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#1
Posted to rec.audio.tubes
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"Deja vu" all over again...
Hi, Vacuumlanders,
I while back I acquired a Pioneer SM-B200 AM(two)/SW/FM stereo, tube-type receiver in a dreadful state (had to rebuild broken tuning drives, replace several dead and missing tubes, etc.. looked like it had been in a landfill.) Finally got it all working except the FM (still needs a complete alignment when I get the time.) On the way I found two of the four audio section coupling caps were leaky (6BM8 triode section plate to O/P pentode section control grid.) I replaced all four, of course. Now, a few weeks ago I picked up a much better looking SM-B200A receiver on eBay ("as is", of course) and justified on the premise that if you have one already then two is better! I tested the tubes, ran it up on a variac and found a very low B+ of around 178 VDC (off a 210-0-210 transformer) and a bad 120 Hz hum. There were two obvious faults: A) a nearly dead reservoir cap at the rectifier cathode (replaced), and B) three of the O/P tubes had -15 to -16 VDC fixed bias (OK) and one had only -5 VDC (not at all OK!) This tube was pulling far too much current. Reason: a leaky 0.05 MFD coupling cap - I've replaced two of them to date and will replace the other two a.s.a.p. when I can get the required "round tuit" tool... g Again, FM needs also needs fixing, has a MPX addition, too... this stuff is sent to try us! These caps in vintage Pioneer receivers are all of the same general type. The recent ones I replaced were marked: "SUZUKI OIL JCP-D 0.05 uF WV 400V DC" (on the grey plastic finish, over aluminum body, one black end.) The ones I replaced in the first chassis were marked similarly but, IIRC, the brand was ELNA (same physical size.) This chassis also has SUZUKI and ELNA brand 0.05 uF and another value caps - all suspect, IMHO. In Pioneer tube chasses, these caps are not only used for coupling but for decoupling, too, all over the place . However, they seem less dangerous in decoupling service so I have not replaced them (yet!) It's just crossed my mind that I've had to replace more of this types "for cause", i.e. failed, than I have the wax covered, paper tubular caps in real old radios, the one we all replace for insurance, i.e. the wax ones that are not actually dead yet (but we should still replace them, of course.) So, perhaps it is common knowledge, but I conclude that these little grey, oil filled 0.05's are very bad news! Expunge them! Cheers, Roger |
#3
Posted to rec.audio.tubes
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"Deja vu" all over again...
I've rebuilt a number of Pioneer, Sansui, and Kenwood receivers. I replace ALL of the grey coupling / bypass capacitors. If you take them out of circuit and measure the leakage under high voltage, you will find that virtually all of them are bad. For that reason, rebuilding these vintage Japanese receivers is a very time-consuming process. Sometimes barely worth the effort . . . Indeed, I did some work on a Sansue 500 ( a lot of hours), and still had a ways to go. After a couple years of eyeing my Sansui and Pioneer amps, I ended up parting them out for the very nice iron they possess. I don't see the justification in effort on these amps, as there are a bunch of other classics which are much more reasonable to work on, and worth more. Bob H. |
#4
Posted to rec.audio.tubes
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"Deja vu" all over again...
"Engineer" said:
So, perhaps it is common knowledge, but I conclude that these little grey, oil filled 0.05's are very bad news! Expunge them! Cheers, Roger They're universally used in Japanese amps of that era, and they're leaky wthout exception. I routinely replace them *all* before even turning the amp on. A notorious firestarter is the one that's soldered across the power transformer from one mains lead to ground. -- "Due knot trussed yore spell chequer two fined awl miss steaks." |
#5
Posted to rec.audio.tubes
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"Deja vu" all over again...
Thanks for the comments... pity about the Pioneer chasses.
I think I may be going through a "bad cap" spell. Another project on my bench is the refurbishing (and improving!) a Heathkit A-9C amplifier. It's full of green SANGAMO tubular caps. The worst ones seem to be the 0.05 uF, marked: "SANGAMO Type 33 ..05-600 ..05-600 ..05-600" The larger, green 0.1 uF ones may not be so bad, but who knows? They have rounded ends and are marked: "SANGAMO Type 33 ..1-600 ..1-600" Anyway, I've replaced two, so far (coupling to 6L6 control grids.) They tested OK out of the circuit but at low voltage (on a Fluke 111.) IIRC, they were putting a couple of hundred mV positive (above ground, not cathode) on the grids - but I was not paying attention so did not write it down! Cheers, Roger |
#6
Posted to rec.audio.tubes
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"Deja vu" all over again...
Engineer wrote: Thanks for the comments... pity about the Pioneer chasses. Actually, the Pioneer chassis was so well built that I spent hours stripping all the stuff off of it, and putting it and the cage on ebay for a few bucks in order to avoid trashing it. All I got was one deadbeat bidder. Hey, I tried... BobH. |
#7
Posted to rec.audio.tubes
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"Deja vu" all over again...
Bob H. wrote: Engineer wrote: Thanks for the comments... pity about the Pioneer chasses. Actually, the Pioneer chassis was so well built that I spent hours stripping all the stuff off of it, and putting it and the cage on ebay for a few bucks in order to avoid trashing it. All I got was one deadbeat bidder. Hey, I tried... BobH. Bob, Yes, they look so good that you can't trash them! BTW, got any bits left? I'll pay the mailing... g Very best. Cheers, Roger |
#8
Posted to rec.audio.tubes
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"Deja vu" all over again...
I should have posted them. They're gone now. I'm condensing my toobie
shop into 1/3 the space, so a lot of stuff had to be compressed or parted out. I've got my remaining classic stuff shoe-horned into shelves. Between that, an extension, and a rental house restoration, I've been away from DIY for almost a year, now. Back to it soon. I do have a Bogen amp chassis if you'de like it. It is the heavy guage screen cover type with a standard sized case. It's in the shed now. cheers, Bob H. Engineer wrote: Bob H. wrote: Engineer wrote: Thanks for the comments... pity about the Pioneer chasses. Actually, the Pioneer chassis was so well built that I spent hours stripping all the stuff off of it, and putting it and the cage on ebay for a few bucks in order to avoid trashing it. All I got was one deadbeat bidder. Hey, I tried... BobH. Bob, Yes, they look so good that you can't trash them! BTW, got any bits left? I'll pay the mailing... g Very best. Cheers, Roger |