The AM Forum

I feel like I did my best friend wrong....

Tonight I did the dirty deed of not muting my SP600 reciever before I keyed up the 4-400 rig.  The Mute relay went out on me so for the last couple weeks I have been muting manualy.  

Besides the loud squeal.... The SP600 now has a loud HUM on incoming signals.  

There is ZERO Hum with no antenna and the audio turned up.
There is No hum between stations.
There is loud hum WITH a big AM signal into the reciever 75 or BC bands.
The incomming audio is a bit distorted now on loud signals also.
The reciever has the same sensitivity, I can hear everyone and all the normal static levels.

If I turn the AVC OFF and back the RF gain way way down, the hum is Very very loud. The hum is less with AVC on and RF gain cranked up.

I swapped tubes around for 15 minutes before I realized I was not going to get off easy on this one.  Any ideas on what I screwed up?

C

Its a lost cause.  Send it my way and I will swap it straight across for a super working, mint S-38.   ;)

Best first assumption would be a diode; they're the most fragile. I'd be hard pressed to think it would have damaged a tube; very unlikely.
Are you running an outboard audio amp or the internal one? If outboard, I'd say untie it and try just the receiver itself. Then, go with a signal tracer from the AF gain pot working back toward the front end and see where it stops. A little digging, you'll find it.

73, Phil

 

i had almost the same thing happen to my nc183. it was my filter cap on the hv. changed them and the hum was gone.

This one got new caps last year along with the HiFi mods.  I was thinking maybe I damaged an IF transformer or cap in that section.  I am going to pull it out and put an SX101A in for now and then toss it on the bench for repair. I will check the filters. Thats easy. Past that, This is a real bear to service so I would love some guidance so I can make a list to check it out.

C

As someone else said, check the detector and front end.

If you're lucky, a leaky diode :)



--Shane
KD6VXI

Also give a looksee at any bypass caps in the AVC line.  A really leaky one caused the same symptom in one old radio I have.

73DG

Ok.. I am going to swap in an SX101 today and then we can have a look at this thing. I think its AVC system.


C

The 6AL5 bi-ass rectifier shorted, or one of the bias filter caps is bad.  The clue here is that the hum is bad with the RF gain DOWN.  That puts max negative bias on the 6BE6.

Another possibility is you fried the 6BE6 and it has a short in it.  I'd get out the scope and look at the bias line and AVC lines.  If you see a lot of hum on the AVC line there may be a shorted tube somewhere else, perhaps the AVC rectifier.

Maybe you fried an input coil, they are usually connected to the antenna terminals.  Thinking maybe an ungrounded coil end might cause the problem you describe, hum only on incoming signals.  Just my thoughts,  but the AVC circuit (as mentioned) seems a more likely cause.  Hum on the AVC line maybe from a shorted tube.

Put a scope on the AVC line and you should see something.  You could also use a DVM on an AC scale and you will see any AC on the AVC line.

In the event you can't fix it and want to throw the set out the window, feel free to send it to me.  My address is good on QRZ.  Just think, you'll save breaking the window.

Fred

Spent an hour checking caps. No bad ones found. however, I did not go into the IF deck as I cant see a way to get inside there.  All other caps under the chassis are "ok".  Alot of them are Vitamin Q caps. Are these known to last or go bad?  I used an old eico cap checker.

Here is what I found while listening to a BC station. 

With AVC ON, Signal is S9 with some decent slight him with the signal. Audio is clear.
AVCS OFF, the audio is distorted heavy and the hum is VERY LOUD.  NO amount of adjustment to the RF gain lets the audio come in clear.  Even with teh RF gain cracked open to the lowest possible setting and with teh AVC OFF, The audio is distorted heavy and the hum is louder then the Signal. I tuned around and even removed the antenna.

Any idea what to check now?  I set a working SP600 next to it and one by one swapped every tube over. Nothing seems to help.

C

FIXED!!  I am so happy.  When I flipped the RXer over I noticed ONE last bathtub style cap 10 UF. I figured, What the heck.. Might as well test it.. It was Leaky.

I took an old 20 UF Sprauge Atom cap and some clip leads and went across it and BINGO, NO hum. Dead silent. I am going to now replace them all.

This cap is a 10UF and it sides near the audio section bolted to the chassis. There is a 022 that runs to a resistor then over to the audio stage.   

I have no idea how this blew out when I keyed up but after not muting the RXer, and I keyed that one time, The hum was there super loud in the room. 

C

Unreal.. EVERY bathtub cap in the radio is bad.  ALL three 10UF bathtubs on the back panel where bad. The ones that are chained up together. I used a 40 UF there. The 10UF near the audio long with a 022 that was next to it was open. 

DEAD silent now. Now hum at all with AVC on or OFF.

I am so pleased that I actualy fixed this radio myself.  It sounds wonderfull and there is not one hint of hum out of the speaker.  Audio is clear with the AVC OFF and RF gain down. 

I cant wait to install it back in the shack!  I used an old SX101 Mk3 for the last few weeks and that radio does not compare to a hifi modified SP600.  I really missed that 8KC filter and smooth sound.

C

Another update.. The Load resistor was actualy the cause of all this. Its a big ohmite in the back.  It was open. The Voltage at the bathtubs was TWICE the 100V rating.  I put a brand new ohmite in and replaced all the bathtubs with 22UF and 47UF 350Volt caps.  This radio is now DEAD Silent. In fact it has never been this clear and clean. 

I have it all back installed with the AADE Freq counter listing to some east coast AM on 85.

C

Coincidence, don't you think, the caps going bad just after an operator error?  Congratulations on no serious damage and on the (relatively) easy resurrection of the beast.

Clark,

    Is this is the same SP600 that crapped out last Jan 15th 2010 after installing LED dial lamps? Here is what you said then:


"Well thats nice..   The 600 just died.  Massive HUM out speaker.. Then Total overload of Reciever on all stations. Audio tube is no longer glowing a slight blue.  Even with the Standard bulbs back in place.
Gotta rip it out of the rack and start checking tubes and Caps!  There goes the rest of my day!"

http://amfone.net/Amforum/index.php?topic=22602.0

 If this is the same radio, I think your more recent discovery could rewrite the Jan 15 thread to a different conclusion.

  I enjoy your posts; always learn something too.

Jim,
WD5JKO

It is the same radio. That time the is caps went. I think I was running this thing with no load resistor for a while now.  The hum started right after I unkeyed. I think I stressed that 10uf cap with the massive squeeling.