Kenwood 700M vs. the newbie

Tonight I pulled the right amp board back out and checked all the resistors again. I got good readings on all of them. Even the ones that I had initially marked as questionable when read with all the bad caps in circuit.

I decided to turn my attention to the remainder of the diodes since I have them on hand anyhow. Even though they read OK, I changed out all the signal and zener diodes and reinstalled the board. Probably should have done that in the first place anyhow.

It would be nice if I had enough knowledge and experience at this point to be able to take specific voltage readings on the board and deduce more specifically where the issues might be. But I'm well aware that skill level is quite a ways off for me. I've been doing lots of reading, and I'm getting glimpses of theory. But much of the specifics of the circuit function still elude me.

So at this point I'm using what I call the 'poke and hope' method.

Offset now reads half of what it did before, down to -.104vdc. At least I'm pokin' in the right direction. :tongue:

Next I'll change out the drivers. I've already got them on hand. We'll see where this poke takes things. :scratch2:
 
Here's what I've got installed so far.

C5,C14 330uf 25v
C6 100uf 50v
C12 220uf 100v
C17 330uf 10v
C11,C18,C19 47uf 10v

Q1,Q2 ZTX795A
Q3,Q4 2SC3902T
Q5,Q8 ZTX795A
Q6,Q7 ZTX694B

diodes 1N4148
zeners TZX14B

R16 71-RL20S561J 560ohm 1/2watt

I'm also running all the original outputs. They all tested ok with a diode check so I wanted to give them a run.

At this point I'm able to listen to music. :music:
Albeit a bit distorted.

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Voltage measurements won't help much...we're only off 0.100V and that's just not enough to take a measurement here or there and say 'this is good' or 'this is bad'.

Got a small film cap laying around someplace? 1uf would be enough. Replace Ce11 with it (as a test) and see what happens to the offset. If that makes no difference, then I'd be looking closer at some of the ceramic caps (Ce2, Ce3, Ce7, Ce16, etc...)
 
...we're only off 0.100V and that's just not enough to take a measurement here or there and say 'this is good' or 'this is bad'...

Understood. I'm sure I can dig up a small cap and give that a go. I recently bought a grab bag of stuff from a local guy that was retiring. Got a nice soldering station and a well worn Hitachi oscilloscope in the deal. Never used a scope before, but I played around with it a bit and managed to get a signal on the screen. Of course, I'm more like a kid with a fancy toy seeing how there's the whole learning curve that goes along with that as well.
 
Last night before inserting the amp board back in the chassis, I gave the pins another once over polishing. Didn't change any components. Let the amp warm up a while and played a tune through it. Checked the offset again and it was reading -.07v. Today I installed the 1uf cap at Ce11 and the offset still held at -.07v.
 
Hmm...70mV now.

What kind of cap? Film, I hope (the idea here was to eliminate cap leakage as a possible source of the issue...electrolytic's leak current, film's do not).

I'd also thinking of grounding problems. Dirty connector, bad or loose connection to chassis ground...
 
…What kind of cap? Film, I hope…

Whoops! ...And the Ref calls an unintentional foul on the newbie and let's him off with a warning!

Yeah, I put an electrolytic in there. :twak:

I'll remedy that and try it again.

…I'd also thinking of grounding problems. Dirty connector, bad or loose connection to chassis ground...

That crossed my mind as well. I might be seeing improvements in my readings not because of any component change, but simply because of pulling the connectors on and off I'm exercising an iffy connection. Seen it enough in the harsh shop environment when a machine starts having the farts. If no obvious tell-tale, the modus operandi is to pull the boards, clean and reseat. Many times it's the magic pill.
 
There are several places (one of them near the back of the transformer) where the grounds are either secured with a screw or soldered (a screw, IIRC). Needs checking. And look the thing over for more.
 
... (the idea here was to eliminate cap leakage as a possible source of the issue...electrolytic's leak current, film's do not)...

Ok, I think I see what you're getting at. I put a film cap in there and the offset now reads -30mv.

I also went and reseated every ground strap and offset is holding steady.
 
Still scratching my head over this offset issue and looking things over noticed something. The dc offset on the speaker outs starts out at a reading of
-17mv when I turn the amp on cold. As it sits and stabilizes to temperature, the reading ratchets up to -30mv where it peaks.

I figured I'd get out the oscilloscope and play around with it a bit. First I hooked it up to the speaker outs along with my DMM to make sure I was reading the DC scaling correctly. Then I moved the scope over to Ce11 and took the notated readings with the film cap in circuit.

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30mV is a reasonable figure....

Put a NEW electrolytic cap in there and see how it behaves.
 
Yes, I can see we're getting to the point where we're down to splitting hairs here. But I'm a patient person and I'm in no particular hurry to get this done. Just want it to be right. And I so much appreciate your patience, EW, looking over my shoulder on this.


...The only other one that really matters is Ce11. If Qe1 and Qe2 are replaced with some modern high-gain transistors (like the 2SA970BL or the ZTX795A), then even the stock value is more than sufficient. Still, I usually put in a 220uf 16V cap here...

Ok, since I'm replacing it anyhow, I decided to follow your lead and installed a 220uf 10v cap here.

And at this point, I'm quite elated at the numbers I'm seeing. :yippy:

Other than a Bourns trimpot that I've got here, I'm not inclined to mess with the right board any more.


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And a look at the solder joints I'm laying down. Please overlook the pools of flux, as I have yet to clean everything up. I started off using .062" solder which gave me good shiney joints, but a bit bulbous. I switched to .031" that layed down a bit nicer. I feel I'm getting good wetting.

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I use the .031" stuff. Easier to control the amount of solder.

Some acetone and a Q-Tip (actually, a bunch of Q-Tips) cleans up excess flux nicely.
 
Acetone it is then. I'll have to get some.

In the mean time, I took a look at the speaker select switch and removed it from the chassis. Wondering just what the heck was going on with it, I pulled it apart and found that the plastic contact carriers were shattered.

Pictured is the better one of the two. The other came out crumbled.

Until I get my knob project buttoned up, I decided to temporarily bypass the switch and wired the A-speaker terminals direct, just wanna hook up some speakers in the mean time.

Looking at the broken plastic, I can't help but think it sure would be neat to have a 3D printer that's been on my wish list. :scratch2:


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Thanks, EW. I actually already purchased that switch. I got the part number from one of your posts in a different thread.

I do intend to install it. I'll probably save if for the very last. A bit of a quagmire of wires to solder in, so for now I guess I'm just postponing the inevitable.

That cdkands site is very nice!
 
Btw, on the same subject, I have a question about removing the main power switch.

I wanted to unbolt the power switch from the front faceplate in order to drop the faceplate completely out of the way. This would allow me better access to work on the speaker select switch and give me some slack in the wires. The round plunger on the switch, however, does not fit through the hole in the chassis.

My question is how to safely remove this plunger. It seems like it is just pressed onto the end of the switch, but I pulled somewhat hard and I hate to force things. Is there a trick to getting these off safely without undue stress on the switch?
 
The plastic plunger on the switch is too damn easy to break. But you're talking about removing the front chassis piece, not the faceplate...? Faceplate (aluminum) comes right off. And I haven't found a good reason yet to remove the entire front chassis. Elevate and support the center of the amp (I have some heavy foam pieces for this) and take some screws loose and it pivots down very nicely...no need to completely remove it.
 
Yes, you would be correct. I was refering to the support panel behind the outer face plate. I did actually get the panel tilted down. Enough in fact that I was able to disassemble the original switch still in place.

I suppose, at the time, my thought process was to try to remove the power switch and the terminal strip from the panel in order to give me some slack in the leads to the speaker switch. I thought I could get away with unsoldering the leads from the original switch. I abandoned that tact after attempting it, especially with the leads that are snaked through more than one terminal. I pretty much stopped after having to put enough heat there that the insulation started to cook. I ended up just snipping the leads from the switch as close as I could. I imagine at this point I will either have to extend or replace some of the leads to get enough length to work with.

It also occurs to me that asking the expert on an ex post facto basis is probably not the smartest thing I've ever done. :whip:

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