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Kr-7060 (7600) intermittent loud pops and constant faint hiss

G

Gertjan90

Member
Hi everybody,

After some unsuccesfull troubleshooting last year I've decided to tackle the 'big one' in my collection again, the Kr-7060(=gunmetal kr-7600).

The problem is as follows:
After the unit is has been playing normally for about 1h-1.5hrs, occasionally I'get a loud pop from one of the channels. I think the right one but it's difficult to hear as it's so sudden. Following the pop! you can hear some crackling in the sound but it'll continue to play. At this point I'd turn the unit off for fear of breaking something.. Sounds like a transistor failing maybe?
The other, much less scary, problem is a faint hiss through both channels, regardless of input or volume.

With the help of some very nice AK members, I rebuilt the power supply board and replaced Qi1 and2 on the control amp board, but to no avail.
DC offset is good with less than 20mV on either channel. Pots have been troroughly cleaned.

I've been doing some reading and it seems that the original 2SA733 and 2SC945 transistors are prone to going noisy, and they are all over the power amp board in this receiver. So maybe replacing these might fix it?
As replacements I've found KSA733CYTA and KSC945CGTA.
Does anyone know if there are any other transistors in the amp that are prone to failing? I'll recap the power amp board as I'm working on it for good measure, but I also could replace all other trannies on that board while I'm at it, save for the TO-3s.
As for the outputs, these are the (original?) Toshiba 2SB555 and 2SD425. Schematic says these should be 2SA747 and 2SC1116. Are these suspect?

BTW, when the small TO-92 transistors in the power amp are replaced, should I reset the bias?

On back of the receiver there are jumpered rca connectors marked ' adaptor' are these pre-in and outs?
That way I could make sure the problem is indeed in the power amp section. Next would be inspecting the power amp board for bad solders.

I'd like to hear your input on this question, as I know there are some Kenwood wizards on these boards.

All help is appreciated!

Gertjan
 
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Yes, I would suspect a failing transistor. I don't know if your unit has any, but 2SC458s are notorious for going noisy. The consensus seems to be that they should be replaced with KSC1845 or equivalent. I think there is a thread here on AK with known troublesome transistors and suggested replacements.

I always check voltages, including DC offset if not a cap coupled unit, against service manual values (you do have one, right?) after working on an amp.

Jumped RCA connectors on the back panel are usually preamp out / main amp in. I don't know what "adaptor" would mean. The schematic should tell you.
 
- Use KSA733CYTA and KSC945CYTA - proper complementary pairs.

- The several I've worked on had the 2SB555 and 2SD425. Not an issue and doubtful they are causing the popping

- Yes, recheck bias and reset if needed

- The adapter jumpers are not pre out/power in unfortunately. They were intended for a Dolby NR or quad adapter, and are at the input to the tone/vol/bal/filter sections. See the block diagram in the manual, P5.

If you choose to replace all the little guys

- Qe1-4 (2sa841) KSA992FBU(bulk) or KSA992FTA (ammo tape) Hfe match these, they are the differential pairs that are responsible for DC offset

- Qe7,8 (2sa620) KSA1015GRTA These are the bias transistors - if you replace these bias will almost certainly change.

These subs were recommended to me by a kind AK'er - I'm just passing along the info. You've probably seen this http://www.audiokarma.org/forums/in...istor-sub-help-please-kenwood-kr-7600.697456/
 
It's been a while but I finally got my parts in. Both Mouser and Digikey only ship out from €50 and above so I try to get as much spares and usefull parts before I order.

So far I've only replaced the differential pairs and caps on the power amp while I was in there, but it seems to have done the trick!
I read somewhere on the Marantz forums that differential pairs are usually to blame for random pops, and especially when they're 2SA872's, so I started there.
For some reason bias was way off on the good channel, 100mV instead of 40mV, but this never caused any problems luckily.

The Kr-7060 has been playing beautifully ( and somewhat loud) for about 4 hours now and all is well. A small hiss persists on both channels but somehow my other Kenwood amps also do this so I'm not to worried.

Only problem now is where to put it.. the living room is kinda centered on my Ka-6100/Kt-6500 combo and I've run out of shelf space.

Big thanks for the help and suggested replacement parts!
 
OK, seems I spoke too early.

The unit runs fine on aux and phono for several hours, but if I start it up on fm, I still get those pops from the left channel..
Any idea what could be causing this? I have the parts to replace more transitors on the amp board if needed.

EDIT: I'd like to add that these pops now start from start-up, whereas before they would only start at 1.5 hours of playing.
 
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There are 2SC1345s in the unit in various places. Perform a search of the AK archives and you may find several instances of those generating noise.
 
The 2SC1345 have all been replaced, except for the two in the mic amp, but I doubt there is a problem there as it has never been used.
That would rule out the control amp and power supply ( completely rebuilt). As far a I can tell there are no seperate transistors in the pre-amp for left/right channels so I'll rule that out for now.
Any idea if 2SC945 and 2SA733 go bad? Plenty of those in the power amp.
 
Neither of those devices is on my "Top Eleven" list of suspects as routine replacements, but that doesn't mean they cannot fail in a noise-making mode.
 
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