Kenwood KR-4200 Repair Notes

[istvi]

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.


INITIAL REPAIRS & ADJUSTMENTS
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.

CLEANING SWITCHES/CONTROLS
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.

INPUT SIGNAL TRANSISTORS
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.

DIAL ADJUSTMENT
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.

LAMPS

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 WJOE.com 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...

Hey!

terrific job on the Kenny.
Can you please tell me what is the capacitance of the receivers phono stage?

thanks a lot !

 

[ChrisPB]

Hi
I purchased one of these, new, in 190s. And used it until about 10 years ago. Since then it has sat in a dry garage. But of course it can get cold in there, and when temperature drops below dewpoint then problems can start.

About 6 weeks ago I decided to use it as my garage music system (what took me so long, you ask). Anyway it worked fine, receiver tuned in well, I could run an old CD player using an aux input etc. No obvious static, balance etc all worked fine for me.

But I am not into electronics and was not prepared for, and cannot deal with, what happened a few days back. After running well for about 10 hours in total (longest continuous time would have been ~45 min), I heard a little 'click', rather like a phonograph needle meeting a sharp scratch. This occurred at ~10 s intervals about 5 times, then the amplifier cut out. I could still tune the receiver, but no sound. It wouldn't work for the next few days. So I left it alone.

Yesterday it worked, and I ran it for about 20 min before it cut out with the same click sequence etc. I shut it down right away and left it for about 10 min. It worked again when I switched it on, but cut out after about 5 min. Left it again for 10 min, then turned it on; worked for a minute or so then click, click, cut out.

I haven't tried it again. It's not the sort of amplifier I'd take to a local electronics repair service. The cost of the diagnosis wouldn't justify the end result. And while I have learnt from this thread that it is a nice little amplifier (I knew that from 40 years of use), I learnt that it will likely have lots of things that are only marginally working.

Any suggestions.

 

[Watthour]

The first step would be to disconnect speakers and power, then open and clean the unit of any accumulated debris, contamination, insect carcasses, etc.

Once cleaned, measuring the power supply voltages at the large (2,200µF/35V) filter capacitor terminals. If those are within reasonable range of ±27VDC, measure the voltages (in millivolts) across R25 through R28 on the power amplifier stage. This will indicate the idle current (bias) of the output stage. It is likely that this has drifted out of tolerance and the protection circuit is detecting this, dropping out the protection relay. That's the click you are hearing as the sound cuts out and returns.

It would also be a good idea to start a new thread for this specific unit and its problem(s) so it doesn't get lost in the confusion of a 7 year old thread.

 

[ChrisPB]

The first step would be to disconnect speakers and power, then open and clean the unit of any accumulated debris, contamination, insect carcasses, etc.

Once cleaned, measuring the power supply voltages at the large (2,200µF/35V) filter capacitor terminals. If those are within reasonable range of ±27VDC, measure the voltages (in millivolts) across R25 through R28 on the power amplifier stage. This will indicate the idle current (bias) of the output stage. It is likely that this has drifted out of tolerance and the protection circuit is detecting this, dropping out the protection relay. That's the click you are hearing as the sound cuts out and returns.

It would also be a good idea to start a new thread for this specific unit and its problem(s) so it doesn't get lost in the confusion of a 7 year old thread.

Thank you for that.

There's a good chance that insects and spiders have found the unit a nice place to hole up. So I'll follow your advice to the letter and see what I find. Opening up a new thread makes good sense. But, I note, you found this question very quickly. . . .!

 

[70smidfiguy]

Hey all, just a follow up on this. I saw there was no discussion of the relay which is an Omron MH 2.
The protection circuit is not clicking in and the relay looks "bad".

Any thoughts on a replacement?

 

[70smidfiguy]

Just to follow on this thread for future reference, I changed Q1-Q4 with reasonably matched pairs of KSC1845FTA (note tr's go in reversed from original 2SC1345) and my relay clicked in.