Desperate Cry for HELP with a Kenwood KA6004

[selmafrog]

And. Just when you thought it was safe to go back in the water. I'm back. Couldn't do it. I wanted to power it on and listen to my 70's vinyl; just could not bring myself to do that without checking some voltages first. I start on the main amp board, and everything is looking good - then I get to the junction of Re24/Re26 which I think should be reading 0V, since the junction of Re23/Re25 is shown as 0V on the schematic. That leads me to the power board. From a post on this site about the power board on a KA-6004, I read this, "Measure the DC voltage at pins 16 and 20 on the same power board. They are the right and left channel outputs. If the voltage at these pins is low (less than 0.5V), the relay circuit is most probably in trouble. But if the voltage at any one of these pins is higher than 0.5V, the problem is within the power amplifier section."
I read pin 16 @-12.4mV (.0124 V), pin 20 @ 31.6mV (.0316 V).
This indicates an issue with Relay 1 (RL1).
When I read the resistance of the power amps to ground (with the Main Amp board removed) I get (using E for Emitter, etc.) from these pins on the power board (first reading is with the gnd lead of meter to gnd, 2nd reading is with pos lead of meter to gnd):
E Q2 pin 15 1.787M/O.L
...........pin 16 1.788M/O.L
E Q4 pin 17 1.789M/O.L
...........pin 18 0.0/0.0 (expected)
E Q3 pin 19 O.L/O.L
...........pin 20 O.L/O.L
E Q1 pin 21 O.L/O.L
I then take readings between the Emitter, Base and Collector - (reversing the leads on the PNPs) and when I reversed the normal leads on each transistor only Q3 showed a reading between Base and Collector of 213k. Q1/Q2 = npn, Q3/Q4 = pnp
Q1 E-B 2.215M E-C O.L B-C 2.037M Q3 E-B 2.670M E-C 2.74M B-C 98.5k
Q2 E-B 2.289M E-C O.L B-C 2.118M Q4 E-B 2.750M E-C O.L B-C 2.107M
Looking at these reading makes me think it is time to change Q1-Q4 since Q3 appears weak, and the others are just as old with probably the same heat buildup during the years. I have a new relay I ordered from a German supplier several months ago, so I will change the relay to see if that has a positive effect while I wait to see if anyone has a different perspective on the issue. I am attaching a part of the schematic showing the Main Amp and Power Board - let me know if this violates any posting rules/policies.

 

[selmafrog]

What have I done now? I purchased replacement caps for C301, 302, and 303. Today, I replaced C302. I made sure the ground wire went to the positive side of the electrolytic (6800uF 50V) and the wires from pin 3 of the power board and the other to the main amp are on the negative side of the electrolytic. I checked the wiring before removing them from the original cap, and the gnd wires went to the pos and the two gray wires went to the post with a black dot.
After I made sure no wires are where they should not be, I hit the power button - nothing happens. I push the power button to the off position, and check for obvious wire issues, but do not see any. I push the power button again, and I get a spark where the amp is plugged into an extension cord. I immediately push the power button to off and remove the power cable from the extension cord. Nothing has obviously smoked in the amp - meaning I have not seen any smoke coming from any component in the amp.
Is it possible this has happened because the cap has not had time to build a charge?
I am thinking I will use the DBT I built when I first started working on this and see what that shows. If the DBT shows okay, then I will connect the amp to my Variac and start at low power and slowly build up to 110 volts.
The big question is: Am I reading the schematic correctly, and remembered the wiring correctly? Or am I reading the schematic wrong and just imagined the wiring matching what I see in the schematic?
The drawing from my previous post shows C302 and C301 and the "+" side of those two caps.

 

[selmafrog]

Weird is weird, right? I hookup the DBT using a 60 watt bulb. The bulb flashes and the lights on the amp come on, the relay clicks, no smoke.
I hookup the variac and start at 30 volts, after a few minutes go to 45 volts, the 70, and 90.
I let it sit for a few hours and connect the variac again at 30 volts - NOTHING, no lights, zilch. I raise it up to 90, in steps, and nothing. I re-connect the DBT, and nothing.
I go back to the variac, set it to 117 volts. No sparks, lights are on, and the relay clicks - no smoke. I power off, power on several times with the same results.
I am going to let it sit for a few hours and then try the wall plug to see what happens.

 

[selmafrog]

C301 replaced with no issues.
My question today is: I think Qe9 and Qe11 make a complementary supplementary push-pull as does Qe10 and Qe12. If this is correct, then the junction of the emitter resistors (Re23 Re25, and Re24 Re26) should be at 0 volts with no signal applied. If this is correct, then how do I reconcile setting the bias at 10mV, or 20 since pin 20 (power board) goes to pin 4 (main amp) and then to the junction of emitter resistors, and pin 16 (PB) goes pin 18 (MA) and to the emitter resistors. Since those junctions of the emitter resistors are shown as 0 volts, shouldn't pins 16 and 20 of the power board also be at 0 volts?

 

[selmafrog]

Does anyone have a good replacement for THe1 and THe2. The "5T-32L" Thermistor? I have looked, but have not found any info.

Also, when I touch VRe2, the voltage showing on my meter goes immediately to zero. Sometimes it will return, usually at a lower value, but that is seldom. I normally have to cycle power. I can make changes to VRe2 with the power off and the value changes when I power on; then the value is different when I let it sit and then power it on to check the voltage on the emitter resistors of the power transistors Q1 and Q3.

Thank you for any suggestions. I am going to try reading the voltages on Qe6, 8, 10, and 12 today and compare to Qe5, 7, 9, and 11.

Maybe I should swap VRe1 and VRe2 to see if the trouble follows the variable resistor?

 

[selmafrog]

I have a new request for your opinion on what I am seeing on a few transistors on the Main Amp board (still wanting to know if anyone has any info on the thermistors THe1 and THe2).
I am seeing the DC voltages (VOLTS unless marked wih mV) on these transistors:
Transistor_Emitter_Base____Collector_EB_____CB___EC____Vce_-_Vbe_=_Vcb
Qe1-pnp___.626__-75.9mV_-36.9____.562____37.9_-39.1_-39.1_-.562=38.538 (I am thinking 38.538 is close enough to 37.9)
Qe2_pnp___.658___.878_____-37.8___-570mV_voltages fluctuate wildly on Base to Collector and Emitter to Collector
Qe9_npn__-.568___1.1_______39.5____.700____38.3_39.1__39.1_-_.7___=38.4
Qe10_npn_-.600___1.18______39.5____.575____38.2_38.7__38.7-.575__=38.125
Qe11_pnp__.573___1.1______-40.1____1.124___38.3_39.7__39.7_-1.124=38.576
Qe12_pnp__.529___1.08_____-40.1____.562____39.2_39.7__39.7__-.562_=39.138

The service manual does show, for "Bias voltage variation", faulty diodes De1, 2, and 12, and transistor Qe1 ~ 4. I have previously replaced De1, 2, and 12 and saw some improvement in the Bias Voltage. However, I was still seeing some variation of the Bias Voltage on the same side as Qe2. I can get the one side adjusted to 10mV and it will stabilize there; it is the other side where the voltage goes to zero as soon as I touch the VRe2 to adjust the Bias.

Watching the meter on Qe2's Emitter to Collector, it appears to me, the voltage would ramp up to 300mV (the top end of the meter's mV scale) briefly show OL (overload), then drop to something like -40mV and start running up until it showed OL again on the positive voltage scale. This repeated the whole time I was observing it and happening very quickly.
I see something similar on Qe1, but it counts up very slowly, and then pauses on the readings I have across the EC and CB to assume that is maybe the voltage being seen by the circuit.

I have ordered some KSA992 transistors hoping they will be a suitable substitute. I guess it was pretty naive of me to think it would be easy to find replacement semiconductors for a 50-year-old device. I have ordered some other transistors which I found really difficult to locate any inventory for, and I think I will start looking at ordering spares for the 4 power transistors for the future. Still hoping someone has an idea about the THe1 and The2 thermistors.

Will it come down to trying to adapt an IC to replace whole sections of the lo-filter, hi-filter, pre-amp, etc.?

 

[selmafrog]

Mac started a separate thread, and after reading his post I was convinced I needed to check for cold solder joints. Well, there must have been at least one because the issue with the VRe1 and VRe2 losing voltage when I touched the resistor to adjust it is now gone - on top of the possibility of a cold solder joint, I also found the lead on my multimeter was broken near the tip where the wire is soldered to the tip.
Anyway, Mac also had an Re48 resistor which was "fractured" in several places, like the one I encountered on my Main Amp board. If Qe18 reads good, and the voltages are close to what the schematic shows, how would this resistor become fractured by heat, current, or voltage? Is there any way old age would cause a resistor to be fractured?
And why are some of the resistors marked with an "IP" on the schematic? Are these resistors some type of special material?
Finally, a shoutout to Mac for a good deed done good.

 

[selmafrog]

Let me get your opinion. Based on what I see in the schematic, there appears to be a discrepancy. In the screenshot I am attaching, Cd16 has a connection off the plus side which goes up and connects to Cd8 and other items. In the lower portion Cd15 does NOT have the connection which connects to Cd7. Looking at the drawing of the board which shows the lands, I believe the lower portion is correct, and the connector going from Cd16 to Cd8 (and others) should not be there.
What do you think? I am looking at this because the sound on my phono inputs is slightly distorted. Backup singers sound like Alvina and the Chipmunks, and the bass is strung out. I have not tried just one input from the turntable (left or right) at a time to see if it is just one side, but I will be doing that soon.

 

[selmafrog]

Hello?

 

[leesonic]

I recently purchased a Kenwood KA6004. The one I had in 1974 was stolen the night before I left the Army in Fort Riley, Kansas. I have purchased a Marantz, an NEC A10II, and a BOSE surround system, and none of those have come close to the sound I enjoyed from the Bose 501s, Dual 1229 Turntable, and the Kenwood KA6004.
I have not powered it on. I did open it and it had a coat of dirt, dust, grime on almost everything. I cleaned some and then pulled the preAmp board - X07-1120-00.
Using an ERS meter on the capacitors returns what appear to be values close to expected,
However, when I look at the backside of the board there are 5 resistors there which, to me, appear to be add-ons.
I have included pics.
Has anyone seen this type of add-on from the factory, or an update after production? If you have, what is the purpose of these? I am not an engineer, but the placement of a couple of these makes me wonder if these are bypassing the function of the diodes. Also, doesn't the placement of the ones in parallel with the original resistors lower the resistance to less than half of what the original resistor provided. Like: 18,000 X 15000\18000+15000 = something like 8100 ?
If this a manufacturer update, do you have an updated schematic to show the correct placement so i can try to verify?
I have attached a copy of my crude drawing of, what I think is the affected part. The upper part is what I think is original, and the lower part is with the added resistors - the dotted lines, in pink are my attempt to show them as add-ons.
Thank you for any assistance.View attachment 3104018View attachment 3104019View attachment 3104020View attachment 3104021

I have the very same amp at home, it's currently sitting on my bench. I can check, but I think mine has the resistors as well.

Lee.

 

[selmafrog]

In my March 15, 2024 post I said this, "Looks like the factory converted the -10 board into a -14 by adding two 390 ohm resistors (Rd7 and Rd8) to the back of the board but did not change the labeling on the board."
I believe I need to make a correction. While working to replace the transistors on the pre-amp board I noticed two circles. Looking closer it appears those two circles, with dotted lines, are Rd7 and Rd8 - two 390-ohm resistors on the back side of the pre-amp board.
I will attach a drawing. Have a good day.

 

[selmafrog]

I have the very same amp at home, it's currently sitting on my bench. I can check, but I think mine has the resistors as well.

Lee.

I said I was going to solder them back in on my 6004, but I could not bring myself to do it. It just seems to me that is one way of trying to get the input to the other channel, bypassing the circuit before there. I have had it on/off a number of times, especially recently while troubleshooting my Pioneer PL-670 turntable that was dragging, and it has not popped, smoked, nor has the left channel failed to sound comparable to the right channel. With the differences in the on-board labels, I am inclined to think someone has taken a board from a similar Kenwood and placed it in the 6004, of course this is wild conjecture, but no one, to this point, has provided a Kenwood service update to validate it, nor has anyone provided an analysis of the common schematic showing the voltages, etc. If you hear anything let me know please.

 

[selmafrog]

My left channel continued to go out after being powered on for just a couple of minutes. All of the voltage checks were in range. I then took the balance knob all the way left and started ratcheting it up/down rapidly, staying in the leff channel range on the knob. Suddenly my left channel came on. I let it play a while, then my right dropped out. I did the same to the right and the sound came back, but it was noisy, not clear. I then started moving other knobs extreme left to extreme right, and then back. The tone control started making a difference in the sound. I manipulated it a few more times, and the sound started being clear at certain spots.
I had to leave it at that point, but I am going to use the cleaner to re-clean all the switches and then give it another go.
If anyone hears from Mac Lessig, let me know he is okay, please.

 

[selmafrog]

I have heard from Mac in another posting.
I used my old Heathkit IO-4105 scope and saw one channel had some "blooming" in the waveform. I started back-tracing and would up at the main amp board looking at the two variable resistors which I think set the bias for the power transistors. One had 7mV and the other was 0mV. I had to purchase a Volt-ohmmeter with a range less than 300mV (I think that is what is in the documentation). I started adjusting the voltage up to 20mV and re-read the document and dropped that down to 10mV. I have run it for some time (it appears to take right at 10 minutes to warm up and stabilize) and then let it sit overnight for the past couple of days. The sound is so much better, neither channel has dropped off, scratchy noise is gone, and that voltage is staying at 10mV after reaching warm-up level.
If, after a couple more days with the sound good and clear, and that voltage staying at 10mV, I am ready to sand down those side panels, re-finish them, and then put it all back together.
Thank you for all of the input - even if this is not a wrap on this one.
If it is, and my Gibson GA-5 amp does well tomorrow, it is on to a 1957 RCA Victor radio before I start on my NEC A10II and finish up with my Marantz 1060.