Blown fuse on KA-6100 upon power up.

[beer_me]

I finished recapping and cleaning my very dirty newly acquired ka-6100. The fuse in F1 blows every time I power on. I tried twice just thinking the first time was a fluke. It’s a 250V, 4A fuse.
I have tested the transistors, they at least meter out good. Should I replace them even if they seem good? I made sure Cm 81 was replaced with a bipolar cap too. Can anyone point me in a direction to troubleshoot? Thank you in advance

 

[Duffalora]

Suspect power supply issue. Check all diodes on the F1 side.

 

[mgkfifty]

Check for solder bridges/caps in backwards.

 

[beer_me]

I'm going to have to build a DBT. The fuse only blew after I worked on it. I only tested the main power transistors so far.
All caps are properly oriented. Checked for solder bridges, haven't found any.
I just noticed that Cd27 (.047uf) is missing from the pre-amp board. Per disassembly instruction, pictured below, that should be removed to take apart unit. It never got put back and the two points on the PCB were just soldered.

 

[beer_me]

yes they are, and they tested out fine.

 

[petehall347]

make sure output transistors collectors are isolated from heat sink

 

[beer_me]

The transistors are isolated with new mica insulators.
The diodes on the F1, I read 5.7 from anode to cathode, a slow climb to OL from cathode to anode using my diode function on my meter. Are these ok? They all read the same way, from F1, F2 etc.
I've started testing other diodes. found two to not be bad - Dm19, & Dm18 which is a W06B shows 0.0 at anode and cathode. Neither side is OL.
All of the other diodes function as normal.
Still wondering what's going on with the cap from the disassembly pictured above too. Could that be giving me problems?
Thanks for the help thus far guys!

 

[gort69]

I don't trust in-circuit diode tests - are you lifting one leg to test?

 

[beer_me]

Only lifted the ones where I got bad readings to double check. Those were d18, 19. I didn't lift the diodes on the Fuse circuits.
Also found that the ground wire from cap c27 to the chassis (that I referred to above) was missing so I resoldered that.

 

[beer_me]

Uh oh. Did you get it back up and running?
I ended up pulling all of my diodes and testing. Most are being replaced. Two each from the rectifiers.
Also going to replace the output transistors I suppose. Should I even if they test out fine? I now understand they can have leaks at the junctions and low current gain after so many years. Now that I think about my relay never clicked when I first powered up.
All I'm finding for equivalents are: for the 2SA980=NTE281, 2SC2260=NTE280. Anyone have any others theyve used to replace?

 

[Blue Shadow]

The NTE are possibly unknown transistors that kinda meet the specs of the NTE numbers. I don't know if they are the best for the amp. I did look up the c2260 here on ak with a search and there were a few recommendations for the MJ15something transistors. I would look into those first as they are a specific transistor, not that the NTE isn't but the NTE could be something else by someone else the that NTE sells to fit the c2260 and probably a few others.

Just search ak for either of the original Kenwood transistors and get the numbers for the MJs.

 

[Blue Shadow]

Don't know why u say they aren't the best for the amp, they are the cross reference part for the transistors in the 6100, 7100.

I believe you mean that are the NTE cross reference part for the transistors used in these amps.

My concern is from the way that NTE normally works. They have transistors they have bought and they match them to spec sheets for different devices and then claim they are the cross referenced part (of their offerings) that will work. I would expect most times they work just fine. Others maybe not so well. But NTE not manufacturing the devices vs. current production from one of the parts suppliers where you match up the specs and select one...could be the way to go. I'm glad to see that the NTEs worked for you. I would only consider them if there wasn't something from a manufacturer available, well I mean, unless I didn't find a good substitute vetted by some here on AK or elsewhere that is a recommended part such as the MJ15015 and MJ15016 in this case. Just that NTE is a latter resort then Mouser/Digikey new parts.

 

[petehall347]

i would like to see the data sheets for those NTE transistors

 

[petehall347]

It's readily available on the NTE Cross-reference site :http://nte01.nteinc.com I use this constantly & have for years to check what component is the same as another, some u can't find but most u can. Just type in the part # u have then a cross will come up, not 100% accurate on everything but close. It's another tool.

Error 400
HTTP Web Server: Unknown Command Exception

 

[petehall347]

i want to see manufacturer data sheet thats all ..

 

[petehall347]

all i see is this .....
Error 400
HTTP Web Server: Unknown Command Exception

 

[beer_me]

To reference above posts...

2SC2260 to NTE280 = http://nte01.nteinc.com/nte\NTExRefSemiProd.nsf/$all/8836EEACA5EA8BEE85257910007EC381?OpenDocument

2SA980 to NTE281 = http://nte01.nteinc.com/nte\NTExRefSemiProd.nsf/$all/C980035AEC69BA7D85257910007E887E?OpenDocument

he original specs for NTE's are much lower than that of the suggested MJ15015,16. What to do for replacement if needed?



update 4/8/2018-
I'm leaving them for now... showed no fault...less current leakage due to age...
Previously I fired up the amp under DBT and it lit up under full brightness.
Today...
I fired up the Amp under dim bulb test and the bulb dimmed after I replaced diodes 15, 17, 19, (all were shorted, either OL on both sides or voltage reading on both sides after I removed from circuit) those diodes are replaced and good.
I pulled the rectifiers (Dm 05-12) to make sure all were ok. They all tested out normally.
Fired it up under DBT, bulb went dim..amp lights up, no relay click, no smoke, thinking this is my best result so far....

Fired it up again a few minutes later with no DBT, with tuner, speakers hooked up.
Have amp lights, no smoke, plumes, kablooies,, but also no indication of power from amp dials, no sound blah blah..Still no relay click either.

At this point are the output transistors at fault? Or Pre-amp? Relay (it is a new relay btw omron my4)

 

[beer_me]

Much appreciate the quick reply JG,
At this point, testing is beyond what I have done before.
Are these tests done under dim bulb test? From what I remember rail voltages are from AC to DC?
but from where to what diodes to get +- 43DC. Or +-24 DC on input board. (I cant seem to find pins 5,6)

 

[beer_me]

I'm not blowing a fuse anymore since I replaced a few of the diodes mentioned before.
Thanks for the instruction! So I tested per your instruction,
D11 & D9 i get a reading of +43VDC,
D5 & D7 I get -43VDC.
D10 & D12 I get +43VDC
D6 & D8 I get -43VDC
Looks good!

But...on the input side I get
Pin 1= -3.8VDC
Pin 2=+24VDC
Pin 3= 0
Pin 4= 0
Pin 5= 0
Pin 6= 0

 

[beer_me]

You have to have that -24vdc for it to work, comes off of Q30 the middle leg, the 2 outside legs have the -43vdc, & gets regulated down to the -24vdc, if you have the -43v on those & not -24vdc in the middle leg then u might have a bad transistor there, try disconnecting the wire that goes to the input board that is supposed to be -24vdc, at the ribbon cable on the input board, there might be something going on on the input board, especially if you get the -24vdc when you disconnect it. that voltage comes straight off of Q30 (Qm30 on schematic), nothing in between.

Thank you so much for your help thus far JG! It's very much appreciated. I disconnected the -24vdc leg from the input board. Still got same readings of -3.9VDC Tested Qm30 and low and behold I had 0 VDC across all E,B,C tests on Qm30. So I'm going to hopefully track one down soon and let you know of my progress.
Thank you again for your guidance thus far! I hope to report back with good news...