Magnavox 175-67 - channel 2 help needed

D

Dig22

Active Member
Hi,
I'm a newbie, and aware of the high voltage dangers.

I picked up a Magnavox 175-67 TT console with pre-amp/receiver. I replaced a few of the caps, added a fuse and 3-prong plug. Channel 1 works fine. But Channel 2...I'm stumped. The balance pot doesn't do anything.

I'd appreciate advice on where to begin tracing the problem, and if someone had schematics to share, that would be grand.

Thanks!
 
Hi & welcome! Some pis would help.

Make sure that the speakers are connected properly. With most Magnavox amplifiers, the speaker terminals are "C", "1", and "2" where "C" is both speakers Common or negative and 1 and 2 are the left & right speaker positives respectfully.

Swap the tubes. Swap two (one channel) of the 6V6GT's and the one 12AX7 tube to the other channel and vice versa.

I can do these amps in my sleep. Maybe try to take a good pic of the underside and I or someone else might be able to point out a discrepancy of some sort.

Maybe try a phone or CD player into the two main amp inputs to see if the problem is either in the tuner/preamp or the amp itself.

Is your balance pot an actual balance pot? I ask because my 175-67 had the actual channel balance pot on the tuner/preamp hidden under the bass control. My amp only had the hum balance control on the amp itself.
 
century tek said:
Hi & welcome! Some pis would help.

Make sure that the speakers are connected properly. With most Magnavox amplifiers, the speaker terminals are "C", "1", and "2" where "C" is both speakers Common or negative and 1 and 2 are the left & right speaker positives respectfully.

Swap the tubes. Swap two (one channel) of the 6V6GT's and the one 12AX7 tube to the other channel and vice versa.

I can do these amps in my sleep. Maybe try to take a good pic of the underside and I or someone else might be able to point out a discrepancy of some sort.

Maybe try a phone or CD player into the two main amp inputs to see if the problem is either in the tuner/preamp or the amp itself.

Is your balance pot an actual balance pot? I ask because my 175-67 had the actual channel balance pot on the tuner/preamp hidden under the bass control. My amp only had the hum balance control on the amp itself.
Click to expand...
 
Hi,
I've tried swapping tubes and different input sources. Same result: Nada. I'll check for balance on preamp/tuner tomorrow.

-D22

 

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Take a meter and measure the resistors in the dead channel. Sounds like one of them could be open.

Its rare that they go bad, but also ohm out both the primary and secondary windings of the output transformer in the left channel.

Are you using this amp as a stand alone or with the console?
 
Channel 2 is back! It was a a couple 470k resistors. And now the balance pot is working. So thank you Century Tek.

But a couple more questions and issues:

1. How do I "ohm out both the primary and secondary windings of the output transformer" as you suggested? I don't understand much of the basics.

2. What are voltages for the disk capacitors? They're not listed in schematics. And do you recommend re capping the disks? And can I replace with mica caps? I have many 500 & 600 v mica caps.

3. is there a step by step guide to turn the unit into a stand alone? I've come up with bits and piece on this site but it would be nice to see it all in one place.

4. and this is a biggie, but the tuner/pre amp is out in one channel. It's consistent with all inputs and outputs. I changed the capacitors, but it looks really complicated. Is there something easy I can check? The cables seem to be ok. I'm ready to leave this unit behind, but would be nice if I could get it going...

Thanks again for your help. It was driving me nutty trying to figure it out.
 
Since it was just a resistor or two, the output transformers are fine and don't need to be checked with a meter.

The disc capacitors you can leave alone. Normally they are anywhere from 100 - 600 volt on their ratings.

Preamp may also have an open resistor somewhere. Check around the preamp output tube. It's the 6EU7 or 12AX7 tube on the radio chassis.
 
No open resistors on preamp. But I can tell this unit was serviced by a few of the capacitors spliced in. The two 25@50v on the 12AX7 in the picture... is that right? I haven't found a model number the unit to help me find schematics. I've posted a few photos. Can someone identify it for me?

BTW, I'm actually amazed at how good my iPhone and amp sounds with the 15" speakers in the console. I wasn't expecting that at all. And the record changer plays 78's well.
 

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Its either 57 C3 21 or there is another number on the side somewhere. The one I have will be a different number as mine had the tv and tv tap on the selector switch.

So the amp is operational and the tuner is now the focus of attention. Maybe wiggle the 12AX7 tube and see if you hear any crackle from both channels of the amplifier.
 
It's stamped 57 C3-21 on the back as well. Other numbers appear to be serial numbers. In the lower right corner of face plate, there's the number D150534.

Wiggling the 12x7 doesn't produce crackle.

I checked continuity on the output cables and plugs and they're ok.

Re: The Amp, I used some 1/2 watt resistors with 5% tolerance instead of 10%. Should I redo?
 
I finally solved the channel 2 Preamp/tuner problem: a dead resistor. That little guy was driving me batty. So all's well in Magnavox 175-67 land for the moment. The unit sounds amazing, and probably extra amazing because I brought it back from the dead. Adios for now.
 
FYI for anyone working on the tuner...I couldn't find the exact photofact for the preamp/Tuner, model 570321. But I found fotofact for 570321. Sort of close. Looks the same. Same tube configuration. That photofact number is 505-07.
 
I changed all capacitors except the multi value capacitor can.
The unit is running well, but is it recommended to change the capacitor can with single electrolytic capacitors? I havent been able to find the values 30@450V and 40@450V. Would it be ok to subsitute 33@450V and 47@450V?
Thanks!
 
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