KR-7400 Offset

[klamsoss]

I have a KR-7400 that is sounding pretty good aside from the fact that the balance/volume control has an issue (going in, the signal is nice and even... signal out isn't, and yeah, I have cleaned it several times), and the DC offset is.... okay but not great.

My question is:
with dc offset at 43mV/23mV, is it worth replacing the transistor pairs that govern the DC offset? or am I likely to only get a relatively marginal improvement?
If the improvement would most likely be meaningful, what are the recommended replacements since almost every semiconductor in this thing no longer exists.

The balance thing isn't a big issue to the owner (who is a good friend, and the guy who owns the record store I frequent), BUT if there were cheap and readily available replacements, I'd surprise him with it.

Many thanks in advance, all!!!

 

[dly66]

DC offset of 43 and 23 mV is good. Some service manuals of that vintage I have looked at generally used 200 mV as the benchmark as time to look for a problem inside. If it is not adjustable or has not been recently adjusted (if adjustable with a trimmer) don't expect it to be near 0. Depending on the match of the transistor pair you could replace and have an even bigger offset. If it sounds good I would leave it alone. I restored a KR-7400 don't remember that one as having an adjustment to the DC offset.

 

[spark1]

It does not...and I agree that your offset numbers are pretty good.

i would not think about changing the diff pairs as long as it is below 75-100mV.

What does it use for the differential pairs?

 

[klamsoss]

No offset adjustment on this big boy.

I don't really worry TOOOO much if the offset is below 50mV but this is borderline. For my ears, I can start hearing offset distortion around 100-150mV depending on the speakers, so this is clearly well below that.

I'll check the service manual for what it says about the drivers but I have doubts about whether that is actually what is in there. I have already found a bunch of errors or revisions that weren't covered in the service manual.

 

[klamsoss]

To be honest, Without actually looking inside to trace the signal path, I'm not sure which differential pairs are the ones I would target, but attached is the list from the service manual. Like I said above, I'm pretty sure that some of these aren't accurate to what is actually in there because there were substitutions or updates that never made it into the service manual.

I've buttoned things back up, but whatever input folks have is welcome. It will probably help someone else out in the future, and it will definitely help me out if/when this receiver comes back to me at some point down the road.

 

[Semanon]

I agree that its probably not worth changing them out. But keep an eye on the offset. It does change over time.

The transistors in the differential amp are 2SA620s. They can be replaced with KSA992 or ZTX795. The ZTX are about a buck each and it took me about 20 to get 1 well-matched pair and 2 OK-matched pairs. The KSA are less than 20 cents in quantities of 10. I used the ZTX is my KA-6004 with excellent results going from ~350/450 mv to 3/5 mv and nice sound, but I'm not sure if it was worth the expense.

 

[klamsoss]

@Semanon thanks for the input! I have a bunch of ksa992FTAs so if I ever see this back, I'll be able to replace them. historically I have not had a problem finding reasonably matched pairs of the 992s. I'm only using a little atlas meter, though, so probably not as well matched as they could be. That said, my goal is rarely perfection, but rather functional long term reliability within the manufacturer specifications. My budget doesn't have the tools for absolute perfection. ha!

 

[hopjohn]

KSA992FBTA is what you want. Higher typical gain than KSA992FBU or KSA992FTA which often people don't realize is just KSA992F in bulk (BU) or tape (TA) packaging. The KSA992FB I think may only be offered in tape packaging (TA) according to the datasheet. In quantities of 100 they are around 8 cents each. 100 should result in more than a few nicely matched pairs.

 

[klamsoss]

As luck would have it, all the 992s I have are FBTA types. I prefer the tape roll parts for keeping my supply stores tidy so that was part of my motivation, but I now remember ordering specifically the higher gain version the last time I placed the order. As luck would have it, that version only came on tape roll.

When matching, what is/are the most important parameters to match?
I have always just tried to match all of them, but I'm sure there is probably some hierarchy of importance.