Service Data Request: Kenwood KA-5700 Integrated Amp

[bob p]

I've got an old Kenwood KA-5700 integrated 40W stereo amplifier from 1981 that bit the dust today. I'm looking for tech info (maybe a schematic and a service manual). If anyone has anything that might be helpful, please let me know.

photo

I don't really think the amp is worth pumping much time or money into, but if service data is available I might give it a shot.

TIA.

On a side note, it really makes me wonder sometimes what manufacturers are thinking when they say that an amp is rated at 40W RMS:

Today I was using the amp to send a 30 Hz sine wave to a speaker that was being refoamed. (One channel of the amp was powering a JBL woofer, and the other channel was hooked up to a 5R power resistor.) I had the volume set to a value of 1-2 out of 10, which corresponded to a power level of less than 0.1 to 0.3 W according to the amp's power meters. The power resistor never became detectably warm, and the sound output from the other channel was pretty quiet. There wasn't much of any motion in the speaker's cone. After a minute or two, I nudged the power up a tad on the amp, to about 1.0 W of power on the meters and CLICK! No output.

Sure the amp is old, but this makes me wonder about how they rated "HiFi" amps back in the 70s-80s -- They used all sorts of meaningless numbers like "music power" and "peak music power." When they said that an amp was rated at 40W RMS, that number seemed meaningful. In this case I'm wondering how long can the amp actually survive at its rated power level? In my case it didn't last 5 minutes at less than 1/10 of the amp's rated output.

[Enzo]

Why are we assuming this is a design weakness or a marketing problem? If the amp kicked off at one watt, then none of that music power RMS power peak power and so on is at issue. Clearly the amp is good for more than a watt, regardless of how inflated the ratings might be.

What are the power rails? What are the output devices? +/-25v rails will allow a little less than that as peak. 25v peak signal means about 18v RMS, which is abot 40 watts into an 8 ohm load. That requires about 2.25 amps. Can the output devices handle 2 or 3 amps? Can the power supply? If so, the amp can produce the 40 watts. I'd be betting the amp has more like 30-35v rails.

In fairness to the amp, a 30Hz sine wave is not a good test. In fact that is asking a lot from an amp. What is the amp output spec at that freq? How often does anyone listen to a 30Hz sine wave? That an amp will handle 30Hz does not mean it will handle a steady 30Hz tone at full level. A totally honest 40w RMS amp spec does not mean it will sit there and produce a tone like that for sustained periods.

[bob p]

In fairness to the amp, a 30Hz sine wave is not a good test. In fact that is asking a lot from an amp. What is the amp output spec at that freq? How often does anyone listen to a 30Hz sine wave? That an amp will handle 30Hz does not mean it will handle a steady 30Hz tone at full level. A totally honest 40w RMS amp spec does not mean it will sit there and produce a tone like that for sustained periods.

I guess my description wasn't clear. I wasn't testing the amp with a 30 Hz sine wave. I was using a temporary 30 Hz signal to align the surround while I was gluing a new foam edge onto a woofer, as that's what the instructions in the woofer repair kit called for.

I also wasn't asking the consumer grade amp to produce a sine wave at its full rated power for an indefinite period of time, like the mil spec torture tests would require. I was only asking the amp to produce a sine wave at less than 1/100 of its rated power for two minutes, and at 1/40 of its rated power for a second or two while I verified the alignment by assuring that there wasn't a voice coil rub. I don't think that type of performance is unreasonable to ask of consumer-grade gear. Maybe I'm wrong on that.

Why are we assuming this is a design weakness or a marketing problem? If the amp kicked off at one watt, then none of that music power RMS power peak power and so on is at issue. Clearly the amp is good for more than a watt, regardless of how inflated the ratings might be.

Well, if the full truth has to be told, I lost TWO low powered amps today in exactly the same fasion: a 15W Pioneer SX-434 receiver and the 40W Kenwood KA-5700 amp. Both of these had been in perfect working order, as I routinely used them to break-in speaker cabinets at deafening volumes for 72-hours non-stop. Both of them died today as soon as they saw a low powered sine wave. Rather than assuming that two perfectly good amps would coincidentally die at the same time, I considered that it might be possible that neither amp was all that it was cracked up to be.

Both of these companies were famous for absurdly hyped power ratings in the 1980s, and both of these otherwise healthy amps died today under the same circumstances -- when experiencing a sine wave transient at less than 1/40 of their rated power. As we know music signals and sine waves are very different things, but any amp should be able to work at a level that's 1/100 to 1/40 of its RMS rating for such a short duration. But the monent that these questionably rated amps saw even a tiny sine wave that was less than 1/40 of their rated output... POOF!.

I finally got tired of killing off the "cheap" low powered gear, so I tore apart my stereo and hooked the signal generator up to a high quality conservatively rated 600W American-made amp that was every bit as old as the Jap crap. Its never had any problems pumping its full output power across a swept spectrum into a purely resistive load. It handled today's duties without any problems.

Regarding the Kenwood, I'm not sure what the "death click" actually indicated (ie: which parts actually died), but I'm assuming that the click indicated something bad had happened. I'm assuming that the amp has no protection circuit.

What are the power rails? What are the output devices? +/-25v rails will allow a little less than that as peak. 25v peak signal means about 18v RMS, which is abot 40 watts into an 8 ohm load. That requires about 2.25 amps. Can the output devices handle 2 or 3 amps? Can the power supply? If so, the amp can produce the 40 watts. I'd be betting the amp has more like 30-35v rails.

The output devices are two NEC D587A and two NEC B617A. I'm assuming that they're obsolete/unavailable.

I have no service data and I don't know how I'd get the PSU specifications without it. The PT has no specs printed on it.

As far as voltages go, I can rearrange thigns on my bench and take some measurements after the glue dries on the woofer I've been working on.

By any chance, would you happen to have any applicable service data in your library for either of these amps? If not, I'm not sure if its worth spending a whole lot of time on them.

thanks for your help, Enzo.

[Enzo]

When I wonder about PS, I look at the filters. Whatever their rated voltage is, I knock 5-10 volts off. That is crude, but close enough. If there are and secondary fuses, I figure the amp might draw as much as 2/3 of the fuse rating at max.


2SD587: 120v 6A 70W

And indeed the 2SB617 is its spec compliment.

MCM no longer lists them, but ANY reasonable pair will work fine. Are they those funny tab types, like a TO220 with side wings? Look at some TB34 types like C2921/A1215. MCM likely has a data sheet. Are those dimensionally close? C2921 is $6

On teh other hand, some plain old TO3P or TO247s might work, just drill a new mounting hole for the screw.

That assumes you want to dump parts in it at all.

I might actually have the Pioneer in my warehouse, and the Kenwood? I'd google the model number and see if free info society of someone has it.

I realize you were not testing the amp per se, but it failed for you, and I meant that particular use was not a fair test of its operation, and by extension its ratings.

I am just thinking aloud in print now, but perhaps that subwoofer at that low freq represents a super low impedance that the amp choked on, but would have been fine running into a normal stereo speaker playing music. Current spikes your hefty amp could handle were more than it could take. I don't doubt the 600 watt amp can handle something a 40 watt amp cannot

Holy Cats! I have the SX434 HERE even, not back home in the warehouse. It is a large fold out and may not scan well. You just need the power amp section?

[bob p]

When I wonder about PS, I look at the filters. Whatever their rated voltage is, I knock 5-10 volts off. That is crude, but close enough. If there are and secondary fuses, I figure the amp might draw as much as 2/3 of the fuse rating at max.

regarding the PSU and fuses, here's what I found:

15W SX-434:
25 VDC caps
+/- 25 VDC rail voltages
Secondary fuse: 3A/250V fast blow

40W KA-5700:
42 VDC caps
+/- 42 VDC rail voltages
Secondary fuse 5A/250V fast blow

With my 124 VAC line input both boxes are running the PSU caps at 100% of their rated DC voltages. (I guess vintage HiFi gear has overvoltage problems just like vintage tube amps, though I've never really thought about that.)

Both boxes also had 1 or 2 open fuses mounted internally on the PCB in a non user-servicable area. I didn't even know that they were there until I opened the boxes to peek at the PSU caps. Maybe I'll get lucky and this is just an internal fuse problem. Does it surprise you at all that these fuses are fast blow fuses and not slow blow fuses? As my luck would have it, I don't have enough fast-blows on hand in the right values, so I need to go shopping before I can say more.

2SD587: 120v 6A 70W

And indeed the 2SB617 is its spec compliment.

MCM no longer lists them, but ANY reasonable pair will work fine. Are they those funny tab types, like a TO220 with side wings?

Side wings? Yep. These are funny looking little guys -- Like a TO220 but instead of having the tab with the mounting hole on the top, its been replaced with a pair of tabs with mounting holes, one on each side.

Interestingly, the current rating for the 2SD587 is 6A for 70W, but the amp is fused at 5A and is rated at 40W. Does that make sense to you?

I Googled around for docs on the Kenwood before I asked for help here. I wasn't able to find a free set of documents, though I did find a service manual that could be purchased for $15 to $20. Factoring in the cost of docs and parts, the amp might not wouldn't be worth repairing if I needed to buy both.

[Enzo]

Interestingly, the current rating for the 2SD587 is 6A for 70W, but the amp is fused at 5A and is rated at 40W. Does that make sense to you?

Yes. A couple reasons. First, if the part is rated 6A, I would want the fuse to open before it got right there. Like I don't drive my car around town revved to the red line. But there is also SOA - safe operating area. Power transistors have max voltage and max current ratings, but you can't usually do both at once. If you plot voltage and current on a graph, the corner where both are maxed will be lopped off.

Of course the 40v rails don't stress the 120v rating, but there are times. I don't have the curves on the parts, just the specs. Got a Japanese xstr manual. It presents xstrs like the GE book presents tubes.

The fuse is not there to do anything under normal operation, so the 40 watts won't stress it. But if there is a short on the speaker line, or junior connects 8 sets of speakers and cranks it, we want the fuse to open rather than the xstrs.

For testing you can always use smaller fuses to see if the thing works, just don't draw a lot of power out of the thing. I do that in the shop all the time - put a 5A fuse in place of a 8 or something. Once it works, then I restore the proper fuse and test under full load.

Mught be hard to find those parts, but is there room for a TO247 or TO3P part? And a place to drill a new hole in the heat sink? If you do much solid state, you probably already have reasonable xstrs in stock in those packages.

[bob p]

I am just thinking aloud in print now, but perhaps that subwoofer at that low freq represents a super low impedance that the amp choked on, but would have been fine running into a normal stereo speaker playing music. Current spikes your hefty amp could handle were more than it could take. I don't doubt the 600 watt amp can handle something a 40 watt amp cannot

you bring up a good point that i hadn't thought about -- this may be more of a load / current issue than a power issue. at that low of a frequency the load that killed the MidFi amps could have been pretty far off from the JBL woofer's nominal rating of 8R, especially since the driver was running in a free-air environment and was not mounted in a cabinet. an odd load like that could represent a challenge that most amps couldn't deal with.

coincidentally, my home stereo speakers (a different brand of speaker) have a reputation for being "amp killers" because their Z drops down to 1-2 ohms at some frequencies. to protect little girlie amps from blowing up (or going into protection), the crossovers even have a 1R resistor that can be switched-on in-series with the 12-in woofer array to bring the sysem Z up to a level that isn't as hard on most amps.

i have to use a 600W magnetic field amp with those speakers because it essentially behaves as a constant current device in spite of the low Z load. with a constant current amp i get more linear bass response out of the system by keeping that extra 1R power resistor switched out in the crossover. i'm able to get away with this because the amp is one that can handle what is admittedly a very odd load.

i have to admit that i didn't consider this sort of low-Z high-current problem while i was performing the speaker repair. there's a big difference between those cheap little japanese MidFi stereos from the 1980s and the magnetic field amp. when the mag field collapses and needs to be restored , that amp pulses so much current out of the wall that it will dim the room lights. long long ago i monitored the line voltage while i was running the amp hard and i saw the wall voltage get pulled down to below 90 VAC! putting things in perspective i guess i'm not at all surprised that the big amp could handle the job that the little amps couldn't. its a monster.

you know its funny -- the woofer repair kit that i was using comes with the 30 Hz test tone CD, and with a set of instructions that warns you about being careful not to fry the speaker with the 30 Hz test signal. but it doesn't warn you about being careful not to fry your amps!

[bob p]

Holy Cats! I have the SX434 HERE even, not back home in the warehouse. It is a large fold out and may not scan well. You just need the power amp section?

maybe if i could just get the power amp section and the PSU that would be all I need. there's no reason to suspect that the tuner or preamp functions should have any problems.

thanks again!

[bob p]

The fuse is not there to do anything under normal operation, so the 40 watts won't stress it. But if there is a short on the speaker line, or junior connects 8 sets of speakers and cranks it, we want the fuse to open rather than the xstrs.

well, as i mentioned in that earlier post, by presenting those amps with a really low-Z load, i might have accomplished the equivalent insult of Junior connecting 8 sets of speakers and cranking it!

Mught be hard to find those parts, but is there room for a TO247 or TO3P part? And a place to drill a new hole in the heat sink? If you do much solid state, you probably already have reasonable xstrs in stock in those packages.

this is the point where i have to sheepishly admit that although i've worked on a bit of tube gear, i don't work on anything SS except my car. anything that i would need i would have to order.

[Enzo]

I used to service a lot of Carver, we sold a lot of the PM1.5s. Had one in for service, and I fixed whatever was wrong with it for the guy, and then put it on the burn in bench. I was not all that familiar with the amps at the time. I applied a sine wave - probably 100Hz, my all the time favorite freq - ran it up to 2/3 to 3/4 power and left it there warming my loads. A few minutes later the fan goes a mile a minute as the thing tries to cool itself off. I coulodn't figure out what was wrong. It seemed to meet all the specs and seemed adjusted right. Customer was getting impatient as I kept his amp another day, trying to fathom this thermal problem. I called Carver for advice, and they told me - SINE WAVE??? No it won't put up with a sine wave for very long. Music isn't a sine wave. SIne wave is a lot harder on an amp than music. The amp was working perfectly all along after my initial repair. Live and learn.

1 ohm resistor? That was one of Bob Carvers tricks. When someone told him his amps sounded OK, but they preferred a tube amp, he told them to add a 1 ohm resistor in series with the load to represent the resistance of the transformer winding, and it will emulate the tube amps. SO I wonder if your speakers sound more "tubey" with the switch thrown... Carver thinks so.

How about the possibility of some sort of back emf from the woofer voice coil?

[bob p]

back EMF? I guess that could be the case. Because I was using a stereo amp and the test tone CD was made in stereo, I hooked a load up to each channel. one of the channels had the JBL woofer that was being serviced, and the other had a 100W 5R resistor. to be honest, i'm not sure which kind of load would be less tolerated by the little solid state amps. looking back on what i did, a transistor amp probably doesn't like a purely resistive 5R load. i probably should have grabbed a 10R or 15R resistor instead.

[bob p]

regarding that 1R resistor -- i've never heard about the 1R resistor trick to simulate "tubeiness." that's interesting.

the infinity speaker cabinet has that 1R resistor built into the woofer section of the crossover. (i posted the schematic about a week ago in the infinity crossover thread.) its funny, the switch has two positions that are cryptically labeled "normal" and "extended." i would have guessed that the positions did exactly the opposite of what they actually do -- the "normal" position engages the resistor, while the "extended" position bypasses it.

AFAIK the whole idea behind placing the resistor in series with the woofers was to present a more tolerable load to the amps that don't like driving a low-Z speaker. i guess that the low Z load will send a fair number of amps go into current limiting, and the added Z is necessary to keep them from going into protection.

the infinity woofer nominal impedance is 2R, so they had to wire two of them in series. adding the 50W 1R resistor in series doesn't actually seem to do that much; it converts the woofers from a 4R nominal load to a 5R nominal load. i guess that this can make a significant difference to some amps. my carver doesn't seem to care which setting gets used. i can drive the speakers pretty hard with it and my ears will give out long before the amp gets anywhere near clipping. i never use all of that power. its just there to prevent me from sending clipped waveforms to the tweeters. so far so good...

[bob p]

from a service perspective Enzo, what do you think of carver amps? have you serviced a lot of them? i'd be interested in hearing your opinions on them, as i ran across a concert sound company that is changing out their backline and is selling off racks of PM-1.5t amps, and i've been thinking about getting one for a bi-amping experiment.

i've had my carver in my home stereo for 20+ years and i've never had any problems with it other than a truly annoying thump at power on/off. i swear, some day i'm going to do something about that.

i've never had a chance to look at the schematics for those amps. do they modulate the power output by dynamically adjusting the voltage rails, like a Class H amp?

[Enzo]

I think you can still get schematics from Carver Pro.

The PM1.5 and I guess most other Carver SS amps are commutating amps, I don't know what class to call them, after class C a lot of class names are made up by marketers.

If I recall - my files are not handy - the main output of the 1.5 runs on about35-40 volts, then there are a couple more rails at like 75v and 120v or something like that. The control circuits follow the waveform and switch the extra rails on and off as needed to follow the peaks. Commutators are the term for electronic switches.

That is why they don't like full power sine waves. The amp has to run off the 120v rails contstantly.

If I recall, the PM1.5T added a couple extra output xstrs on little auxilliary heatsinks, as compared to the original PM1.5

I have serviced quite a few, and they are to me QUITE inconvenient to work in. There is a service bulletin on the filter caps. The dual ones in cans for the higher rails are no longer available, so they made a littel board work around. FOur caps replace the two duals. OK< except extracting the old cans is a serious pain.

I think they aer great amps, but wouldn't be my choice for subwoofers, nor would they be my choice for touring.

I have a couple of my own, I think an M175 and a PM600 if I recall. None of the large ones though.

If you ever want a serious pain in the ass, take apart one of the original Carver CUbe amps. I forget the model, something 400 I think. It is a cube about 8" on a side and it kicks out 400 watts. No fan. No knobs. The Borg in stereo. Imagine a couple rack space amp folded up into a cube.


As to the normal/extended, just a guess, but with the resistor, the speaker puts out the "normal" amount of lows, while the removal of the resistor add to the power in the woofer to "extend" the lows. I am thinking a 1 ohm resistor and two 2 ohm drivers in series will have the resistor sucking up 20% of the power applied.

[bob p]

I think you can still get schematics from Carver Pro.

The PM1.5 and I guess most other Carver SS amps are commutating amps, I don't know what class to call them, after class C a lot of class names are made up by marketers.

If I recall - my files are not handy - the main output of the 1.5 runs on about35-40 volts, then there are a couple more rails at like 75v and 120v or something like that. The control circuits follow the waveform and switch the extra rails on and off as needed to follow the peaks.

Thanks! then as I understand it then, the M-1.5t is a rail-switching amp.

As I understand the Carver story, Bob Carver wanted to design an amp with a continuously variable voltage rail when he was designing the cube, but couldn't do it. He ended up making the Carver amps as amps that switched among a number of fixed voltage rails, and the made-up term for this sort of operation is Class "G". (I honestly don't know if that's a "real" class name or not, but that's what I found this type of amp described as when I searched for rail-switching amps on Wikipedia.)

According to Bob Carver, 20 or so years after designing the original Carver amps he revisited the "floating rail" idea (my term for variable rail), and he was able to design a new set of amps in which the voltage rail is continuously adjusted so that the rail voltage is always 6V above the voltage needed to amplify the signal. I think that he was finally successful with the original plan that he had had for the Carver amps because electronic components had improved to the point that the necessary support devices were available. But I'm just guessing about that.

The new "Sunfire" amps use what Bob Carver calls a "Tracking Downcoverter." Cutting through the proprietary BS marketing lingo, the amps use a TI PWM digital comparator to force the voltage supply rail to track the envelope of the signal being amplified. Because they can make the voltage rail track the musical signal, the amp's voltage rail can be modulated so that its always 6V above what's needed to amplify the signal.

This sort of design does seem like it would increase the thermal efficiency so that the amps would produce even less heat than the old rail-switching designs. I read an article on the Wikipedia about amp Classes that lists the rail-switchers as Class G, and the rail-adjusters as Class H. Of course, the only real amplifier classes are A, B, C, and D. Hence the confusiong alphabet-soup marketing lingo. Interestingly, I've never heard Carver or Sunfire use the "Class G" or "Class H" lingo, so I wonder where it comes from.

There's an interesting interview with Bob Carver where the talks about the new amp design. http://www.vxm.com/21R.69.html