Kenwood KA-3500 distortion left channel only! Guidance needed.

[Longhorn262]

Let me walk you through what I've run into and done since receiving this amplifier.

At first I had 3volts dc on the left channel speaker terminals so I replaced all out put drivers with the recommended replacements in another topic (FJP5200/FJP1943 (230V, 15A, 100W, 30 MHz)) also replaced all zener diodes, electrolytic caps and bias resistors.

I now have music out the left channel but viewing a 1khz tone on the scope it's very distorted. The right channel sounds very very clean. I even bypassed the pre amp by cutting the blue and yellow wires from the pre amp out to the amplifier board and ran wire straight from the back of the aux input jacks just to rule out the pre amp but it didn't change anything. Still going to recap the pre amp as soon as I find a solution to the power amp.

I have a decent scope so if I could get some guidance on where to look I could report back with my findings.

Some of the early transistors have higher voltages than what's listed in the schematic. And distortion starts around qe1 and qe3 on the emitter I believe. I'd have to double check just going off memory here.

 

[mbz]

I'm a little confused about the need for cutting wires etc... You have a scope, ideal for a problem like this.
I think you are saying the problem/distortion is in the left channel, power amp stage.

Confirm by checking for distortion at either leg of Re3 (2.2k) or pin 1 (normally a post or solder pad for wire)
on the left power amp, expect no distortion.

Check for distortion at
Qe5base (alt Re9 junction)
Qe17 base (alt Re19)
Qe19 base (alt Re23)

I don't like measuring at transistor legs, small space, one slip... So alternative components listed. Measure at the common node/junction
of the transistor and resistor.

Is the 1kHz sine wave distorted on the top half cycle, bottom half or both top and bottom.

 

[Longhorn262]

I'm a little confused about the need for cutting wires etc... You have a scope, ideal for a problem like this.
I think you are saying the problem/distortion is in the left channel, power amp stage.

Previous owner already cut the wires so I figured I'd double check to be sure. Pre-amp has a clean signal. So yes distortion is present only in the left channel of the power amp.

Check for distortion at
Qe5base (alt Re9 junction)
Qe17 base (alt Re19)
Qe19 base (alt Re23)

I will check these today and report back.

Is the 1kHz sine wave distorted on the top half cycle, bottom half or both top and bottom.

The top half of the sine wave is skewed near the early stages of the signal. I will probe those points today and report back with my findings.

 

[Longhorn262]

There is distortion at the base of Qe5. About .4v almost a square wave top and bottom. Does this mean qe1 and qe3 are bad?

 

[Longhorn262]

Here is the output for both channels. Also shows some dc voltage from both channels.

 

[Longhorn262]

Next two photos are the base of qe1 and base of Qe5.

 

[mbz]

Next two photos are the base of qe1 and base of Qe5.

Input to Qe1 looks reasonable, output from Qe1 to Qe5 is not ok. Suggest replace Qe1 and Qe3 with closely matched hfe pair, the
service manual has them as 2SA620 however actual components may differ.

 

[Longhorn262]

Yeah after looking in the service manual they do not look like the original 2sa620wn that is listed for either channel. They are stamped A640 F7Y.

Also here is how I mounted the fairchilds for now using bits from the old batwing. Will probably do it right once I fix all the faults.

 

[mbz]

2SA640. I think KSA992 would be a suitable replacement, suggest waiting for others to chime in and confirm...

 

[Longhorn262]

Thanks that's what I figured. Would you suggest replacing both channels or should I leave the right channel alone. Yeah hardest part is finding and choosing replacements. It's been almost 10 years since I went to school for EE but I've been teaching math and everything I learned about transistors is all but gone except the basics lol

 

[mbz]

I don't see the need to replace the transistors in the right (working) channel. Most multimeters can measure hfe, best to buy the KSA992
in tape/ammo format and measure each writing the hfe on the tape underneath. Try to get under 10% variation, buy at least 20 KSA992's, they
cost like 10c and are widely used.

 

[Longhorn262]

I don't see the need to replace the transistors in the right (working) channel. Most multimeters can measure hfe, best to buy the KSA992
in tape/ammo format and measure each writing the hfe on the tape underneath. Try to get under 10% variation, buy at least 20 KSA992's, they
cost like 10c and are widely used.

Awesome mouser stocks 4 or 5 different ones by looking at the documentation they separate them based on gain and packaging.

Looks like this 512-KSA992FATA would be a good option. I read on here over 300 would be good and FA designates 300 to 470.

 

[mbz]

Looks like this 512-KSA992FATA would be a good option

This would be fine.

I think most people would go for the KSA992FTA to get a slightly higher gain 300-600, the FATA is probably better as it's a closer
group. Both are ammo style with bent legs.

 

[Longhorn262]

This would be fine.

I think most people would go for the KSA992FTA to get a slightly higher gain 300-600, the FATA is probably better as it's a closer
group. Both are ammo style with bent legs.

Okay finally got around to replacing q1-4 with matched sets. Still get distortion on scope above 4v at the speaker outputs. Below is a shot of emitter voltage of q1 and 3 with volume knob below half and second as you turn it up past half way.

 

[Longhorn262]

Here is a shot of the collector for q1

 

[Longhorn262]

Nvm figured it out input signal to power amp is good if I probe before C1 ç3 and r5 but after these it is distorted at the base of q1

Ignore this I thought q1 and q3 base was tied together

 

[Longhorn262]

Base of Q1 looks good to 1 volt but the base of Q3 is what's distorted above .3 volts

 

[mbz]

The base of Qe3 is effectively the speaker line/wire. The distortion might be injected by the outputs Qe21, Qe23 however the cause
may be before then. Suggest start at outputs and work back, check for distortion at,
Qe21base (measure at Qe17e/Re41)
Qe23base (measure at Qe19e/Re43)
Qe17b (Re19)
Qe19b (Re23)

I would be a little surprised if you get 0.3Vdc at Qe3b, likewise 1Vdc at Qe1b, both should be near zero Vdc.

 

[Longhorn262]

I replaced both sets of outputs I'll work my way back and post the results. Thanks again.

 

[Longhorn262]

I get 32mv DC on right channel and 34mv on the left channel.

I was talking peak AC at Q1b is 1v 1khz sine wave. When Q3b gets above .3vac it starts to distort. This is using the volume knob.

Just checked all the points suggested above and there is distortion at every one of them.