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PostPosted: 26 Aug 2016, 11:18 
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Location: Arizona, USA
Hi Everyone, Even those of us with a bit of experience can get fooled. :eek: My back up Groove phono preamp dropped a channel. :( So doing the logical thing I took it to the bench and started testing. 8-) Sure enough the one channel was dead. There was noise (the normal minimal amount) on the output but nothing went through. I isolated the problem to the first stage. OK, out comes the board (BTW all my prototypes are modular...it makes like a lot easier). Check it over, nothing. :confused: Look at the interconnections, nothing. :confused: Duh what now. So I took of the tube shields (yes I use them on high gain stages)...and...and.. one tube had lost vacuum. :eek: Good brand, unknown reason...just full of air. Put in a new tube and yup...it worked. :thumbsup: So in addition to verifying a device is plugged in, or the device has good batteries (if it uses them), now I'll check the tubes before spending an hour taking something apart. :idea:

BTW I had some fun with the Smilies.

Good listening
Bruce

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PostPosted: 26 Aug 2016, 14:11 
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Some "simple" issues are *very hard* to track down.

I've had more than one power amp fail due to an active device (eg, a driver transistor) test good, even giving the standard hfe, but when voltage was applied, simply turned into a varistor :eek:

Only discovered it by doing a "shotgun" repair (when in doubt, replace EVERYTHING) and with some free time, do a post-mortum on the components... testing the driver on a zener tester, the normally Vce of 160V had dropped to 25V :confused: :confused: :confused:
(die contamination? Doping migration? It was a 40-year-old device)

Cheers!

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PostPosted: 27 Aug 2016, 17:07 
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Hi, Yes, I have done that. A real bummer, especially if you don't find anything and with new parts the device works. On the stuff I have designed I usually can pin point faults very closely. It helps when someone builds a project and it misbehaves. Other stuff, goes back to tried and true diagnosis processes. I have found though that over time the best piece of test gear I have resides between my ears. :) Spending a bit of time thinking about what something is doing is usually a quicker way for me to find a problem than hooking up the troublemaker to all the test gear I have (lots).

Good listening
Bruce

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PostPosted: 11 Sep 2016, 02:50 
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:D When I was an apprentice<<<longer than the dinosaurs ago,

I had an amplifier to repair.
Not knowing about half split I did the usual test from the input to the output<<wrong but I didn't know that at the time.
After a few hours I found the PP output transistor stage was faulty I de- soldered the output transistors and tested them.
The first one I tested was faulty so I replaced that transistor and put it back together.
Guess what it didn't work<<<so I was testing the PSU the resistors etc, etc.
Then one of the technicians said, "what are you doing? you have been testing that **** circuit all day!"
I said well the output transistor was duff but it still won't work.
He said change both of them again..BUT<<<just do it!
So I changed both of them again<<power on...it worked...
The technician started laughing..then he tested the transistors.

He said the new transistor is faulty and the other one has an open circuit on the base...
So did you test the new part before you fitted it? and try tapping the one with the open circuit base<<sure enough when I bashed the transistor it started working...He said I bet it was working before you de-soldered it!
The lesson of the day he said remember this..on a PP circuit if one is duff change both of them every time! And test before you fit!
He said if one has failed the other probably isn't far behind it..
He said on a critical piece of kit you could spend a few hours finding then dismantling and putting on heat sink compound refitting and installing to then rip it out all over again..learn now not in the field!

Then he said there are three more in the back have a go at them<<my heart sank.
He started laughing and said you can go home when they are done. (you did want some over time didn't you) it was Friday at 4.00.
It was one of many learning occasions.. :bawling:


Regards
M. Gregg

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PostPosted: 11 Sep 2016, 16:49 
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That's another one I've fallen for :blush:

Quote:
So did you test the new part before you fitted it?


Good advice! I did a recap of an old Sansui. Worked, but one channel was distorted and got worse as the volume went up. Power stage tested perfect. Separating the pre and power, each worked perfect, just not together.

The cause turned out to be the brand new bipolar on the "Connected/Separate" board (not a cheapie, a Nichicon UES). Was a lemon that slipped through QC. Replacing it and all was well :thumbsup:

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 11 Sep 2016, 21:18 
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Hi Everyone, I've been there with the output transistors as well. Now my rule is always replace both in a push pull output stage. Another thing to look for in a dead commercial product is a fuse soldered inside. Same for bad polarity protection diodes inside.

Good listening
Bruce

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PostPosted: 26 Sep 2016, 08:52 
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In the middle of a long listening session the left channel dropped out with a click. The Class D I was listening to had an attenuator so I bypassed the preamp. All good.

The next day I pulled out the pre to I search for the issue. Everything tested perfect and the NOS GE tubes were both OK. Long story short a commercial interconnect went open for no reason. Swapped in one for my wire-wrap marvels to the whole system back.

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PostPosted: 26 Sep 2016, 10:27 
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mwhouston wrote:
....a commercial interconnect went open for no reason.


I've had that in everything from a generic Monster to Clear Audio cables. No one seems immune :bawling:

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 26 Sep 2016, 21:34 
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Cables got me too...About three years ago... a batch of Audiquest ones that after a few months started to detach inside the RCA jacks. I trashed them and now use ones I make. I use Belden 8412 and REAN RCA plugs. If something goes bad now I know who to blame and how to fix it easily.

Good listening
Bruce

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PostPosted: 26 Sep 2016, 22:42 
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Mine were Van der Hulls. The ends kept unscrewing.

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