HH Scott 200

New 1k5 resistors in parallel brought it up to 87v.

Took out the 8k2s and put the new 1k5s in and now getting 83.5v and 79.7v

Edit: Updated on occasion

How could this have been operating for all of these years with a design flaw? And would this cause any sonic changes? Surely techs over the years have seen these voltages.

Would tubes make a difference? I have two pairs of tubes and they behave the same when I swap them around.
 
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At worst, what would happen is the phase inverter output was not ideally balanced. It will run like that, it will just tend to produce more distortion than it could if everything were exactly right. Tubes and amplifier circuits are actually fairly forgiving in terms of how not perfect things can be and have them still operate in some fashion.

If you wanted to get overly fancy with it, a 1K pot in series with a 1K resistor instead of the 1.5K would give you an AC balance control. You'd need a distortion analyzer to get it adjusted ideally though.
 
Well, that's a mystery. Scott's engineers might have flubbed this one. If they had tubes that worked okay with the 8.2K cathode bias resistors, then why do we find correct bias conditions in the other stages? It would be most interesting to measure amplifier power output and distortion for comparison to the numbers in Scott's specification for this model.

Service technicians are not expected to do an engineer's job. Most of them will simply assume that wrong voltages in functioning equipment are due to documentation errors. Distortion analyzers are seldom seen outside of audio specialty shops and the secret laboratories of underground AK operatives.
 
Nice outcome here--an improvement on the original Scott design. It would be interesting to hear from another Scott 200 owner on the voltage they measure at the triode cathodes of V2, 102.
 
This may be going off topic, but I've been thinking about engineering gaffes getting into production. I once took a job with a company who had just fired its senior engineer. The second in command had resigned, leaving only a PCB designer in the department. I worked overtime to finish the electrical design of a new product, which happened to use a number of transistor array chips, and handed it off to the PCB designer, who did a terrific-looking job with the artwork. The boards were rushed into production, but the production test guys couldn't make any of them work. After several days of struggle, they finally asked me for help. It turned out that the PCB designer had swapped transistors in some of the array chips. Now if you're not familiar with these devices, understand that the transistors are matched and interchangeable except for a single detail: One of the transistors has a leg tied to the substrate. If it's an NPN array with one of the emitters tied to the substrate, for instance, then this emitter lead must be at the most negative potential in the device to avoid current flowing through the substrate to other transistors.

After initially blaming myself for this fiasco, I realized there's a more nuanced explanation: company culture. The PCB designer had no authority to alter anything in the schematic drawing, but he was unwilling to request an engineering decision because the previous engineer had been an autocratic bully. In addition, the PCB guy was working around the clock because top management was desperate to have the product for an imminent trade show. The production test technicians were busting their butts for similar reasons, which only delayed the resolution.

I haven't got a whisper of a clue about the culture at Scott when the 200 was designed, but my little story might suggest an explanation for the wrong resistor value we discovered. This stuff really happens, even at NASA.
 
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Got tired of waiting on a resistor (its only 10% out) so brought it in other room for source and speakers.
Disappointed.
Like an old radio. Or a slashed driver cone.
All switches and pots worked great. No background noise. Good volume.
I turned the bias pots both ways but no test points (click only) up top to monitor. Speakers liked it when I turned them down to where there is the lowest voltage. But it lost a lot of instruments. Tested to be .110v and .210v. I had it set to .35v per the design drawings.
Did swap out the pre tubes and no change.
 
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Lost a lot of instruments? Not sure what you're trying to say here. It's probably best to restore correct bias in the finals and then describe the symptoms in plain language. Is it equally distorted at all volume levels? Equal distortion in both channels?

The only thing that comes to mind that might not have been discussed is screen stopper resistors at the finals (pin 3). Replacement of power supply filter caps with modern electrolytics can sometimes provoke RF oscillation, which can be intermittent and sensitive to the audio waveform. If this is causing the distortion and you cannot confirm it with a scope, then it's best to install 100R resistors with near-zero lead length in series to pin 3 of every output tube as a diagnostic and preventative measure. I recall some discussion of DC voltage measurements that were sensitive to physical position of your body, which is a strong indicator of parasitic oscillation. Is this phenomenon still evident?
 
Will do further testing this evening as there are a couple of things that I didnt try. It was sounding fine before I "tweaked" it.

The sensitivity to physical position relative to the unit is there but it is a small number. I was incorrectly trying to measure measure current or voltage at the test points when I commented. The test point may be 1 mV when three feet from the unit and jump up to 40 mV when i get near but then falls back down...and up...and down until it stabilizes.

AC voltage does the same thing moving from about .04v to about .20v. When I flip the wall plug, the same thing occurs except in a different range (one way higher than the other).

I also feel a very slight charge when I touch the chassis on occasion.

The only place that I can see the "ground loop" go to chassis is at the bias test switch.

The wall plugs are not grounded in the house.

Dont have a scope.
 
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Went back and checked every component I replaced. Found that R204 was a 3k3 but design drawings show it to be 3.3ohm.
But the original build sure looks like a 3k3. Am I missing something or maybe not that critical? Those two 1W resistors in photo are coming off of C202 square.
bettershot.jpg 33.jpg
 
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Put on the tube shields, put speakers on the 4ohm taps and let it warm up for 4 hours. Sings again.
Maybe the new cold tubes were the problem?

Calling it good. With all of your help I learned that the schematics incorrectly call R20 and that R35 should be 1.5k in lieu of 8.2k.

Thanks again to all.

Oldman
 
Good show. I hope this thread isn't too convoluted to be helpful to other folks who encounter problems with the same model.
 
After a mishap, I am getting about twice the current at the output tube cathode. The schematic:
http://www.hhscott.com/pdf/fs/200-C1.JPG
has no voltages shown on the bias supply from the rectifier to the grid so I have no test points to compare what the bias voltages should be.
Is there a voltage chart or something that I am missing?
BTW, I am getting -6.3v at the grid (pin8).
 

Since I cannot get under about .6vdc in lieu of .35vdc at the test point (10 ohm cathode resistor), I figured that I have a problem with the bias voltage. However, there are no voltages shown anywhere from the diode to the tube on the bias supply in order for me to try to track down where the problem occurs.
It would have been nice if a voltage was shown after the adj. pots.
Sunday, February 11, 2018.jpg
Thanks for reply.
old
 
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