Re: Encouraging beginners: What are we accomplishing?


Eric
 

Hey Bruce,

    The 485's are fun and you get in to 50 Ohm inputs if you are hoping to do a full cal on it most of the signals are the same as the 475 but the high frequency compensation of the verticals gets a little interesting. you need a fast clean square wave at better then 700pS is spec I think. 1nS is 350 Mhz so you are in to TD pulsers. Does ether scope need deoxit? The attenuater boards in the scope are a interesting material to work with Tek manual says don't use anything on the boards other then alcohol MG has 99.9% in an spray can which is great for flushing things.

I am most likely going to start a storm with this comment but here goes. Deoxit is designed as a cleaner and lubricant/ protectant. I don't think it is harsh enough to strip the gold plating on the circuit boards. However it IS designed to leave behind a contact lubricant to prevent corrosion in the future. What is left behind i expect will very much mess with the high frequency response of the attenuaters and switches. I have an EDC DC voltage reference in the lab and deoxit and shield it are called for in the manual. But this is for DC where the impotent spec is contact resistance and nothing else.

My advice is stick to the cleaning in the manual looking forward to hearing how it goes!

Eric

On 6/12/2020 9:39 PM, ciclista41 via groups.io wrote:
Hello again, folks!

I read this thread several days ago from start to finish. I had no idea my 475 thread had stirred up so much discussion! Some of your posts made me feel really good. Others were interesting, and in spite of the controversy, none made me feel unwelcome. You are all gentlemen!.

I took a break from posting as some posters seemed to question whether the extreme length of a thread such as mine was a good thing. Besides, I had to wait for my capacitors to arrive before making further notable progress, and Digi-Key screwed up part of my order by sending an inductor in a bag labeled properly for the capacitor it was supposed to contain. They were very good about correcting it--even offering to pay for overnight shipping to get me that one capacitor! I told them just to send it Priority Mail, as I am in no particular hurry, having been advised to proceed "glacially." ;-) Also, having typed more words in that thread than anyone, I was ready for a break.

I just reread this entire thread, and thought I would thank everyone for sticking up for novices, which even the original poster did, as well! I want to assure you all that I do have a lot to learn and I may yet do damage to the 475 in question, but so far, I have replaced 5 of the 6 large capacitors (waiting on a sixth round PCB from Alex on eBay), as well as the two tantalums I pulled. I have not turned the scope on since pulling the caps, so can't say for sure, but continuities are all good with the + and - leads of the caps and the ground points that the PCB's restore, so I believe all is well. I was also able to check all of the associated rectifiers, and they all seem to be in working order. I bought a used Fluke 87-V, so I am not suffering from the use of ancient analog multimeters any longer.

The advice you all have given has been invaluable! I intend to allay all of your concerns by posting photos of the repairs made as well as a nice, crisp trace on the 475 when all is well. I just bought a 485 yesterday that has issues, but at least is able to show a trace from my function generator on both channels. I paid $75. Good thing I don't live close to Tustin, CA, or I'd be giving Reed a visit! Ha!

I promise the thread from the 485 repair will be mercifully shorter than the 475 one! Luckily for you, any thread regarding the Philips PM-3218 will have to appear elsewhere. Both channels are working, by the way, but it's waiting on some Deoxit to show up so I can get rid of most of its remaining issues, I hope!

Wishing everyone a pleasant and safe weekend!

Bruce




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