Magnavox 175-67 Rebuild Questions

[_fred_]

hope this comes through... here's what I took out...

https://audiokarma.org/forums/index.php?media/maggie_feedback_circuit.21444/

 

[_fred_]

ended up cutting the two white wires...

 

[Dswankey]

Hello,..I'm working on a 175-55 Magi....Yesterday I pulled the OPT's and replaced them with some Hammond 1650E's. When I was first done and plugged her back in she sounded like a police siren.

I did some trouble shooting and found that it would run fine in mono, but not with both channels without 'sounding off'. Based on that I pretty much knew it was something in the 'input' as with both AX7's out, or with one in (either channel) it was fine, but with both it was not...
I saw what looked like a previous owner mod going from OPT speaker outputs back into the input (neg feedback) and as part of trouble shooting just disconnected all of it... and bamm -- all good...
So -- do I need to research and put in a 'better' feedback circuit, or with the larger/better OPT's do I need to even bother?

@_fred_

Here's the picture you're having trouble posting, hopefully this will help others help you to figure out what's going on with the wiring

What was wrong with the original OPT's? I'm sure some of the Maggie folks will be along soon to assist in the meantime here's a schematic if you don't already have it

https://audiokarma.org/forums/index.php?threads/revamping-a-magnovox-175-67-amp.684303/

 

[century tek]

Fred, the reason you are getting the siren (high pitched oscillations) is because you need to reverse the plate (pin #3) wires on one channel of your 6V6 tubes.

You should connect back up the feedback circuit so you don't get high amounts of distortion. Then you can tweak it from there.

 

[_fred_]

@_fred_

Here's the picture you're having trouble posting, hopefully this will help others help you to figure out what's going on with the wiring

What was wrong with the original OPT's? I'm sure some of the Maggie folks will be along soon to assist in the meantime here's a schematic if you don't already have it

https://audiokarma.org/forums/index.php?threads/revamping-a-magnovox-175-67-amp.684303/

View attachment 1819317

Besides being tiny (about 2"x2" - not much bigger than the size of the choke, open bobbin) nothing.
Was hoping to get better bass with some larger iron, and also be able to set it up for multiple speaker impedance options.

 

[_fred_]

Fred, the reason you are getting the siren (high pitched oscillations) is because you need to reverse the plate (pin #3) wires on one channel of your 6V6 tubes.

You should connect back up the feedback circuit so you don't get high amounts of distortion. Then you can tweak it from there.

Thanks for tip on switching up plate pin wiring, I need to look into that more to know exactly what to do -- are you saying all I need to do is switch which tube (for one channel) the blue/brown wires from the OPT go?

What's baffling me is why the old OPT's didn't cause this problem but the new ones do and why I'd have to swap connections at this time. any ideas?

 

[century tek]

Pick either the left channel or right channel, doesn't matter but pick one channel only and just reverse the #3 leads of both 6V6's. I believe it has a lot to do with the feedback circuit whether it is negative, positive, or global.

Mono amps don't suffer this.

The new transformers are not the same and are not wound the same as the originals.

 

[_fred_]

Hey, just a little follow up on this as I finally got time to get back into it...
Switched the pin#3 hookup on one channel (tested still working), then hooked up the feedback circuit and this time I got a low 'rumble' vs 'sirens'... hmm. Swapped the pin#3's back and got sirens again...
Looked harder and saw that the feedback circuits were feeding back into the opposite channels (thanks to previous owner mod as wires were 'cut' so there was no way to mistake which was which) -- Edit: due to current owner hooking them up backwards
Corrected that and now I have a clean working sweet sounding amp again... Thanks for pointing me at the right stuff!! Now to think about other 'upgrades'

 

[Dswankey]

Hey, just a little follow up....Looked harder and saw that the feedback circuits were feeding back into the opposite channels Edit: due to current owner hooking them up backwards
Corrected that and now I have a clean working sweet sounding amp again... Now to think about other 'upgrades'

Congrats on finding the source of your problem and correcting it, as to "Upgrades" plenty of documented ones in threads here at AK. The link below is a an advanced search query of threads you should find interesting

https://www.audiokarma.org/forums/i.../&q=Magnavox+Amp+175&t=post&o=date&c[node]=14

 

[rkt88]

we rebuild/restore all magnavoxes here so yu've reached a wealth of knowledge in that venue.
enjoy the ride.
glad you found us. welcome

Hi there, I realize this is an older thread, I’m hoping some of the knowledgeable folks here might still be around to help me out.
I’ve just acquired a magnavox 1st207L console with the amp175 67.
I’m trying to find a schematic and parts list that’s not too blurry to read.

The 2nd channel on the amp isn’t putting out, I get some faint lows out of that side but that’s it.

Someone has already gone through and replaced the caps and resistors, but I don’t know wether they’re the correct values or wired right.
The amp has 2 pots, one for Bal and one for hum.
the wiring on the bal pot confuses me a bit, as it seems to just hook to 1 channel…
I’m green on restoring/ rebuilding-
if anyone has a good source for parts they can share that would be appreciated as well.

I mainly want to get it into original working order, am not opposed to enhancements or modifications, especially for safety and longevity’s sake.

I think I’m going to replace the power chord with a fused IEC chord-

 

[Dswankey]

I’ve just acquired... the amp 175 67...I’m trying to find a schematic and parts list that’s not too blurry to read.

Should be able to find one here, it might take a little time but definitely worth the perusal

https://www.audiokarma.org/forums/i.../&q=Magnavox+Amp+175&t=post&o=date&c[node]=14