Kenwood KA-4006 "new to me" not working

[Oilmaster]

Nice job ! Yep, I did similar measurements on resistors of several 70s Kenwood and Sony amps, and they most often of very good quality. Only the ones emitting heat get worse over time. I concluded that not many resistors need replacement. However, now you are on it, I would replace the ceramic disc capacitors Ce7/8/9/10 by decent NP0/C0G multilayer caps. The ceramic discs have a very wide tolerance and are very shifting value by little temperature variations. Notably Ce9/10, because they keep the amp stable (Miller caps, or Cdom caps), and equal stable values between the 2 channels keeps the stereo image better.

 

[roger2]

Oilmaster: thanks for your input and your suggestions make sense. But at this moment I am preparing two more posts to document other component changes already done. Then, before any further changes I need to do some testing. But stay tuned, I may have some questions regarding those ceramics later.

 

[roger2]

amp board component replacements - part 2

capacitors

All electrolytics on the amp board have been replaced. My first choice was Panasonic FM, but Digi-Key had only two of my needed values in stock. Second choice was Nichicon PW and DK had all needed values so the bulk of the new caps are from this series. One bi-polar cap required got a Nichicon SU. And in the amp input locations I have temporarily installed Panasonic FC's (1uF/50v) that I happened to have on hand. Those will later be replaced with polypropylene. < EDIT: main amp input caps have been replaced with Panasonic polypropylene as detailed in this thread: EchoWars' KA-7300 cap mod and my KA-4006 >

As with the resistors, I measured all caps, old and new, with the B&K LCR meter. I checked both the capacitance (uF) and the dissipation factor (DF).

Re uF: The old caps measured from approximately -4% to +9% higher than nominal. Most fell within the +1% to +5% range. The new PW's measured from -9% to +3% with an average just below nominal values. The single SU cap, nominal 100uF, measured 92.3 .The 2 FM caps both read around 4% below nominal value.

Re DF: The old caps on average read higher than the Nichicon PW's (lower is better for DF). In two sizes, 220uF/6.3v and 47uF/10v, the old caps read notably higher DF than the the new PW's. Possibly a sign of the old caps aging. The 2 Panasonic FM's also had notably better DF readings than their older counterparts, but this might be attributable to the fact that FM is low ESR by design.


transistors

Input differential pairs (Qe1/Qe3; Qe2/Qe4) were replaced per EchoWars' suggestion (post 36) with KSA992FBU. I bought enough to match all four at hFE of 479. hFE values of the original 2SA620's were Qe1-209/Qe3-229; Qe2-225/Qe4-271. If the DC offset requires further adjustment I will use the method suggested by ecluser (posts 26, 28).

Qe25, originally a 2sc1416, was replaced with 2sc1845F. I replaced this transistor merely because the outer metal case was showing rust. Unnecessary? Future KA-4006 rebuilders (if any ) note that the the pin-out for Qe25 on my PCB is EBC instead of ECB. I had to carefully spiral the B and C legs around each other as shown in pic below. In the pic it appears that the legs are touching where they emerge from the body of the transistor, that is an illusion. There is good spacing between the legs.

Qe26, 2sc1213A, was replaced by the same part number still available at B&D Enterprises. This transistor is part of the relay control circuit (subject of my next post).

 

[Oilmaster]

Future KA-4006 rebuilders (if any ) note that...

I've got two of them in my stock pile.... so I follow your findings.... keep going

 

[roger2]

relay control circuit modification

As suggested by ecluser, post 49, and further discussed in posts 57 and 58.

As seen in Pic 1 below, left to right:
- new Omron MY4-02-DC24 relay, 24v DC (same part# as original relay)
- new 2sc1213A transistor, Qe26 <schematic attached to post #77 on page 6>
- new 6.8k resistor, Re58
- circuit modification a 2w, 360 ohm resistor replacing 11v zener diode De8
- (also seen in pic - new FM and PW caps)

And in pic 2:
- circuit modification a new standard diode (1n4002, 1 amp 100v) attached in parallel with the relay coil

calculations
< if I am off base here somebody please let me know>

The original De8 zener dropped 11v from the 34v rail so the original circuit was giving 23v to the relay coil.

In making a decision as to the size of the resistor which would replace De8 and also determine the voltage applied to the relay coil I considered these factors:
- the original coil received 23v (seems too close to max to me)
- the relay datasheet has a spec called " Must Operate Voltage" stated as "80% Max". This I interpreted as .8 x 24v = 19.2v, and also made the assumption that the coil would operate correctly at any voltage above 19.2v. Is this correct?

So I played with some numbers and settled on 360 ohms for the resistor to give the relay coil around 21.72v.

The coil's DC resistance, according to the datasheet, is 636 ohms. Rail voltage is 34v. This gives:

- circuit total resistance = 636 + 360 = 996
- total amperage of this circuit = 34/996 = .0341 amps
- voltage drop across the resistor = 360 x .0341 = 12.276v
- power dissipated in the resistor = 12.276 x .0341 = .418w
- voltage drop across relay coil = 34 - 12.276 = 21.724v

Based on the these calculations, I may have gone a bit overboard in selecting a 2w resistor and (especially) a 1 amp diode. Better to err on the side of too much capacity than too little I guess.

<note the pic shows a 390 ohm resistor, it was later changed to 360 ohms>

 

[roger2]

there is a problem...

Before describing the symptoms, I have triple checked the following:
- solder joints all good and no solder bridges
- all of the new components were tested prior to installation
- orientation of all caps, transistors, and the new diode all correct
- no neighboring component legs touching
- new relay tested OK on power supply cap (with resistor in series)

Is it possible that a problem could have developed from either of the following?
- heat damage to an old diode or transistor when reflowing the board
- repetitive repositioning (i.e. flexing) of the PCB caused the lead of some component to break firm contact within its body

In any case, I am at a loss as to how to trouble shoot :sigh:



The symptoms:

After the new work (posts 60, 63, 65) the system has been in one of two states:
(1) "relay open"
(2) "relay closed"

When in the "relay open" state the following all apply:
- speaker relay contacts open
- DC offset (from power transistor emitter resistor to ground) high in both channels. Ranging from -76mv to -117mv
- Bias (voltage measured across driver emitter resistors) is unbalanced within each channel. For example, at one reading the voltage across Re42 was .191v and Re44 was .503v, both in the left channel. Right channel readings similar.

When in the "closed relay" state the following all apply:
- speaker relay contacts closed
- DC offset low and stable in both channels. Eg 5mv/3mv
- Bias voltage readings almost identical across driver emitter resistors within each channel. One reading, for example: Re42 .341v and Re44 .343v
- listening through headphones sounds very good, no noise


Then there's this...whenever the system is working, in the "closed relay" state described above, physical movement of the PCB causes the system to revert to the "open relay" state. Even a slight movement.

I should explain that the PCB is floating suspended by 9 wires attached by wire wrap connections. When the heatsink is removed the PCB can not be firmly mounted to the chassis. The combined strength of the 9 attached wires is greater than the PCB itself, so I suspect that there is some flexing of the PCB each time it is repositioned.

I have performed some troubleshooting in the form of replacing components one at a time and recording results. I have been unable to make any firm conclusions. As this post is probably pretty long already I will refrain from posting my troubleshooting efforts unless requested to do so.


Where do I go from here?




EDIT: when I made this post originally I forgot to mention that the output transistors are removed from the amp

.

 

[ecluser]

...- Bias (voltage measured across driver emitter resistors) is unbalanced within each channel. For example, at one reading the voltage across Re42 was .191v and Re44 was .503v, both in the left channel. Right channel readings similar.

When in the "closed relay" state the following all apply:
...- Bias voltage readings almost identical across driver emitter resistors within each channel. One reading, for example: Re42 .341v and Re44 .343v
...I should explain that the PCB is floating suspended by 9 wires attached by wire wrap connections. When the heatsink is removed the PCB can not be firmly mounted to the chassis.

If you have ~400mV across Re42 and RE44, this is too much. You should have ~25mV at most.

Do you mean that the power transistors are not heat sinked? What kind of thermal feedback do you have with this set-up, between the bias stage (Qe8, THe2) and the power transistors?

 

[roger2]

Neither the heatsink nor the power transistors are installed. Sorry I did not make that clear.

This amp does not have a removable bottom cover. Access to the solder side of the amp board requires removal of the heatsink (and power transistors). Yes, I had power transistors installed briefly for testing (post 47), but then removed the heatsink and power transistors again to install new capacitors and the other new components as described in my posts #60, 63, and 65.


Everything described in post 66 was without power transistors installed. This is similar to earlier in this thread when you instructed me to do several tests, including listening with headphones, without power transistors installed.

I was probably incorrect to refer to one of my readings as a "bias" measurement.

 

[ecluser]

So, your voltage measurements are across Re38 and Re40 (not Re42 and Re44) ?

I think that your protection circuit (relay) is reacting to the small DC offset that you have sometimes. The question is... why do you have this offset?

The problem seems to affect both channels at the same time. Can you monitor the voltage at the +14.0V line (De5) ?

 

[roger2]

No, the voltage reading I referred to is from Re42 and Re44, the driver emitter resistors in the left channel. Right channel readings are similar. I was duplicating the measurement you instructed me to do in post #16.

..... Measure the DC voltage drop in the 330 Ohms resistor (one X07-1280-12 board this is Re38, and on X07-1350-10 board this is Re42) and turn the bias trim pot for the minimum voltage. Once the power transistors are installed you will need to readjust the bias for the correct value.
.....

Similar voltage readings across these resistors did not seem to concern you earlier (see post #42) when the power transistors were not in the circuit. I only mentioned it in post #66 because it was unequal across driver emitter resistors in the same channel.

Also, I want to be certain the we are looking at the same schematic, board X07-1350-10 from the 2nd page of the link you provided. Further, my PCB is (factory) altered from X07-1350-10 to X07-1350-12 by some component changes also noted in the parts list on the 2nd page of your link.

I mention this because it may affect some of the voltage markings on the schematic. For example, on the schematic the cathode side of De8 is marked 14.0v because that version of the board used a 14v zener whereas my amp came originally with an 11v zener in the De8 location.

----------------------------------------------------

De5 readings:

anode side reads 10.7 v to ground
cathode side reads 24.6v to ground

(is it odd that these do not add up to 34v which is what i get across the power supply caps?)

While taking these readings :
- speaker relay is open
- DC offset is -74mv in left channel; -112mv in right channel

.

 

[ecluser]

I am so sorry, I was not looking at the good schematic.

From the schematic, the anode side of De5 is connected to ground (terminal 9 on the right of the schematic). Could it be that a trace is broken? It's not normal that you have 10.7V at the anode of De5.

In principle the voltage drop in Re51 plus the voltage drop in De5 should be equal to the voltage at the positive supply rail (34V).

I am concerned too by the fact that you have unequal voltage drop in the emitter resistors of the driver transistors, and a sudden change in offset, when the speaker relay opens.

 

[ecluser]

Inspect the ground connection at terminal 9. If the ground is bad the speaker relay will open, the tail current in both differential stages will be higher than expected and this will change the offset.

 

[roger2]

Inspect the ground connection at terminal 9. If the ground is bad the speaker relay will open, the tail current in both differential stages will be higher than expected and this will change the offset.

Bingo!!

Terminal 9 had the same 10.7v to ground that was found at the anode of De5. That can't be a good thing (although, it is very, very good that the problem has been found)

I jumpered a wire from terminal 9 to actual ground and could hear the relay closing. Given that several components are connected to ground at terminal 9 I think that could explain all of the anomalies that I reported in post 66.

ecluser, thank you

I will investigate further this evening to see which end of the connection is bad. One question: if I need to eliminate the wire-wrap connection is it OK to simply solder the ground wire to the pad on the PCB?

 

[roger2]

Well, my investigation did not take long. The wire is broken where it exits the bottom of the wire-wrap.

Can the wire-wrap pin be un-soldered and removed from the PCB? If so, can I then simply solder the wire to the PCB, possibly through the hole where the pin was originally?

 

[EchoWars]

Strip the wire back 1/2" or so and solder it onto the pin.

 

[roger2]

Thanks EchoWars. That is a better solution than the one I was considering.

.

 

[roger2]

other transistors - any candidates for replacement?

Quick summary of where the project stands:

Only the amp board has received any attention...
- new output transistors installed, amp tested successfully, then outputs and heatsink removed to provide work access
- all electrolytics replaced with FM and PW
- differential input transistors replaced with matched KSA992F's
- bias trim pots replaced with Bournes
- new speaker relay
- speaker relay control circuit modified per ecluser's advice
- intermittent ground wire connection repaired
- DC offset is stable at @ 5mv Left and 3mv Right (when warm)
- headphone listening tests all good

------------------------------------------------------------------------

Now, before I replace the output transistors and heatsink for the final time I would like to know whether replacing any of the remaining transistors would be beneficial. In terms of longevity, the electrolytics are all fresh as well as many other parts listed above.

Are any of the remaining transistors, listed below, known to be trouble makers? Or simply due to age (approx 37 years) would it be good to replace any of them?

Transistor - Part #
2sc1451 - (Qe5,6)
2sc1746A - (Qe7,8)
2sa733 - (Qe9,10,15,16)
2sc945 - (Qe11,12,13,14)
2sc1212A - (Qe17,18)
2sa743 - (Qe19,20)

NOTES:
1) the 2sc1451's run very warm. I measured (unofficially) a steady 44C and 45C on the surfaces of these at idle. These have a cylindrical heat sink attached.

2) the 2sc1746A's are small metal cans. These are NOT what the schematic lists as the original part. According to the schematic Qe7 and Qe8 should be 2sc1416



Any thoughts? Would it be beneficial to replace any of the transistors in the list above?



<Pictured: amp board schematic, 2sc1451, 2sc1746A >

 

[ecluser]

I would keep it as it is actually. If you change Qe5 and Qe6, your offset may change because it is related to the actual Vbe drop in these transistors. As Bob Dylan said... "Don't think twice, it's all right" !

 

[roger2]

All right then...we'll go with Bob's Wisdom on this one



"Well there ain't no use to sit an' wonder why babe
If'n ya don't know by now"

 

[roger2]

main amp board access

This post (and the next one) will hopefully be useful to future KA-4006 rebuilders and/or novices (like myself) considering this project.

As mentioned previously, this amp has no removable cover on its underside. There are only two ways (that I could think of) to get to the solder side of the amp board: (1) removing heatsink and main outputs then moving board with wires attached, or (2) removing the 9 wire-wrap pins attached to the PCB, in which case the heatsink could ( I believe) remain attached. Pros and cons of each approach:

Option 1 - removing heatsink:
-Pros: can replace smaller numbers of components and test as you go
-Cons: Repeated moving of the PCB stresses the solid core wires at the point where they exit the wire-wraps. In my case I did develop one problem (discussed below)

Option 2 - removing wire-wrap pins:
-Pros: much easier access to both sides of amp board
-Cons: (1) all components must be replaced in one shot with no live testing possible (2) it might possibly still be necessary (don't know) to remove the heatsink to get the soldering iron on the bottom of the wire-wrap pins


I chose option one. I do not know whether it was the best choice.

Pics 1-3: To make it a bit easier to work on the amp board I duct-taped a couple wood blocks on the face of the amp so that I could stand it on its face without affecting the knob shafts and switch levers. With the amp in this position it was possible to rotate the amp board forward to provide better access to the solder side.

Pics 4-5: My preventive measures intended to avoid breaking any connecting wires by providing strain relief. The board was moved dozens of times during the process of locating/removing/replacing components. As a novice, being overly cautious and a bit disoriented, I am certain that I rotated the board many more times than was necessary locating and verifying component leads. The vulnerable area of the connecting wires is at the bottom of the wire-wrap where the wire exits...this is where the wire bends with each movement of the board. Pic 4 is Household Goop (craft glue) applied between the bottom of each wire and a small sand paper roughened area of the PCB. I realize this may be controversial and it certainly is unattractive as well as not original. I was careful not to allow any glue to bridge across any conductors. The glue held surprisingly well and did its intended job better than I had anticipated. Pic 5 is a small tie-wrap around one wire (connected to a main cap) that was moving a lot with each movement of the board

Even though I took preventive measures, I did have one of the ground wires break from repetitive movement of the amp board. Had I not applied the glue I believe the problem would have been more extensive. As it was, the single issue was easily resolved (discussed in posts 66-75).

________________________________________________________________________
EDIT November 2012: further observations regarding working access

1) Main amp board Something that I had not noticed when I posted originally is that the designers of this amp apparently did provide a method of access to the underside of the main amp board, and removal of the heatsink (and TO-3 outputs) is not required.

The front section of the chassis (not sure what else to call it), which is basically a flat metal sheet which, on each end, bends around 90 degrees where it is attached to the bottom part of the chassis. In the stub ends on each side of the front chassis there are slots cut which can be used as alternative mounting points. When fastened this way the front chassis moves forward and up relative to normal mounting. When in this position it is possible to rotate the main amp PCB forward, like I show in the pics here, but with the heatsink still attached. If you have a KA-4006 you will see what I am talking about. I used this method later a couple times later in the project. Access is not as good as when the heatsink is removed, but for small jobs it is a good option. I also drilled out the appropriate holes and used 10-32 hex head bolts with star washers and nuts for greater holding strength.

2) Tone board Access is a pain on this one. Take a look at the pic in post 86. To get to the tone board it is necessary to remove the front section of the chassis. This results in several PCBs, pots, switches, etc hanging loose. These various parts are still connected to each other (mostly) via small solid core wires emanating from wire-wrap pins. These wires will break if they are moved too many times. I had to re-solder 4 or 5 of them to pins. I did get pretty good at making this repair. And to be fair, if I had not revised the tone board several times this might not have been an issue.

To get to the solder side of the tone board I had to pull the board above it, the one with filter/tone/mute/loudness switches, up and back and tape it in that position while working.

3) Power supply board Again, having no removable cover on the underside of this amp makes access to this board difficult. There are wires entering this board from all four sides. To gain access I temporarily removed, on one side, the power supply wires feeding the tone and phono boards and the power indicator light. And on another side removed the wires to the main PS caps.