DM500s 22khz tone circuit board fix

[mickeydebo]

Hi I have a DM500s with a 22khz tone problem or no high band.
I know its the receiver cos I have tested 5 other dm500s with exactly the same image flashed to them, and they work fine.
So before I get check your lnb and all that. (DONT BOTHER PLEASE)
Im asking if anybody knows what components on the motherboard to test. or replace for the fix?
I get a reading from the LNB ouput voltage with a mutmeter.
+13.08 volts vertical and 17.88 volts horizontal and it dosent change when I go the a highband frequencie?
All I managed to find is test R712 (15 ohm) resistor wich is reading 6ohms.
And the Q72 (the FET-transistor) wich I can read from the drain side with a multimeter but not from the other.
What you reckon shall I bin it hehe.

 

[HB13DISH]

My guess is that the 22 KHz signal is built into the tuner circuitry, so testing the tuner might be the next step.
I could be wrong and it might be something else.

 

[mickeydebo]

Thanks for the suggestions and your time.

I noticed there is a small LED on the motherboard (D72) I assuming that this comes on when the tone is selected or the high frequencies (SKY ONE),I havent got a spare DM500s to test this anymore.
Can somone please let me know if this is correct?

Also looked at the circuit diagrammes and 22khz tone seems to be generated from the motherboard.

Can someone also help me with this one im getting some weird readings from resistors.
(R715) Schematic says should read 472 ohms i assume but im getting a reading of 4.7Kohms
OK so it could be me so I checked resistor R710 & R711 wich schematic says 333 ohms and I get a reading of 33.3kohms.
Could you pls tell me wich one is wrong? (If any),
I would like to fix it just to help someone else next time this crops up.
Ok it definatly getting nearer the BIN heheh.

 

[mickeydebo]

See attached Circuit daigramme for the DM500s LNB circuit

 

[HB13DISH]

According to the resistors code for 5% tolerance
333 = 33.3 Kohms
472 = 4.7 Kohm
So the schematics is probably using the code for value and not absolute value.

How is the tone being generated?
Is it using a crystal oscillator or a resistor/Capacitor circuitry?
Don't bin it. If the problem is on the PCB, you just need to find out which part is faulty.

 

[HB13DISH]

Thanks for the schematics.
Did you test Q71?
It looks like it is the one used for the oscillator circuit using the R-L-C network above it.

 

[mickeydebo]

Hi.
OK what shall i test next?

So if its a code say R72 472 = 4.7K if i find a resistor with the same code should it be the same resistance?

I am not a electronic engineer but I know what componentis are what.

How would i go about testing these transistors for faults with just a multimeter? I have forgoton all I have learnt dont play much with them anymore just replace units with new ones.

Ive attached more Scehmatics for you for the dm500s hope they help.
I have got a few more schematics for the dm500s to upload wil do it next time.

 

[Llew]

On your original schematic, the way I read it is as follows - Q71 is a switch, and the 22kHz output line to the base of the transistor will be a voltage on/off signal from the microprocessor. With an on voltage, the LED lights up, indicating low band. This voltage will switch on Q71 and remove the 22kHz signal from the LNB 13/18V line (presumably generated by the LC oscillator C726/L712).

With no voltage to Q71, the 22kHz signal remains active.

A voltage check on the 22kHz output line (really an input) when changing high/low frequency channels will show whether this signal is present.

Ignore resistor value readings when they are in parallel with coils, they can be misleading when checking in-circuit.

Llew

 

[mickeydebo]

Thanks for the info about checking the transistor base voltage I will check it and let you know what the voltage result is.
Also about the resistors
Is there any other checks I can make whilst im at it?

 

[confal44]

HI mickeydebo

Sorry for my english language

You write:

So if its a code say R72 472 = 4.7K if i find a resistor with the same code should it be the same resistance?

I don't find R72 perhaps Q72


On DM 500 we can read R715 number of SMD resistor
472 = 4.7K ( 47 and the last number 2 = two 0 ----> 4700 Ohms = 4.7 KOhm)

For an even value you have more SMT boxes (size) 0603, or 0805 or 1206

On DM500 R715 is a 0603 boxe

Best regards

 

[mickeydebo]

Hi Thanks for all your help on this problem, its a good challenge for me.
Im not sure if i have the skills to overcome this problem but ill give it a go.
I have managed to check the voltage of the transistor Q71 and found that there is O volts reading from two of the three legs
Im not shure wich is the base,emitter and collector so will explain them from looking a the top wth the tuner above.
Tested all legs with the negative attched to the chasis.
I also made sure I was on a channel that uses high band (sky news)
Top left leg O Volts
Top Right leg 0 Volts
Bottom leg 13.08 Volts or 18.08 volts dependng on polarity.
I have checked this with another dreambox 500s and the readings were as follows
@ 14.26 Volts
Top left leg 0.54 Volts
Top Right leg 0.91 Volts
Bottom Leg 8.86 Volts
@ 18.08 Volts
Top left leg 0.59 Volts
Top Right leg 0.94 Volts
Bottom Leg 12.14 Volts
Am i going to put it in the bin or is there another problem from another transistor?

 

[andmac]

Hi.hope this will help if you did not bin the box yet.
Q72 is a swithing transistor try connecting a cable from bottom leg as you say to the diode near the input of psu top left of the box on cathode [bigger black smd diode,side with the line]this solved many problems for me especialy
voltage drop for diseq motors.

 

[JamesBank007]

I have same problem with you !!!
My DM500s has no 22KHz and no DiSEqC command.
LNB signal pass only DiSEqC ch.1

I try this but still not work

Hi.hope this will help if you did not bin the box yet.
Q72 is a swithing transistor try connecting a cable from bottom leg as you say to the diode near the input of psu top left of the box on cathode [bigger black smd diode,side with the line]this solved many problems for me especialy
voltage drop for diseq motors.

My DM500s didn't has L714 and F71 !!!

Q71 check

Top left leg 0 Volts
Top Right leg 2 Volts
Bottom Leg 13-18 Volts

Sorry for my english language.

 

[learning]

Same here the odd thing i found was that the high band worked as a receiver when looped through my tecnomate to the dreambox, that is I got channels but I had to tune through the tech when it was on high band to get high band channels on the dreambox

 

[mickeydebo]

Thanks for the tip but it did not work.
nevermind I will put it on the shelf until someone who knows what they are doing can look at it.

 

[dev_kfree_skb]

Hi,

I had the same problem, no 22KHz present, so no band selection was possible, and here is my story.

The "22KHz output" label shown in F.pdf is actually a signal coming from the tuner (tuner pin 15 out of 28, counting from F-connector edge). This signal is not just and on/off for any RLC network. This signal IS the 22KHz pulse chain (frequency) itself. It actually modulates the amplitude of the 13/18 voltage for a deviation of 0.75 V using Q71, so the LNB can tell which LO to use.

In my case, the pin header that joins this signal (among all tuner signals) from main PCB towards tuners' PCB had been poorly soldered, and I just heated it up a bit and that was it.

The receiver was a clone one, so the description of the actual route (from R715 towards the tuner) of this signal may vary.

 

[Llew]

Thanks, that should help owners of the box. I don't recall a tuner itself providing 22kHz. Don't have the STB myself, but checking the circuit with a 'scope would have helped to find it.

Llew

 

[hajduk.sam]

Hi there and thanks for the useful topic.

I am in Bulgaria and recently had experienced no LNB power and missing 22kHz tone on two of my three boxes (dm500s-clones). This happens when after arrangement of my system I have cut the LNB cables without power off the two receivers.
Replacing the Q72 (mosfet) doesn't solve the problem and thanks to this theme I replaced the Q71. With one of the boxes this was enough and it is working fine now. But the second one doesn't - much more I realised that when LNB cable is connected the Q72(mosfet) is extremely hot ... when LNB cable is disconnected measuring of LNB power on the F-connector is OK.
I will appreciate any advice or opinion shared on my case.

 

[barlas]

hello dear.
it seems that you have good knowledge of dm500s. mt dm500s was working perfectly ok but a sudden electric trip has generated a fault in it now i can hear the sound i can change the channel but no picture same probelm happened to 4 of my boxes and all is because of a sudden electric trip. i have reflashed them but still no picture at all.
will you please tell me which component could be dead and which parts are for picture.
thanks

 

[anata82]

Dear All,

I m encountering an issue similar as this one.

I have 1 DM500S. When I start an automatic/manual transponder scan.
Some transponder are skipped eventhough the SNR is about 60-80%.
Somehow the receivers will lock few transponder from Astra2.

The DM500S seems to get all data from the other satellites (Astra1 and Hotbird)

Do you think the replacement of Q72 will fix the problem?