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Kenwood 700M restoration

DanTana said:
So, let me gets this correct, putting say a .01 uf 100v cap across the + and - terminals of the power supply caps will effectively keep ESR down to very low levels? I would imagine then using a good metallized polypropolene cap is the ideal choice in this situation? I think I would like to try this on a couple of my amps and see what the results are.
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Too small. A 1µf would be a minimum. I'd suggest a 2.2µf cap. Rated for 250V.

And a metal polypropylene is fine here. Certainly you can get away just fine with a cheaper metal polyester.

The Panasonic ECQ-E(B) is a 250V metal polyester. The Digikey PN for a 2.2µf is P10983-ND, and they are $1.31 each, or 10 for $9.84.

For contrast, the cheaper of the Panasonic metal polypropylene, the ECWF(L) in a 2.2µf 400V are $3.00 each, or 10 for $25.76. The premium metal polypropylene, the ECWF(B) in a 2.2µf 250V is $4.60 each (!) or 10 for $39.39.

These are not 'really' signal path caps, and the expensive ones are really unnecessary unless you just have cash burning a hole in your pocket. The metal polyester caps are peachy here.
 
gonzothegreat said:
Sorry if I'm annoying anybody with this thread. I'm posting multiple daily updates with every bit of progress just to try and inspire somebody else to rescue an abused amp, a wrecked receiver or trashed tapedeck. Mind you EW provided me with a lot of mods he performed on his but I don't think this would have been impossible for most AKers.

More importantly, I have an amp that's impressive in every sense of the word. It has a certain physical presence with the backlit blue-green VU meters. It just screams quality. Machined metal knobs and faceplate. The switches and pots click and turn with a very solid feel. I think I'm in love...

I replaced the factory bias pots and set the new ones to the same value. In both channels, the bias was at least 12mA low. :yikes:

My brother stopped by today and I tried to get him to put down his iPod and listen to something he doesn't have to stick in his ears. He barely turned the volume up above the normal TV volume. Oh well...

I just realized that this wasn't as expensive a task as it could have been. Others might have waited for a working one to show up but look at it this way. The caps almost certainly would have needed replacement, the OnSemi outputs are far more reliable and the input diffs would have drifted on any "working" unit from the average ebay seller. Besides, with all the mods EW suggested, she probably beats the original factory specs. Speaking of which, I need to print out something EW sent me. I'll have to do one last photo when everything is buttoned up.

One last question for EW. In one of your emails you suggested bypassing the filter caps on the high current supply with a Panasonic ECQ-E(B) metal polyester in about a 4.7µf 250V size, #P10987-ND. Exactly what is the benefit of this mod?
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You certainly haven't annoyed me with this thread. I've found it informative & inspiring. You've done a real nice job on that amp and it's always nice to read Echowars messages of guidance - we're real lucky to have someone like him on this forum who isn't afraid to share his knowledge. :yes:
 
Way back in July, I *finally* replaced the input diff pair as per EW's sagely advice.

Before transistor replacement
Left: 9 mV
Right: 44 mV

After replacement with new Zetex 795a (matched within 5 points on a POS do-everything DMM)

Left: 3 mV
Right: 5 mV

Far better than I hoped it would be. I put off doing this for a *long* time because I wanted to see if I could get a local place to run a nice complete distortion analysis before & after to see if we could quantify the improvement most of us hear. Then I decided to leave that for somebody with more time on their hands (and better gear than I have)

One tricky part about the replacement of the original 2sa810 trannies was the pinout for the zetex units messed with my brain. Specifically, my ancient brain kept thinking the labeled side of the Zs was the wide side. Had I installed like that, no telling what would have happened.

Finally a big fat razzie to the guy who laid out the artwork on the driver pcb. Why the hell did you use some little semiconductor shorthand glyphs to denote transistor leads instead of the perfectly usable B-C-E convention? OK, a short line and a dot is the collector and the long line is the base? Oh, now *that's* intuitive!
 
I found some at Mouser...lemme see if I can find a part number.

Edit: Made by Kobiconn. Mouser #161-0715-E. Polarized, and friction-fit via plastic spring-loaded fingers. As opposed to the original phenolic held in with two screws. I mount the new ones in, then put the screws in on either side (with the nuts in back, of course) to cover the unused holes.
 
Ho yeah! My technician made a refurbished of my 1th 700 M with this tread 3 years ago.
And he did a great job and a crazy refurbished on my parts demands !
Onsemi, Mundorf cap. Mundorf filtaration with bypass…. was a good choice

dc ofser 4mV,
I’ll
said 0 mV kick ass ! (the Fluke is at 2-4mV when is not connected ;)

Just found two more 700M waiting the bench but more normal refurbish
 
Nomademix said:
Ho yeah! My technician made a refurbished of my 1th 700 M with this tread 3 years ago.
And he did a great job and a crazy refurbished on my parts demands !
Onsemi, Mundorf cap. Mundorf filtaration with bypass…. was a good choice

dc ofser 4mV,
I’ll
said 0 mV kick ass ! (the Fluke is at 2-4mV when is not connected ;)

Just found two more 700M waiting the bench but more normal refurbish
Click to expand...
Glad Echowars' wisdom continues to help others. Congrats on getting that 700M running. IIRC there were PSU caps I left in mine that EW later suggested replacing.
 
Late 2027, still referencing this thread myself for an upcoming 700M rebuild. Shout out to EW for their contributions, wherever they might be.

-0.6v to -0.8v present on the right channel, merely 42mV on the left. Working, but not healthy.

All electrolytics, several other caps, resistors, relays, trimmers and more planned. Transistors, likely, but after in-between testing. Just from removing the right driver board, I already found conductive glue and heavily tarnished pins. No blatantly bad electrolytics as of yet, but confident I can exceed their specs.

Cleaning the male pins, won't be an issue. I've got several options. Abrasion, ultrasonic, chemical. The female end however... We'll see. Don't want to use the standard DeoxIT and countless mating cycles at risk of wearing them out. Thinking miniature brass pipe cleaning brushes, if I can find some. Would like to install new terminals, but the wires are rather short as-is.

Servicing this guy isn't as straight-forward as the P-266 thus far, but close.

To those reading, I struggled for months to dig up a free service manual - it had to be out there. Well, it is. Archive.org, shared by Sketch The Cow. Despite that, searches refused to pull it up. I've since reposted it on some expansive Russian repair site I discovered - loads schematics and much more (they had an unobtainium Toshiba SC-665 SM I was seeking, so in return I shared the 700M SM which they were missing). Credited Sketch, but they removed that part.

The last rebuild was reviving a dead Carver TFM-25 (PITA) with phenomenal results (Polarizing amp. I'm a fan, least of mine). Got even higher hopes for the 700M. If I remember, I'll try to post updates or least the final result for those in the future. Lots of projects.
 
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