Kicking It with a Kenwood KA-2000!

[jmccanna]

Hi everyone, James is back with his next project.

I picked up a Kenwood KA-2000 at a garage sale for $3 last summer. I cleaned it out and it worked fine. Yesterday I noticed it had developed a "hiss" in the speakers even at rest. I did some research and found a schematic on HIFIengine.

I studied the schematic and compared it to the unit and there are differences. The schematic is for the Trio version of the amp (British market) and has circuits for one phono and two aux channels. My unit has two phono - phono 1 and phono 2.

And, there are differences in what is in my amp on the pre-amp board and amp board component-wise.

First question is does anyone have a service manual for the American version of the KA-2000? I have searched the internet and find only the Trio one.

My best guess right now is a transistor is failing and I originally thought it must be in the phono circuit because the hiss is louder when set to either phono circuit with phono 2 being quite a bit louder than phono 1. Whereas, the aux and tuner settings are quiet.

Thoughts and suggestions are invited. I pulled the four transistors in the pre-amp section (2sc458) (schematic calls for 2sc350r and y) and they tested fine though one side had a slightly lesser hfe than the other. Each of the legs on the transistors was quite black. (I got a Peak DCA55 tester for my birthday).

The transistors on the amp board are four 2sc984 (little metal cans) where the schematic I have calls for 2sc734 transistors.

The one common factor is the schematic has a 2sc458 at Q9 and so does the unit.

?

James

 

[jerluwoo]

The old 2sc458's are known to become leaky and noisy.

 

[spind]

Over on AudioKarma the 2sc458's have a reputation as being quite the 'troublemaker.' They don't appear to age well


Edit-ooops, jerluwoo beat me to it while I was verifying what I remembered.

 

[jmccanna]

Thanks for the replies. I pulled and tested each of the transistors with my nifty Peak transistor tester. Each one tested fine and none had any leakage. That said, given their age it may be time anyway.

Any suggestions as to a replacement for them?

Thanks James

 

[jmccanna]

The schematic calls for 4 2sc350 on the main board. On my unit there are 4 2sc458s all of which tested fine. They are not terribly similar though if I compare the data sheets and no online database names them as equivalents.

It looks like someone was in here before as I see indications as minor scorch marks on some wires where a soldering iron touched and the like. Does it make sense that since this unit has two phono options whereas the schematic only has one that is why I have different components? Confusing I must say.

 

[Max Headroom]

You can save your 2SC458s. The problem is silver oxide needles that form between the 3 legs. Note the black legs? With a fine stainless steel tool scrape the area between the 3 legs, and most transistors will be magically restored!

www.tapeheads.net/showpost.php
Conflicting info in the post, I have always just changed out 'black leg' transistors and no more troubles.
That said the noise caused is flicker noise and not 'hiss' IIRC.

Dan.

 

[prairiemystic]

I use freeze spray to find the noisy component. Just spray one part at a time and in a minute you'll find it.

 

[jmccanna]

Hi, Thanks for the reply. When you say "flicker" noise is that the same thing as oscillation? Oscillation seems to point to my power capacitors which look to be original.

Thanks, James

www.tapeheads.net/showpost.php
Conflicting info in the post, I have always just changed out 'black leg' transistors and no more troubles.
That said the noise caused is flicker noise and not 'hiss' IIRC.

Dan.

 

[jmccanna]

freeze spray

I guess I will need to get some freeze spray and give that a try. That helps identify the culprit but not what I should replace with I think?

Does anyone have thoughts on the difference in components I have mentioned?

I use freeze spray to find the noisy component. Just spray one part at a time and in a minute you'll find it.

 

[jmccanna]

Hi, I just found another schematic over on AudioKarma. Problem solved. Now, I just need to get replacements for the 458s and new caps. I smell better sound!

James

 

[Max Headroom]

Hi, Thanks for the reply. When you say "flicker" noise is that the same thing as oscillation? Oscillation seems to point to my power capacitors which look to be original.

Thanks, James

Flicker noise is lower frequency erratic rumbling and bursting sounds.
Freeze spray is the way to go...use a hair dryer to warm up pcb area and lightly chill suspect components.

Dan.

 

[jerluwoo]

Seems Hifi Engine has added the dual phono schematic recently.

 

[anatech]

Hi James,

Each of the legs on the transistors was quite black.

This is quite normal and will not cause any problems unless you're soldering to those areas. In that case, just clean the legs off with pliers or steel wool (careful to get rid of all fibres).

The 2SC458 transistors should be replaced for sure. Check the specs, but I think the 2SC945 will work well as a substitute. That's going off memory, so please do check before using them.

-Chris

 

[Max Headroom]

This is quite normal and will not cause any problems unless you're soldering to those areas. In that case, just clean the legs off with pliers or steel wool (careful to get rid of all fibres). -Chris

Black legs is a well known fault mode amongst service technicians.

Dan.

 

[anatech]

Hi Dan,
In what way? It's just an oxidised coating on the surface of the metal. I run into it all the time and have never had any problems with it unless I'm trying to solder to it.

-Chris

 

[Max Headroom]

I am not certain of the failure mode, suggestions I have been told/read is that silver oxide migrates across the package between pins causing leakage currents.
Other explanation is that the silver oxide transmits up the legs and causes leakage across the die.
Either way transistors with black legs may be ok, but are definitely suspect in situations of abnormal circuit noise.

Dan.

 

[jerluwoo]

The subject of blackened leads goes round and round on nearly every forum I've seen. Many say it's simply oxidation and others say it's a sign that the encapsulation seal has failed. Whatever the reason for it, it seems it's always associated with noisy circuits. As anatech said the 2sc945 and 2sc1815 are common subs. I would personally go for ksc1845 or, if you can get "real" ones, the 2sc2240 since they occupy a phono pre and other parts where noise should be a concern.

 

[jmccanna]

Yeah that was me

I found it on audiokarma in a post and uploaded it for everyone else.

James

Seems Hifi Engine has added the dual phono schematic recently.

 

[jmccanna]

Interestingly, I have the Peak DCA55 tester and I pulled each of the sc458 trannies on the pre board. They tested fine with zero leakage. That being said does not mean that they have not become overly noisy right?

James

The subject of blackened leads goes round and round on nearly every forum I've seen. Many say it's simply oxidation and others say it's a sign that the encapsulation seal has failed. Whatever the reason for it, it seems it's always associated with noisy circuits. As anatech said the 2sc945 and 2sc1815 are common subs. I would personally go for ksc1845 or, if you can get "real" ones, the 2sc2240 since they occupy a phono pre and other parts where noise should be a concern.

 

[jmccanna]

Comparison

Hi, I have seen a recommendation on audiokarma to replace the 2sc458 with a ksc1845 and here I see one for the 2sc1815.

The 1815 specs are:

Current - Collector (Ic) (Max)
150mA Voltage - Collector Emitter Breakdown (Max) 50V Vce Saturation (Max) @ Ib, Ic 250mV @ 10mA, 100mA Current - Collector Cutoff (Max) 100nA (ICBO) DC Current Gain (hFE) (Min) @ Ic, Vce 70 @ 2mA, 6V Power - Max 400mW Frequency - Transition 80MHz
Whereas the KSC1845 specs are:

Transistor Type
NPN Current - Collector (Ic) (Max) 50mA Voltage - Collector Emitter Breakdown (Max) 120V Vce Saturation (Max) @ Ib, Ic 300mV @ 1mA, 10mA Current - Collector Cutoff (Max) 500nA (ICBO) DC Current Gain (hFE) (Min) @ Ic, Vce 300 @ 1mA, 6V Power - Max 500mW Frequency - Transition 110MHz
To my uneducated eye, either one will work as the pre board is low voltage/low current. What I do not understand in my non-electronics brain is if there is a significance to the transition frequency?

Thanks, James

The subject of blackened leads goes round and round on nearly every forum I've seen. Many say it's simply oxidation and others say it's a sign that the encapsulation seal has failed. Whatever the reason for it, it seems it's always associated with noisy circuits. As anatech said the 2sc945 and 2sc1815 are common subs. I would personally go for ksc1845 or, if you can get "real" ones, the 2sc2240 since they occupy a phono pre and other parts where noise should be a concern.