Hi all, I've been using my dad's old KA-3500 for some time now and it started to have some channel imbalance a few months ago. Just finished a full recap today. However, when I powered it on I saw a glow in the reflection of the heatsink plus a bit of smoke. I immediately turned it off and tried to figure out what caused it, moved some wires around as I thought maybe they had gotten too hot touching something. I turned it back on and saw no visual problems so tried to play some music through it and there's no audio, only a hum. Looking back inside, the resistor at R52 is completely toast - black in the middle and almost looks like it split a bit on top. I have no idea where to even begin with this. I don't imagine it's a problem with a cap as all my joints on the caps were fine and I triple checked polarity on each one I put in. Something tells me it may be due to my rewiring as I took the wrapped wires off the posts to get each board free and although I took photos beforehand and all wires are where they belong, I wonder if one or some weren't making good contact, but that's just a theory. Any help is greatly appreciated!
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Kenwood KA-3500 blown resistor after recap
- Thread starter Joolian
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Since this was a power up post a recap, numerous issues are suspect. Even though you checked the component(s) were installed incorrectly, recheck that. A splash of solder bridged a trace or component. You may have a crossed wire.
I would investigate the components that burned and everything in that path, starting at R52. Trace your wires. Look for burned traces on the board. Use the schematic to assist.
Most important, build yourself a dim bulb tester. You can get parts at your local home center. I would say a 75 watt incandescent bulb (not an LED bulb) will work for this amp. This will save you from burning up the unit. The light will flash on power up and then glow very dim. If it stays bright, you have an issue.
I would investigate the components that burned and everything in that path, starting at R52. Trace your wires. Look for burned traces on the board. Use the schematic to assist.
Most important, build yourself a dim bulb tester. You can get parts at your local home center. I would say a 75 watt incandescent bulb (not an LED bulb) will work for this amp. This will save you from burning up the unit. The light will flash on power up and then glow very dim. If it stays bright, you have an issue.
petehall347
the brandy coffee man
can you point to R52 on the schematic ? i can't find it ..i see it's listed as 330 ohms .
How about a picture of the area with the burned out resistor.
My 3500 schematic shows several "R52's" Kenwood uses a designation for different sections or maybe circuit boards. I found Re52 in the power amp section.
It would kill the -39 volts to the amp front end if open. Confirm the decoupling cap Ce35 is installed with correct polarity. (in this case the + is grounded)
I know you said you checked polarity but we have all done it and overlooked it. I once installed a transistor incorrectly, found the problem and then put it back wrong two more times in a row.
Did you take pictures or take notes of all of the caps original polarities? Silk screens on PC boards are not always correct.
My 3500 schematic shows several "R52's" Kenwood uses a designation for different sections or maybe circuit boards. I found Re52 in the power amp section.
It would kill the -39 volts to the amp front end if open. Confirm the decoupling cap Ce35 is installed with correct polarity. (in this case the + is grounded)
I know you said you checked polarity but we have all done it and overlooked it. I once installed a transistor incorrectly, found the problem and then put it back wrong two more times in a row.
Did you take pictures or take notes of all of the caps original polarities? Silk screens on PC boards are not always correct.
I’m doing a scan of the power board right now and don’t see any reversed caps. I only took photos of a couple caps but I removed and installed every cap one by one and confirmed each time that the originals were matched to the board printings. I’m going to get the board back up to check for bridged solder (these wires drive me crazy!).
That is definitely the resistor and cap I was looking at. C35 appears to be installed correctly. Have a peek at C13 & C14 as well.
petehall347
the brandy coffee man
I’ve found two issues so far (without removing wires to get the board fully out again): I never rewired the ground cable from the speaker terminals and it was sitting under the board, and there’s a solder bridge between the leads of C21.
Edit: I don’t see any other issues underneath the board.
Edit: I don’t see any other issues underneath the board.
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I'm circling back to this thread as I still haven't had any luck with this amp. I delayed getting back into it just because of how tedious it is to unbutton everything to work on it then reconnect all the wires to test again. However, I decided to give it a go again. Both issues from my previous comment have been fixed, all wires are in their correct places. I'm assuming that power comes in from the transformer, to the filter caps, then under the heat sink to the rest of the board. Following this path I found transistor Qe19 just ahead of Re52, pulled it, and it was shorted. I replaced this, as well as the bridge rectifier just for peace of mind (both oriented correctly) and assumed I might be on my way. Powerup on DBT immediately starts frying the resistor again. I really don't know what else to look at, I pulled the zeners by the filter caps and they all test fine, and all the original resistors in that area are reading fine. At this point, my only assumption is either the safety cap or I suppose the transformer. I will get a photo of the underside of the board uploaded tomorrow in case someone can see something I can't. Thank you in advance for any help!
Keeping this thread updated as I go along in case anyone needs it in the future. I had read a lot of things saying that smoking resistors can usually come from shorted output transistors. I pulled Qe21 and Qe23 (same side as Qe19) and they both tested fine, but then I pulled Qe17 which seemingly feeds into Qe21 and Qe19 and that had a dead collector. I don't have the knowledge to know what purpose Qe17-20 have, but I'm just going off of the schematic. After this, I checked Qe26 as this seemed like the only other missing piece pointing into Qe17, but it tested fine. My best guess at this point is Qe17 caused all of this. I've ordered replacements for Qe17 and Qe18 (2SC1735 -> KSC2383OTA) and Qe19 & Qe20 (2SA850 -> BC640TA), as well as replacement output transistors as I was thinking about replacing them someday in the future but as I already have two out I figure I might as well do it now. These are 2SB617 -> 2SA1943N and 2SD587 -> 2SC5200N.
petehall347
the brandy coffee man
keep looking for bad transistors .
I suppose I lucked out then if the drivers in that channel are done but the outputs are fine. I've yet to test the drivers and outputs in the other channel as I was focusing on the channel closest to Re52. I will pull those tomorrow. Either way all drivers and outputs will be getting replaced. Is it possible for drivers to go on their own or should I still be looking upstream?
I will go ahead and pull all transistors on the main amp board tomorrow and see if anything else needs to go
petehall347
the brandy coffee man
is it 33 ohms ?
Yes, granted it's a pack of 10 I ordered on Amazon but the reviews were fine, markings are correct and they test as 33
petehall347
the brandy coffee man
something is still shorted then . its on ce35 side of re52 . check with your meter ohms to ground power off .
On the Ce35 side of Re52 I’m getting 0 ohms (33 on the other side)
petehall347
the brandy coffee man
dead short to ground then . you need to find it