G-7700 near-basket-case where to start?

willray

New Member
Greetings AKers,

I come seeking wisdom on G-7700s and their ilk.

I have in my possession a sadly mistreated G-7700. eBay purchase. The seller appears to have tried to disguise an internal fault by doing a completely inadequate job of packing, hoping to then claim shipping damage and insurance.

The thing was shipped in a toilet-tissue-packing box that was about 10 inches too large in every direction, with two pillowcases - not pillows, pillowcases - thrown in, and no other padding. There were pieces of the shattered wood case sticking through the box, and a fair-sized hole ripped through one end of the box, through which some of the trim and knobs apparently leaked during its journey. The aluminum faceplate is dished in in the middle, and the backing plate that carries all of the controls is now shaped kind of like a steel drum.

Miracle of miracles, the thing is /not/ electrically dead. Pried bits of the chassis back away from the boards and made sure that nothing obvious was shorting, figured "what the hell, how much worse can it get", and brought it up on a variac. Who'd have believed it - no smoke. She still starts up, comes out of protection, and lights up her "safe to operate" lamp - so maybe this thing is still worth fixing!

On Aux input, I've got both channels! One is /way/ down compared to the other, but other than static from the dirty controls, doesn't sound horribly distorted. Quite a lot of 60Hz hum bleeding through at zero volume, but it doesn't change with the volume control, so it's getting into the output stage rather than the preamp.

Didn't have the service manual handy when I was poking around in the mangled remains, so I wasn't sure where to check idle (and after reading the SM, and a half-dozen threads that disagree on the topic, I'm still not sure), but I did check DC offset on the speaker terminals. One's "not bad" (everything considered) at 22mV. The other, is showing _negative_ 120mV. Seriously - negative - I know which way to hold the probes…

I'm not even going to screw with trying to adjust things with the original trimmers - will wait until replacements get here from DigiKey - but does this ring any bells for anyone? Any obvious components I ought to be looking at first? I'll probably rebuild a bunch of the F-2980 board anyway - what else is there to do while I wait for the snow to melt - but I'd like to make sure I don't miss any likely suspects.

Thanks for any advice,
Will
 
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If it's that beat up I would absolutely demand a full refund and that the seller pay to have it shipped back - no ifs ands or buts about it. Be prepared to get eBay involved because there is a good chance this seller will not be cooperative.
 
why don't you simply send it back for a full refund ?
the dc offset doesn't matter if + or - ,swap the meter probes over it bothers you .
 
why don't you simply send it back for a full refund ?
the dc offset doesn't matter if + or - ,swap the meter probes over it bothers you .


Ah, I should have mentioned, there's a bit of a silver lining to the eBay story: Seller tried to refuse a return, eBay said "send it back" and gave me a shipping label, so I wrapped the remains up nice and sent it all back home.

The idiot seller refused the delivery, so the thing came back to me, eBay issued a refund and said "heck, if they don't want it back, do whatever you want with it".

So, now it's a badly-beaten, but (other than any repair parts I drop into it) free to me, G-7700. I'm sad to see a machine that once was this nice, now in this condition, but, over all, it's not a bad deal.


(and seriously? DC offset polarity doesn't matter? I'm going to have to scratch my head about that.)
 
Ah, I should have mentioned, there's a bit of a silver lining to the eBay story: Seller tried to refuse a return, eBay said "send it back" and gave me a shipping label, so I wrapped the remains up nice and sent it all back home.

The idiot seller refused the delivery, so the thing came back to me, eBay issued a refund and said "heck, if they don't want it back, do whatever you want with it".

So, now it's a badly-beaten, but (other than any repair parts I drop into it) free to me, G-7700. I'm sad to see a machine that once was this nice, now in this condition, but, over all, it's not a bad deal.


(and seriously? DC offset polarity doesn't matter? I'm going to have to scratch my head about that.)

Pete is quite right, polarity doesn't matter, however magnitude DOES matter. :yes:
 
I hope you have taken a boat load of pics, and lots of notes to establish a baseline. Would you like a template for the F-3058 bd?
 
Heh this sounds like fun

Post up the sellers name so we can steer clear.

Sounds like this 7700 traveled 3x more than it needed to!

Must see pics. Would love to see how it arrived!!

Pretty sure the guys here can help - they are gifted.
 
You have your work cut out......

These things can be a complete nightmare, and require a lot of patience and blowing up new transistors......


Get some pics up so we can see what sort of a nightmare it is...
 
Greetings all,

I'm only down at the shop where the receiver is located on weekends, so I'll get some pictures up, probably early next week (unless I can find the photos I already took of the box when it arrived).

I already ran the seller off of eBay (hint - swearing at eBay customer support isn't a way to win cases :), so their original ID won't help, but, if you ever run across someone with an ID that might translate to "little corbe", caution would be advisable.

A couple questions in the mean time, while I wait for the weekend to roll around and another go at the beast:

Is there any sane reason not to bypass the bias-adjustment trimmer with, for the sake of argument, a 100 ohm resistor, and then use i.e., a high-turns 1K trimmer? You'd still get a 90 ohm adjustment range, and it would have much better immunity to wiper noise/drop-outs on the trimmer. Clearly, you'd lose resolution on the low end of the trimmer, but even down at the bottom, with a 25 turn 1K trimmer you'd still get the lowest 50 ohms in 2.5 turns, which is better than the original.

A smaller-ohms bypass should be even better, if the full 100 ohm range isn't needed, and a maximum safe resistance could be worked out at less than that 100 ohm value.

Of course, I'm /not/ a EE (physicist, instead), so perhaps I'm misthinking something here.

Any thoughts on the low channel? No obvious changes in level (un) balance from wiggling controls and pushing buttons. Chase component values, or contacts first? (Yeah, I'll inject a signal and start chasing it around. Would have done that last weekend, except it looks like the kids "borrowed" most of my grabbers - time for another order of those too).
 
If I have done the math right:
100Ω in parallel with a 1,000Ω would give you 90.91Ω.
A 25 turn 1,000Ω pot would be ~40Ω per turn.
One turn down from 1,000Ω should be 960Ω, combined to yield 90.57Ω.
But this is not linear as 23 turns yields 44.44Ω and 24 turns yields 28.57Ω.
Hopefully you can set your bias with the mid range of your 1kΩ pot, or your 100Ω pots will arrive soon.

Stability of these circuits is flakey at best, so go slowly. Let it warm up for 30-60 min before adjusting bias. The feedback loop can run up so fast that it can blow your OP TRs PDQ.

I have no suggestion for your low channel without additional information. Is it low for all inputs? Any data points that you can supply may be useful. Too much is better than not enuff.
 
If I have done the math right:
100Ω in parallel with a 1,000Ω would give you 90.91Ω.
A 25 turn 1,000Ω pot would be ~40Ω per turn.
One turn down from 1,000Ω should be 960Ω, combined to yield 90.57Ω.
But this is not linear as 23 turns yields 44.44Ω and 24 turns yields 28.57Ω.
Hopefully you can set your bias with the mid range of your 1kΩ pot, or your 100Ω pots will arrive soon.

Right - zero to 50Ω in the parallel combination would live in the 0-100Ω range of the trimmer (2.5 turns), but the bypass resistor would absolutely prevent wiper noise on the pot from ever appearing as a momentary open in the bias adjustment, which if I understand correctly, is instant death for the outputs.

I'm kind of a belt-and-suspenders sort of guy, and I can live with an adjustment that's more fiddly, if it's also less lethal.

Of course, it wouldn't be hard to tune a setup like this to have decent turns-resolution across the actually needed Ohms range, if we knew the true useful adjustment range on these things. I have a hard time believing that some of them dial in at 1Ω and others at 99Ω...

More data on the wreckage forthcoming, as soon as I get back to it.

Thanks!
 
I believe that the bias current required is a function of the parameters of the components in the circuit. Change a cap and you may have to adjust the bias. Time and use will change the parameters of most components and even slight changes in values are not always offsetting and the composite total can be noticeable. This is why it is wise to check the DC Offset and Bias from time to time.

Even if the only thing you change is the trimmer and you set it to the same value as the old one, you will probably need to adjust the bias.

Just my humble opinion . . .
 
Even if the only thing you change is the trimmer and you set it to the same value as the old one, you will probably need to adjust the bias.

Just my humble opinion . . .

… Now I'm not sure if I'm miscommunicating terribly - have I appeared to suggest otherwise?
 
Just my humble opinion . . .

Bits from Digikey should be here soon, and I'll try to get some more details on the current state of the system this evening, if I'm still awake once I get down to the farm.


… by the way Steve, the system says your mailbox is full, so I can't PM you...
 
"Change a cap and you may have to adjust the bias."

No. Bias is DC. the only way a cap affects bias is if either the old one or the new one (but not both) has electrical leakage. (not electrolyte leaking out, but conducting DC)

Anyway, there are standards for shipping these things. I work at a music store and I ran acrosss a thiing they printed up on how to pack an accordion for shipping. There ight be something for audio equipment as well.

Not long ago we rehabbed and sold a nice Tandberg 9400 on eBay. It arrived, the guy said it was dmaged. Wasn't too cooperative. We gave him the refund to maintain 100 % possitive feedback. Plus the fact it sold for about half what is was worth.

Although eBay is pretty popular because of extreme growth, I believe their policies are making it less attrtative for both buyers and selers. As such there is a new policy. Item will be photographed as it is packed to prove the packing is adequate. Buyer will cooperate in cae of damage and that means sending pictures of the damage. Buyer is paying shipping of course so they are paying insurance on twice the value of the unit. If it is damaged we split the money and you get it for free.

However in THIS case from what I've gleaned, the seller did not issue the refund, eBay did, correct ? In that case he is jammed up. They will sanction him. they will hold his ooney for a time before releasing it. He will find he made a big mistake by not being cooperative. What's more, eBay money comes through Paypal, so it is coming straight out of his bank account.

but the problem is, because of assholes like that, eBay policy has changed so much in favor of the buyer it is unfair to the seller. We firmly believe the guy who bought the Tandberg scammed us. I don't have all the details because I don't handle that end of it, but I got the idea.

Anyway, if none of the PC boards are broken (even if they are), almost anything can be fixed. You'll have to take every part off those front and back panels to properly straighten them out. But since you got pretty much a working unit, it may well be worth it.

I would screw with it electrically. Just get it working, forget the caps for now. Oncew everything on it works then get to the cosmetics.

If you really can't make it look good, it can be buried on a shelf as part of a biamped system or something. There are also valuable parts of course.

Pillowcases. tha tis so stupid I would highly suspect he was trying to scam the shipper. There are alot of dishonest people out there.
 
Tandberg 9400? Is that something from the data storage or video conferencing product lines, or just a typo? No such model in the audio lineup...


John
 
"Tandberg 9400? Is that something from the data storage or video conferencing product lines, or just a typo? "

Oops, it's a "remembero". It is a 9200.

http://www.hifiengine.com/manual_library/tandberg/9200xd.shtml

Sorry.

"I was being anal and inferring that any change, even the ESR of a cap, could cause the bias to need adjustment"

The problem here is that his is simply not factual. The ESR of a cap is SPECIFICALLY referring to its AC impedance, not its DC impedance.

YES, it is proper, but not normal to express DC resistance as impedance because it is.

Impedance is the vector sum of the DC resistance and XL of a coil or the vector sum of the ESR (effective series resistance) and XC of a capacitor. Each number is affected slightly by other factors, the capacitance between the windings of a coil or the DC leakage of a capacitor, but these are generally considered insignificant in all but the most critical cases. They are insignificant when talking theoretically of course unless you want to. Fact is, ESR has absolutely nothing to do with DC. To say otherwise is WHAT ?
 
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