KA-7002 Nearing the Finish Line...Just Need a Little Push, Please

[spark1]

Hello again. With the friendly help of expert members along the way, I'm just about done updating my Kenwood KA-7002 amp. I've replace all the electrolytics, all of the 2sc458 and 2sc871 transistors, the trimmers and a few other components. Also put new paste and insulators on the outputs. It's been quite a project - getting access to some of the boards has been a trial for an amateur like me. In fact, with regret, I finally gave up trying to do the high and low filter boards; I just could not figure out how to get at them without cutting a LOT of wires. I rarely use the low filter and never the high, so I can live with it, although it goes against my obsessive nature.

Anyway, I'm getting ready to turn it up on my DBT, and I have several questions related to adjustments (center voltage and bias current). The directions in the manual are very confusing to me, and I sure could use some help.

First, as regards center voltage, the manual says to connect a meter to the output terminal and adjust voltage to +/-100mV of zero, with an 8ohm dummy load on the circuit. In other amps I've worked on, center voltage was always stated as a discrete voltage (ie 30V), or sometimes half of rail voltage. I've seen threads around the WWW talking about confusing and even wrong directions in the older Kenwood manuals, so I just wanted to get some additional insight about whether this seems right. Also, I can't figure out where, physically, to measure the voltage. And finally, is the dummy load really necessary?

For the bias, the directions become even more confusing...they require a voltmeter, an oscilloscope (don't have one) and a signal generator (no go there either!). However, I found another more generic Kenwood document that describes adjusting the bias by measuring across one of the .47ohm emitter resistors on each side and then doing the math to figure out the correct idle voltage. This same document shows the bias current spec for the 7002 to be 50mA...so the target voltage for the adjustment would seem to be 23.5mV...right?

The page of the document showing bias current for various Kenwood amps has a repeat of the instructions...but in this version they show measuring across both emitter resistors. This would imply a voltage of 47mV to get to 50mA (if I'm thinking about this correctly).

Looking forward to getting schooled. Thanks again for any help/guidance you can provide.

 

[UncleBingo]

Echowars is the resident Kenwood expert (afaik) but I'll say that if by center voltage you mean DC offset as measured at the speaker outputs, you want it as close to zero as possible as the zero-line represents the halfway point on the sine wave. For example, if you have +10 mv dc offset, a 40 millivolt sinewave would measure 30mv above the zero line and 10 mv below. It means during a no signal condition, your speaker cone (assuming it is polarity correct) is pushed and held outward- making heat without any motion to dissipate it.

 

[spark1]

I did not realize that center voltage is the same as DC offset. I remember doing some basic servicing of several other amps with a center voltage adjustment, but it was via a single pot and to a specific, much higher voltage. I always thought of DC offset as having two adjustments...one for the left channel and one for the right. I'm surprised to see such an acceptable range (100mV). Also, I've not seen the use of a dummy load when measuring DC offset....which I generally measure at the speaker output, even if the manual shows internal test points. In fact, I was under the impression that cap-coupled amps don't really have DC offset.

In the case of the KA-7002 there are two trimmers, so it makes sense that, at least in this case, center voltage may well be a synonym for DC offset. I'm not sure that is the case in the other amps I mentioned (those with a single adjustment and much higher voltage spec). I assume this means for the 7002 I can just get the voltage at the speaker outputs and adjust the pots to as close to zero as possible...business as usual for DC offset adjustment.

If it's helpful to know, the 7002 service manual mentions neither DC offset nor center voltage - instead it addresses an adjustment under the heading of "differential amplification stage" which I (mistakenly, I now know) referred to as center voltage. I didn't recognize it as DC offset because of the +/-100mV spec and the requirement for the dummy load.

Thank you! Now I just need to wait for the next kind soul to come along and educate me on the bias current process for this amp. Sure am looking forward to wrapping this project up and getting the amp back into use.

 

[UncleBingo]

Found this in a thread. It says the 7002 is in there but I couldn't find it...
http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Kenwood-Audio-Adjustment.pdf

 

[spark1]

That's the same document I referenced. Basically, my question regarding bias current is whether I should measure voltage across one or both emitter resistors?

The document shows the measurement being done both ways. I get how to calculate the correct idle voltage in either case, but I would think only one or the other method is correct for the KA-7002. I just don't know how to determine which one it is.

 

[spark1]

Regarding the "differential amplification" adjustment...I believe that this is an adjustment to assure that the output voltage is centered, or close to it. In turn, this assures that the amp will swing the same voltages at its negative and positive peaks, thereby providing maximum power before clipping. Effectively, I think one could call it a center voltage adjustment, since cap-coupled amps don't really have DC offset. Technically, it is not the same as DC offset, but in practice its effect seems to be quite similar.

Regarding bias current...I ended up just measuring across one of the emitter resistors (one on left channel, one on right) and adjusting to 23.5mV (.47ohm resistor x 50mA bias current specification). Seems to be working fine, although interestingly the heat sink on one side seems to get a bit warmer than the other.

I'm guessing that I need to take a closer look at the main amp board, measuring resistors and perhaps replacing 2sc627s (since I have some 2N3440's on hand). I hope it's not an issue with the thermistor on that channel, as these are apparently quite difficult to match and source.

Over and out!