Alternative to Faceplate/Preamp unit for Table Radios...

[Millstonemike]

I'm on a satellite link today and posting pictures isn't working well. It's on page 4 of this link.

We may be thinking two different things.

Single supply bridged amps: in the general case when there isn't DC buck circuitry increasing the nominal 12 V car electrical system. Most all head units use the 12 V as-is and implement Bridged Tied Load (BTL) mode.

On the positive half of the output wave, the power out circuits use Bat- as output Gnd and Bat+ as output Vcc+. On the negative half of the wave, the power output polarity is reversed. Bat+ become output Gnd and Bat- becomes output Vcc-. Thus the output signal can swing between the two extremes of Vcc+ and Vcc-. This increases the maximum output power by a factor of 4 over non BTL mode.

The result imitates a dual supply amp with Gnd, Vcc+ and Vcc-. except Gnd is ever changing with each half cycle. That's why car stereos have two leads for each speaker. Neither of which can be ever be connected to the car frame (i.e., battery ground). The output would short on each negative portion of the cycle. When the circuity reverses polarity as Bat+ would be connected to chassis ground.

So the output swings from Bat+ to Bat-. without a constant ground and never connected to chassis/battery ground. A "virtual" ground in the output wave would be 2 x Vcc / 2 (e.g., Vcc.).

On another note, the line level signal in single supply amps use a DC blocking capacitor to bias the input signal to Vcc/2. That allows the signal to vary from it's + peak to - peak without ever going negative. It's viryual ground being Vcc/2.

I just don't see how to convert the BTL output to a line level signal given that the RCA output conductor is tied to system ground. That would be like trying to use chassis ground for one side of the speaker connection in my explanation, above, albeit via some resistor network.

[neildavis]

22ga wire is way overkill. 30ga wire-wrap is much easier to work with, and it you have the expensive teflon-coated stuff, the insulation won't shrink back when you solder it. You can tack it down into place with a dot of hot melt glue or silicon glue, and the repairs will look nice and last forever. 30ga wire is the equivalent of a 1.5mm trace of 1oz copper, according to one online calculator. That's plenty for these signals.

[tomzarbo]

Thanks Mike, that's good advice.

I did forget to flux up the wire before I tinned it but I didn't really have any trouble getting it to stick. It's probably more of an issue of getting things lined up.

I magnified this with a loop to take the pic on my phone so it's looking way larger than normal, but it's really kind of tiny stuff to fiddle with as you know. I have a cool flux pen that works well, and I have a few rolls of 'speaker tinsel lead'... um, I mean desoldering braid copper line, along with some solder suckers too. It all comes in handy when you need it.

I had turned up the heat a little because even when tinned, the very tip of the iron doesn't get quite as hot as a few millimeters up the iron. What degree heat would you suggest for a fairly fine pointy tip?

The #14 and #15 pins did produce sound, so that checks out. Well, I forgot to check if my audio output was front or rear on the fader... next time.

So I got a few things off the board...


the antenna and RCA jacks. I'm a little hesitant to tackle the biggest blob of plastic on this thing... the dual connector for voltage input and speaker output. Any ideas on how to get this off without borking the board? I do have a heat gun that I could use.

Also, here is the faceplate with the rest of the metal frame removed....



It's about 3/4" thick minus the four mounting tabs that will serve as an attachment point for the rear board at some.... um, point.

TomZ

[Millstonemike]

... I'm a little hesitant to tackle the biggest blob of plastic on this thing...

TomZ

In the absence of a Dremel cut-off, I would try a fine tooth hacksaw blade to cut it off close to the board. That might allow the individual pins to be removed one at a time.

+1 on Neil's wire size post. I like reclaiming wire from Ethernet cables. The wire comes in twisted pairs, each pair has a signal and a ground in the Ethernet application. Seems perfect for audio line level signals to minimize noise.

[tomzarbo]

In the absence of a Dremel cut-off, I would try a fine tooth hacksaw blade to cut it off close to the board. That might allow the individual pins to be removed one at a time.

+1 on Neil's wire size post. I like reclaiming wire from Ethernet cables. The wire comes in twisted pairs, each pair has a signal and a ground in the Ethernet application. Seems perfect for audio line level signals to minimize noise.

I often just use pliers or cutters and smoosh/break apart the plastic bits until it kind of comes apart in bits, but that may be dangerous with this. I do have a Dremel, maybe I could give that a go. Maybe Dremel off what I can get to and snip off the rest, whatever works and is safe.

I was wrong, I actually used 24 gauge wire for this. I have various colors of this finer gauge for hook-up duties, so I used what I had.
I do have some twisted Cat 5 type wire around somewhere from when I wired up my other house, but I thought multi-stranded may have a bit more durability to it.

Thanks for the suggestions. I'll see what Dremel blades I have!

TomZ

[tomzarbo]

I was able to get the big black plastic blob cut off without (I think) damaging the board or any components. The Dremell tool was a good idea.
I did just touch the big cap with the cutting wheel, but it just barely broke through the plastic outer case...


If you look closely, you can see that I also nicked the board in a few places as I got rid of the last of what was there, I probably should have left that little bit on there, it wasn't taking up much space, just wanted to clean things up a bit. Looks like that part is solid copper underneath so no issues there I don't think.

Next step, clean the rest of the leads up, and run some wires to it and make sure I didn't mess it up somehow. Onward!

TomZ

[Millstonemike]

I was able to get the big black plastic blob cut off without (I think) damaging the board or any components. The Dremell tool was a good idea.
I did just touch the big cap with the cutting wheel, but it just barely broke through the plastic outer case...

Next step, clean the rest of the leads up, and run some wires to it and make sure I didn't mess it up somehow. Onward!

TomZ

Nice going. I wouldn't worry about the big cap. That's the bypass cap for the power amp - reserve energy close to the amp itself to handle peak demands. It might even be removed, possibly replaced with a small axial cap to further reduce the the board height.

[tomzarbo]

Well, I borked it up. I'm not sure which of my recent maneuvers did it, but it's not powering up...

I made a diagram by testing continuity for the power leads coming in, ground, antenna, 12 amp power-on, etc... I'm pretty sure I did that right.

I did dig into one lead and the actual amp chip just a bit when cutting off the big plastic connector blob... I don't know if that did anything or not....




I also noticed in the process of getting the blob off, I think I managed to pull off a trace from the board. (see pencil mark on end of board) The other pair of connections that have that little raised bridge under the green coating between them had continuity, and this one did not, so I made a little piece of wire and jumped the connections from the other side.



Still, no dice. After messing with this for awhile, I feel like I need a break for today. I'll give it another crack maybe tomorrow when my eyes uncross.
I have another unit coming from Aliexpress so I can use that to help me troubleshoot this one when the time comes. Ahhh, learning by failure!

TomZ

[Millstonemike]

Well, I borked it up. I'm not sure which of my recent maneuvers did it, but it's not powering up...

TomZ

Did you account for the fuse that's no longer in the power circuit?

There's the constant battery power input (e.g., remember settings), and, a separate +12 V input to turn the unit on (e.g., turn the key on, the radio comes on).



If I still lived in Millstone, I'd have to take a ride out to the country and watch you wield the iron. Geez . To be fair, I've been soldering well over have a century. My first iron was a trapezoidal hunk of metal with a pointed end . It was attached to a wood handle via a steel rod. You heated it on the stove burner.and then quickly tried to solder a connection. Wish I still had it.

[tomzarbo]

Did you account for the fuse that's no longer in the power circuit?

I checked continuity between the various leads, and the respective node on the board. I figured the fuse was 'in line' with the main 12V+ feed on the big black plastic connector.

[Millstonemike]

I checked continuity between the various leads, and the respective node on the board. I figured the fuse was 'in line' with the main 12V+ feed on the big black plastic connector.

Tom, I updated my previous post to add the "Ignition" turn on input.

[tomzarbo]

I did hook both yellow and red lines to 12V positive.
did the amp chip look okay to you?

I'll try to double check the nodes I've soldered to, compare against the schematic, just in case I picked the wrong nodes
TomZ

[tomzarbo]

I got a few more of these radios in recently to play with. I have another boom box/table radio type of thing to make with this head unit.
I think I'll test with the built-in amp to see if it's up to the task of powering the speaker I plan to use (a set of Wolf's Rubyk's which I have the parts for)

If I don't go with the built-in amp, I'll use one of the 30 to 50 watt at 24 volt Class D amps I have with a delay on/off unit that Mike suggested above.

I won't modify the head unit this time... I just don't think it's worth the risk for the small gain in space and amperage; I just don't seem to have the 'knack' to 'hack'. I think this will be a 'corded' radio for the most part. I have someone in mind that I plan to build it for, and I'll ask them if they want cordless or corded, but I'm hoping for corded.

Anyway, I didn't forget about this, and I think there were a few of you who were wondering how it worked out, sorry I just left it there. I hope to start messing around with this thing in the next few months.

TomZ

[Millstonemike]

I got a few more of these radios in recently to play with. I have another boom box/table radio type of thing to make with this head unit.
I think I'll test with the built-in amp to see if it's up to the task of powering the speaker I plan to use (a set of Wolf's Rubyk's which I have the parts for)

If I don't go with the built-in amp, I'll use one of the 30 to 50 watt at 24 volt Class D amps I have with a delay on/off unit that Mike suggested above.

I won't modify the head unit this time... I just don't think it's worth the risk for the small gain in space and amperage; I just don't seem to have the 'knack' to 'hack'. I think this will be a 'corded' radio for the most part. I have someone in mind that I plan to build it for, and I'll ask them if they want cordless or corded, but I'm hoping for corded.

Anyway, I didn't forget about this, and I think there were a few of you who were wondering how it worked out, sorry I just left it there. I hope to start messing around with this thing in the next few months.

TomZ

Remember to get the 12V version. The circuitry is the same in both. The difference is the relay coil voltage. Both have losses in a 5V linear regulator and 555 timer circuitry. But at 5 V, there's not enough volts left over to trip the relay coil control transistor. Note, I never tested the 12 V version, I just reverse engineered the 5 V and updated it. If the 12 V has the same issue, just shunt the trip trigger LED. That's where the most losses are.

[tomzarbo]

Remember to get the 12V version. The circuitry is the same in both. The difference is the relay coil voltage. Both have losses in a 5V linear regulator and 555 timer circuitry. But at 5 V, there's not enough volts left over to trip the relay coil control transistor. Note, I never tested the 12 V version, I just reverse engineered the 5 V and updated it. If the 12 V has the same issue, just shunt the trip trigger LED. That's where the most losses are.

I may have already picked up some of the 12v ones awhile back.
I have a design sketched up for this already, Curves and angles and such. Plus... white paint! Taking a break from veneer.

TomZ