Zenith record player amp conversion

[MCM1910]

Good evening everyone. I'm dead a newbie but have some small amount of experience with simple audio circuits. I found this zenith solidstate record player in my wife's grandfathers garage. It's an all in one unit with speakers that hinged out. The phono was not functional so I dug in and started to try to see if the amp was functional and discovered that at least one side of it was. I was able to tap into the circuit and plug my guitar in. Since then I have been reading about RIAA curves and I'm wondering if this has it and if I'll be able to bypass it. So my questions are as follows...

1. Where should I start on the channel that doesn't seem to be working.
2. To bypass the RIAA should I just trace the circuit back to the power amp and then attempt to use a probe to see if audio comes out? Is there a way to protect my sound source in case accidentally probe some place carrying power?


I sincerely appreciate the help. I hope that I am not real far off base. I will try to snap some pics of the circuit for reference some time tomorrow if that is helpful!

Thanks again

[DrGonz78]

In addition to picks let us know what is the model of the zenith phonograph. We'll help dig up a schematic as well.  :dbtu:

[MCM1910]

Thanks for the suggestion.  I finally found the model number inside the compartment that held the amp.  It looks like it is a Model number x560-2.  It lists wattage as 50 watts.  I am still working on pictures and will see if I can come up with anything but the schematic would be incredibly helpful.

Thanks!

[DrGonz78]

Very hard schematic to find online. For $22 you can get a copy on the SAMS page, but that seems pretty expensive to me.
https://www.samswebsite.com/en/photofact/details/index/id/27667

You could also post to this thread and ask kindly for the same file... Most people are willing to help someone out. It does involve some effort trying to sign up and ask. No harm in asking as they say.
http://www.antiqueradios.com/forums/viewtopic.php?f=9&t=267198

A schematic will be the most important key to allowing other people here to participate in the repair process. Also, google a search for signal tracer to learn about tracing the audio.
https://www.google.com/search?q=audio+signal+tracer&gws_rd=ssl

[MCM1910]

Thank you for the suggestions.  I posted on that forum and hopefully will have some luck there.  I'm looking at the signal tracers online as we speak, and will look a bit more during my lunch break.  Thank you again for the information.

[Enzo]

A signal tracer is nothing more than an amplifier with a probe connected to its input.  if you have some sort of amplifier already, then connect a cable to its input, add a cap in series with it to block any DC.  Now with the free end of the cap, probe the circuit and listen to what is there.  You will also need to connect the ground of the listening amp to the ground of the unit under repair.

[nosaj]

What tubes does it have in it?  Is it stereo or mono?  If stereo maybe swapping the output tubes around?  to see if other side starts working?

First and for most if the filter caps are original you should probably be looking at replacing them first.

That schematic is pretty hard to find.  You might try PM'ing him.

Jason

[MCM1910]

A signal tracer is nothing more than an amplifier with a probe connected to its input.  if you have some sort of amplifier already, then connect a cable to its input, add a cap in series with it to block any DC.  Now with the free end of the cap, probe the circuit and listen to what is there.  You will also need to connect the ground of the listening amp to the ground of the unit under repair.

I see!  I was kind of doing the reverse.  I was putting an audio signal into the amplifier of the phonograph player and poking through the circuit.  I will try this too!

A signal tracer is nothing more than an amplifier with a probe connected to its input.  if you have some sort of amplifier already, then connect a cable to its input, add a cap in series with it to block any DC.  Now with the free end of the cap, probe the circuit and listen to what is there.  You will also need to connect the ground of the listening amp to the ground of the unit under repair.

It isn't a tube record player.  It is all solid state. 

[Enzo]

I think your second quote was meant to be from Nosaj.

What you were doing by putting a signal in is called signal injection.  Signal tracing is listening to the circuit at various points.  All are valid techniques.  You can inject a signal at the input and follow it through the amp with a signal tracer, or you can listen to the amp output and inject a signal at various points through the circuit.  In other words if you inject a signal halfway through the circuit and it sounds OK out the speaker, then the last half of the amps is OK.  But if it sounds bad, then the problem is after the injection point.

[MCM1910]

I think your second quote was meant to be from Nosaj.

What you were doing by putting a signal in is called signal injection.  Signal tracing is listening to the circuit at various points.  All are valid techniques.  You can inject a signal at the input and follow it through the amp with a signal tracer, or you can listen to the amp output and inject a signal at various points through the circuit.  In other words if you inject a signal halfway through the circuit and it sounds OK out the speaker, then the last half of the amps is OK.  But if it sounds bad, then the problem is after the injection point.

Sorry about that you were correct that was for Nosaj.  Sorry for the error.  Thanks again for that info that makes a lot of sense. I really would like to learn the ends and outs of trouble shooting and I know I'll get much more out of it if I experience it like this. 

[Enzo]

I tell ya, I have been in electronics as a hobby and profession for over 60 years, and I have learned a zillion tricks of the trade.  Experience is a great teacher.  But the real amazing thing is that I continue to learn to this day.  Not a day goes by I don't learn something I didn't know before.

I am surprised my brain has not run out of RAM.

[MCM1910]

I tell ya, I have been in electronics as a hobby and profession for over 60 years, and I have learned a zillion tricks of the trade.  Experience is a great teacher.  But the real amazing thing is that I continue to learn to this day.  Not a day goes by I don't learn something I didn't know before.

I am surprised my brain has not run out of RAM.

It is really a lot of fun, and you are right there is so much to learn!

The nice folks over at antique radio provided me with the schematic that includes a parts list and "circuitrace."  So much useful information in 6 pages.  I am tempted to post it here to get suggestions on circuit modifications but I don't want to be disrespectful to the kind person who provided me the file out of what I am assuming is his personal collection.  Would this sort of thing be frowned upon?

I will say this from the schematic I'm kind of thinking of bypassing the baxandall in preamp for some drive?  I'm assuming the baxandall has the same signal loss as the fender style tonstack?  I also think that there is a negative feedback loop.  There is a connection with a 22k ohm resistor (with a bypass cap) from the output transformer back to transistor.  I'm wondering if changing that resistor to a pot might be useful.

[MCM1910]

Okay so I've poked around a little bit more with it tonight.  I'm going to attach the document I was given and if I find out that I am in the wrong let me know and I will take it down.

I injected signal right before the volume control at point 35 and 14 on schematic.  With an mp3 plugged in it gave a good strong loud signal at point 35.  Point 14 didn't do much other than crackle a little bit. So I am guessing it is in the left side power amp where my problem is.

I also tried plugging my guitar into these spots hoping to bypass any preamp filtering it is doing.  Signal was much weaker than when I plugged in the front end, so I am guess ing that at that point in the circuit it wants to be fed more like line level?

Looking at the tone controls, I'm thinking that this is a baxandall tone circuit?  I've read that with the Fender tonestack you can put a switch where the mid goes to ground to bypass the controls.  Can the same be done by disconnecting the ground on the bass and treble control here?

Thanks for any help!

[Enzo]

Make it work before you start trying to modify it.

14 and 35 are the outputs of the preamps.  I would assume injecting a signal there would result in less output from the speakers than connecting to the input, after all you just bypassed the gain of the preamp.

So 35 works, and 14 doesn't, so yes, the problem in the dead channel is after 14.  So the natural thing is to move your injection point to 17 and 38, eliminating possible bad volume control.

Like any good Photofact, this schematic has oodles of voltage references all over it.  The whole power amp is four transistors a channel.  Go through the dead channel and check those DC voltages.  For example collector voltages on Q2, Q3 are -1.8 and -20.  Got those?  Q3 for example, I don't care if the -20 is really -18 or -21, what I don;t want to see is -2 or something.  Gross errors.  No amp is going to not work because some voltage is off a couple percent.
Output xstr Q4 has point 1 wired to its collector, at -27v.   Point 22 is the amp output, and should sit at about half that, so 14v more or less.  Is it?

And what about the preamp?  We have to repair the power amp, but does the preamp channel work?  Apply an input and trace it through the amp.  Does it look/sound good at point 14?  if so, continue to trace it through teh power amp, Q2, Q3, etc.

The key to this whole thing is to isolate the problem.

[MCM1910]

You are absolutely right I'm getting ahead of myself. I will continue to trace the circuit and report back what I find and if there are voltage problems

Thanks again!