No "Loudness" on Sanyo Big Ben volume control.

So here's the problem...

My Big Ben has no bass hardly until the super woofer kicks in (around 6 on a volume scale of 1 to 10.) The bass pot is still working as you can hear a slight rise in bass when you turn the it from 1 to 10. I have swapped both it and the volume pot out and still have the same problem with the replacement.

Is anyone familiar with Sanyo's Auto Loudness circuit? It doesn't appear on the block diagram in the service manual. I'm assuming it has an auto loudness feature because it has the word "Loudness" written around the lower scale of the volume knob, like so:



Anybody ever see this problem before?

Well, firstly, where did you get the replacement pot from? Is it a 7/8-pin pot or is it a true 8-pin pot with loudness compensation circuit? I ask because the center tapped stereo pots are extremely hard to find, and there are some 7 or 8-pin pots out there but the 8th pin isn't a true center tap or the center taps for the two channels are tethered together internally. You need to check.

Secondly, there is loudness compensation circuitry there attached to the center tap. Make sure the appurtenant circuitry are within specs and there is no break in that part of the circuits although because it appears that the left and right channels have their own separate bass compensation circuitry, it would be less likely to have components in both channels fail exactly the same at the same time.

I'm not going to pretend I understand completely how the loudness compensation, center-tapped pot works. From looking at the Big Ben's schematic it seems that the wipers are tied to connections at the top of the case? That's to go with the six pins across the bottom of the pot. I do kind of figure that with lines, grounds, and the wipers attached that seems to leave 2 pins for a pair of center taps? Not sure which ones, though.

As a replacement pot I used a Sharp 100kXA2, same number of pins across the bottom (6) and the same case connections on top so that wires can run to the circuit board (2). It was the volume control out of a parts GF-515.

I agree it seems unlikely that the compensation circuitry would fail for both VR803 & VR903 at the same time. I guess to directly answer your question no, I don't know if the replacement pot I used was a true 8 pin, center tapped pot with loudness compensation circuitry.

Hey Ken,

I'm not suggesting that you should know these of course. But you asked the question about how it works and what the issues could be so I'm just saying it's an area you should look at, and also that the pot needs to be a true 8-pin pot with separate center taps for each channel. Some pots have the center taps for both channels tethered together, or to ground as some circuits are designed that way but the sanyo requires those to be separate.. Oh and just because something is unlikely doesn't mean can't happen. If you are sure the issue is the loudness circuit, that's where it is.

As for how loudness works, it's like how I said. There are additional circuitry (usually a couple of capacitors and a resistor) connected to the center tap, which those additional components tether the center tap to the signal line, and to ground. If you want to see how it works, just look at the schematic for the big ben, and then look at the schematic for the GF-777 (volume control circuit). On the GF-777, there is a loudness switch which connects the center tap to the signal line through a cap. On the Sanyo, it's always connected and relies on the position of the wiper to determine if there is any bass boost. Obviously, the rollover point is determined by the wiper and it's relation to the center tap.

How bassy is the sound supposed to be on a Big Ben with the super woofer switched off? Those main speakers do look modestly sized.

I've only heard one a while back and can't quite recall how they sound except to say that it took a bit of knob fiddling to get it to sound good.

James....

I was never a big fan of the big 3rd woofer type of speaker system that some blasters went with. The Big ben is boomy because of it's amphitheater woofer bay but won't really match an M90 & its traditional 2x8" woofers & 2.75" tweeters....I bet even a Lasonic 931 or 975 would probably own a Big ben for volume & bass, etc. but the Ben will dazzle with better clarity with cassettes & radio, line in etc....

I think I see what you mean, Norm. It looks to me as if the volume goes up and down on both channels then that means the wiper works and the problem should be in the associated circuitry. I see where a cap ties each channel off of the center tap to the downstream Main Amp (IC 703.) That would mean the resister & cap tying the center tap to ground on the other side is acting as a High Pass Filter? I have the original pot back in now.

I wondered what "HPF" meant on the block diagram. Sorry.

I guess I had better take this bad boy apart and examine all of the loudness circuitry.

jimmyjimmy19702010 said:

How bassy is the sound supposed to be on a Big Ben with the super woofer switched off? Those main speakers do look modestly sized.

I've only heard one a while back and can't quite recall how they sound except to say that it took a bit of knob fiddling to get it to sound good.

James....

Click to expand...

It still would sound better than it does now. I just suddenly lost all my bass one day. And that's with the superwoofer turned all the way up. That's how I run all tests. You just can't hear it until you get to 6 on the volume scale now. :annoyed: It has it's own Low Pass Filter & Amp downstream of the volume control.

Ken, I have a similar issue. Did you get to the bottom of this please? Looks like the headphone socket and external speakers are common to both channels before hitting the IC. I'll give those a bit of a clean.

In my case the bass pot read close to 50k as it should but when measuring the adjusting resistance from wiper to end, both left and right readings were off.
So I took the pot out (pain in the ass! thanks Sanyo) and opened it up and found what I thought looked like brushes out of alignment, and a little nudge later the brush fell out so was definitely loose inside and not providing the resistence needed. The pot board looks nice and clean, no corrosion.
I'll put a spot of superglue on this brush so it aligns with the other and fingers crossed we will have resistance restored.
Hope this might help others.







After being glued back in place.

The sub woofer speaker cone jumps, but no relation to the bass pot. It is kind of the same when you turn it all the way up.

Big Ben Sanyo, Sanyo M in general are very fragile units. I am scared to explore more on this beauty.

docs said:

In my case the bass pot read close to 50k as it should but when measuring the adjusting resistance from wiper to end, both left and right readings were off.
So I took the pot out (pain in the ass! thanks Sanyo) and opened it up and found what I thought looked like brushes out of alignment, and a little nudge later the brush fell out so was definitely loose inside and not providing the resistence needed. The pot board looks nice and clean, no corrosion.
I'll put a spot of superglue on this brush so it aligns with the other and fingers crossed we will have resistance restored.
Hope this might help others.

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After being glued back in place.

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Click to expand...

I keep a tooth pick under the curved fingers and tape it all down on a card board, then gently clean from tail joints to the tip. I used 2000 grit automotive quality emery paper. One back stroke you do by accident can destroy all those fingers. It needs surgical fingers.

docs said:

Ken, I have a similar issue. Did you get to the bottom of this please? Looks like the headphone socket and external speakers are common to both channels before hitting the IC. I'll give those a bit of a clean.

Click to expand...

I need to clean the headphone and external speaker sockets. Good advise!

The cleaning of the headphone and external speaker sockets gave an additional 1/3 volume.

It got bass, but the subwoofer volume control has little or no effect.

The subwoofer volume control pot show no effect. I sprayed deoxit to clean it, but didn't open up to clean the feelers and carbon disc.
Sorry to confuse with Ken's post.

Hmm, I have an M7900K that might have this same problem. It goes loud, but the bass is VERY weak, and the bass and treble knobs only have like 10-20% of the effect they're supposed to have. Something the seller failed to reveal. Perhaps someone busted the loudness feeler inside the volume knob. A lot of people are guilty of damaging their knobs (myself included) by turning the knob too many times in quick succession after it's been sitting in the closet for 30 years. The internal grease often dries up after that much disuse, which increases friction and burns/breaks the feelers when you do that.