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Magnavox 9304 DCG troubleshooting help

RUmad

RUmad

Super Member
Apparently I messed up my Maggie today. It’s a 9304-00 that I modified to the DCG circuit, but with original OPT’s and no buffer. I don’t use it VERY often, but when I do it’s been perfect. Always reliable, sounds great, runs nice and cool. I finished building a prototype preamp a couple weeks ago and for testing/debugging I switched between this Maggie and my 8802 (also modified in DCG/thorpeJ fashion). Figured they could use the play time.
So anyway. Today I decided to remove the 100k pot and just retain the 470k resistors. After that, while still on the bench, the left 6EU7 must have been crooked so I pushed it straight, heard some crackling and saw a small arc in the tube. Long story short, after thinking the socket was faulty, I found that the power supply wire (pin 7) was a cold joint. Soldered it proper and figured I was good to go. The cackling and arcing is fixed. Here’s my current trouble, that I can’t figure out and frankly don’t know how to test/troubleshoot.

I left it on the bench to warm up and make sure nothing else going on, and after a short while I noticed the transformers were all very hot. Like I said this runs very cool normally. I decided to check the bias, mostly because I mounted the EFB pot in a stupid spot and I easily could’ve bumped it. Turning the potentiometer that controls the EFB does absolutely nothing. No voltage change whatsoever. I shut it down immediately. No red plating, I just didn’t want to chance burning up my tubes or trannies. I’m totally baffled. I wasn’t working anywhere near the EFB regulator, but I assume I did something. I would really appreciate some input here. I don’t see anything out of place. Nothing looks or smells smoked. I don’t know how to test these things. I can get some voltages if anyone needs them tomorrow, probably get pics if necessary tonight. I’m having an ‘all thumbs’ day today and I’ve packed it in.
 
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I meant to also say that I bought a whole bunch of the regulators when I put these Maggie’s together, but I don’t like just replacing components willynilly untill the problem (hopefully) goes away.
Also that it’s amplifying fine (I guess) but the speakers on the bench are 5” point source, so I can’t really tell HOW well. There’s sound anyway. Thanks y’all.
 
check the voltage output of the pot wiper. If the voltage there varies, but the voltage at the cathode does not, it pretty much has to be the regulator at fault. You can do this with the tubes out if you'd like.
 
Excellent. As always, gadget, thank you. I’ll get right to it in the am. Could it have been the arcing that caused it?
 
Yeah. Difficult to get in right without moving wires around. If this doesn’t help, let me know specifically what you’d like to see. I’ll try and get in there.
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Thanks RU, I have a moto HS711 that I'm going to install Daves EFB in and I'm just researching as much as I can. I'm a relative noob to the tube amp world and trying to figure out the best way to install in my amp. Would you happen to have a pic of the general underside? Thank you.
 
I’ll find out tomorrow if that’s what it is. Whatever the case, I sure would like to know how or why it happened so I can not do it again.
 
oh, I should mention, the rectifier does need to be in for this, but the other tubes can be out.
 
Ok gadget. Things got complicated cuz the wife stayed home sick. I’m about to set up. Always fun. :confused:
 
Ok so, measuring from chassis ground to the wiper I have movement. All the way clockwise it’s at 1.967. Counterclockwise it’s 2.002. Not a whole lot of swing but it consistently moves with the pot. That seem right?
 
Ayuh. I bought a whole package of em when I built these amps. Actually because I figured I’d find more Maggie’s to rebuild. Wasn’t thinking I’d need to replace em lol. Whatever the case, she’s back up and running. Appreciate the help gadget. Now, let’s see if I can remove the pot from the 8802 without toasting anything.
 
Hm, that sounds awful low voltage at the wiper, the reg must be shorted. I'd expect around 18v at the wiper, +/- a volt or so. SS things tend to fail in a rather less than considerate manner when they die, so no surprise there.
 
The new regulator did the trick. Don’t know what the hell I did to it, but it’s fine now. I managed to do the 8802 without hurting anything. Both back to power amp status, the way it is designed to be. You always jump in and help, gadget, hope I can get you back one day. Thank you.
 
RU -- Documented regulator failure (that is, those events I'm aware of either here on AK, or made aware by PM or client correspondence) is actually quite rare -- obviously not completely absent, but very very small compared to the estimated number of installations. All of the occurrences have involved either a significant catastrophic tube failure (most rare), or a very sharp transient spike, much like you had with your loose connection (almost all events).

As a result, I now recommend bypassing the regulator with a small (typically ceramic) cap (.005-.01 uF) connected directly between the IN and OUT terminals at the regulator. There is already an electrolytic bypass cap connected across these terminals within the EFB™ circuit for the purpose of stabilizing regulator operation, but with values as large as 100-120 uF, these caps aren't very effective in bypassing supersonic transient spikes, which if large enough, can then take out the regulator. By adding a small cap as recommended, the regulator is then effectively bypassed against such spikes, making for even greater long term dependability.

I hope this helps!

Dave
 
My guess is the hot output transformers were caused by high current flow. With ~2v of bias the output tubes should have been glowing red, but either way they were definitely drawing a lot more current than normal.


Dave, would a film cap be OK? Just asking because thats what I have on hand in that value range.
 
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