Kenwood KR-4200 Repair Notes

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by hopjohn, Dec 4, 2014.

  1. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Edit: After careful consideration I've decided to just make this a full fledged restoration thread. I'll document it for all Kenwood lovers to see.

    Since this receiver is only 19 watts a channel RMS and doesn't really command much money on the secondary market it's probably not the type of thing you're likely to read about here very often. In some ways that inspired me to buck the trend and move forward with this.

    That being said , I just love the look of this unit. The styling just says 70's to me in a way I'm favorable to. Funny enough the larger receivers in this same series aren't quite as appealing to my eye. There's something about the visual balance of the front panel with this model that seems just right. So for those reasons I'm going to keep this thing for a lifetime and I suppose I'd like it to keep working for that long too.

    Some backstory: I got this off of craigslist from a guy in Garland, Texas who said he was the original owner. He used it quite often, but said, "it just didn't sound they way it used to" so he replaced it with some black plastic POS. This was actually my first vintage receiver purchase since joining AK back in 2010. I was pretty clueless back then regarding model numbers so I guessed on it. I figured it was around 40 watts a channel so you can imagine I wasn't thrilled to learn it was half that. It's been in the closet getting unfairly ignored for about 4 years. No longer.

    Here were my initial issues:
    1. The Right channel had no audio, except for some annoying white noise, which persisted even when the balance pot was all the way to the left.
    2. Left channel was intermittent and seemed to come and go while working the Tape 1 monitor button, even with monitoring off.
    3. Major Crackling in Bass adjust.
    4. Offset in left channel either wasn't giving a reading or was pretty close to 0mv.
    5. Offset in right channel was 560mv.
    6. FM Dial reading inaccurately.
    7. Dead lamps in the dial.

    The switches are all accessible just by removing the bottom plate. There are two rotary switches (not shown), four pots, and six push switches. I start with a thorough flush with some CRC contact cleaner on every control and working the control 20-30 times. One by one a spray of Deoxit D5 in the switches and Deoxit Faderlube in the pots I again work the controls 20-30 times. This brought back the left channel and took the noise and crackling out of all the controls.

    Switches and pots after a fresh cleaning. Rotary switches not shown.

    The common culprit for constant hiss in receivers across brands is the small input signal transistors, so I started there.

    Q1-Q4 2sc1345, pinout ECB (outhouse style case with numbers on the rear) were replaced with Fairchild BC550B, pinout CBE. The solder side of the amp board has pinout ID for Emitter and Base which is helpful. The hfe was matched for each channel. Q1 & Q3 right ch. @ ~312 hfe. Q2 & Q4 left ch. @ ~306. During the install of Q3 I discovered a loose capacitor C5 47uf 6.3v so I installed a new cap > 47uf 6.3v Nichicon PW. Apparently I'd also installed one other cap 220uf 35v on this board back when I first got it.

    A quick check of offset at terminals, with no waiting was -11mv in each channel and moving toward zero. An audio check confirmed the white noise issue had been resolved. the culprit was with either Q1 or Q3. I replaced them all at the same time so its tough to say which one in particular, but one of the two measured bad with a DMM six-way diode test.

    Edit2: Just rethinking some of the work from above. The B550B (I didnt have any C rated parts) I used to replace the 2SC1345 at Q1-4 are lower in gain to the originals. I had some matched pairs of Zetex ZTX694B on hand so I removed the B550B at Q1, Q2, Q3, Q4. (they seemed to be working fine) in favor of the ZTX694B (CBE pinout) which have hfe figures closer to the original 2sc1345 (the good ones measured between 425 to 550 hfe). Using the cheapo Harbor Freight DMM with the hfe mode I measured out a couple of matching ZTX694B pairs. Q1, Q3 were 610 hfe +/-5 Q2, Q4 620 hfe +/- 5. Once these were installed bias was adjusted for the optimal 18.8mv (0.47 + 0.47 x 20ma) in each channel measuring across both emitter resistors. Then offset was measured at the speaker terminals and is now well under 5mv in each channel. Nice!

    Purple arrows show the location of 2SC1345 Q1,Q2,Q3,Q4 of the Main Amp Board and pin locations on the board which are in a straight row ( X07-1030-00 shown). Yellow arrows indicate where to connect leads to emitter resistors for each bias check and adjustment. The two left arrows for the left channel and the two right arrows for the right channel. Optimum bias gives a reading of 18.8mv in each channel, (0.47ohm + 0.47ohm) x 20ma

    With the main issues dealt with I checked the remainder of the board voltages I could find from the pdf schematic. Voltages appeared normal.

    DISCLAIMER The following adjustment contained in the paragraph below is not rookie/newbie friendly. If not done properly and with the proper tools it will screw up the tuner section of your KR-4200. Do not attempt this if you don't know precisely what you are doing. I AM NOT RESPONSIBLE AND YOU HAVE BEEN WARNED

    FM and AM tuned strong, though the FM dial was off by a bit more than I care for and needed some tweaking. I dealt with this by tuning to the actual 90.1 FM signal here and looking at how far off it was on the dial. It read to the high side at around 91.5, so approximately 1.4 Mhz off. I then detuned the station to left side in the same direction I wanted the dial to adjust to and then retuned it to maximum signal with the top right core of the RF section (with the appropriate tuning tool) which adjusts the low end of the dial. I keep nudging it a little a time until I'm actually about a .5 Mhz lower than where I want it to end up so around 89.6 or so on the dial. Then I tuned to the 106.1 signal which is now just left (low) of where it should be. Using the high side pot (a lot more touchy than the low side adjustment) located at the top middle of the RF section (red on mine) I nudge the 106.1 signal over to exactly 106.1 on the dial. Checking the dial position of the 90.1 signal puts it spot on. It's now accurate across the entire dial. YMMV.



    The KR-4200 has 6 incandescent lamps. (Left to right) Dial lamps are 8v 300ma x4, stereo beacon lamp is 8v 50ma, and the the signal meter lamp is 8v 200ma.

    You could probably use lamps from dwojo or get 8v 250/300ma wedge based Pioneer/Sansui lamps from ebay but I chose to use what I had on hand. Dial lamps and meter lamps were replaced with #51 7.5v 220ma lamps, which sacrifices some bulb life for cost savings and a more original fit. The stereo beacon was replaced with an 8v 50ma that had presoldered leads that I got from when I restored a KR-9600 previously. Dial and meter lamps must be removed by desoldering the lamps to keep lead length to a maximum. The original leads are very short so DO NOT cut them. A third hand tool is likely required for the installation of the new lamps and it's a good idea to tin the bulbs first and use a little flux if you use the #51 lamps like I did.

    Next up is the filter caps...
    Last edited: Apr 26, 2015
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  2. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    KR-4200 OVERVIEW

    Top View

    Bottom View

    Rear panel

    The two stock filter capacitors of the KR-4200 are 3300uf 35v (25mm W x 60mm H) These were replaced with Nichicon KG 3300uf 50v snap-in caps of the same diameter (25mm). I soldered ring terminals on to the snap-in pins to install them.

    While the caps were pulled it was a good opportunity to deal with the small amount surface rust in this area. You can see the rust spots in the original cap photo. I used several applications of Brasso and whole bunch of elbow grease and it cleaned up pretty nicely. The results can be seen in the new caps photo.

    Parts Used:
    2 - KG 3300uf 50v Mouser 647-LKG1H332MESABK
    2 - TE Connetivity Ring Terminals Mouser 571-32185
    2 - .047uf 50v metalized polyester film capacitors used for bypass surplus-electronics sales 120-1087
    1/2" - NTE Heat Shrink 2:1 Assorted Colors/Sizes 4 160 Pcs Amazon

    (Left)Original Elna 3300uf 35v filter caps. (Right) Stock filter cap wiring.

    New Nichicon KG 3300uf 50v filter caps install process. (left to right) Crimp the ring terminals to snap-in terminals, solder on the ring terminals, apply heat shrink tubing, fit the capacitor bracket (new/old size comparison), wire in the new caps with bypass caps, install complete.

    The old Elna caps are measured for capacitance. Neither showed signs of leaking and are still within 20% tolerance of 3300uf.
    Last edited: May 19, 2016
  3. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Power Supply

    Power Supply X00-1200-10 or 01

    Power Supply Schematic

    Power Supply before (left) and after (right)

    Electrolytic capacitors replaced on the power supply:

    Ck1 470uf 63v (gray) axial > 470uf 63v Panasonic FC 667-EEU-FC1J471
    Ck2 1uf 50v (gray) > 1uf 50v Panasonic ECQ 667-ECQ-V1H105JL
    Ck3 10uf 50v (gray) > 10uf 50v Panasonic FC 667-EEU-FC1H100L
    Ck4 3.3uf 50v (gray) > 3.3uf 50v Panasonic FC 667-EEU-FC1H3R3
    Ck5 10uf 63v (gray) > 10uf 63v Nichicon PW 647-UPW1J100MDD
    Ck6 33uf 50v (orange) > 33uf 50v Nichicon KL 647-UKL1H330MPDANATD
    Ck7 .01uf 630v (gray) axial non-polar > .01uf 630v CDE Type 150 539-150103J630BB
    Ck8 330uf 16v (orange) > 330uf 16v Nichicon KL 647-UKL1C331MPDANA
    Ck9 10uf 16v (gray) > 10uf 25v Panasonic FC 667-EEA-FC1E100H

    For the Ck1 470uf 63v I put a new hole through the PCB in order to use a radial cap in place of the stock axial cap. I used some clear RTV silicone adhesive sealant on the bottom of the capacitor to secure it in place.

    Transistors replaced in the power supply:

    Qk2 2SC945(Q) hFE 135-270, ECB > KSC945CGTA hFE 200-400, ECB 512-KSC945CGTA
    Qk3 2SC945(Q) hFE 135-270, ECB > KSC945CGTA hFE 200-400, ECB 512-KSC945CGTA

    Fuse holders:
    I also replaced the chintzy fuse holders with some parts I had from my junk parts pile. The fuse holders I used had solder tabs on the back that are spaced just right to go through the existing slits in the PC. I'm not 100% certain, but I think these would work in the same fashion. Keystone 3526 534-3526

    Power Supply voltages

    Here are the power supply voltages I had after completing the board:
    Pin 01 5mv
    Pin 02 24.85v
    Pin 03 24.85v
    Pin 04 14.65v
    Pin 05 -0.2mv
    Pin 06 0v
    Pin 07 32.65v (35v Nominal)
    Pin 08 44.9v
    Pin 09 10.24v (10v Nominal)
    Pin 10 0v
    Pin 11 0v
    Last edited: Dec 30, 2014
  4. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Tone Amp

    Tone Amp X11-1100-10

    Tone Amp Schematic

    Tone Amp before (top) and after (bottom)

    Capacitors replaced on the Tone Amp:

    Ci1, Ci2 1uf 25v (white) > 1uf 50v Panasonic ECQ 667-ECQ-V1H105JL
    Ci5, Ci6 10uf 25v (orange) > 10uf 25v Nichicon KL 647-UKL1E100MDDANA
    Ci7, Ci8 100uf 6.3v (gray) > 100uf 6.3v Panasonic FC 667-EEU-FC0J101B
    Ci9, Ci10 .01uf 50v (green mylar) > 0.01uf 50v Polystyrene (not pictured) 23PS310
    Ci11, Ci12 .1uf 50v (green mylar) > 0.1uf 50v CDE DSF 598-DSF50J104-F
    Ci13, Ci14 2200pf 50v (green mylar) > 2200pf 100v Wima FKP2 505-FKP22200/100/5
    Ci15, Ci16 .027uf 50v (brown ceramic) > .027uf 50v CDE DSF 598-DSF050J273
    Ci17 100uf 35v (35v pulled, 50v schem) (orange) > 100uf 35v Nichicon KL 647-UKL1V101KPDANA

    Transistors replaced on the Tone amp:

    Qi1, Qi2 2sc1000(BL) ECB hFE 350-700 > 2SC1815(GR) ECB hFE ~550 2SC1815

    Resistors replaced in the Tone Amp:
    Ri1, Ri2, Ri3, Ri4 10K Ohm 1/4W Carbon Film > 10K 1/2W Metal Film 71-RN65D-F-10K

    Pin 6 32.5v (35v)
    Qi1 Collector 10.67v (10v)
    Qi1 Emitter 4.07v (4.5v)
    Qi2 Collector 10.87v
    Qi2 Emitter 4.04v

    There's been a load of necessary rework on the joints of this board and others. I've found loads of cold joints, some weak, and a even a few that are cracked. Be sure to look these over really well.

    Last edited: Dec 30, 2014
  5. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    I could use some advice on the transistors in the KR-4200.

    There are some 2SA620 and 2SC1416 in the pre amp are there any concerns with those? If so, any recommended replacements?

  6. hatman

    hatman Active Member

    Northern New Jersey
    As for concern, lets see what the experts say.
    RE:2SA620 This novice noted the recommendations I got for the amps' diff inputs on a KR-5200; ZTX795A; KSA992'F' per elcuser & avionic @ AK.

    For the 2SC1416 (55V), Jap sub book shows 2SC732, 2SC1010, 2SC2389. B&D should have C732 (50V)


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  7. DrumminDaddy

    DrumminDaddy Hit it, Baby !!

    Lost in musing circumstances
    Good stuff.
  8. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Pre Amp

    Pre Amp X08-1080-13

    Pre Amp (X08-1080-13) Schematic

    Pre Amp (X08-1080-12) in stock form

    Pre Amp (X08-1080-12) with new capacitors and transistors.

    In progress:

    Capacitors replaced on the Pre Amp board:

    Cd1, Cd2 1uf 25v (white) > 1uf 50v Panasonic ECQ
    Cd7, Cd8 4700pf 50v (brown) > 4700pf 63v Kemet PFR 80-PFR5472J63J11L4
    Cd9, Cd10 .015uf 50v (brown) > .015uf 63v Wima FKP2 505-FKP2.015/63/2.5L
    Cd13, Cd14 10uf 25v (orange) > 10uf 25v Nichicon KL
    Cd15, Cd16 10uf 25v (orange) > 10uf 25v Nichicon KL
    Cd19, Cd20 100uf 6.3v (gray) > 100uf 6.3v Panasonic FC
    Cd21, Cd22 10uf 25v (orange) > 10uf 25v Nichicon KL
    Cd23 10uf 50v (gray) > 10uf 50v Panasonic FC
    Cd24 100uf 50v (orange) > 100uf 50v Nichicon KL

    Transistors replaced on the Pre Amp board

    Qd1, Qd2 2SA620 > KSA992F 512-KSA992FTA
    Qd3, Qd4 2SC1416(BL) > KSC1845F 512-KSC1845FTA

    Pin 1 32.75 (35v)
    Cd23(+) 27.9v (28v)
    Qd1 Emitter 10.4v (12.5v)
    Qd3 Collector 12.6v (15v)
    Last edited: Dec 24, 2014
  9. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX

    Tuner Board (X05-1080-00)

    Tuner board (X05-1080-00) in stock form

    Tuner board (X05-1080-00) with new capacitors

    There is a total of 10 electrolytic capacitors on the tuner board. The 0.33uf 25v electrolytic is on the bottom side. There are no electrolytic caps beneath the shield cover of the front end. FM muting needed tweaking and was accomplished by trimming VR5 to about the halfway point.

    Capacitors replaced on the Tuner board:

    C47 10uf 25v (orange) > 10uf 25v Nichicon KL
    C49 100uf 16v (gray) > 100uf 16v Panasonic FC
    C54 3.3uf 50v (gray) > 3.3uf 50v Panasonic FC
    C58 10uf 25v (orange) > 10uf 25v Nichicon KL
    C63 10uf 25v (orange) > 10uf 25v Nichicon KL
    C69 10uf 25v (orange) > 10uf 25v Nichicon KL
    C71 1uf 50v (gray) > 1uf 50v Panasonic ECQ
    C76 1uf 50v (gray) > 1uf 50v Panasonic ECQ
    C78 10uf 25v (orange) > 10uf 25v Nichicon KL
    C79 47uf 6.3v (gray) > 47uf 6.3 Nichicon PW
    C?, C? 0.47uf 25v (white) > 0.47uf 63v Wima
    Bottom of board 0.33uf 25v (white) > 0.33uf 50v Panasonic ECQ
    Last edited: Jan 1, 2015
  10. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Main Amp

    Main Amp Board (X07-1030-03)

    Main Amp Board (X07-1030-03) Schematic

    Main Amp Board (X07-1030-00) Before component installation

    Main Amp Board (X07-1030-00) After component installation

    Output transistors Qe11/12/13/14 are removed. The old compound is cleaned from the transistor bodies and mica insulators. New compound is applied to each side of the mica insualtor, it is refitted, and the output transistors are reinstalled.

    The Ce22/23 .022uf 630v gray axial electrolytics reside behind the heat-sink of the main amp board. These are replaced by Panasonic ECQ film caps of the same value. The new caps have a lead spacing of 10mm and require a new through hole drilled to one side for proper fit.

    Capacitors installed on the Main Amp:
    Ce3/4 1uf 50v (gray) > 1uf 50v Panasonic ECQ ECQ-V1H105JL
    Ce5/6/11/12 47uf 6.3v (gray, Ce11/12 schem 100uf 3.15v) > 47uf 6.3v Nichicon PW
    Ce13/14 47uf 35v (orange) > 47uf 35v Nichicon KL
    Ce15/16 0.22uf 50v (green, mylar) > 0.22uf 50v Panasonic ECQ ECQ-V1H224JL
    Ce17/18/19/20 100uf 6.3v (gray, schem 100uf 3.15v) > 100uf 6.3v Panasonic FC
    Ce21 33uf 50v (orange) > 33uf 50v Nichicon KL
    Ce22/23 .022uf 630v (gray, axial, rear of board) > Panasonic ECQ .022uf 630v ECQ-E6223KF
    Ce24 220uf 35v (orange, previously replaced by PW) > 220uf 35v Nichicon KL

    Transistors installed on the main Amp (discussed in initial repairs):

    Q1, Q2, Q3, Q4 2sc1345 > ZTX694B (matched pairs)

    Trimmers installed on the Main Amp:

    VRe1, VRe2 500ohm carbon trimmers > 500ohm 3/8" Bourns trimmers 3296Y-1-501LF

    Trimmer installation : Since the bias has already been set previously I use the following method for installation of the new multi-turn trimmers. Remove one trimmer. The center pin will be shorted to one of the pins on the solder side of the board. Measure resistance between the lone pin (non-shorted) and the center pin and record the resistance. Adjust the new trimmer to the recorded value and install. Repeat for other channel. Pins will need to be formed slightly for proper fit in the wider through holes. After trimmer installation a final bias adjustment is performed after a 20 minute warm-up. Leads are connected as explained in the photo in the initial repair section with the yellow arrows, 18.8mv for each channel.

    Voltages (nominal):

    Pin 1 -2.3mv
    Pin 3 0.2mv
    Pin 4 0v
    Pin 5 0v
    Pin 6 0.1mv
    pin 8 1.6mv
    Pin 9 0.2mv
    Pin 10 28.20v (26v)
    Pin 11 .02v
    Pin 12 28.21v (26v)
    Pin 13 -28.21(26v)
    Pin 14 0v
    Pin 15 28.23v (26v)
    Pin 16 0.2mv
    Pin 17 16.8v (15.5v)
    Pin 18 28.24v (29v)
    Pin 19 -28.27v (-26v)
    Pin 20 -28.27v (-26v)
    Qe1 and Qe3 emitter -0.55v (-0.6v)
    Qe1 Base -0.04v (-0.05v)
    Qe3 Base -0.05v (-0.05v)
    Qe3 collector 27.69v (28v?)
    Qe5 emitter 27.4v (27.8v)
    Qe15 collector 28v (0v schem error?), emitter 7mv, base 1mv
    Qe17 collector 28v (0v schem error?), emitter 4mv, base 5mv
    Re9 (Re10 side) -14.29v (-14v)
    Re35 (Ce17 side) 4mv (0v)
    Re49 (Ce21 side) 0v (0v)
    Last edited: Jan 2, 2015
  11. Watthour

    Watthour Electron Rancher - JS3600

    Good job in getting rid of those ugly old trimmers.


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  12. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX

    Those old carbon trimmers do get mighty nasty looking. The ones on the tuner board got cleaned up some since they weren't going anywhere.

  13. HVHiFI

    HVHiFI Hudson Valley HiFI Subscriber

    Chester, NY
    Lots of love for the baby receiver but why not if your gonna own it forever and want to enjoy it at it's full potential. Great work!
  14. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Project Complete

    Isn't it a handsome little devil? :banana:

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  15. Mayank

    Mayank AK Subscriber Subscriber

    Falls Church VA

    This is a very well documented thread.
    Your meticulous work is really awesome & inspiring (I need to dig out the sad & dusty KR-6600 from my project rack now). Great leads on the lamp alternatives.
    How does it sound?

  16. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Thank you, I appreciate it.

    I have great admiration for what the engineers of the day were able to do with some of the smaller units of the mid to late seventies. So many of them are greatly overlooked by the masses. I think this whole line of Kenwoods is greatly unappreciated, really. This unit in particular I find to be incredibly revealing for what it is and the recap further emphasizes this. Every small room or office needs one pf these matched with an efficient set of bookshelves that can comfortably reach the sub-regions. It's an incredibly tasty little treat.

    I really like the fact that this little guy accepts two auxiliary inputs along with a phono input. That gives you the option of connecting it to say a computer or phone/mp3 device for casual tunes and then a nice transport for more serious listening and still have your vinyl option too. On the downside everything is routed through the tone controls with no way of defeat which is a major deal to some. Fortunately, I find the tone controls to be more helpful on this unit than a hindrance, thus forgivable.
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  17. Ludovit

    Ludovit New Member

    Perfect job, I´m just preparing to do similar action with my KR5200... Re input signal transistors, I´m used to use BC 173 (my old Siemens stock...) or C2240 from Toshiba. Nice replacing of filter capacitor - good idea with the ring terminals...
  18. Vicente Neto

    Vicente Neto New Member

    We used to have one of these units in our home in the 70's/80's.
    You've encouraged me to start my own restoration. I've already ordered the parts.
    Can you describe how you cleaned the trimmers in the FM section? They all look pristine.
  19. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Dallas, TX
    Sure. Firstly I photographed them so I'd know precisely where to set them should they move. Then I just dabbed a very, very small amount of Brasso to it with a q -tip then scrubbed it lightly with a fine microfiber cloth like the ones used for cleaning eyeglasses. It took a 2-3 applications, but it's pretty straight forward. I was sure not to get the Brasso down in the trimmer, just the upper surface.
    Vicente Neto likes this.
  20. KeithD

    KeithD AK Subscriber Subscriber

    La Crosse, WI
    Great thread and great reference. I picked up a 4200 about a month ago. It does have static on the right channel, much like yours did. Measured DC at speaker outputs and R channel has about 500mV. Left channel is at 0.1mV. Is there a way to adjust the DC offset on this receiver? I'm not seeing anything obvious. Idle current was also a bit high, but adjusted that using your instructions. Thanks!

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