Kenwood KR4070 went silent....

[quaddriver]

'bout 3 christmases ago. Finally got to it, no sound from speakers, some sound from headphones.

Taking it apart, I found that it got a bug...something that crawled inside and made a cocoon under the R55/R56 resistors for the headphones hatched in the warm (this was in my garage as a daily driver for over a year) and it splorched one of the resistors. one reads the spec 330, the other now half at 160. Right side, and coincidently, the right side has a slight distortion. Since kenwood placed these resistors exactly on the drive output, Mr Bug got full output current.

He didnt survive.

Anyways, this has a simple protection circuit described in the SM as 'ASO' (avoid senseless overload?) on page 11 of the SM (my copy from Rick in hand, or here at the AK or on HFE.

I figure some stuff is hurt - or not - so am building a BOM for the 4070. probing around, I note that neither emitter resistor for the outputs has any flow on it (spec 15mv). The ASO circuit is to detect over voltage on the emitters of the NPN outputs and turns on or off a cascade of stuff such that it eventually grounds out via Q23 the power supply if the input pairs. It would seem to me that as soon as overcurrent quit, it would turn back on, at any rate, I have no current. I have no reason to believe that any outputs have any current thru them other than the faint signal I heard.

I have not started taking apart the amp yet as I am getting my order list ready, I have just about everything BUT, Q21 is listed as a 2SA750(1) (e) or (f) suffix, or a 2SA872 (D) or (e) or 2sa893 (d) or (e)

Im not finding realistic subs for them. 2SA970 has been mentioned in the past but mouser no digikey appear to have.

One AK post from years back, for a KR 7300 list the KSA992 as a sub - in the audio path, this is not, its the protections. However, I have read that the 2SA750 (e) or (f) is a very high 350-700 hfe and other subs like 1815s etc are no high enough.

Any recommendations?

It could be that the ASO circuit got torched and now Q23 is always on, I guess a simple test would be remove it, and if music comes back, voila, BUT, if something else downstream is bad, I risk really blowing the crap out of what is there...

Sadly, not everything is cut and dried in here, for some unknown reason designed to aggravate me, half the amps resistors are made out of carbon deposits on the board, kinda hard to lift those legs...

 

[quaddriver]

I poked and prodded and was convinced that the ASO was no longer operating as a functioning ASO so I removed Q23 and powered up and bingo - Left channel in all its glory, a hyper faint right channel with faint hum on it.

Rebuild time. Both sides, I am sure after nigh onto 40 years new silicon will behave differently than old...so the next post will be a BOM of sorts simply because I wanna put something out there for posterity. I have owned at least a half dozen of these from my silver hoarding years and usually its just the tuners that need some tickling...these amps were always fairly resilient.

Since I tend to discover ancient AK wisdom via innocent google searches, perhaps someone else can benefit. Enjoy

PS, I can see nothing in the ASO that indicates Q21 needs to be anymore special than anyone else so a 1815 might get the call to fix it...

 

[quaddriver]

Lets get to the matter, as I discover, I will simply edit this particular post over and over and comments and suggestions are welcome.

KR 4070 Audio board X09-1220-10

Outputs:
Q15, Q16 Outputs 2SD588 NPN t0220 batwing? bce 150v 7A 80w 8Mhz hfe min 40
replace with: 512-FJP5200RTU npn tp220 bce 230v 15a 80w 30mhz hfe 55-110 $1.60

Q17, Q18 Outputs 2SB618 PNP t0220 batwing bce 150v 7A 80w 8mhz hfe min 40
replace with 512-FJP1943RTU pnp to220 bce -230v 15a 80w 30mhz hfe 55-110 $1.82

Both are avail from on-semi in some beefed up current/wattage ratings in other packages with similar other characteristics

Drivers:
Q11, Q12 output drivers: 2SD438MP(e) npn 2006a ecb 100v 700ma 900mw 100mhz hfe 100-200
replace with 512-KSC2690AYS npn to126-3 ecb 160v 1.2a 1.2w 155mhz hfe 160-320 $.50

Q13, Q14 output drivers: 2SB560MP(e) pnp 2006a ecb -100v 700ma 900mw 100mhz hfe 100-200
replace with 512-KSA1220AYS pnp to126-3 ecb -160v 1.2a 1.2w 175mhz hfe 160-320 $.45

I matched the drivers PNP/NPN on HFE as best I could AND L to R

Differential inputs and mirrors:
Q1-4 differential inputs: can be 2SA640 or 2SA841, found 2SA640
old: 2SA640 pnp to92 ecb -50v 50ma 250mw 50-100mhz hfe 225 min
new: 512-KSA992FTA pnp to2 ecb -120v 50ma 500mw 50-100mhz hfe 200 min $.22

Q5-8 current mirrors: can be 2SC1775 or 2SC1980, found 2SC1980
old: 2SC1980 npn to92 ecb 120v 50ma 250mw 200mhz 180min hfe
new: 512-KSC1845FTA npn to92 ecb 120v 50ma 500mw 50-110 mhz hfe 300-600 $.21

Note: I matched these carefully, I had 8 transisitors Q1-8 with hfes as close as I can get them. Can you say "Zero DC offset"? ;-)

Protection sense:
Q19 an Q20 are the over current sense devices. According to the SM they can be 1 of 4 items: 2SC1399(e or f), 2SC1400 (e or u), 2SC1980(s or t) or 2SC1890(e or f). In this unit I found 2SC1400e's

old: 2SC1400E npn to92 ecb 100v 50ma 250mw 50mhz hfe 2225min
new: 512-KSC1845FTA npn to92 ecb 120v 50ma 500mw 50-110 mhz hfe 300-600 $.21

(512-KSC1845FTA currently out of stock at digi and mouser with many coming in 4 weeks) I know the gain is a bit more but the voltage to turn this one is based on a resistor divider literally etched into the PCB and this just servers in turn to turn something else off

Bias:
Q9 and Q10 are the bias devices. According to the SM they can be 1 of 2 items: 2SC1681(G) or 2SC1345(d). In this unit I found 2SC1681(G)'s

old: 2SC1681G npn to92 ecb 60v 50ma 200mw 65mhz hfe 200min
new: 512-KSC1845FTA npn to92 ecb 120v 50ma 500mw 50-110 mhz hfe 300-600 $.21
(I hate replacing these, will I still have idle current in range?)

ASO (aka protection):

Q21 front end of the ASO, 2SA750, replaced with:
512-KSA992FTA pnp to2 ecb -120v 50ma 500mw 50-100mhz hfe 200 min $.22
Rather than a KSA1013 or KSA1015 because the 992F series I have, have a gain more in like with the G model 2SA750. At any rate, it works.

Q23 Part of the ASO, 2SC945 or 2SC828A, replaced with a common 512-KSC945CGBU

Q22 and Q24, also part of the ASO, can be 2SA564A or 2SA733, I found 2SA564A, replaced with common 512-KSA733CGBU

Diodes:
D5, 6 and 11: "W06B" glass diode. 150piv, 750ma, replace with 1N4004/1N4005 (get from anywhere 400+piv, 1A Mouser#: 512-1N4004 - set to be discontinued so if you dont have them, go for 1N4005)

D2, 7-9, 15-17 1S1555 use simple 512-1N4148
(a purist will note that D7 is listed as a 1S1553 - yea so what, still use the 1N4148)

D10 and the missing D1: YZ-140 14v 500mw zener, use Mouser 512-1N5244B

D12, D13: WZ-197, 20v Zener 500mw, use Mouser 512-BZX79C20

C37 is a 4.7uF, *100*v unit. wow. If you are like me, you are not likely to stock that one...

The machine works and sounds good without finals, using headphones. I have worked the phono EQ board and now the tuner. I am waiting on the finals as I am making brackets to hold the TO220 outputs in where batwings used to be so I can reuse the screws, which I think are #4-32, I am unsure if I have a tap for that.

 

[quaddriver]

2SA750:
http://www.ebay.com/itm/2SA750-Tran...611814?hash=item5d6e9b05a6:g:Vs8AAOSwxp9W8A2h
2SA970:
http://www.ebay.com/itm/5x-Transist...027580?hash=item282ae0313c:g:ZwQAAOSwZkJUUCr~

OR: ZTX795A (different pin config)
http://www.digikey.com/product-deta...73?WT.srch=1&gclid=CNi89M_s6NICFUeCfgodQwAPlg

boy, they are proud of those transistors! thx

 

[blhagstrom]

NEVER buy so called NOS from eBay unless there is NO other replacement.
Little Diode is an OK outfit but may take 6 weeks to get the parts to you.
I found one on-shore seller that appears to be legit.
ALL Asian parts are suspect. DO NOT pay premium for ANYTHING from Asia.

Find a modern replacement part and buy from places like Mouser.com or Digikey.com.

I have repaired so many units that the owner put fakes-from-china-from-ebay into that its no longer even a question that the parts are genuine.

 

[quaddriver]

Oddness noted (please refer to the schematic)

D1 (YZ-140 aka 14v 500mw zener) is in the parts list, its in the SCM it has a spot on the board screen print but was NOT there. nor any evidence it ever was.

It seems to limit voltage to the emitters of the input pairs, but that task may be handled in the split voltage power supply. Dunno.

Anyone have one with this installed? (by no means is this urgent, it was not in there before, it wont be in there now)

 

[quaddriver]

Does anybody have....J32-0237-04 aka a 'boss' from the service manual that simply slides over 2 of the controls shafts and extend the threads about an in so a simple nut can hold the faceplate to the chassis. they are plastic and were broken by the PO. without them the case screws hold the faceplate on, but not rock solidly.

And the heat sink screws, for future reference, are M3 in either .5 or .6 thread pitch. I have the tap. (whoo hoo)

 

[quaddriver]

Ok, remember how this had batwing outputs? They look like thus:


They will be replaced with TO220 style units that are much smaller. Will they do the job? Refer to my substitution:

Outputs:
Q15, Q16 Outputs 2SD588 NPN t0220 batwing? bce 150v 7A 80w 8Mhz hfe min 40
replace with: 512-FJP5200RTU npn tp220 bce 230v 15a 80w 30mhz hfe 55-110 $1.60

Q17, Q18 Outputs 2SB618 PNP t0220 batwing bce 150v 7A 80w 8mhz hfe min 40
replace with 512-FJP1943RTU pnp to220 bce -230v 15a 80w 30mhz hfe 55-110 $1.82

The new ones dissipate as much heat (W) at twice the current flow. The machine is a 40W unit into 8ohms so this being the future, lets hope the engineers at fairchild remembered to 'carry the two' in their calculations.

Not knowing at the time if I had the proper fine tools in my shop, I planned on making aluminum brackets to hold the to220s OVER the batwing area, but I would have needed longer screws (bolts) which are M3-.5, and slightly harder to find in 12-14mm long than honest politicians so....plan B

Given that I have drills and dremels and taps and stuff I cleaned off the old grease and micas to get:



holding the old, up to the new, I find that to make the legs stick down as far as I need to, the center of the new hole will be the center of the upper edge of the batwing left to right, and EXACTLY on the batwing edge for the up/down or vertical position.

Use a 4d finish nail and a tack hammer to make a slight indent on where you want to drill.

Start out and make a pilot hole with the smallest dremel you can, and go farther than you have to by at least 1/8th of an inch as your tap is a starter tap and likely not a bottom tap (flat bottom for blind holes.)

On the 4070 heat sink, there is a rib more or less exactly below this point so if you are dead on balls perfect, you WILL stay within this rib, but if you let it walk a wee bit, either accidentally or intentionally, you will break out a side which coincidentally helps get rid of chips.

FWIW hole #1 was dead on balls perfect and it was harder to tap, so holes 2-4 I might have helped walk a wee bit

an M3 screw translates into .118", slightly less than 1/8th (.125) and dremel bits come in 64ths sizes so pilot with a 1/16th ish to a 5/64

You want some threads, dont need a lot, you are not holding an alternator bracket to a small block cylinder head and the torque is minuscule so a final tap of 7/64 (.109) gives you .009 for threads aka 9-thou so that might be too little. so perhaps a 3/32 might be too much.

You will know from how your tap feels. it wont take much to break it off in the hole and be screwed so go slow, back it out a half turn for every 3/4 turn and every 2 full turns remove it and clean it. And use a drop of very light oil. I have a syringe looking thing full of oil used for ho scale choo choos and it works well. The oil is a cutting lubricant and makes it go faster.

if it works out you get:


And after a good clean, some grease, new micas, we get



"But they dont stick below the heat sink!" you say. Remember the 4070 heat sink bolts to the bottom of the chassis below the PCB.

If I get some time after the scouting hike tonight, might get to hook-er up and see if the idle current set is attainable...sweating bullets on that one...

PS: I wrote in sharpie B D D B on the sink to remind me where the 2SB and 2SD devices were. Might suck to mix those up later...

 

[EchoWars]

Read through quickly...did the outputs really need replacing? Perhaps a moot point at this juncture, since HS is now drilled and tapped.

The Zetex transistor is a fine sub electrically, but its pinout makes it a PITA to install unless you've done this a number of times.

 

[quaddriver]

Honestly, I think they do work (albeit hfes in the 20-32 range) as it was the R channel drivers that went but for the cost and peace of mind....

(as always, anyone can have the outputs for the asking...)

More coming, gotta get a car load of stuff catalogued for auction tomorrow (I decided I am not going to live long enough to fix all the projects I accumulated so I have been sending them off to a local house.)

Welcome back Glen, I just heard today via reading. My condolences.

 

[quaddriver]

Ok, we have a receiver. In the interest of full disclosure, take my heat sink mounting procedure above and STILL mount them lower. Apparently the batwigs had wider leg spacing so it was easy to make the spread that the 4070 has, depending on where you are, the B or E is far off to one side.

Dammit. I should have test fit better, so while it WOULD have worked, it would have barely made contact on the one leg and lets be honest, they get so hot they desolder themselves if nt glued in good, so what I did since I had a nice drill and tap, was to cut off 3/16 on the ends only of the sink, lowering it in the cradle - sorta like lowering the motor in your street rod...then you end up having to lop 3/16th-ish off the collector leg as it is now too long, but I bent the outers into a sort of a rams horn shape, like an old gen 1 SBC exhaust manifold (there goes the car reference again)

I let her run for a while and set the idles (thankfully) at like 14.7mv (15 spec, I like to err on cooler) and the DCO is not settable, but it was 1.2mv left, -2.1mv right.

the LED light conversion is done, Im thinking about adding another 1000uf to the power caps, they were 7500uf-50, I was able to find 6800uf-50, which is in tolerance spec, but I found a genius way to add 1000uf each half using the B+/B- pin outs, but odd coincidence, they have the same form factor for pin spacing as the leads, just push caps thru the holes and solder the leads to the bottom...then its time to recase.

Id like to find the 'boss'(es) from the service manual that allow you to nut the face plate down...I fear that will be chance luck for someone to have a 2090, 3090 or 4070 scrapped somewhere and Im not that interested to throw $20 over to oaktree...

Lastly, there is a boss and a hole for the right side of the dial (facing) for a second fuse style dial light, in fact, the SCM shows 2 lights off that circuit, but there is no fuse light holder or wires, such that I believe it was never there, in fact, there is reflective tape over the end of the dial 'glass' to terminate light transmission so I think its factory. Toying with the idea of adding one, I got a crapload of fuse holders and still have half a dozen 8v led lights in warm white...

So I only listed above the silicon subs for the amp. Looking at this forum it seems in all the years it has been around, no one ever had a zorched one.

For the caps, replace what you see. Even modern general purpose caps are better than the 40 year olds in there and the resistors - what you see. All the flameproof resistors are marked well in the SCM and they are standing off on the board.

The 330ohm 1/2 watt ones on the headphone circuit that damn near lit this up, SHOULD be flameproof. They are not technically IN the amp, but they can get all the amp output and when shorted to zero...well here we are.