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More Fun With Magnavox: The 9300 Series

petrnyc said:
would some sort of circuitry stop it?
Click to expand...

There is no substitute for using the correct components in your builds. Any sound suppression circuitry you could possibly try to use, would change the audio dynamics of your amp.

It's not a difficult or expensive fix to unsolder the old switch and install new MBB switch. A lot easier than trying to figure out a way to suppress a sound that needn't be there in the first place.

You can purchase the correct MBB switch here for $2.25: https://www.tubesandmore.com/products/switch-rotary-3-poles-4-position
 
petrnyc said:
Its produces very loud clicks if I don't remember to bring the volume down.
Click to expand...

I'm guessing that petrnyc already knows that turning the volume down every time he wants to change input channels, is an available option, but not one he chooses to live with. Especially when (not if) you accidentally forget to do it.
 
Hooray!
After drinking around with my build for about a year (! ), I finally finished it last night. It works.
But I discovered something that bothers me: I built using a two prong cord, thus the chassis is floating. And I measure about 15 volts AC between the chassis and my house's ground.
Given that every one of these I've seen has a naked chassis exposed, doesn't that bother us?
I know that 15 volts is reasonably safe, but it seems inelegant at best. And since it's floating, it may fluctuate up a bit?
I've read that grounding chassis' can introduce ground loops, but this doesn't seem right either.
 
its at essentially no current. Put a 100k resistor from chassis to ground and measure across that. Fair bet it will go to nothing.
 
Hello,

I have changed the input selector on my pre amp and while doing/testing it, I inadvertently sent huge full on volume blast to the Mag.
Now its red plating, I think I fried the LM337 unit. Any other suggestions on how to trouble shoot to find whats wrong?

My Stupid mistake...


thanks
 
Pull the output tubes and measure the voltage at pin 3 of the 6bq5 sockets. If its not ~17 volts, the LM337 is probably dead. Confirm voltage input from the screen supply first though. Should be ~18v at the wiper of the bias adjustment pot.
 
Gadget, thank you, I think lm337 is ng there is 0v between ground and pin 3.

on the 5K pot : wiper has 6,97V,
off the dropping resistor it’s 10,80V and out to ground it’s 6.13V.

But 0v on pin 3.

between + hot On third Cap and pin3 there is 414V.
 
I'd expect more on the wiper but it depends on position. Either way 0v output is not right. I popped an LM337 on mine too. Forget what I did, but it was something dumb and it died exactly the same way.

Might be worth confirming that resistor chain is on value too, just for S&G.
 
Thanks, I will take reading on the resistors, good point.
one positive thing is, I have a spare for the LM, I bought 2 so don’t have to wait to get it... guess Was in the know what’s going to happen hahaha....
thanks again!
 
New LM 337 in all is well.... when I was setting the bias again, I have a "Matched" Quad of EL84's, but the bias varied between the two sets as much as 20mV. I switched them around and got it within 8mv close between the two pairs. Guess thats normal operating procedure with two pairs on single pot?

thanks Gadget!
 
Gah...
Hooked my amp up to the living room speakers and there's a serious problem. With no input, it makes a rapid puttering sound, like an old movie projector.
Volume is proportional to my volume control.
Music sounds great through it, other than this.
This amp has Dave's buffer, and a volume in front of the buffer.
Here's a link to the sound on YouTube.

 
exactly which schematic is it built to? There are several in here.

The hum sounds like maybe a ground issue. The puttering is an LF oscillation problem. Pull the buffer tube and see if anything changes.
 
gadget73 said:
exactly which schematic is it built to? There are several in here.

The hum sounds like maybe a ground issue. The puttering is an LF oscillation problem. Pull the buffer tube and see if anything changes.
Click to expand...
I'm using Dave's base schematic - the one with the stock output transformers, and his buffer. I've added a volume control as pictured below.
Is LF oscillation low frequency oscillation? I'm just a hacker with a soldering iron.

20180625_124019.jpg

20200814_212837.jpg
 
gadget73 said:
exactly which schematic is it built to? There are several in here.

The hum sounds like maybe a ground issue. The puttering is an LF oscillation problem. Pull the buffer tube and see if anything changes.
Click to expand...
Test results:
1. Pulled the buffer tube and powered up. The puttering continues, but now at a constant volume regardless of where the volume knob is set.
2. Swapped V1 preamp tube with buffer tube just to see what happens, and the fuse blew.
 
Archie,
I had a similar situation, when hooked up to speakers with no input, It was making similar noise.
The LM337 was bad. Tubes ran hot and red plating.
check out the voltages Gadget told me to do few posts back.
 
LF = low frequency, HF would be a shrieking noise.

Its not making a horrible high pitch scream so I'm not inclined to think the output transformer leads are reversed. Usually its some sort of power supply problem where the stages are interacting with each other, but it could be a problem with the LM337. I did have some odd behavior from one that I damaged. It was making the screen voltage bounce up and down and it made a noise. When its power supply related, usually thats bad filter caps. The stages don't get decoupled from one another and the whole thing starts going stupid.

Try removing the 6EU7 tubes. If it keeps making the noise, it has to be something at the output stage. Another clue would be odd voltages measured across the bias test point resistors.


if none of that turns up anything, maybe some pics of the underside?
 
gadget73 said:
LF = low frequency, HF would be a shrieking noise.

Its not making a horrible high pitch scream so I'm not inclined to think the output transformer leads are reversed. Usually its some sort of power supply problem where the stages are interacting with each other, but it could be a problem with the LM337. I did have some odd behavior from one that I damaged. It was making the screen voltage bounce up and down and it made a noise. When its power supply related, usually thats bad filter caps. The stages don't get decoupled from one another and the whole thing starts going stupid.

Try removing the 6EU7 tubes. If it keeps making the noise, it has to be something at the output stage. Another clue would be odd voltages measured across the bias test point resistors.


if none of that turns up anything, maybe some pics of the underside?
Click to expand...
I put in a new Lm337 and tested voltage at pin 3 of EL84s (with tubes out). It's sitting at about 3V DC.
The wiper of the bias adjust pot is at 8 volts, and the bias resistors are both 8-10 volts.
The amp still makes the movie projector sound.
I verified a bunch of resistors and found no problems.

This amp has a new 40/40/40/40 cap can and all new components.

B+ is about 450V with all output tubes removed.

Here's some pics. The Lm337 connections are hard to see but they're correct according to the schematic.

Let me know if you need pics of anything else. I really appreciate the help!

20200815_143941.jpg

20200815_143912.jpg


20200815_143929.jpg
 
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