My First Recap - Kenwood KA-7100

Well, to start off with, the folks on AK have been so helpful, it has motivated me to post a learning thread. - Thank you!

Ok, quick story. I have always been a bit of a fix-it guy and have done a number of simple electronics fixes for myself and friends. I've always wanted to step-up and learn some more advance electronics repair skills. I'm not sure I'm quite ready for a re-cap project, but given the circumstances, I think now is the time to jump in.

A buddy of mine asked me to take a look at this Kenwood KA-7100 amp he has had for many years. Looking at the physical layout, my thought is, there's not a lot of caps, they seem to be relatively easy to access and there is no real pressure to get it done at some customer agreed estimate and deadline. So here goes...

When the power switch is thrown, I get nothing, no lights, no sound, nothing. I checked the fuses. They appear to be un-blown. I cleaned the fuses and contacts, put them back in threw the power switch and still got nothing.

I put my multimeter on DCV, put the black leg on the chassis and the red leg on each of the output leads coming from the power transformer. Aside from a couple small numerical flutters, they essentially read zero on all 6 leads. Assumptions - the output leads are in fact DCV? As I stated, I am a beginner. Is there a recommended test for power transformers?

Reading a few threads and articles, I kept seeing "dual power DC". I was thinking the first few times I saw that, it mus have been a mistake, as obviously, there is only one power transformer. Then I read a little more and discovered the transformer is a "dual wrap" transformer providing a pseudo dual power output. I have some opinions on the validity of that design being a "dual power" system. That's for another day.

OK, so there is the basis for the project. Questions would be such as, is the simple test I performed on the output leads a reasonable test for the condition of transformer? What would be the nest step? I can't imagine addressing the transformer first, was some how wrong. I need power to test anything further, right? I have a desktop DCV variable power unit. Would I be able to hook it up to the transformer output terminals to power up for additional testing? I don't have a variac (which I assume is a particular make). In reading and learning, it seems to me to be a required tool for the serious electronics tech guy. I have pictures referenced in the post, but I'm not sure they are going to work. Still learning the ins and out of AK post.

Kind regards,
Steve

sstudley said:

I put my multimeter on DCV, put the black leg on the chassis and the red leg on each of the output leads coming from the power transformer. Aside from a couple small numerical flutters, they essentially read zero on all 6 leads. Assumptions - the output leads are in fact DCV? As I stated, I am a beginner. Is there a recommended test for power transformers?

Click to expand...

The output of the transformer is AC, just like the input. That's how transformers work.

Still, if the main line fuse is OK, and you get 'nothing', it may be a power switch issue. The power switch is a trouble point for these units, but they usually weld themselves in the 'on' position. Most of the time the switch can be disassembled and made to function well enough to trigger a thyristor, thus saving a lot of trouble hunting a replacement switch.

But the 7100 has only one visual indication of power on, and that's the little red LED pilot lamp. I'd not trust it to be telling me everything is not functioning. Measure DC at the big filter caps (neg to chassis ground). Each of the four big caps should have something like +47V or -47V on them. Check all four. If you have DC voltage there, then time to dig deeper.

Oh, and your pics are a no-show.

ok, at the suggestion of a private email, I measured the voltage of the output leads of the transformer.
They read :
(black + red1 = 6.7 V)
(black + red2 = 5.9 V)
(brown + org1 = 5.9 V)
(brown + org2 = 5.8 V)
after connecting the transformer output leads to the pcb posts
(black + red1 = 2.8 V)
(black + red2 = 3.0 V)
(brown + org1 = 3.0 V)
(brown + org2 = 3.0 V)
That seems strange to me, but I don't know what the appropriate reading should be.

In an effort to rule out the power switch, I read the voltage on the switched ac power socket. It read 123V with power on and 60V with the power switched off. That seems strange to me. Possible short?

I then did what you, echowars suggested. The reading in ACV was negligible at all four big caps, 2-3VAC.

For grins I threw the meter on the fuses. F1 = 88 VAC, F2 52VAC and F3 and F4 were basically nothing, 2.4mVAC.

BTW: All those readings are in AC.

This is attempt at linking a pictures... this is just a test...

EchoWars is helping you now, please disregard my pm's. Don't confuse the thread by reporting on stuff that EW didn't ask for. And please go back and measure DC, not AC, on the big filter caps as described...

EchoWars said:

........................

Measure DC at the big filter caps (neg to chassis ground). Each of the four big caps should have something like +47V or -47V on them. Check all four. If you have DC voltage there, then time to dig deeper.

Oh, and your pics are a no-show.

Click to expand...

oops! Sorry, the DCV was zero for all 4 big caps.

Where is the transformer?

The black square thing in the bottom right of the following pic? It's a somewhat critical part.

I have it, it's on my desk. I removed it for testing.

Anything you needed to do could have easily been done with the transformer in place. And cutting wires is never good...at least in the secondary side of the t-former, you couple have desoldered the wire-wrap pins and made a clean job of it.

EchoWars said:

Anything you needed to do could have easily been done with the transformer in place. And cutting wires is never good...at least in the secondary side of the t-former, you couple have desoldered the wire-wrap pins and made a clean job of it.

Click to expand...

Yes they enhanced a lot of wire wrap pins and at times i do have to cut away those bundles inorder to access the boards for a rebuild.

Probably after it i will wrap a few round and solder it for better connection.

well I was convinced at the time I would need to replace the transformer. And cutting the wires, even though they just slide off the posts was just a simple way of noting which wires went where. I also noticed there was some rust around the top of the transformer. I wanted to sand blast the top and re-paint it, if I were to use it again. I have a part donor unit coming from leesonic.

Can anyone tell me where I find the specs for what the various components should read? Do you guys just know from experience or is that something that can be read from the schematic?

I took apart the transformer and sandblasted the top and bottom, primed them and put down a first coat of chasis black. I figured while I'm waiting for the donor unit to arrive and would do a little sprucing up. Once I'm done with that, I was going to desolder the filter caps and the output transistors. I used leesonic's parts list for my mouser order.

Kind regards,
Steve

sstudley said:

Can anyone tell me where I find the specs for what the various components should read?

Click to expand...

Like what?

Do you guys just know from experience or is that something that can be read from the schematic?

Click to expand...

Mostly, I suppose. But your question is too general to give a meaningful answer.

yes, it is a very general question. my focus right now is on the transformer. What should the output leads, from the transformer, read? I see 6 wires, black, brown, 2 red and 2 orange. Looking at the pcb, where the post land, it appears to me (assumption) that the brown lead and the orange leads represent one side of the power out and the black and 2 reds represents the other side.

thank you for your help!
Steve

Take a look at the schematic and see if that doesn't answer your question.

It does indeed!

Trasformer Input/Output Value:
Input:
RED = 120V
WHT = N
There are 4 input leads to the transformer, YEL, WHT, BLU, & RED. Two sets of 120VAC+NEUTRAL?

OUTPUT:
ORG = 33V (5)
GRY = 0 1.4A
ORG = 33V (6)
RED = 33V (7)
BLK = 0 1.4A
RED = 33V (8)

So the transformer output leads should have read 33VAC, across BLK+RED, GRY+ORG, yes?
The numbers in the parens are noted in the schematic. Do you know what they represent?

Kind regards,
Steve

I thought the numbers represented pins on the PCB, but checking my amp I can see that is wrong. Maybe they are there merely to help you identify the many parallel lines on the schematic? I don't know. You got the other part right. Note that RED-to-RED or ORG-to-ORG will be double.


http://education.lenardaudio.com/en/12_amps_7.html
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part5/page1.html

Well, if they (the numbers in the perans) aren't used for identification, they should be. My hunch is your are correct.

As an exercise, I wanted to follow the circuits around to the filter caps, noting where the transition from AC to DC happens. I see the circuits through the fuses and the what appears to be (8) rectifiers, De19-De24? At the other side of the DeNN symbols, the voltage changes to -47V (DC?). Would that be correct?

That's right they rectify the AC into DC. Both of the links I gave you have pictures demonstrating this exact topic. On the second link don't be discouraged by the technical explanations on the first couple pages, on the third page you will get to something similar to your amp but without the dual rectifier groups/dual filter caps.

EDIT: where the power is rectified into DC there are two sides, one positive and one negative, and these are called "rails" and in this amp they are ±47 VDC (not exactly, depending on the AC voltage at your wall socket). Also in this amp, each channel has its own separate pair of rails. Many amps do not have this.

Electronic Components - identification and basic functionality. Link on first page to a page about soldering cautions
for different components

Learn About Electronics - components and various circuits with lots of illustrations. The section on resistors is good.


All About Circuits - nicely written text book style that beginners can understand. Covers basic DC, AC, components, commonly used circuits.


How and WHY to Solder Correctly - best 6 minutes on this subject
.