Question on Spectra Control PCB

[njscan]

The receive audio amp chip (on the command board) in my UHF Spectra (45W) appears to have reached the end of its useful life.

(Link below to a photo of the board)
http://www.repeater-builder.com/motorol ... _board.jpg

In my junk pile I have two 900 mhz (trunking) Spectras. Visually the boards look the same except for two minor differences:
1.The 900 mhz radio's board has a HLN6704A p/n and the UHF is 84D80085P02. I suspect these might be two different numbers for the same board.
2 The other minor difference is the audio amp chip. The 900 radio uses TDA7255, while the UHF radio uses TDA7256. These appear to be pin for pin compatible, so I don't see this to be a problem.

If I were to switch the MLM board that plugs into the control board, are control boards interchangable?
How about alignmant data? I think this would be stored on the MLM board, but I'm not sure.

If the above boards can't be swaped, does anyone have
1. The Motorola p/n for the TDA7256 IC so I can order it?
2. A copy of the schematic for this PCB (in PDF)?

Thanks

[jackhackett]

Part # is 5180065C19

[Will]

I will bet the caps went bad and that is why the audio is bad, common problem. Try replacing the caps first.

[njscan]

Nope, tried the caps first. You're right, usually they leak. I think this one may have been fried when the two spkr leads touched in a previous life. Looking at the cable that connector into the ACC plug, who ever soldered it didn't do such a good job.

I'll wait and see if anyone knows if these boards are interchangable before ordering the IC.
Thanks!

[Will]

I'll wait and see if anyone knows if these boards are interchangable before ordering the IC.
Thanks!

Not without a lot of work. Values are stored in the Command board memory so when you change the Cmd board you have to do a complete alignment and some bit banging too.

You need to change out two resistors and two more caps also. And that may not fix the problem.

[akardam]

I'll wait and see if anyone knows if these boards are interchangable before ordering the IC.
Thanks!

Not without a lot of work. Values are stored in the Command board memory so when you change the Cmd board you have to do a complete alignment and some bit banging too.

You need to change out two resistors and two more caps also. And that may not fix the problem.

Correct me if I'm wrong, but if you write down your tuning values, and use the F2 service menu board replacement proceedure, isn't replacing a command board a fairly straightforward proceedure? Been a while since I've poked around in that section of Spectra RSS...

[jackhackett]

If the audio amp doesn't show physical signs of being fried, you might want to do a few checks...

pin 2 is audio in, you can check that with a scope for signal
pin 10 is enable, that should have about 12v on it when the radio unmutes
pin 12 is Vcc, it should have A+ voltage on it (if not check fuse at front of board)
pins 11 and 13 are the outputs, they should have around 6.5v on them when the radio unmutes

if a board swap is a problem you could always pull the audio chip off one of the good boards, from what I understand the TDA7256 in a Spectra is really a mislabeled TDA7255 anyway.

[njscan]

if a board swap is a problem you could always pull the audio chip off one of the good boards, from what I understand the TDA7256 in a Spectra is really a mislabeled TDA7255 anyway.

Not really...
I've been doing some searching and found the spec sheets on both 7255 and 7256.
"7256" is an 11 pin device, whereas "7255" has several additional pins.
Your advice does help as I can do a pin for pin check to see if these signals are present.

DigiKey has the IC's in stock at $5.67 each, so I think I'll order two just in case it is the IC.
Thanks for your assistance.


pin 2 is audio in, you can check that with a scope for signal
pin 10 is enable, that should have about 12v on it when the radio unmutes
pin 12 is Vcc, it should have A+ voltage on it (if not check fuse at front of board)
pins 11 and 13 are the outputs, they should have around 6.5v on them when the radio unmutes

[jackhackett]

I think you'll find that the chip in the spectra marked TDA7256 is NOT an actual TDA7256, it has 15 pins and from what I've read is a 7255 that Moto had marked as 7256 for some reason, the ones from Digikey will be the real 11 pin 7256s and won't work in a spectra.

[Will]

Correct me if I'm wrong, but if you write down your tuning values, and use the F2 service menu board replacement proceedure, isn't replacing a command board a fairly straightforward proceedure? Been a while since I've poked around in that section of Spectra RSS...

There are enough differences in componets that even with the same values, the tuning/alignment would not be correct.. Close, but no cigar!

[Will]

I think you'll find that the chip in the spectra marked TDA7256 is NOT an actual TDA7256, it has 15 pins and from what I've read is a 7255 that Moto had marked as 7256 for some reason, the ones from Digikey will be the real 11 pin 7256s and won't work in a spectra.

Remember, there are three versions of the audio IC, one marked TAD7255 and two marked TDA7256 used in the Spectra and Astro Spectras. The caps are different values/type and one is missing on some boards. 6 caps...

The TDA7255 uses only 10 pins
The two different TAD2256 uses 11 pins.
The TDA7256 has some added circuitry in the IC.

So, the marked TDA7255 is NOT the same as the TDA7256 marked IC.

[njscan]

I think you'll find that the chip in the spectra marked TDA7256 is NOT an actual TDA7256, it has 15 pins and from what I've read is a 7255 that Moto had marked as 7256 for some reason, the ones from Digikey will be the real 11 pin 7256s and won't work in a spectra.

Thanks for the heads up.
The hunt goes on..

[jackhackett]

Remember, there are three versions of the audio IC, one marked TAD7255 and two marked TDA7256 used in the Spectra and Astro Spectras. The caps are different values/type and one is missing on some boards. 6 caps...

My manuals show different cap values on the Astro boards but same part number for the IC, maybe there's one I don't have a manual for? In any case none of them are the 11 pin variety, and I'm pretty sure he could use the chip off one of the 900 boards.

[akardam]

Correct me if I'm wrong, but if you write down your tuning values, and use the F2 service menu board replacement proceedure, isn't replacing a command board a fairly straightforward proceedure? Been a while since I've poked around in that section of Spectra RSS...

There are enough differences in componets that even with the same values, the tuning/alignment would not be correct.. Close, but no cigar!

Was actually referring more to the bit banging part, in that the service proceedure takes care of all that for you... of course you'd want to check the alignment when you're done.

[Motrar20]

Wow. Now I'm confused. I have a 900MHz spectra, with an audio IC marked "TDA7256", 15-pin IC.
I already made the mistake of ordering a TDA7256 from Newark, only to find out that it is an 11-pin IC.
(Unfortunately, I hadn't stumbled across this thread until after I had made this mistake).

So now I see TDA7255's, with 15-pin packages, available through a source on eBay. So, to be clear,
I can order the 7255 and put it in my Spectra?

Thanks,
Bob

[Will]

Wow. Now I'm confused. So, to be clear,
I can order the 7255 and put it in my Spectra?

Thanks,
Bob

Not if your Spectra has a TDA7256 in it. TDA7255 is a different circuit.

[Motrar20]

It's marked TDA7256, but it has 15 pins. The TDA7256 data sheet indicates it should have 11 pins, and the part I
got from Newark matches the data sheet, 11-pins. I can't find a datasheet for the TDA7256 that shows a 15-pin
version. So what gives?

[Will]

Motar,

You can use the TDA7256 with 11 pins to replace the 15 pin TDA7256...if you do a little "pin bending" to get the correct pin in the correct location on the Command board.

4, 5, 14, & 15 are not used on the command board. = 11 pins!


The 11 pin TDA7255 can be used to replace the 13 pin 7255 in older radios.

[jackhackett]

Thanks again Motorola, for making things so simple.
So there are at least 3 different versions of the 7255? 11, 13 and 15 pin?

Now I remember why I always go with this audio IC replacement method:
Step 1) Remove TDA72xx and surrounding UHF Spectra
Step 2) Replace with UHF CDM1250

[Will]

I now have 15 pin TDA7256 DIRECT replacements for the Spectra audio IC. We were paying $19+ for them at service shop price from /\/\. Motorola customer price is over $ 22. Much less than Motorola wants for them, my price is half that to BatBoard members.

These also replace the TDA7255's.