Got the Kenwood KA-3500 Working

[roger2]

Spenser...I will do my best to try to help. But I have to tell you that I am very methodical and slow moving. There are too many things going on in this thread that make me confused or uneasy.

A couple of random responses to some things you mentioned previously-


Again, probing for voltages while the DBT is in place makes me uneasy. I noticed that you reported the DBT glowing brighter when you checked voltages on certain legs of a transistor. To me this feels like chaos. To an ultra knowledgeable tech who understands amp circuits backwards and forwards, maybe something could be made of this. But I am not that guy. I am a pragmatic hobbyist who likes to focus in on one variable at a time, so if I am able to help you it will have to be done in baby steps.

Static is not the same as hum or buzz. Technically, I believe that hum is 60Hz, and buzz is 120Hz, in either case a continuous low frequency heard on the speakers.. Static is something else. Also, you could have static, hum, or buzz while also hearing music signal. One does not preclude the other. To say that the signal is intermittent means that sometimes you are able to hear music on the outputs, other times not. This is a separate issue than any type of distortion (hum,buzz, static, etc).

IMHO, problem definition seems to be the thing most needed at this point. Problems need to be defined clearly one at at time, then (if there are more than one) prioritized. Basic DBT test seems to be OK now. But please describe the issue(s) you are having with the input signal, how you have tested, and what you have observed.


You also have a KA-5500. Is it fully functional? Do you have a known-good signal source, CD player (first choice), tape deck, or tuner?

 

[Spenser]

Sounds good to me.

I'm a hobbyist, too. I've only been at this a few months. I'm trying to read material and learn as I go.

Here is the problem: Signal going in. No sound coming out. Channels are dead silent, at least as evidenced by 8 ohm speakers.

There was, at one brief point, signal getting through to the outputs. That was right after I discovered and fixed the source of hum. A loose power connection dumb me had left between the power amp and preamp/control boards after removing from the chassis. However, when I powered the amplifier off and then back on again, the signal no longer was getting through.

I have a tuner, a CD player and turntable. No tapedeck.

I do have a fully functional Kenwood KA-5500.

I have an oscilloscope, signal generator and 8 Ohm dummy loads. I have a couple digital multimeters and an analog one. I have a $25 Chinese ESR/LCR meter.

Brace yourself there is one last clue I must throw out there before we let the DBT go. When you flip the "high filter" switch "on" the DBT goes bright, then dims, then in 4-5 seconds goes bright for a moment, then dims, repeats; and there is static, on these otherwise-dead-silent channels, associated with the brightening phase.

Signing off now until tomorrow evening.

 

[roger2]

Spenser I haven't a clue what that means when your DBT is attached and you flip the high filter.

I only know of ONE proper use for, and means of interpreting the behavior of, DBT devices. That is powering up an amp whose health is unknown either because the amp is unknown to you, or because it has had significant work done to it. The whole point of DBT is to minimize or prevent excessive collateral damage in the event that output transistors (or drivers possibly) are shorted. You should have researched DBT usage before employing one. I am still not certain that the DBT is being used as it was intended. No probing for voltages, no flipping switches to see what may happen. Again, what you describe above may mean something to somebody, but it is lost on me.

To be frank, the way you have that amp disassembled and cobbled together by jumper wires and such, any number of issues could be present, some of which could have been existing before you took it apart, and others manifesting because of the state of the amp as it sits now. Grounds could easily be disturbed or broken, and shorts are a definite possibility.


Here is what I mean by methodical approach:

Test your CD player on your known working KA-5500. Assuming ALL is good with that test, using the same CD player and same RCA cable you just tested and proved currently working, plug into EACH input on the KA-3500 and test. On the KA-3500 try different positions for the Tape Monitor and Tape source switches, the loudness control, the attenuator switch/button (or whichever of these the amp has, I don't know that amp personally). Try with speakers and headphones. Do all of this for each input, i.e tuner, AUX, Tape Play, etc. All of those inputs are essentially the same and can accept input from a CD player. Ignore Phono inputs for now.

As you go through these steps observe and report back the results including the process that you used, not just "no sound"

 

[Hyperion]

At the junction of D13, D14 and R21 is 29.5V where it should be 35V.

Emitters of Q11, Q12 are -0.3V and -0.5V respectively where should be -1.2V.

These are probably Ok - on DBT, not far enough off to be a concern.

DBT goes from dim to medium bright when probe is attached to these transistors and others.

Something I thought interesting. Collector of Q2 is in a dodgy place. I had to reach the probe in while the amplifier was running in order to take a reading. As the probe began to get close in, the DBT would begin to glow even before the probe made contact.

As I said, DBT also went to 'glowing' when probe was attached to some of the other transistor test points. I don't know whether this is typical.

Yes, that is a bit of a worry, unless you are putting your probe on a very sensitive part of the circuit, or early stages - when amplification will be greatest, so it might be normal to see the DBT bulb brighten a little as the amp amplifies the hum & noise thus injected. I have not studied the schematic, so I don't know where you are probing, (what 'probe' is this? a multimeter, or an oscilloscope, or something else?). The bolded part above is a concern which might lead me to think the amplifier is oscillating. On the other hand all this might be nothing, and the effects you see are simply caused by the use of the DBT and injudicious 'probing' in sensitive parts of the circuitry.

Note: the use of a DBT is for when you suspect you have a short circuit or other fault which might cause additional damage to a unit while you measure voltages to find the fault. If you have no short circuits, overheating devices, or fuses blowing, then you almost certainly don't need to use the DBT.

 

[c.coyle]

My two cents:

DBT starts bright, then quickly goes dim: This is what you want. There are no shorts or near-shorts that are drawing dangerous amounts of current. Don't test or measure anything with the DBT in line. Take it out until the next time you fire it up. If it glows then dims at the next fire-up, take it out and do more work. If all is OK on the next fire-up, remove it until next fire-up. And so on and so on, until the unit is working properly. When I am working on a unit, I use the DBT at every fire-up until the work is completed and I trust the unit. This is probably overkill, but that's just me.

You had sound, now you don't: Something is intermittent. I would start by cleaning (or re-cleaning) all the controls and switches, especially input selector and tape monitor type controls. De-Oxit and then work the switch 50 or so times. The other thing I would do is a careful visual inspection for bad solder joints and loose/broken connections. Get the schematic out and start by checking along to "signal path." If I recall correctly, the KA-3500 schematic highlights the signal path in bold. Poke connections with a pencil eraser or something non-metallic and see if the sound comes and goes. Carefully and gently flex the boards and see if the sound comes and goes. That is often a sign of a bad solder joint or loose/broken connection. Do a Youtube search on bad solder joints.

I would focus on doing these two steps, carefully and thoroughly, before moving on.

 

[Hyperion]

And so on and so on, until the unit is working properly. When I am working on a unit, I use the DBT at every fire-up until the work is completed and I trust the unit. This is probably overkill, but that's just me.

I don't think this is overkill, I do this and think it is the best way to proceed.

Carry on gentlemen.

 

[Spenser]

Hi Roger 2, C. Coyle and Hyperion:

Great feedback and instruction. My thanks to you all.

(what 'probe' is this? a multimeter, or an oscilloscope, or something else?

Probe was was a minigrabber attached to a digital multimeter. Should instead have approached this collector from the foil side.

OK so anyway I understand your comments and suggestions. I have my marching orders; and will post results in the next day or two.

Thanks again!!!

 

[Spenser]

Cleaned controls. Prodded components along signal path. Gently flexed boards. No discernible change.

I'm getting some sound out of speakers briefly when power up and after turning the power off. I've got problems now with voltages.

I'm going to check some suspect components, identified earlier during initial inspection.

Going to huddle with a local friend on this one until I can be in a more productive posture with respect to spending people's time here.

Thank you for your help and patience. I will be back later.

VTY.

 

[Spenser]

Hope this finds you well. Good here.

For any who might be interested, good news. Completed next moves, and now the unit IS working. Following is more than you ever wanted to know for any who care to read on.

Phase III: replaced all remaining electrolytic capacitors. Replaced all remaining diodes. Substituted new bias trim pots. Went back and replaced one suspect resistor in the power supply.

Retested everything as it came out. All capacitors tested not-horribly-off-spec, (close to in-circuit results).

Some of the diodes however did again yield suspect readings (as they had in-circuit).

Properly reconnected two jumper-wire-wraps that had been unwrapped by prior owner.

Note that all the original transistors remain. In-circuit testing continued to show all good, and -- as mentioned in an earlier post -- I have another working amplifier that has nearly the same power amplifier circuit (Kenwood KA-5500), which I used for side-by-side comparison. Double checked by pulling and testing three key transistors, which were then reinstalled.

Started it up on variac and DBT.

Outcome: Working. One possible problem is when you turn the power off the amplifier continues to run for a moment or two.

Sort of a hollow victory, in a way, beause I am not 100% certain precisely what was causing the last remaining issue, or earlier issues, for that matter. Not precisely.

On the other hand it would have taken a lot of time, and perhaps could have added unwarranted risk, to make a single change, fire it up. Make another change, fire it up. Maybe I will proceed more incrementally next time around.

My theory is that the final issue was a bad rectifier diode, one of a pair. It may also have been due to another diode that was malfunctioning. Who knows! It could have been a ground fault; bad solder joint; capacitor(s); on and on.

In any event, reset bias. Running the amp. Watching for things getting hot where they shouldn't.

All further thoughts and recommendations welcome.

Thanks again!

VTY.

 

[hopjohn]

Outcome: Working. One possible problem is when you turn the power off the amplifier continues to run for a moment or two.

That is perfectly normal for this amp which has no relay.

 

[Spenser]

Hey good to hear from you. Understood. Thanks for the information and assurance. Take care.