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

[tomzarbo]

You should be able to turn off the power amp by grounding pin 4 of the chip (Standby). In standby mode, the chip draws very little current. But check to make sure pin 4 isn't tied to 12V. The data sheet shows that this pin is usually tied high with a 10K or so resistor and a 1uF to ground, which gives a "virtually pop-free" turn-on (or so they say...).

The Bluetooth is provided by a different chip than that other decoder board. The chip is made by Buildwin, part of the AppoTech Group, and it is probably a CW6690G with a custom part number for that code. It is probably decent enough quality, even though it is intended for the low/moderate priced market. That chip shows up in several makes of Bluetooth speakers and the only "reviews" I found are favorable.

This decoder/preamp would be better if that TM2313 chip was on the main board rather than the amp board. It's a preamp chip with analog switches, tone and volume controls with decent specs, but putting it on the same board as the power amp means you can't get rid of the amp. Not a huge deal, but it it means you can't just unplug that amp board to make the assembly smaller and lighter..

Great, you answered the main question we had about this piece, Neil. Thank you for digging into this for all of us.

So it looks like the rear board needs to stay connected to operate.

Since most would probably be thinking of using this as a table radio/boombox head unit like myself -- where the entire radio may be 6 to 10" deep, I wonder if the ribbon cable that connects the front and rear boards could be replaced with a longer cable to facilitate the rear board being placed independently in the cabinet. As-is, the unit is maybe three inches deep, and that cable is maybe 4" long at the most; the rear panel can't be moved very much currently -- I can just barely get it perpendicular to the front plate. The actual metal 'case' could be removed leaving a 3/4" thick front panel to mount, and that would allow for more leeway when mounting the rear board. The rear circuit board can be removed from the total rear panel/heatsink, which makes it a good 2" shorter. If one were to not use the built-in amp and had disconnected it, loosing the heatsink wouldn't hurt anything.

I suppose any lengthening of the cable would make it more susceptible to noise though, and I haven't tested this out with an amp board in close proximity, only a Lepai 2020 amp 2 feet away. I should hook up the 320-606 TDA7492 amp that I frequently used next to it to see what the noise issues are.

Anyway, thank you again Neal for clearing up some questions for us. If the click on/off issue can be eliminated, The more I tinker with this, the more I think this may be a real nice unit to mess with for small radios and such. As much as I like the feel of knobs and buttons, the touch screen on this seems like it would work well for a close-proximity radio unit in the kitchen or what-have-you; and all of the other small faceplate preamps don't have this knob/touchscreen combination.

TomZ

[Millstonemike]

You should be able to turn off the power amp by grounding pin 4 of the chip (Standby). In standby mode, the chip draws very little current. But check to make sure pin 4 isn't tied to 12V. The data sheet shows that this pin is usually tied high with a 10K or so resistor and a 1uF to ground, which gives a "virtually pop-free" turn-on (or so they say...).

The Bluetooth is provided by a different chip than that other decoder board. The chip is made by Buildwin, part of the AppoTech Group, and it is probably a CW6690G with a custom part number for that code. It is probably decent enough quality, even though it is intended for the low/moderate priced market. That chip shows up in several makes of Bluetooth speakers and the only "reviews" I found are favorable.

This decoder/preamp would be better if that TM2313 chip was on the main board rather than the amp board. It's a preamp chip with analog switches, tone and volume controls with decent specs, but putting it on the same board as the power amp means you can't get rid of the amp. Not a huge deal, but it it means you can't just unplug that amp board to make the assembly smaller and lighter..

Yes, while the TDA7388 consumes ~200 ma while idling, in standby mode it consumes 20 ua. Standby is Pin 4 (Pin 1 is left, count front and back pins as if they were all in one row).. Standby is used to control on/off pop via an RC ciruit that allows the standby pin to come up once the power chip has settled down after power up. I would bet that the large SMD cap to the left of the TDA7388 is in that RC circuit. If so, one side should be connected to ground, the other side to pin 4. If that's true, shunting that cap with not allow the chip to come out of standby mode.

Do a continuity test on the cap to ground and pin 4. If I'm right, hook up a speaker and play something. Shunting the cap should force the amp into standby and stop all output to the speaker.

Here's a pic of the standby circuit. The standby input wants to see a minimum of 3.5 V to turn the amp on. On power up, +5 V is fed to via a 10 K resistor to a capacitor connected to ground and on to the standby input. The voltage at the cap and pin 4 will begin to rise as the cap charges eventually reaching 3.5 V thus turning the amp on. About 12 ms delay to turn-on. By shubting the cap, the 5 V source will be connected to ground via the resistor and Pin 4 will never rise.

[Millstonemike]

I wonder if the ribbon cable that connects the front and rear boards could be replaced with a longer cable to facilitate the rear board being placed independently in the cabinet. As-is, the unit is maybe three inches deep, and that cable is maybe 4" long at the most; the rear panel can't be moved very much currently -- I can just barely get it perpendicular to the front plate.
TomZ

It seems the cable is readily available in a wide variety of pins and length (I counted 20 pins from your pic). And I believe the PCB connectors provide the termination for the cable and the existing cable can be removed and a new cable inserted. You have to determine some parameters like pitch (measure the total width of the cable and divide by 20) . And you need to determine if you want the end connections on the same side or on opposite sides of the cable (e.g., pay attention at the existing orientation in the connector).

AND you need to verify this as I can't clearly seem the terminations from the Pic.'s.

Ali has them much cheaper. But you need to search the term "FPC FCC". Price low to high. Then click on one and scroll down to the sellers other items for you specific cable. An 8 " cable can be had for ~$2.

[tomzarbo]

Man, you are amazing with this stuff Mike. I really appreciate you and Neil giving us so much info with this.

So I actually understood your explanation, instead of 5v going right to pin 4, it goes through a 10k resistor, then to a cap through to pin #4. The cap's other terminal goes to ground. Upon turn-on, 5v goes through the 10k resistor, then to the cap/ground, and it begins to charge, once it charges sufficiently to let at least 3.5v through, which is enough to turn on the amp via pin #4, the amp will power up. That brief delay while the cap is charging is enough to allow things to normalize so there isn't a bunch of clicking/popping upon turn-on. (I can't be sure I've understood something until I can explain it to someone)

Well, I guess I might need some clarification on "the large SMD cap to the left of the TDA7388." I don't really see it... unless you're being funny? C34?



Thanks guys,

I'll do more again as soon as I can. Thanks for the ribbon info too.

TomZ

[Millstonemike]

You got it. That's my guess - C34. If C34 isn't it, you can find the cap by testing for continuity to pin 4 with nearby SMD caps. The large 47 uf Can cap is Vcc+ bypass/filter cap.

[tomzarbo]

Mike,
you mentioned this Time Delay Relay before in my Toni Table Radio thread and elsewhere....



The 12v version is cheap enough, but one thing I can't seem to find out is: does it turn off immediately after power is cut. I assume it does and that it's too obvious to mention, but I just wanted to check before I order up a few along with some ribbon cable.

I'm going to see if I can mess with the cap tonight if I can get my eyes uncrossed from Tuesdays workday goings-on.

TomZ

[Millstonemike]

Yes, cut power and the relay goes back to it's default position. Note energizing the relay will add some 40 ma draw on the power source.

I noticed in another pic that the resistors near the C34 are 100K ohm. That would extend the turn on delay 10 times over the mfg's design. Somewhere in the neighborhood of 120 ms. Of course, there's no way to tell the value of the cap short of measuring it.

[tomzarbo]

So I was able to do a little messing with this.

There was continuity between Pin #4 (standby) on the amp chip and the right side of C34.

There was also continuity between the left side of C34 and ground (I used the black line on the 12v input on the back of the unit for this)
It looks like you were right Mike.

Now I have to carefully shunt the cap while the amp is powered up and playing... the amp should go silent as it will be in 'standby mode.' I'll check the amp draw while I'm doing this if I can to see what the drop is.

Regarding the ribbon cable, you were right, 20 lines, really hard to count something that small. It does disconnect from the rear board, but looks to be solidly attached to the front board and is not removable.

Thanks again for the help, I'm kind of enjoying messing with this.

TomZ

[neildavis]

I'm guessing that amp chip is from UTC. UTC is a major semiconductor house and they have a lot of parts that are unique to them, but they also sell "old gold" parts that were originally developed by other vendors such as ST and National. The block diagram in their datasheet for the TDA7388 shows some more details of the "standby" function. As you can see, it does what you wanted: it removes power from the amps:

The standby voltage is compatible with CMOS voltage levels (1.5V/3.5V), but the pull-up resistor on C34 is probably connected to 12V (Vs), not 5V, It doesn't really make a difference, but in case you are buzzing out the circuit, it might help to know that. And as Mike said, experimenting with a larger value of C34 will slow down the rise time and possibly make that power-on pop less annoying.

[Millstonemike]

[QUOTE=neildavis;n1433931... but the pull-up resistor on C34 is probably connected to 12V (Vs), not 5V, It doesn't really make a difference, but in case you are buzzing out the circuit, it might help to know that. And as Mike said, experimenting with a larger value of C34 will slow down the rise time and possibly make that power-on pop less annoying.[/QUOTE]

That may explain another issue I see. The pull-up resistors near C34 are 100K ohms. The datasheet lists input impedance at 100K ohms (the datasheet does not specify whether that impedance is for signal inputs, control inputs or both). If the control input impedance is circa 100K, feeding the 100K pull up resistor with 5 V would not be able to bring the input up to 3.5V to turn on the amp.

[tomzarbo]

Now that I've looked at it... the cable does have 20 pins, and is exactly 2 centimeters, 1 millimeter wide.
So I think that's a 20 pin, 1.00 mm pitch cable, just like you referenced above. It would be 'forward direction, oriented since I want the exposed metal side on the same side as it already is.

I found it on Aliexpress for 10 of them for a few dollars. I'm going to get some extension boards as well to connect the existing ribbon to the new ribbon, since it doesn't seem to disconnect from the front board.

TomZ

[Millstonemike]

Now that I've looked at it... the cable does have 20 pins, and is exactly 2 centimeters, 1 millimeter wide.
So I think that's a 20 pin, 1.00 mm pitch cable, just like you referenced above. It would be 'forward direction, oriented since I want the exposed metal side on the same side as it already is.

I found it on Aliexpress for 10 of them for a few dollars. I'm going to get some extension boards as well to connect the existing ribbon to the new ribbon, since it doesn't seem to disconnect from the front board.

TomZ

Not sure, but I believe the connector on the amp board is made to be opened to replace a cable. But an extension is foolproof so for a buck or two more, that's likely the best way to go.

Even with the longer cable I'd be looking at removing what ever I can from the amp board - the power out speaker connectors and the RCA line out connectors are "low hanging fruit" (you can solder the L-R signal wire into the original holes). That's got to remove a lot of bulk from the amp board. After that, the power amp itself. And without using the amp, that 35 V 2200 uf cap could be replaced, perhaps with a much smaller value. How much "surgery" depends of the arrangement of the target system.

[tomzarbo]

Not sure, but I believe the connector on the amp board is made to be opened to replace a cable. But an extension is foolproof so for a buck or two more, that's likely the best way to go.

Even with the longer cable I'd be looking at removing what ever I can from the amp board - the power out speaker connectors and the RCA line out connectors are "low hanging fruit" (you can solder the L-R signal wire into the original holes). That's got to remove a lot of bulk from the amp board. After that, the power amp itself. And without using the amp, that 35 V 2200 uf cap could be replaced, perhaps with a much smaller value. How much "surgery" depends of the arrangement of the target system.

I was able to remove the cable from the rear board by sliding out the plastic 'shim' and just pulling it out. Where the ribbon connects to the front board, however, I don't see a way to 'release' the cable, unless I'm missing something.

I just purchased some 8" cables, some connectors to join the two ribbons, and a few of the 12 delay relay boards.
I purchased from Aliexpress, so it's probably going to be mid-April before I get them.

So do you think the amp chip could just have the leads clipped to remove the space it takes up, and things would still function properly then?

If the RCA outputs, two plug-in cable connectors for power/speaker output etc, and the antenna jack were removed along with the amp, that's a pretty tiny little board that could be shoved in a small sliver of space. Maybe if noise or interference isn't an issue, it may even be able to be mounted with a few plastic bits to the front panel via the four plastic 'ears' that connect the black face to the metal frame.

TomZ

[Millstonemike]

Maybe this will shed some light ...

https://www.youtube.com/watch?v=PimDML9YHoY

[tomzarbo]

Maybe this will shed some light ...

https://www.youtube.com/watch?v=PimDML9YHoY

That's interesting. Thanks for finding that. I've messed with a few of these video board thingy's and have had to remove the ribbon cable a few times. A few of the other video players used the push in and tilt down method. This car radio uses the style where you push in the ribbon... no resistance at all... then you push in a plastic piece underneath which (I think) wedges the whole shebang closed tight.

I took some pics of the ribbon connected to the front board and I don't see any clips, buttons, or anything or any way to remove it. This one must be a permanent, solder-in type of connection. I took some pics, but I'll have to post them when I get home.

Hopefully I can get some time this weekend to tinker with this. I suspect in a few weeks, I'll be getting a break weather I want it or not if this Corona virus keeps up. :(

TomZ