D67 is dead... please help
« on: December 05, 2019, 08:03:22 AM »

I built my D67 6y ago. 3y ago i had a project and i was stupid enough to bring the D67 with me on a trip to record sound... it didn't like that and i had some nasty noise in the mic.
I have not used it for 2y and now i plug it but i have very little to no sound and annoying hiss. i changed tube.. same.

Bad news is i can build a kit.. but not debug it. Would anyone here be able to tell me if there's few step by step simple things i could mesure to have a rough understanding where the problem is ?  :-\

thx for help


Re: D67 is dead... please help
« Reply #1 on: December 05, 2019, 08:59:38 AM »
A famous stereo diy’er (kegger) told me once to “assay your constants”.

These mics are not very complicated.  You have a power supply delivering two kinds of voltages, an amplification circuit and an eq circuit, and a microphone capsule.

Do you have voltages in the power supply?  Do they make it to the mic?  Be careful with a hot opened mic, I like to put them on a stand gripping the headbasket or something else that is stable.  Don’t want a 100vdc hot potato flying around.

The tubes don’t fail like that. They can get noisy, the pins can get oxidized and cause noise or the tube can actually go.  This sounds like a dirty vinyl record - constant shhhhshhhshh sounds with some ticks and pops.  I’ve never seen one just poof in a mic.

The circuit is also unlikely to just go bad, however you absolutely could have a cold solder joint. Inspect it.  99.5% chance  that there is not a bad resistor or capacitor, that is too rare to consider, or at least way,way down the list.

Capsule - capsules do a lot of weird things.  Easiest check is to just replace it with a known working one.

They can take on moisture from being left out, you should be storing the mic with silica packs.

Some of my mic troubleshooting finds over the years:
Forgot a solder joint in the circuit
Hiz section needed cleaning
Capsule took on moisture, needed silica
Used slightly wrong screws to attach capsule, capsule fell off stand and was punctured
Cable solder joint shorted (resoldered, improved strain relief)

My guess is hi-z, cable problem, or cold solder joint

Re: D67 is dead... please help
« Reply #2 on: December 05, 2019, 02:47:06 PM »
You said it all.  Most of the trouble i have had over the years is with capsules.  Even the best ones will go bad.  The membrane loses tension and that's all it takes.  Silicon gel pack is essential.  Must keep the capsule dry.  Same with singing into the mic... use a screen, breath on the capsule condenses and the integrity of the insulation is gone..and you have noise... Never heard of an output transformer going bad unless the coupling capacitor shorts and puts DC on the primary.

Occasionally a tube or resistor will go.  There was a case recently where a EF86 was installed in a radio station console (Gates Dualux) and ran for 60 years before the tube failed.   Be careful when working on the D67...that supply probably furnishes 210 vdc HT to the microphone, and the normal 6 v dc heater voltage. 

Re: D67 is dead... please help
« Reply #3 on: December 16, 2019, 10:01:44 AM »
thx so much for detailed answers.
i will try the moisture first.. if that can be fixed.
i found this post explaining it. and true.. the problem might have started when i recorded some crazy sound in a watermill ! in a summer night in potrugal so... yeah humidity was there for sure.
dam ...  :(


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