Symbol I have found

[Chrisraven4321]

Theory question, if anyone would like to put in their thoughts. I have been out of the electronics world for a while now and my memory fails me on the following. On the attached drawing there are 2 transistors Q119 and Q121 which is marked as a differential preamp. The signal comes in through the "input from the generator" jack and exits into the speakers on the right of the drawing. While the normal path trough an amplifier is fairly straightforward, this little addition of Q121 has me slightly confused as to how the signal is passing through. Some input would be nice especially if you can give me an idea of the amplification levels through the various stages. Plus how the signal is processed via the different stages. Hope I asked this in a logical manner.
Just another quick request. The capacitor that is connected to the speakers is showing a + sign on both sides of the capacitor. Which side do you think should be the positive and which side should be the negative? I have +35 volts on the top and -35 volts on the bottom, this is the basis for my question.
Thanks
Chris

 

[Harald Kapp]

As I see it:
Q119 drives Q120 which in turn drives the output via Q122 (labeled Q22), Q123, Q124 and A125 in AB mode. From the output A121 creates a negative feedback to stabilize the amplifier loop. In simple terms: A positive signal at the input jack will create a positive signal at the output. The positive signal at the output will drive the base of Q121 which in turn increases the emitter voltage of Q119, thus reducing the gain.

The capacitor that is connected to the speakers is showing a + sign on both sides of the capacitor. Which side do you think should be the positive and which side should be the negative?

None. This is an non-polarized electrolytic capacitor.

 

[Delta Prime]

looked deeply the symbol at the Cordovox schematic

Look deeply into post #1.
If the components are original to the 1975 Cordovox accordion then that makes all electrolytic capacitors 51 years old…

You are seeing 38V on a 25V rail that suggests a serious 'No-Load' condition or failed filter caps that are no longer smoothing the ripple.

See post #2…

 

[Chrisraven4321]

I am going to replace all the electrolytic capacitors, I am just trying to get them right now. I had placed an order with SRI and their website indicated that they had them all in stock but then I get an email from them saying they had none of the 18 different caps in stock but they had another source but the cost would go from [imath]500 to over[/imath]750 USD. Somehow I decided that I did not need to deal with that company any further.
I am finding Axial capacitors are seemingly not to available anymore so I have decided that I would make Radial caps work. Will be a little more work installing them but with a little heat shrink and some work I can make it all work.
Question?? Can you replace a non polarized cap with a polarized, without circuit damage?
As to the voltage the readings I have from Drawing #7, Power Supply; I read with a meter off the joint of D78 and D79 says it should be 35 volts. I am reading 40 volts, which could easily be because of damaged filter caps. Almost all the larger electrolytic caps have leakage showing. Hopefully the reading will be closer to 35 volts when those caps are all changed.
Here is a question, how critical do you think the 35 volts is? It is reading 40 volts and that is tying into the Chord Bass Preamp board. Meanwhile the next connection down on P&S Pin 3 calls for +19 volts which is what I measured. And on P&S4 Pin 4 calls for -20 volts and I am getting -19 volts. looking at that group of voltages being sourced from the -19 volts I have a -1volt supply to the Matrix. If I am already down 1 volt then is the -1 volt rail now less than -1 volt, and in the Matrix there are 85 circuits that this -1 volt feeds. (Drawing #1)
I will attach drawing 1 and 7 here for reference.

Thanks all I appreciate all your inputs
Chris

 

[Harald Kapp]

I am finding Axial capacitors are seemingly not to available anymore

I can't confirm that, see e.g. here or any other renowned distributor.

how critical do you think the 35 volts is? It is reading 40 volts

Check my reply to this issue in post #5. Did you measure with or without a load (aka amplifier) attached to the supply? 35 V are unregulated to a tolerance of +1 10% is imho not unexpected.

 

[Chrisraven4321]

Hi Harald: Yes the readings are with the amp connected and sending a tone through the system. Measured my house voltage and it was 117 volts ac.
I will redo the measurements after I have replaced all the caps and see what that gives me for voltage into the other voltage distribution points. Will post when I have those results.
The caps are supposed to be here mid next week some time. Have a surgery schedules for Feb 3 so I will probably not get into changing the caps until the following week when I can get up and about again.
I have checked with Mouser and their prices were way to high for my wallet. The one place I found that had good prices and good stock was Vetco, owner Spenser talked to me directly and was extremely helpful. I would recommend him for parts. IMO
Thanks for the input

 

[Martaine2005]

Question?? Can you replace a non polarized cap with a polarized, without circuit damage?

Yes is the short answer.
But it depends on the circuit.
Certainly not a good idea for bi-directional signals like AC. Although you can put them in series.
I would stay with what the circuit has and replace like for like.
Obviously axial caps can be replaced with radial caps with a little modification to the legs.

 

[Delta Prime]

Question?? Can you replace a non polarized cap with a polarized, without circuit damage?

High capacitance electrolytic Bipolar or non-polarized capacitors are commonly used for inexpensive low-end audio coupling of crossover networks separating high, mid, low, frequencies, terminating to independent transducers (speakers).
Often times people connect two polarized capacitors back to back. Example : negative to negative or positive to positive.
This is a special case “only” to replace faulty bipolar audio capacitors without circuit damage.

 

[barbar56]

mj2955 is not a secret house part; it is a very common PNP power transistor. It's basically the opposite of the 2N3055, which goes with your RCA 480058A. Motorola, Onsemi, and others made a lot of them. You want a TO-3 case with an audio-safe SOA and a Vce of about 60V and 15A.

The MJ2955G (onsemi) is the modern sub that fits right in. If you're stuck, MJ15025 and MJ15024 pairs will also work in that Cordovox amplifier. Just don't mix old and new in the same push-pull stage. Replace the complements in pairs, check the bias again (the diode stack is important), and you'll be fine.

The date codes 8026 and 8102 are just the year and week, nothing more. Don't overthink the regulator section either. After mains drift, your voltages are already where they should be. First, go over everything again, and then only chase silicon if something really tests bad.

 

[Delta Prime]

To avoid confusion.
If one looks at the datasheet for any transistors the proper terminology is “Device Package Style.”

 

[Chrisraven4321]

Thanks all for your input, all good advice. Just had my surgery and all went well so looks like I will be starting the re-cap sometime next week. Have received all the caps, I decided to replace all the axial with radial since they were more common and much less on costs. It will take a little extra heat shrink but I have the time. Looking forward to seeing how this project is going to come out. I will attach some actual photographs in a bit to show the actual project and get some opinions on some of the ,22ufd caps that are not polarized. They look a little blottchy but I have not yet removed one and measured it. There is probably 50 of them in the unit spread all over, so I think it is probably a good idea to take a couple out from various points and test them.
Thanks all

 

[Delta Prime]

Just had my surgery and all went well so looks like I will be starting the re-cap sometime next week

Off Topic: Get well soon!