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Matching transistors & measuring the results
Matching transistors & measuring the results
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Old 31st December 2016, 07:39 PM   #21
cogeniac is offline cogeniac  United States
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It's a 334A. I haven't tried using it yet.
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Old 31st December 2016, 08:21 PM   #22
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Nice,
Auto-nulling and an AM detector you might not use.
The same thing s apply to these that apply to any other electronic thing. Check the capacitors in the power supply. Switch contacts may need to be cleaned. Do not get any liquid of any kind in that large tuning capacitor or any other trimmer capacitor! Clean very carefully.

The tuning knob might be stiff or stuck. Do not force it! There is a nylon coupler that can break if you do. Take the tuning cap knob apart remembering where everything goes. Take pictures. Once you are down to the actual shafts, clean them with solvent, dry completely. After that, apply some fine, single weight machine oil (zero additives!!). Not very much, and you will probably still use too much lubricant. Slip the shafts back into the bearings and work them until they move free and easily. You might have to clean the shafts and relube a couple times. Clean out any excess oil and also from the entrance and exists of the bearings. Reassemble and try it out. The dial should move very smoothly when you are done. You have to push the fine frequency adjust knob in to disengage it from the dial. I'm assuming you cleaned it as well. Use a little grease where the spring pushes on the shaft of the fine tune knob.

These are a pleasure to use compared to any other hand tuned THD meter. Take good care of it.

Do you have the manual for it yet? It can be downloaded from http://bama.edebris.com/manuals/hp/334a/ for no charge. That is a great site for old manuals for test gear. You can also look at 334A Distortion Analyzer [Obsolete] | Keysight (formerly Agilent?s Electronic Measurement) under "Document Library".

-Chris
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Old 31st December 2016, 08:30 PM   #23
cogeniac is offline cogeniac  United States
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Thanks for the info!

I do have a manual, and there is also a you tube video of a guy showing how to use it.

It is really just a high precision voltmeter with a tunable notch filter. So you put in a signal a some frequency, measure the level, set the filter to eliminate that frequency and measure what's left. Super easy in concept, but I suspect hard to implement in practice!!

Mine seems to be in very good condition, no sticking knobs or wonky shafts...
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Old 31st December 2016, 08:37 PM   #24
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Scott,
Good to hear. Your's is better than any I've seen. Best clean and lubricate before it does get sticky if you feel confident to do it.

You're right about how it works, and yes, it is difficult to actually put it into practice. Examine the circuit and specs for the HF cut-off point. Then be amazed.

THe very low noise needed to measure tiny voltages means that the power supply health is very important. Short the input terminals and see what you read on the minimum scale.

-Chris
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Old 31st December 2016, 10:50 PM   #25
cogeniac is offline cogeniac  United States
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Back when I was a baby engineer, I commented to my boss that HP equipment was so expensive.This was 1977, and I was using a 20 GHz signal generator that cost about $20K back then...He said, matter of factly, that you get what you pay for, and if you want the best, then it was best to pay for HP gear. Always superbly engineered! I recently got a pair of nice Agilent 35 volt bench supplies. I’ll see how they hold up. So far I like them much more than my old BK Precision supply.

I recently rebuilt my old Textronix 2213A scope. The vertical drive was wonky. I found one of the RF transistors in the vertical output stage was cooked, so I found the replacement type and replaced them both. Works great now.. But THEN I found a 1990’s 500 MHz digital sampling scope, with four traces, cursors, frequency counter, etc.. So I am using that now. I’ll ditch the little Tek scope on eBay. I needed the big scope for my digital audio project. Hard to work with 25 MHz clocks when you can barely sync to them, and when you do you can barely see them!

I am surprised by the availability of good used test equipment out there..

Have a happy new year!

Scott
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Old 1st January 2017, 12:49 AM   #26
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Scott,
That's the only reason why I have a bench full of HP gear. Yes, it is the best out there.

-Chris
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Old 26th January 2017, 04:33 PM   #27
cogeniac is offline cogeniac  United States
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OK, so I sorted out the new board, and finally hooked up the output stage. I powered it up and blew a circuit breaker. Grrr!! I traced the problem to the negative side of the output stage. The positive side works great!

Turned out I had a bad power transistor and two open resistors (the 0.33 ohm and 10 ohm resistors associated with his device) in the negative side of the output. Took a while to find it as it wasn't shorted, but had 1.2K of collector-emitter resistance both ways.. I suspect that was seriously messing up the bias on all the other devices. They all appear to be fine, and all are very close in junction drops, etc.

I have a new transistor on its way, so I hope to have this running this weekend. Then I can clean up the control board (put in fixed resistors for the current source trimmers), and button this beast up!

I need to make three small changes to the board to better fit a capacitor, to better fit the heatsink and output transistors (the heatsink pins were off by about .05 in), and to fit the SMT stabistors Phloodpants found. I'll do another set of 5 boards, and report back with the performance.

I am planning to go back over my collection of 565 amps (I have six of these!), and will replace the control boards in all of them with this new board.

Cheers!

Scott
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Old 26th January 2017, 08:10 PM   #28
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The base resistor and emitter resistor for the bad transistor were open? That would take it out of circuit, so the amp should work with the remaining 9 transistors unless something else is wrong. Do you know if they popped open before or after you powered up?

Did you locate an authentic 2SB554? They really should be matched as a set, but you can check their matching in-situ, and under load, by checking the RMS AC voltage across the emitter resistors at various power levels. Check for good matching between emitter resistor ohm values or this test is invalid.

To avoid risking any further transistor blowouts while you troublshoot, you could install just one output—something from the junk drawer—and when you've got it working put the originals back. Just take the screws out of the transistors you're not using to disconnect the collectors, or pop out the emitter resistors.
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Last edited by Phloodpants; 26th January 2017 at 08:28 PM.
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Old 26th January 2017, 08:45 PM   #29
anatech is offline anatech  Canada
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Matching transistors & measuring the results
Hi Scott,
Yes, the outputs really have to be matched. I always match them, then check the drops across the emitter resistors to make certain the current is shared equally. You can use MJ15024 and MJ15025. They are fairly inexpensive, but not as well matched unit to unit as the newer MJ21195 and MJ21196. You can also use those, but I haven't tried them yet in an Adcom. While you're at it, replace the driver transistors after that incident. The transistor doesn't have to be completely failed to be damaged. You can guarantee that they have been operated well outside their maximum ratings until the fuse went. It was a race between the outputs and drivers, and the fuse. Even if the fuse comes in first, the drivers have been very stressed.

I know you didn't want to hear this, but replacing the drivers and outputs is the minimum acceptable repair - especially if you are fixing for someone else. You are worth at least that much, so do it right.

-Chris
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Old 26th January 2017, 09:33 PM   #30
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Sorry, corrrection, parts list location is here...

https://www.dropbox.com/s/odyzrt987b...0list.xls?dl=0
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