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Need help with my TDA7088 based Radio

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DaKandEKid

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I had a printed circuit board made for a radio design based on the phillips TDA7088T. It's almost identical to schematic in the phillips datasheet with the exception i used a bb809 rather then a bb910. And am using 68nH surface mount inductors rather then a 70nH and a 78nH. I was hoping this wouldn't completely ruin the circuit.


For anyone who has built this circuit i've been monitoring the voltage on pin 16 which is supposed to vary from 0 to 1.8 volts when being tuned. Each time you press the 'UP' button:
1) a capacitor will begin to charge which ->
2) effects the capictance of the variacap ->
3) which changes the frequency .....

The voltage on this pin stays at 0V always...

I was wondering if anyone had any input on
Differences between bb910 & bb808
* Cd @ VR = 4.7pF (bb808) vs. 2.5pF (bb909)
* Tuning Range
- Ratio = 9 vs. 16
- V1 = 1 vs. 0.5
- V2 = 28 (same for both)
* Rs Max = .6 vs.1
* Matched Sets = 3 vs. 2.5

Which are the critical values that must be matched in varicaps?

Anyone that is familiar with this circuit how tolerant is the Search Tuning to slight variations (ie 68nH inductor rather then a 78nH & varicap differences).

I'm trying to figure out if I have a problem with the chip or another part or if i just need different components.
 

Sebi

Active Member
DaKandEKid said:
The voltage on this pin stays at 0V always...

This is the problem. First no thinking about varicap difference,because it make only frequency range shift. (possible compensating with coil)
 

DaKandEKid

New Member
the voltage on the tuning pin varies from 0V to 1.8V

what's supposed to happen is the voltage is supposed to rise and as it's rising the frequency of the varicap circuit is changing affecting the LO of the radio front end...

What's supposed to happen is as soon as a good signal is found it's supposed to stop increasing the voltage.

I figured that it might not be finding any good stations and thus always statying in the 0V state... but perhaps then it would always be in the 1.8V state rather.

I'll poke around some more tommorow when i go back to the lab.

Thanks,
Blake
 

seeker

New Member
Hi DaKandEKid,

No offense but the inductors are about 14% out of spec, ( I see the schematic calls for variables there, don't know what the tuning range on those are)
The varactors are way out of spec for the circuit...I would try and get something closer to the original first for that.
Do you receive any signal at all with it? Can you break the connection at pin 5 and manually tune the varactor to pick up a signal?

just my 2cents...good luck with it :)
 

DaKandEKid

New Member
found my big problem... pulled the old put a 3.9pF in the 3.9nF capacitor's place :oops:

I'm now capable of receiving about 4 radio stations.

I ordered some closer inductors but i wonder if the problem has to do with the varicap.

If you are trying to replace a discontinued varicap diode which parameters do you pay the most attention to. In my original post i posted the specs of my varicaps.

Thanks for the tips
 
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