Trimmer pots go bad?

M

MyHeresyIsMe

Well-Known Member
I am working on a Sansui 881 that I picked up for a song and it has a lot of good things for it but it's shows a lot of signs of neglect-----like rust on the frame, the bottom cover, and the inside is really filthy. I cleaned it up (not the rust, not yet anyway) but the one thing that I tested for today was the idle amperage. It should be 30ma+/- . It showed 36.1mA on the left, 40mA on the right. The right channel I turned down but only to 38 before it ran out of the trimmer pot stopped. The other channel did not change at all, no matter how much I turned the trimmer pot.
Is this showing that the trimmer is shot or does it point to something else on the driver board? Thanks for any help you can give.
 
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It is highly advisable to replace trimmers in old amps like this as the originals tend to develop dead spots from lack of adjustment for years (or decades) at a stretch. Multi-turns are much better because they permit for much finer adjustment of bias than the single turn types installed by the factory. This makes it much lless likely to over-bias and burn up a channel.

That said, there may be other passive components in the driver circuit (low value resistors especially) that have drifted far enough off spec to make dialing bias in "dead on" impossible. I don't know about the 881, but the 9090 and related models - the successors to the 881 line - have a terrible problem with low value flameproof reaistors on the driver boards going off spec enough to cause catastrophic failure of the output stage. I'd look at that as a potential issuenand replace as neccessary to insure stability.
 
Sansuiman said:
It is highly advisable to replace trimmers in old amps like this as the originals tend to develop dead spots from lack of adjustment for years (or decades) at a stretch. Multi-turns are much better because they permit for much finer adjustment of bias than the single turn types installed by the factory. This makes it much lless likely to over-bias and burn up a channel.

That said, there may be other passive components in the driver circuit (low value resistors especially) that have drifted far enough off spec to make dialing bias in "dead on" impossible. I don't know about the 881, but the 9090 and related models - the successors to the 881 line - have a terrible problem with low value flameproof reaistors on the driver boards going off spec enough to cause catastrophic failure of the output stage. I'd look at that as a potential issuenand replace as neccessary to insure stability.
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I thank you very much, Sansuiman, that was very helpful, especially the part about the low value resistors.I had no idea that there could be a fire threat. The first lack of performance that caught my attention was the very poor FM reception whereas the Sansui 771 that I have sitting right next to it is wonderful in pulling in stations as far as 50 miles with a 300 ohm dipole. Now that I am retired, I can do a little work on amps & receivers, but the tuner board & the variable capacitor is way out of my league. I will order caps and resistors for the driver board as well as the trimmer pots.

Thank you again for you word of help.

Bob
 
I should clarify, not so much a fire threat (though the resistors in question are typically non-flammable type) but more a matter of runaway bias due to their original value drifting and then blowing the output transistors and associated circuitry in tbe driver amp.
 
Okay, yes. I overstating that but I knew what you meant. By the way, I know that the 9090 model as the same tuner board as what I have in my 881. Mine was manufactured in June 1970, whereas the earlier 881s had a different tuner board number. I don't know the exact component difference, but the HiFi Engine schematic does not have info for this F-2549 tuner board (the same number as the 9090). Thanks for response! BTW, I see you are where I lived & worked since 1976 ---- (Webster Groves for grad school, SLU post-grad).
 
A fair amount of similarity from the 881 to the 9090 in terms of basic circuit design. I was unaware of the running change in the tuner, Sansui did that sort of stuff a lot and almost never issued service manual supplememts to reflect the updates. The driver/output stage is actually lower power version of that utilized in the 9090.

I'm St. Louis born and raised, the place suits me fine. Besides, the whole family is here still and I've built up a pretty solid business for myself here in the laat several years. I take it you went to Webster University (College prior to 1981 or so)?
 
I have a fair amount of Sansui units, including one that has a blown channel and keeps blowing fuses --- Sansui 7000, also at the change of the decade, I believe, around the time of the Sansui 5000a which I have up and running in my system now, all original but, as you said, I cannot find a matching schematic for this version. It is one well designed driver circuit that has two driver boards and I will be getting a recapping kit for it tomorrow. It is rated at 55 watts rms / channel, and the FM tuner really shines. It has that 'tubey' sound.

In Webster I attended the 'other' school across Lockwood from Webster U, Eden Theological Seminary. I was ordained in 1980, stayed in the Metro area until 2003, came back 2015 and retired in 2018. Then moved to my birth-town, Evansville, IN, where my wife has a daughter. My "home" though is STL and try to get over there when I can ---get to The Hill and various other places. Charlie Gitto's downtown, when I can. Yep, my 'home' town.
 
Sansuiman, On a side note, do you know anything about Sansui SP-2500 speakers? I've never had Sansui loudspeakers but I found a pair in STL. Just curious.
 
I have a pair of 2500's myself I'm getting ready to replace the woofer surrounds on. The original cloth surrounds harden with age, making movement of the woofer cone exceedingly difficult and really impacting the low frequency output as a result.

At this age the crossovers also need their capacitors replaced to sound their best. This is true of the whole model line (1200, 1700, 2500, 3500, 5500) but is especially bad on the 3500's for some reason. I also have a pair of 3500's and multiple pairs of 5500's.

Personally, I like them. But, Sansui speakers generally take a beating in discussions amongst purists with more refined tastes. Partly for their multi-driver designs that are considered unorthodox (by current speaker design conventions) and are often cited as having poor or inaccurate imaging because of the multitude of mid and high range drivers. They utilize horn mids and tweeters, which seems not to be popular anymore for most serious listeners due to perceived harshness versus cone or dome type drivers.

This stuff is heavily subjective, and while it is possible to build a poorly conceived speaker, most of Sansui's speakers from roughly 1968-1975 are very well built with exceptionally solid cabinets and quality drivers. They just don't comport with present dah thinkng on what constitutes a "good" speaker design.

In short, if you buy an all original pair, they will need the crossovers recapped (easy enough) and likely the woofer surrounds replaced as well (a bit more difficult) to get them to live up to their potential. If the price is reasonable, I'd say go for it and recap them before giving them regular use. Then you can decide if the low end sounds lacking, and plan to replace the surrounds.

The 5000A (5000 and 5000X as well) is a great receiver, and has finally received the recognition it is due in the past couple of years. At only 55 wpc, it delivers well beyond expectations. I have an A and an X both with repro 6013 driver boards installed, and they sound great. Excellent tuners as well, though there is little left on FM anymore I care to listen to.

Any other questions feel free to ask.
 
Again, I thank you for sharing your knowledge about the Sansui products in general, but especially more so about the speakers. My history with loudspeakers has been limited to KLH, Large Advent, Boston Acoustics, and ---a great departure---the Klipsch Heresy. No prejudice in 4-way or 5-way, but simple expedience----I was close to Hi-Fi-Fo-Fum in the late '70s where I could pick up a pair --then another pair-- of Large Advents. I am on the look-out for vintage Sansui loudspeakers (and your mentioning of the years 1968-1975 was very helpful).

My Sansui 5000a came with the F-6013 boards but they are original. Despite the fact they are 50 years old, the receiver does sound very nice and yet I think there MUST be leaking capacitors or out of tolerance resistors somewhere. Those parts will arrive today but I may not redo the boards today; I will replace the bayonet-base lamps.

You have been a well of experience with Sansui and their technical history. Thanks again!
Bob
 
ConradH said:
I think the 881 has about 8 fusible resistors on the power circuit that need to be changed. Also, change C11, as it tends to go bad due to ripple current.
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That is very helpful advice, ConradH. I need to get the value of the fusible resistors, as my schematic is inaccurate in some places. That's why being on AK is such and aid for me, especially the Exclusively Sansui. I am not going into unchartered territory and someone has done this, perhaps years before me. Thanks again!
 
Fusible resistors will usually be lower values, though not always. The usually have a unique visual appearance, flat, not shiny, but not always. They may be labeled on the schematic or legend, but not always. Remember, any resistor that measures high in-circuit, is bad, because you can't put anything in parallel with a resistor that will make the value go up. But not always! If the circuit has any residual voltage from charged capacitors, they can fool the meter into reading high or low ohm values. Be sure caps are completely discharged before making ohm readings.
 
ConradH said:
Fusible resistors will usually be lower values, though not always. The usually have a unique visual appearance, flat, not shiny, but not always. They may be labeled on the schematic or legend, but not always. Remember, any resistor that measures high in-circuit, is bad, because you can't put anything in parallel with a resistor that will make the value go up. But not always! If the circuit has any residual voltage from charged capacitors, they can fool the meter into reading high or low ohm values. Be sure caps are completely discharged before making ohm readings.
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That's a ton of useful information! I need to take pictures of the boards and mark with a circle with a question mark when I have no idea what that is. One question: to discharge a capacitor (when the power is off and even unplugged from the wall socket) is it enough to short with a screwdriver blade, or will that do more harm to other components?
 
MyHeresyIsMe said:
is it enough to short with a screwdriver blade
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Never, never do this, especially as you don't need to do it - just leave the unit for a few minutes.
MyHeresyIsMe said:
or will that do more harm to other components?
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It might do harm to the capacitors you are discharging, (and the poor screwdriver), but also might damage other components like unobtanium semiconductors. AND - what if you do this but have forgotten to turn the unit off? :yikes: - it's just a bad habit to get into.
 
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Hyperion, you just saved another unit from my disastrous ways. I would have unplugged the unit, and I sort of thought other components could get damaged, but I'm walking that thin line between 'an advanced beginner' to 'knows enough to dig in'. I know I'm not there. I worked on Fender amplifiers in the 60's, but no pcbs then, and a VOM did just fine.

Thanks for your advice!
 
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