Kenwood KA-5002 Protection board

[mpr2000]

I'm assessing a Kenwood KA-5002 integrated amp before I do a full restore (spend money). It arrived in non-working condition, and that consists of not coming out of protect. I went though and checked all the usual things; power supply voltages, amp board issues, did a quick ESR of the caps in circuit, checked for critical resistor values, etc. All okay. IE, I can set the bias and so on per the service manual. Idle current is correct at 30mA (14.1mV across the emitter resistors) and the DC offset is about 10mV on both channels (testing points before the relay). So for all intents and purposes, the amp section seems to be working.

However, it will not go out of protect. So I then attacked the protection board (a separate board from the amp board on this revision of the amp).

Pins 1 (unmarked below), 2, 3 and 4 which should be zero volts, start out at about 100mV and drop to about 30mV each with respect to ground after about 3 minutes. So I'm imaging that is why it won't go out of protect.

I did pull the output transistors and they test good for gain and leakage. Transistors on the amp board were checked for gain in circuit.

Anyway, any thoughts on this? Leaky transistors before the outputs? Pins 1 & 3 on the protection board is connected to the output emitters for one side, and pins 2 & 4 connected to the output emitters the other side.

One thing I'll mention, this amp seems to be a low serial number (411386) and the service manual is close but not exact. IE, my amp board is an X07-1020-00, where the service manual shows X07-1020-01, etc.

And from that come a couple of oddities; the relay coil is connected (as found) to pins 7 & 10 below. The manual shows the coil connected to pins 6 & 7 of the protection board. And pin 9 on mine is not connected to anything - and does not appear to ever have been. So - how would the relay coil ever work connected to pins 7 & 10?

I traced out the protection board and mine does match the manual here.

 

[mpr2000]

Quick edit/update, reading across pins 1 and 3, I get 10mV. Same for pins 2 & 4. This as opposed to each pin to ground as mentioned above.

 

[petehall347]

relay will work on pins 7 and 10 .
get voltages 6 7 8 9 10 dc volts

and base of q6 .. check ac and dc here .

 

[mpr2000]

Ok..

8 = electrical and chassis ground. My measurements are between 8 and all pins mentioned above.
6 = 49.6vdc
7 = 49.6vdc
9 = 49.6vdc
10 = 49.6vdc

Base of Q6 = 0 volts DC or AC

I'll mention also that I pulled all transistors on this board and tested for gain and leakage.

Once I get this sorted, it appears that this would send around 50v to a 24v relay coil?

Appreciate the help!

 

[mpr2000]

 

[petehall347]

q5 base emitter and collector voltages ?

 

[mpr2000]

Q5 is 49.6 as well.

Also, pin 5 is -37vdc

 

[petehall347]

what are the other voltages q5 ?
and what does the manual say about the trimmers ?

 

[mpr2000]

Q5 base = 49.6vdc
Q5 collector = 49.6vdc
Q5 emitter = 0v

There is no mention of the trimmer pots in the manual. They are 1k and positions 'glued' in place so they haven't been bumped. Also, all diodes were checked in circuit.

Thanks

 

[petehall347]

try a new c1 10uf 50v .. if you haven't got one remove it and see if relay clicks instantly .

 

[mpr2000]

I did replace that early on with a new low ESR cap that measured 12uf. Not sure how critical the value is. The old cap measured okay for value and leakage.

One thing I still don't understand, is how the relay coil gets a return path. As it was wired (wire-wrapped and looks undisturbed) it goes to pins 7 & 10 as mentioned above. Pin 9 (where I'd expect to see ground?) is not connected to anything. The pin on the top of the board was never connected. So there's a 1 watt, 270 ohm resistor populated but unconnected.


Protection board - prior to any work. Pin 9 is dead center, just above the 100k resistor. The two red wires on the lower left go to the relay coil.

 

[petehall347]

is the cap in the right way round ?
it wont be wired wrong ..
pin 9 has correct voltage ..
power goes through the resistor to relay .
pin 8 is ground and the circuit to ground is made when c1 charges up after q6 turns on .. q6 isnt turning on because of lack of voltage .
q5 might be bad . or c1 is wrong way round .

 

[mpr2000]

I'll double check the cap.

But the relay coil is across pins 7 & 10. Pin 9 is N/C. Pins 7, 9 and 10 all have 49.6 vdc with reference to pin 8 (ground), but the relay coil has no return side. The 270 ohm resistor is just hanging out there...

I do appreciate the help!

 

[petehall347]

so where is the voltage you read coming from ?

 

[mpr2000]

Ok - now I know why I have the reading. I pulled the board back out, and found that there's a jumper between pin 6 (V+) and pin 9. The manual should show a connection point shown between 6 & 9 as drawn and was missed.

Now just need to figure out why Q5 and Q6 don't seem to be working. While they both did test good out of circuit, I might dig through my transistor collection and see if I have something close to try as a temporary sub for the 2SA497.

 

[mpr2000]

Wow - I've been chasing an issue that wasn't an issue? It appears that the protection relay only pulls in under protection. I was under the assumption that it pulled in if the measured mV was below spec and would then apply the speaker load. Here, when the relay is engaged it removes the speakers! No changes or mods appear to have been done.

So I get the turn-on thump and all - which I would have thought for a few more cents, it could have been a modern design with a delay, latch and then release in protection or shutdown.

Am I missing something here?

 

[petehall347]

i will have to study this .

 

[petehall347]

not really sure .but i have just looked at schematic i have here and it looks like the protect is for over-current only .
p.s schematic i have here looks very different to your one .
but one thing in common is the protect is for over-current and not dc . although lots of dc might well trip it off ..
so yes you are correct the relay is normally on at power on . i dont see anything to cause a delay .but looking again ce17 might well cause a delay .

 

[mpr2000]

The relay is energized (opens contacts) when in protection is what I'm seeing.

Another miss on my part - I was looking for a DC voltage to trip the protection - thinking something above 100mV. Makes sense now. Terminals 1 and 3 connect across each far end of the output emitter resistors on one channel. Terminals 2 and 4 do the same on the other channel.

 

[petehall347]

here is a screenshot of the schematic i have here .showing protect circuit hopefully .