Kenwood 700C Restoration

Duffalora

Active Member
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OK, ready to (slowly) go with the EW-influenced rebuild/restore/recap (http://www.audiokarma.org/forums/showthread.php?t=92741). Any pointers/tips/help greatly appreciated.

I bought this from its original owner at the end of last year. General condition is pretty damn fine. Front & back panels and knobs are basically unmarked. A little dirty (with smear marks) but that'll all clean up no problem.

It powers up but was sold as faulty (crackly on one channel) which is likely to be dirty pots/switches.
 
It was a fine spring afternoon in Auckland so I stripped it down and gave it a wash. Under the covers, it was very clean. I'll do the switches and pots tomorrow.

Gotta love those easy access, modular boards. I pulled them out to get the cap values for the (Panasonic) film caps. All parts are ordered but some are going to take a week or so to arrive.

I found the 10uf Auricaps in Australia and they emailed me this evening to say they've been dispatched (that's quick service!).

Recapped all the bigger values and around 75% of the original caps were out of spec.
 

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Getting there -- just about everything has arrived. I'm still waiting on the 4700uf caps (I've selected Epcos for these).

The wait time was not wasted, however, and I swear that cleaning all the connector pins took longer than the recap!

Anyway, here's most of the caps.


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The R9 resistor on the power supply board looked a little worse for wear.

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Tested fine but it obviously gets stressed in there so I replaced it with a 0.5 watt metal film.
 
Boards back in. Those Auricaps make it look like superchargers have been bolted on!

The Tone Amp board is slightly different to the one in the EW pictures. My one has four 10nf capacitors that were also originally designated as film caps so I changed these out too.

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And the bottom:

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Not yet. The faceplate and knobs are still off so I don't know what any of the controls are set on, and I can't turn any of them. I prefer to leave it disassembled until all is finished because it too easy to scratch/damage faceplates when working on them.
 
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Get rid of the 2SC1451's on the buffer amp. Replace with a decent TO-126 tranny (getting tough to find, but Mouser has some new 2SC3902T devices on order from On-Semi...these and the 2SA1507T should be used to replace any 2SA809/810 and 2SC1451/1452 devices you find in any gear you service).
 
Thanks for that EW. My local supplier has them in stock so have put the order in.
A couple of quick questions for you: Do you normally replace the pots on the power supply board or the relay on the output relay board?
 
The relay that worked as a drop-in replacement has been discontinued, unfortunately. There were also two styles of output boards, one where the relay shorts the signal to ground (rare), and one where the relay connects the output to the RCA jacks (common). In any event, I've not yet researched a replacement.

I usually shoot the trimpots on the PS board with Faderlube, and if they appear to do the job OK, I let 'em stay.
 
The main caps are now in. The existing wire connecting the two caps fitted straight on with no modification.

To connect the other two wires, I've soldered speaker spades onto the caps (crimp the spade onto the terminal, put your iron on the outside of the spade and heat until solder flows down the middle). Then crimp and solder on the wires using the same technique.

This provides a good solid connection (and the spades stand no higher than the original wirewrap spikes). They also work well when you need to have more that one wire per terminal.

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Dammit. The 2sc3902T trannies have arrived as replacements for the 2sc1451 but they aren't suitable. Pin arrangement is wrong and pins are too big for the existing holes.

The straight line layout on the boards requires a E-B-C transistor. I was wondering if the 2n5551 (which has the right layout) is a good substitute here.

2sc1451 - 150v, .05a, 0.7w, 150hfe, 130mhz
2n5551 - 160v, 0.6a, 1.5w, 80-250hfe, 100mhz

or maybe

ZTX455 160v, 1a, 1w, 100-300hfe, 100mhz
 
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Dammit. The 2sc3902T trannies have arrived as replacements for the 2sc1451 but they aren't suitable. Pin arrangement is wrong and pins are too big for the existing holes.

The straight line layout on the boards requires a E-B-C transistor. I was wondering if the 2n5551 (which has the right layout) is a good substitute here.

2sc1451 - 150v, .05a, 0.7w, 150hfe, 130mhz
2n5551 - 160v, 0.6a, 1.5w, 80-250hfe, 100mhz

or maybe

ZTX455 160v, 1a, 1w, 100-300hfe, 100mhz

You can bend the leads so the order is right. For the hole size, you could widen it with a small bit if needed. I use 1/32 bit when I need do this. Or something like that. 1/16 is too big. I used the ksc3503 transistors here when I did mine and they fit fine, I just had to bend the leads. It can be done neatly.
 
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Yes, that's probably the way to go. I've got a pin vice with lots of tiny drills so can just enlarge the holes a little (done that before). My main worry when the transistors arrived was the fact that the legs on them are thicker and less malleable than the originals so that even if I enlarged the holes, they still wouldn't bend easily into the required shape. I've had another play around with them today and it looks like they'll bend into line much more easily than I thought so I'll put 'em in tomorrow.
 
All in! Transistors were easy. I was slowed down because when I was putting them in on the EQ board, I spotted a couple of resistors that I'd missed earlier that needed replacing, And, of course, I didn't have any on hand (6.8K 0.5w):

resistors.jpg


So, new relay in and onto the DBT. Clicks in a few seconds. Phew! Checked a few voltages round the boards and all looking good, so onto "live" testing. Front and knobs are back on and plugged into the "rack of black":

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Turn it on, slowly turn up the volume and ...

Absolute silence. Nothing.

Double checked all leads and sources. Changed inputs. Still nothing.

Tried turning a few more knobs and then realised I'd put the Tape Monitor knob on wrong -- so instead of being on Source (as indicated), it was on Dubbing B-A.

Set it correctly and there it is -- sweet, sweet music. Both channels working fine. Everything smooth and silent except for the Mode selector (still had a couple of tiny static noises).

So, I've given that another clean (it's good now), and am about to close this one up. I'll post some audio porn later today and some thoughts on performance when I've had a chance to give it an extended play.

Big thanks to EW for showing the way with the mods on these units and for all the additional helpful info. Cheers.
 
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