NEC/MCS 3236 has a short looking for suggestions

Ric Harris

Active Member
I picked up a MCS 3236 receiver on ebay for 20.00 including shipping and the seller said on channel was out.
Well when I got it yesterday I cracked it open and replaced the fuse for the left channel and it the channel started working but with volume down you could hear crackling. Well I noticed earlier the filter caps looked bad as one had the black covering off the top and the other was lifted. So being I had 6800uf's on hand I replaced them. Fired it back up and sure enough that crackle was still there. So I plugged it into my dim bulb tester and it lit up like the sun. Damn! Well I decided to pull the left channel fuse and powered it back up and the bulb lit up just as bright. I did the right side as well same result. Then I noticed the when it was plugged into the tester the tuner LED didn't have enough juice to light up fully and it no longer worked on FM as it did before I plugged it into the dim bulb unit.
I haven't found any signs of heat damage nor bulging or leaking caps.
I really am unsure where to begin or what to test.
Were still somewhat novice but I won't learn anything if I give up.
 
Greetings;
Since there is no schematic available (nor service manual) I would suggest taking pics as close as possible of the guts. You could search AK via search engine for model perhaps get some hits. You didn't note if you had audio output but you need to resolve the DBT staying bright,....' is power supply issues. (note caps in p.s.you visually saw. but again no schematic you'll have to locate stuff by traces. Most likely all the caps would be replaced aside from tuner but you're going to have test a lot. Inspect for cold solder joints and some component level for transistors crackling.. probably transistor but a leaky cap can make crackles too. I'd make sure the PS is in good shape and locate rails.
 
Greetings;
Since there is no schematic available (nor service manual) I would suggest taking pics as close as possible of the guts. You could search AK via search engine for model perhaps get some hits. You didn't note if you had audio output but you need to resolve the DBT staying bright,....' is power supply issues. (note caps in p.s.you visually saw. but again no schematic you'll have to locate stuff by traces. Most likely all the caps would be replaced aside from tuner but you're going to have test a lot. Inspect for cold solder joints and some component level for transistors crackling.. probably transistor but a leaky cap can make crackles too. I'd make sure the PS is in good shape and locate rails.
I'll shoot some photo's tonight. But can a bad cap create a short I am guessing yes and there's a few caps that have a green film on top. I'll add pics of those as well but I don't know if they're made like that or if it's leaked and rather than being that brownish orange. it green. But will get some photo's posted later. Mama want's to go out to eat so she gets priority.
 
20180112_110914.jpg 20180112_190203.jpg 20180112_190134.jpg 20180112_190045.jpg 20180112_190316.jpg The forth picture has a couple caps that are suspicious and will get yanked providing I have a value that's close. I bought a variety pack of caps from China that has been handy for repairs. On important stuff I use them like temporary crowns until I can get Elna's or the like.
Just another note I kind of doubt the output transistors are faulty in my few experiences the cement resistors that accompany it usually show signs of melting down or at least get hotter than a firecracker. I've got an inferred thermometer and I will measure the temps of all those just to be sure. Nevertheless being I have a full view of the board in the back I'll yank many of those caps just on general principals. The receiver isn't the best NEC ever produced. Build quality is a bit cheap but I've never heard a NEC that wasn't pleasing to the ear. There like Toshiba, Hitachi and Nikko, brands that are not on most peoples radar and really should be.
 
Well I noticed earlier the filter caps looked bad as one had the black covering off the top and the other was lifted
Not and issue if the aluminum top of the filter cap was still flat. Those plastic cap tops are always deforming from the heat of the unit over time.
 
Just another note I kind of doubt the output transistors are faulty in my few experiences the cement resistors that accompany it usually show signs of melting down or at least get hotter than a firecracker.
That doesnt mean nothing.. I've seen hundreds of shorted output transistors that still had serviceable emitter resistors ( cement resistors).And dozens of emitter resistors that looked great on the exterior,but were burnt wide open internally.
 
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I'd start pulling and testing transistors.But in this case it sounds like a short in the power supply. Since you pulled the fuses for both amplifiers and it still was a bright DBT. Unless....you have two shorted amplifiers.
 
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I'd start pulling and testing transistors.But in this case it sounds like a short in the power supply. Since you pulled the fuses for both amplifiers and it still was a bright DBT. Unless....you have two shorted amplifiers.
That's my gut instinct is it being in the power supply as if it's plugged into the dim bulb unit the bulb is getting most of the power and the receiver barely has enough power to run it's LED's. The issue is only on the left channel so an output transistor could be the problem however I do get decent output from that left channel considering it's got that crackle in the back round. What I was hoping to avoid it having to check continuity on everything in hopes of getting lucky. I'm good at replacing things but when it comes to multimeters and knowing where to check and what I am checking for like voltage, ohms and such I'm pretty clueless.
We'll keep grinding on it.
 
Ric, what wattage bulb is in your DBT?

Double check the polarity of the caps you replaced.
Usually I have a 300 watt bulb in it but I broke it and while testing it was a 40 watt as it was the only non-led bulb I had. I picked up a new 300 watt this morning. I'm pretty darn sure I got them in right but I'm going to pull them and test them. I have a plenty of 6800uF caps on hand if ones a dud. These days even the trusted brands will have a dud once in a while. That's why when I order them especially from China I get at least a 12 or more at a time. 6800 and 8200 were pretty common in vintage stuff but 8200's are more expensive than 10000uF and rarely come in voltages or brands I want so I use the 10000's for those apps. Nothings blown up yet;)
Regardless will be yanking those a a couple other suspect ones out tonight. I've got no less than 7 receivers that need a few tweak's so I can list them. The GF scowls every time she walks past my enterprise or in her eyes junk. I can't help it I get a certain satisfaction when someone I sold to on Ebay tells me they love what they got.
In the case of the MCS I have a buddy who collects the stuff and this one he doesn't have. I'll donate it to his Wall-O-Penny's.
 
The 40 watt bulb is probably too small. It may there's nothing wrong.

Have you tried powering it up with the 300 watt bulb?
 
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