AC line Connection Load Connections Model A to Model A Connections Auxilary Outputs External Remote Sensing Ground Connections

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Welcome, Guest. Please login or register. Did you miss your activation email? This topic This board Entire forum Google Bing. Print Search. I have recently been working on a Fluke a with a bad case of the power supply flu.

Several bad capacitors have caused a cascade of problems through the voltage regulation and main amplifier sections of the device. I'm still in the process of tracking down and repairing all the bad components, but while I have most of the a's guts splayed out on my workbech, I thought some people here might be interested in seeing what makes this thing tick.

These supply rails are exposed and easily "bumped into" if the case is opened. Back in the day, the list price was almost 12, USD. Now a days you can pick one up for a few hundred on eBay. But these things obviously aren't the reason you'd invest in one of these bad boys. As many of you with degrees in engineering may find a smile overtly expensive some days, it is worth mentioning that no smile is required to operate a a. Inside the device, you will find 6 or 7 boards that are all connected to a main back-plane.

The functionality of the a is logically subdivided onto these boards. Here's the basic theory of operation: The big goal here is to create a voltage servo-controlled amplifier. This sinusoid is fed into the Power Amplifier Assembly yellow tabs , where, as previously mentioned, its voltage is increased by a factor of around three.

The transformer in operation is selected by the range knob on the front of the device mV, V If you want the full V output, the step-down transformers are bypassed and the main amplifier is cranked to a neck-breaking gain of 30 gasp! The output of the active transformer is fed to the front of the device where it is outputted for your use -- diabolical or otherwise. This reference assembly is built around the device's DC voltage standard a ye olde SZA -- which is really just fancy talk for a temperature controlled buried zener.

The error of this comparison, is fed into the Oscillator Control Assembly blue tabs , which closes the servo loop by directing the AC Oscillator Assembly to correct its output voltage to the main amplifier. Couldn't be easier, right? For the careful observer, the board that has been omitted from the discussion thus far is the RCU Assembly grey tabs. It is the optional seventh board that allows the a to be remote controlled.

Detailed pictures of each section with commentary will be added and linked to later. Stay tuned. JPG Nice 'bench weight', looks nice and clean inside.

Looking forward to some nice pictures. The odd multimeter or 2 or 3 or First things first: the Oscillator Assembly. This is where the magic happens. And by magic, I mean tedious and boring. From the manual, Fluke says this is a "double-integrator type, RC oscillator". For any oscillator to work, you need: 1 power gain and 2 frequency selection. In this case, the output of the amplifier is filtered though an RC network and fed back into the input of the amplifier.

The iterative filtering and phase shifting means only one frequency can exist though the feed back process -- all others are attenuated. Luckily if that kind of thing is important to you, you can phase lock the oscillator to an external reference.

On to the pictures. Nothing really exciting here. You see the quadrature, summing, and gain amplifiers on the left, while on the right you find the FET selectable RC networks. More Oscillator Schematics Vgkid Super Contributor Posts: Country:. Thanks for the uploads, I desire more. If you own any North Hills Electronics gear, message me.

I thought I was the only crazy enough to purchase one of those On mine the amplitude is right on but the frequency is a little bit off. It is not an issue because I can check the frequency by other means I also purchased the A amplifier, that is serious hardware, each time I turn it on it sounds like a nuclear reactor and the whole room warms up rather quickly as it uses vacuum tubes.

The cable to link the to the was impossible to find so I had to build one myself. I tested both bad boys together and I was able to generate VAC, really scary stuff. One of this days I'll open the A to calibrate the frequency. If you need any pictures from mine, please let me know. Frank Super Contributor Posts: Country:. Quote from: CaptnYellowShirt on February 06, , am. Quote from: Dr. Frank on February 06, , pm. This amplifier makes me wish I had paid more attention in school But seriously, I did have to pull out my copy of Art of Electronics and re-read the section "Some Amplifier Building Blocks" to get my head wrapped around it.

Speaking of which, a prize to anyone who can answer a trick question at the bottom of the post. As previously mentioned, the amplifier has a fixed gain of around 3. This gain value is increased to 30 when the a is placed in its "V" mode. There are three stages to the amplifier, the first the "Input Stage" is a "differential voltage-to-current amplifier". Q3-Q8 form a push-pull style current amplifier which provides minimal zero-crossing distortion to its output.

The left side of the current mirror Q3,5,7 forms the positive input branch which responds to the input signal provided by the FET Q1.

The right side of the current mirror Q4,6,8 forms the negative input branch which responds to the DC-zeroing feedback path though U1. This feedback path removes any DC bias from the output of the entire assembly. I'm not sure if there's any reason for this feedback path beyond the obvious removal of DC current and wasted power from the Attenuation Assembly and its transformers hysteresis?

Lastly the "Output Stage" consists several transistors which form a "complementary emitter-follower bootstrap amplifier". The manual says the bootstrapping action is provided by CR8 and CR9 -- frankly, I'm not getting that? Answer: I know I'm glad someone else designed this bad boy. This was playing havoc with Q2 on the input stage -- causing the Overload Detector to fire at around Hz. This gave huge spikes on the output and made a really annoying bz.

But after fixing all of that, I plugged the amplifier back in to find several more problems. Long story short: after futzing with it for about 30mins, I overloaded something in the V regulator section.

This caused a failure of R92, R88, and R87 I forget what order they failed in. R87 and R88 are soldered directly on top of a ground plane. So when they failed, I had a V DC plasma arc eating away at the ground plane and the nearby components. R87 came out in several pieces. Note to self: replace with flameproof components.

More Pictures Q15 the Mid-to-Output stage bridge transistor is the big one, top center with a heat sink that looks like something out of Samurai Jack. Thanks for the pictures, that uf kemet cap is rather interesting. Up next is the Attenuator assembly. To output the correct voltage, a series of precision transformers are used to scale the voltage down.

Three transformers with several taps each. It is worth noting the impressive bank of "DAC" relays does the complex switching. The transformers are both shielded and sealed see pictures. I was excited that the shield might have been some kind of exotic material e. Mu-Metal , but alas it was simply tinned steal.

The foam looks to me like polystyrene. The metal box on the top left houses R8 and C3 which look to be some kind of phase compensation for the high voltage divider. It was soldered shut, so I didn't bother to open it. The manual suggests that during a fault's diagnostic work to de-couple the Attenuator Assembly from the Power Amplifier Assembly.

This helps to logically isolate the fault to a certain assembly board. I found it extremely helpful during my repairs! Now how do we verify the output voltage? Sample, FFT, abs. Oh wait We use an ideal rectifier!


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