Developing the BOM for a KR-6160 restoration

dlucy

dlucy67 (Doug)
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I've finally pulled the first KR-6160 out of the "to do" stack and started the restoration. I'll share some of what's going on here and ask for some help along the way. After it's done and working, I'll post the results and the BOM for a recap as well as a more-thorough restoration.

This unit sounds pretty great as it is, but it started acting flaky and I took it off the shelf for later restoration. That time has arrived.

IMG_1278.JPG IMG_1296.JPG IMG_1317.JPG
 
There are some oddball parts in there already. This should be fun....

IMG_1354.JPG IMG_1355.JPG IMG_1359.JPG
 
Weird parts, nice pics :)

I don't recall seeing any green colored resistors in Kenwood units. Are those fusibles?
 
Weird parts, nice pics :)

I don't recall seeing any green colored resistors in Kenwood units. Are those fusibles?

I'm not seeing references to fusible resistors or lower-than-1/4-watt ones, so I don't think they are fusible. I'd love to have someone more-experienced confirm or correct this, though.
 
I'll start off with the MAIN amp board:

MAIN-SCHEMATIC.png MAIN-UA1384J-IMG_1337_edit_tiny.png

The transistors first:

The schematic lists Qe1-Qe4 as 2SC693U(E or F), but this unit has 2SC458 :eek: transistors installed in those locations, so I'll put in KSC1845F.

Qe5,Qe6 are listed as 2SC627 in the schematic, but I can't read the label on these. The datasheet for 2SC627 is typical and boring until you reach the hFE of "360 minimum".

2SC627 NPN CBO=200 CEO=200 IC=100mA Pd=700mW fT=20MHz hFE=360

So, these will have to be high output and it took a thorough run through alltransistors to find devices that matched and Mouser to find one that was available and matched or exceeded. I'm going to try ZTX696B instead of a Darlington since I really don'[t know enough about them to select and replace them confidently.​

Qe7,Qe8 are listed on the schematic as 2SC960(I, L or M) and my unit has 2SC484Y installed there. These are complemented by the 2SA484Y in Qe9,Qe10, so a little extra care for this replacement.

2SC484Y TO-39 NPN CBO=150 CEO=110 Ueb=5V IC=1.5A Pd=800mW fT=10MHz hFE=30

KSC2073 will handle those needs well and it has a complement of KSA940, so KSC2073/KSA940 will be the ones for these.​

As stated above, Qe9,Qe10 is marked as 2SA607 on the schematic, but my unit has 2SA484Y in those positions. As stated above KSC2073/KSA940 is a complemetary pair that matches these parameters, so KSA940 is what I'll use for Qe9,Qe10.

Qe11 has a 2SC373 on the schematic, but a 2SC984 on the actual board.

2SC984 TO-1 NPN CEO=50 CBO=50 IC=500mA Pd=350mW fT=100MHz hFE=30/300

Either KSC2690A or KSC2383 should work fine in the Qe11 position.​

The strange gold-legged black-capped transistors at Qe12,Qe13 are 2SC715E

2SC715E TO-92 NPN CBO=40 Ved=5V IC=200mA Pd=125mW fT=100MHz hFE=80

IMG_1359.JPG

It looks like KSC2383O will do fine here.​

Lastly, Qe14 is a 2SC734Y

2SC734Y TO-92 NPN CBO=70 CEO=50 Ueb=5V IC=150mA Pd=300mW fT=75MHz hFE=120

And I think KSC2383O will do fine here, too.​

There are only two sets of diodes actually on the board, so they are easy:

De1,De2 are 1N60 Germanium diodes, so I'll replace them with NOS 1N60 or 1N60P diodes

1N60 Germanium glass diode Vrrm=40V If=30mA Ifsm=150mA​

De3-De5 are strange-looking twinkie-shaped devices that the schematic calls out as SM-150-S. I couldn't find specs exactly for the SM-150-S, but I found several places where SM-150-xx were discussed and it appears these are simple 100 V 1A diodes. If anyone knows differently, please let me know. If that is correct, then they can be replaced with 1N4002, but I like the ultra fast soft recovery UF4002 better.

IMG_1355.JPG

The capacitors are almost all electrolytics, so the replacements are mostly easy: bump up the voltage so the replacements are operating at 60-70% rated voltage, use nothing lower than 35 V (where possible), use film caps (for zero leakage) where possible (4.7 uF or lower) or low-leakage caps (Nichicon KL series) for everything else.

  • Ce1,Ce2 1 uF 50 V --> UKL2A010KDD 1 uF 100 V (Mouser doesn't even stock the 50 V version anymore) or ECQ-E1105KF3 (much larger film cap, but it has no leakage, this one is the smallest I can find, and it will fit... it'll just be ugly)
  • Ce10,Ce29 47 uF 3.15 V --> UKL1V470KPDANA 47 uF 35 V
  • Ce11,Ce12 100 uF 50 V--> UKL2A101KHD 100 uF 100 V
  • Ce13 220 uF 25 V--> UKL1V221KPD 220 uF 35 V
  • Ce15 47 uF 100 V Axial --> TVX2C470MCD 47 uF 160 V Axial
  • Ce3,Ce4,Ce14 100 uF 3.15 V --> UKL1V101MPDANA 100 uF 35 V
  • Ce5,Ce6 33 uF 16 V --> UKL1E330MEDANA 33 uF 25 V

I'm crazy, so I'll be replacing the one ceramic cap while I'm in there

  • Ce7,Ce8 220 pF --> FK28C0G2E221J 220 pF 250 V C0G (NP0)

I'll probably replace all the resistors on this board with metal film ones, but I don't want a bunch of "hey, why would you do that" comments just because I'm OCD and have lots of spares from mistaken orders, so I'll just skip putting those in here.

There are two trimpots on this board, so I'll replace them instead of rolling the dice and having them blow a transistor or more:

VRe1,VRe2 are three-legged, single-turn 10K trimpots so they'll be replaced with 3309W-1-103

This board leads directly to the output transistors and their resistors on the heatsink, so I'll cover those while I'm at it:

The output transistors Q1-Q4 are marked as 2SC1115 on the schematic, but 2SC898 are installed instead.

2SC1115 TO-3 NPN CBO=140 CEO=80 Ueb=5V IC=10A Pd=100W fT=10MHz hFE=30-150

MJ21194G will be a fine replacement for these output transistors.​

Power resistors at R171,R172,R271,R272 and R173,R273 may have been stressed, so they'll get updates, too.
  • R171,R172,R271,R272 0.47 R 2-3 W --> CPF3R47000JNB14 0.47 R 3 W 5%
  • R173,R273 4.7 R 2-3 W --> CPF34R7000FKB14 4.7 R 3 W 5%
The output in the KR-6160 is cap-coupled, so C171,C271 are huge terminal-lead caps installed up top, but I'll cover them here.

C171,C271 output-coupling caps handle the amplified signal just before it goes to the speakers, so these will need to be big (to avoid low end rolloff) and great quality (low or no leakage). Sadly, the KL series doesn't cover this much capacitance at this voltage (need 63 V minimum), so I have to look elsewhere. Nichicon makes plenty of snap-in and leaded caps in this size, so I'll choose some options that claim better audio performance but are roughly the same price as non-marketing-crap ones.

C171,C271 2200 uF 63 V --> UKW2A222MRD or LKG1K222MESAAK 2200 uF 100 V

(If Nichicon made ones that had skull tattoos on the side or little glasses of Japanese bourbon, I'd put those in there instead since they'd be "better audio quality" ;))​

Ok, so what of the above is just dead-wrong?
 
This unit sounds pretty great as it is, but it started acting flaky and I took it off the shelf for later restoration. That time has arrived.
Hello Doug,

I do not know how much experience that you have doing this sort of work, but my suggestion is to first figure out what is causing the flaky behavior before changing all these parts. Just the way that I troubleshoot my stuff and what I see other experts doing. I have seen far too often folks taking your approach with good intentions, just burying them selves alive and end up with a basket case.
Start to figure out if the issue is in one of the channels or both, that way you can determine if it is a common source like a power supply issue.
Determine if it is tarnished switch contacts, dirty pots etc first before you start changing comps.
If you are going to heed my advise at least change the ecaps first before changing out semi's.
Does not hurt to review threads about this unit to determine what has failed and what was done to fix them.
I see quiet a few hermetically sealed bjts, they may out last you and me and the new ones you put for replacement.
I would not call them odd ball parts, just the way that they made them back in the early 70's. Many of those bjts were hand assembled before automated packaging came in the late 70's. Who cares what colour the laquer is on the resistors, every mfg has their own processes, has nothing to do if the comp is work properly or not.
The amp is a fairly simple design, single supply, bootstrapped VAS, quasi-complementary, ecap coupled to load.
Do you have any instrumentation, like a signal gen, scope, DMM? If so use them to measure the performance of each section of the receiver.

Good Luck
Rick
 
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I do not know how much experience that you have doing this sort of work, but my suggestion is to first figure out what is causing the flaky behavior before changing all these parts.

Rick,

Thanks, that's really good advice. You guys here on AK have been a big help troubleshooting issues and getting me through to the right answers.

This particular thread is a "build the big list of parts and modern replacements" so the next person with a KR-6160 has a better start. I've got two 6160's, one of which is working fine and the other is just flaky, so I'm taking this opportunity to tear one of them down completely, build up the list of actual components (not just what the schematic says), compile some photos of the boards and notated pinouts, and put it all back together.

I'll definitely change out all the electrolytics caps as they are far past their life expectancy and I won't allow them to fail and take out anything more expensive. It'll also sound better when I'm done, as I know from all the other vintage receivers I've done so far. The known-noisy transistors (e.g. 2SC458) will get replaced, as will any power resistors near the output transistors.

Your points about troubleshooting and reading other threads before replacing are good and solid. Again, thank you!

-Doug
 
This might be as bit late coming to you, but I am in the middle of repairing a few of these KR-6160s and they all seem to have the same problem of failing driver transistors. I have the service manual and within it gives a service bulletin suggesting to replace Q1-Q4
 
This might be as bit late coming to you, but I am in the middle of repairing a few of these KR-6160s and they all seem to have the same problem of failing driver transistors. I have the service manual and within it gives a service bulletin suggesting to replace Q1-Q4

You're not late at all. I made the mistake of starting several restores at once and I haven't moved forward (much) on the KR-6160.

Youre talking about the 2SC693's on the main amp board? My plans are to replace them with KSC1845's. But I'm approaching this with a replace-most perspective.
 
So, I have an emergency: I'm stuck in temporary housing, I've got the unrestored KR-6160 doing temporary tune-listening duty... and the Amazon just delivered the 40th anniversary remastered copy of Jeff Wayne's 1978 musical War Of The Worlds. So, I have to recap this Kenwood receiver quickly!
 
Preamp board first

It's on the topside of the chassis, mounted by itself, and you just have to carefully document the wires, cut them, pull out the board, replace the componets, then put it back in. No problem.

05 Preamp Board 00 location 01 IMG_7543.JPG

05 Preamp Board 00 location 03 IMG_7545.JPG

05 Preamp Board 00 location 04 IMG_7497.JPG

I don't have time to replace all the components, so the trimpots, non-electrolytic caps, and resistors get to stay... even as ugly as they are an as coated in old rosin or whatever....

05 Preamp Board 01 before 01 IMG_7507.JPG

05 Preamp Board 01 before 02 IMG_7516.JPG

Here's a better view of the silkscreening once the transistors and electrolytics have been removed. Easier to see the component numbers and the transistor pinout diagrams this way. Triple-checked the outgoing transistors with the Peak Atlas DCA75 Pro and the silkscreen on this board are correct.

05 Preamp Board 01 before 03 IMG_7523.JPG

The schematic tells us what should be in there:

05 Preamp Board 02 layout 01.png

A quick sketch from before removing the components reminds me where the polarity for the electrolytic caps are, what my transistor check revealed, and what values came off the board:

05 Preamp Board 02 layout 02 IMG_7583.JPG

The two 2SC871 and two 2SC458 all get replaced with KSC1845FA.

The electrolytics all get replaced with higher-voltage Nichicon KL series low leakage caps (except the one value I didn't have).

05 Preamp Board 03 after 01 IMG_7527.JPG

The trimpot gets a good Deoxit-ing and FaderLube-ing and it all goes back in place. I'll have to locate the service manual procedure for setting the one trimpot. Hmmm....

05 Preamp Board 03 after 02 IMG_7544.JPG

To do list:

List all the caps that went in.
List the caps that should have also been replaced
Point to the trimpot adjustment procedure.
 
Pots (THREE in the tone stack!), switches and idiotic balance slider deoxit'ed and fadelube'd and there is still noise.

Preamp board recapped and upgraded, tone board recapped. Still noise.

Tone board recapped and upgraded. Less noise, but still audible noise.

Driver board recapped. Still audible noise. Sigh. I sure hope it's the output caps. That's next.
 
I sure hope it's the output caps. That's next.

Nope. A standard recap of the audio stages and a replacement of the mostly-likely noise small signal transistor culprits lowered the noise, but it's still there... so a deeper restore is next.

IMG_7672.JPG
 
The recap (and replacement of the small signal transistors) on the tone board went as follows.

The tone board is mounted... a little oddly, almost as if it were crammed in the only almost-available space:

IMG_7545.JPG

It has wire-wrap posts along one edge, but it is buried under a bird's nest of wires... and it has a terminal strip... and it has resistors coming up from the foil side to the terminal strip and to the tone pots... weird like an after thought.

IMG_7548.JPG

IMG_7552.JPG

IMG_7567.JPG

Nonetheless, the board is otherwise normal Kenwood:

IMG_7560.JPG

and follows the schematic well:

KR-6160 tone board.png

I marked the electrolytic cap polarities, noted the pinout order for the four transistors being replaced, and measured their gain.

IMG_7585.JPG

While the board was partly-de-populated, I snapped a photo so the silkscreen was easier to see. The pinouts on the transistors match the silkscreening and the cap polarities were all correct.

IMG_7574.JPG

Put the new electrolytics and KSC1845F transistors in.

IMG_7577.JPG

I really wanted to replace the bad-looking film caps (one connecting to the terminal strip was so old it had broken before i started my work), but I didn't have the right values on hand. So, that will have to wait for a less-rushed return project on this receiver.
 
The KR-6160 I'm working on right now is actually a different unit from the one I was planning the driver / power amp board restoration back up in post #5.

For this particular KR-6160, I'm looking for a quick recap, so I've trimmed the to-do and part list significantly. Here's the detail on the work:

The driver board is easy to get to, just beyond the big power reservoir cap and the two kinda-large output capacitors:

IMG_7539.JPG

It is plugged into an edge connector socket underneath, so the two screws up at the top hold it in place...

IMG_7540.JPG

But after you remove the two screws and start lifting the board out, you'll find there are two output wires connected with lugs to the component side of the board. You'll have to use pliers or such to pull them off as you pull the driver board up and out of its edge connector slot:

IMG_7622-whole.jpg

There is another of the odd-design issues here. These two output wires leave the driver board via lugs and then pass through the retaining collar holes for the output capacitor. Weird. Doesn't seem like a great idea, but this is what I found in my unit.

IMG_7622-passthru.jpg

and those wires connect to the corresponding output capacitor on the other side of the chassis:

IMG_7649.JPG

Once the board is out, the layout is easy:

IMG_7614.JPG

and it follows the schematic well:

KR-6160 driver (ppower amp) board.png

There are two wires on the foil side of the board you'll need to watch out for:

IMG_7616.JPG

and reattach properly after you're done:

IMG_7617.JPG

My final result was just replacing the electrolytic caps this time:

IMG_7635.JPG

And @hopjohn was kind enough to explain how to set the bias current after working on this board, so read all about that here: http://audiokarma.org/forums/index....rent-bias-in-kenwood-kr-6160-receiver.811996/
 
The output caps were my last must-do recap for now.

The original caps are huge, smaller only than the single power reservoir cap:

IMG_7534.JPG

It looks like the original design has these caps snubbed already or whatever you call caps and resistors on the output caps:

IMG_7648.JPG

IMG_7650.JPG

The replacement caps are a fraction of the size. In this case I was able to find a reasonable-quality, low-cost, same-diameter replacement for these 2200 uF output caps. Remember to keep the capacitance value identical to the original as this is part of a output filter and altering (upping) the capacitance will hav the effect of over-emphasizing the bass. Ask me how I know this sometime.

IMG_7652.JPG


With the new output caps in place, I replaced the snubber caps and the resistors:

IMG_7660.JPG

The result is kinda funny. The new caps look undersized and out of place when, in fact, they are far superior to the originals:

IMG_7664.JPG

When I drop in the replacement for the power reservoir cap these two won't look so bad....
 
The restoration isn't complete, but it sounds much better now with a partial recap and replacement of some transistors. The MIC input seems to be on all the time and that circuit is definitely introducing noise. Gotta take that apart soon. Only problem is THIS is the current bedroom system, so while it is down I've got nothing to listen to except the laptop's built-in speakers. Waugh.
 
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