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
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:
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!
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
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.
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.
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
Quote from: Enzo on June 08, 2015, 08:38:36 PM
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!
Quote from: Enzo on June 08, 2015, 08:38:36 PM
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.
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.
Quote from: Enzo on June 09, 2015, 12:34:48 PM
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.
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.
Quote from: Enzo on June 09, 2015, 04:30:06 PM
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.
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!
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.
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!
Well so here is the update. I decided that I wanted to unhook this from the record player because as it was set up I had to turn on the old record player motor to turn on the amp. So I disconnected it from all of that stuff and wired up a switch and added a terminal strip to this scrap of MDF to make solid connections while I was testing. Wouldn't you know it in the process both sides of the amp started to work! I have an idea of my error and its embarrassing but anyway it is fully functional now. With an MP3 player feeding it, it gets reasonably loud too!
So now I am a little stuck because I'm not sure if I want to use this for a home stereo or continue with converting it into a guitar amp. If I am to use it for guitar it will need some sort of preamp feeding it or something to boost the output. Anyone have any thoughts or opinions on what I should do next?
Thank you for all the help!
(http://i364.photobucket.com/albums/oo89/unicorneggspam/IMG_5416.jpg) (http://s364.photobucket.com/user/unicorneggspam/media/IMG_5416.jpg.html)
I must have time slipped there, which is easy these days.
I saw Enzo had said for you to check all the Voltages and compare between the channels, which is good advice as Voltages can vary from the ideal on the circuit diagram.
The first thing you do with any repair job is to make sure that there is a supply and the Voltages around the circuit are correct, or at least within 10%. Having 2 channels allows you to compare these Voltages with relative ease.
No PSU on to any part of the circuit, no output.
Once you have done this, then you can start on signal tracing.
As far as the pre-amp is concerned you could of course modify the input or you can just bypass the whole front section up to point 14 and 35, but more about that later if you decide to go down that route.
The main thing is to get it working first :)
As far as I can tell the OP had fixed the amp. Intermittent problem or user error I am not sure but the amp was working. I can't recommend turning it into a guitar amp unless that is what MCM1910 wants to do. Modifying circuit board layouts and changing the preamp to work with guitar will take some doing. Still sounds like some fun tinkering to be done too.
Quote from: MCM1910 on June 13, 2015, 03:16:11 PMAnyone have any thoughts or opinions on what I should do next?
It's your choice :)
That said, for a music player you'd need a couple reasonably efficient speakers because availoable power is not much.
Frequency response might not be that flat either, specially on lowest frequencies, but it depends on your program material too.
FWIW it might be fun to use it to listen to some old ... *vinyl* (gasp !!)
Sorry I missed the last few responses. It has been crazy here recently. The amp currently functions completely as designed except for some slightly scratchy pots. I'm in the process of designing a new enclosure for it now. I still haven't decided what exactly to do with it. I currently have a stereo jack wired to the input and have been listening to my iPod through it in the shop. It sounds decent...has a cool Lofi vinyl vibe...I found a really nice record player that needs some work but should be able to get up and running and I'm considering using this amp for that...
I still like the idea of using it as a guitar amp. If I do does anyone have any suggestions? ROG preamp? P27 preamp?
I just thought I would update everyone on this thing, as I am still working it out...
I repaired a record player and put it into the amp. Very low output so I took the plunge and ordered a phono preamp to see if that would help. It did and I was able to get decent output out of it.
I'm preparing to make the enclosure. I started by obtaining an aluminum bar which I am drilling to be a "faceplate." I plan to make the enclosure to look similar to a guitar head and will be using pine.
One new problem I am trying to work out is how to add an on/off lamp. Looking at the schematic it appears the power coming off the rectifier is 22V? Should I tap into that and then try to drop voltage (with a resistor?) to a more manageable level? It looks like there are some points in the schematic that are labeled as around 12V, could I just tap into those spots?
As always thanks again for all your help...
Stay tuned, if I make any progress in putting it together I will post pics