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Heathkit DX-100 restore




 
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Author Topic: Heathkit DX-100 restore  (Read 1026 times)
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N8NMA
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« on: January 02, 2018, 07:11:24 PM »

Just wondering if anyone on here has a lot of experience with the DX100. I am currently trying to revive one and have the following. CW works great has the Heathkit mod done. AM works but has static in the audio and then chirps a bit. Have replaced all electrolytics in entire radio and also have replaced all the resistors in the audio amp stage. I have 435 volts on the 12BY7A plate which seems high to me but I also have 700 volts at the 5V4G. The 5R4GY's have been replaced with solid state rectifiers. Also I have changed the audio pot because it did not work right. The new one also does not work right. Goes to about 30% and drops off. I hope that is enough info. Now my question is how to get the static and chirp out of the audio ?

Thanks in advance
Tom N8NMA
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #1 on: January 02, 2018, 09:40:27 PM »

Tom,

Welcome to the BB and your first posting!

I'm not a DX-100 expert, but can get ya started with general stuff... Since you have changed to solid state rectifiers, my guess is the higher voltage has put a strain on the ancient audio coupling and bypass caps -  and the audio chirp and noise is from one or more caps breaking down.   It's good that you replaced all of the electrolytics.

If you don't want to do a shotgun replacement of all the remaining caps, then use a scope and put the probe on each cap looking for the break down. Push them with a plastic rod to get them singing. Also, a direct freeze spray and heat lamp on the components can help locate intermittent noisy parts too. Also, the chirp may be caused by a failing bypass cap that has lost most of its capacitance. The path can be restored by simply jumping a good cap across the suspected bad one to locate the problem.

Remember that "divide and conquer" is a technician's best tool.   Split the audio stage in half by disconnecting a coupling cap in the middle of the circuit. Now look at it with a scope and decide which half the noise is coming from. Divide again if needed.

Tom, K1JJ
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

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N8NMA
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« Reply #2 on: January 02, 2018, 09:56:40 PM »

Hello, the solid state rectifiers were in it when I got it but I will probably put the 5R4GY tubes back in it. I really think it is the interstage transformer breaking down with a short inside but will check some of the capacitors. I replaced all the capacitors other than the disc caps at this point. Thanks for the input.
Tom N8NMA
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K1JJ
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« Reply #3 on: January 02, 2018, 10:31:18 PM »

Sure, it could be an inter-stage xfmr breaking down as well.  And I've had many disc caps fail too.

If you have a general substitution xfmr, clip lead it in place of the suspected one.  Before doing this, I would get it running while watching the chirp and noise on the scope.  Then spray freeze cool and heat it and look for any subtle changes. If no luck, then heat the general underside with a hair dryer or heat lamp and watch.  Thermal change can expose many types of failing components.

Let us know what ya find.

T



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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
N8NMA
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« Reply #4 on: January 03, 2018, 03:31:00 PM »

Hello, well you were correct. After chasing my tale several times I got the air can out and sprayed the two 510pf caps in the audio section and it seemed to help. So I replace them with the only ones I had .01uf 600 volt caps and behold the noise was gone and the audio pot works as well.
Thank you for your help Tom.

Tom N8NMA
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K1JJ
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« Reply #5 on: January 03, 2018, 06:11:50 PM »

Well, that's good news, Tom!

One thing to check out -  the 500 pf caps may have been that value to limit the lows somewhat. If you find that your audio is now very bassy with too much mid bass and distortion, it may mean you will need to put in smaller cap values.   Where are these in the circuit... bypass caps or coupling caps?

In order to get a hi-fi sound cleanly, it may require a lot more work to pass the lows.  

However, the DX-100 has perhaps the best stock audio out there - somewhat better than communications quality.  

The best way to see where you are is to connect a padded audio generator to the audio input and sweep it from say, 70 Hz to 7 KHz and see how clean the sinewave looks. Be careful when sweeping below 100 Hz - you can easily pop the mod transformer if modulated much over positive 70% or so.  Do not let it go negative -100% at ANY frequency, whatever you do. Be very careful not to have a loose connection or plug that causes an audio spike. This can kill that iron quickly.

Here's an article I wrote about sweeping your rig:

http://amfone.net/ECSound/K1JJ1.htm

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
N8NMA
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« Reply #6 on: January 06, 2018, 10:06:00 PM »

Well I am not really looking for HI FI audio but have paperwork for making it sound better. Locals that I have talked to say that it sounds great as it is but may lean to the tinny side. It's a work in progress at this point. Just glad it's working now.

Tom N8NMA
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