, which can get pretty hot. Electrically the same though, as the cathodes for these tubes are connected.

Btw, with plenty of room under the chassis there is no reason to mount the main filter caps above deck. Up there, they are a real shock hazard. A few terminal strips mounted underneath and you'll be much safer.

RWood

 

[FlaCharlie]

I don't know I can help but I have a question. I am getting ready to do the same job you are working on. Can I ask where you mounted the 4th capacitor from the can capacitor? I have been really worried about this part of my project since i ma new to this. The schematic says its supposed to have 30/40/40 400V; 100 25V? I see three on top but did you mount the 4th underneath? Maybe the black colored one underneath?

Welcome to AK.

I'm sure won't get a reply from the OP since this thread is over two years old and he quit posting on here a few weeks after he joined.

I'd suggest using individual replacement caps and mounting them all under-chassis. You can just disconnect the original can and leave it in place to retain the original appearance.

The advantage to using individual caps is that it is MUCH cheaper and you can get caps with better specs than those typically found in replacement cans. I suggest buying caps that are rated at 105 degrees for 10,000 hours. Can caps are generally rated at only 85 degrees and sometimes less.

Go with 47uf and 33uf (which is the modern standard) rated at 450v. The 100uf 25v is the cathode bypass cap on the center channel and it can be mounted anywhere convenient. Just don't put it too close to the cathode resistor so it stays cooler. It looks like there are a couple of 8uf cathode bypass caps on the left and right channels that were used in some versions and not others. They're not listed in the parts section. I'd suggest using 10uf parts rated at either 10v or 16v.

Shipping costs for small orders can get expensive. Here's how you can get free shipping:
http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

 

[JamesR]

Thanks,

Im new and didn't realize this was such an old post. I appreciate the advice. My knowledge is limited yet. My plan is to read a lot and study the schematic before I do anything. I do like the idea of mounting the capacitors underneath. Ive already ordered capacitors for this before making my post. I didn't know I should have purchased different rated ones that what is original. I was able to find ones rated to the original rating but the voltage rating may be higher like 600? Antique radio supply sells a can capacitor that has 30/40/40/40 but no 100 25V. I thought about buying that one and adding the 100 uF because the price was about the same and I thought it would be easier but i was not sure if I could leave one of the 40s unused. Probably better to use individual capacitors.

 

[JamesR]

Hey FlaCharlie,

Just a couple questions. I am in the process of replacing capacitors on our unit now. I see on the schematic there is a C14 and a C16 which are 8uf on the left and right channels. Ours does not have these installed. Should they be there? I did have one 8uf in the tuner I have not replaced yet. I had already ordered the capacitors for the can capacitor before posting on here and they are the Sprague 40 a 30 and a 100. The 40s and 30 are pretty big. I can fit them underneath but I also wanted to ask does it matter where the ground point is? I just want them to be not close to anything too hot. I was thinking of shimming the amp up a quarter inch or so for cooling when I mount it back into the cabinet. There are no visible markings on the can capacitor bottom only on the side of it but the schematic and your advice helped with figuring out which ones are which. Thanks for the advice. I pretty much just have these three large ones to replace and one other in the tuner and its on to the next part.