Meter Squelch

[smithy959]

My R7000 has today developed a fault namely the Signal meter is reading +20 as though the squelch control is in use but even if I turn the squelch control fully anti clockwise the meter doesn't move so consequently nothing can be received even the local FM station cannot be heard,
any thoughts on this one Gents?

Thanks :-Ian

 

[KE5MC]

My R7000 has today developed a fault namely the Signal meter is reading +20 as though the squelch control is in use but even if I turn the squelch control fully anti clockwise the meter doesn't move so consequently nothing can be received even the local FM station cannot be heard,
any thoughts on this one Gents?

Thanks :-Ian

Not familiar with R7000, but on an HF transceiver the rf gain will move the S-meter indicator upscale as the gain is decreased.

Good Luck,
Mike

 

[cmdrwill]

Not familiar with R7000, but on an HF transceiver the rf gain will move the S-meter indicator upscale as the gain is decreased. Good Luck, Mike

A good idea.

 

[majoco]

The R7000 has a sort of "dual use" squelch control. From fully CCW to about halfway up it works like you'd expect squelch to work but it does not affect the RF gain - then as you continue to turn CW, the RF gain portion steps in and RF gain reduces - moving the S meter up scale as you'd expect until it reads S9+60dB with the control fully CW.

Page 15 of the Owners Manual.

I suspect that something is introducing a voltage onto the AGC line - if the S meter reads S9+20db Then your receiver is going to be as deaf as a post.

I'll scrutinise the schematic and get back to you.

 

[majoco]

I suggest that you go around all the PC board plugs/sockets and check that they are fully seated - don't just look at them - pull apart and then replace to break even the slightest bit of corrosion. The ones that will affect the squelch/gain are J2/P2 and J1P2 on the main board and J3/P2 on the IF unit.

Did you get the DC-DC board and the display fixed?

 

[smithy959]

I replaced the Caps and Diodes and still no 24v line or -7v line i was thinking it might be a rouge transistor
going to look at it again when I have some time,
Thanks for your help

 

[smithy959]

Today I checked the 5 transistors and all seemed OK so i'm stumped as to what to check next,I also checked the connections on the main board at J2/P2 J1/P2 and J3/P2 on the RF board

 

[majoco]

So you never actually fixed the power supply problem from your other post - did you fix the display fade-out problem?

 

[smithy959]

Yes I replaced caps C 19 and C 20 Display been OK since replacing those Caps,Even though the voltages were wrong the receiver worked OK it just started with this squelch issue whilst scanning the Airband channels,it certainly has me thinking.

Thanks for your help.
Regards:-Ian

 

[majoco]

Lets go back to the dc-dc board post......

 

[smithy959]

Looking at the schematic today now correct me if i'm wrong but does the
dc - dc unit get fed with 13.8v,-7v and 9v from the main unit?
Then do its magic and output 24v etc to the RF unit?

Regards:-Ian

 

[smithy959]

I bought the Receiver a couple of months ago on E-Bay so it could have been played with Before
I will check the Diode D1 and see if someone has put it in wrong way round.

Thanks:-Ian

 

[majoco]

Right - lets talk you through this thing - get the schematic in front of you.

+13.8 V DC from the main power supply is applied to the centre tap on the primary of the transformer via a bit of filtering, C1, C2 and L1.

The two transistors Q1 and Q2 along with R1/C3 and R2/C4 form an astable (unstable?) multivibrator - one transistor is conducting heavily while the other one is turned off hard, but it can't stay in this state. The highish voltage on the collector of the "off" transistor is conducted through the R/C combination the base of the other transistor turning it on and so on continuously at a frequency determined by the values of R1 and C3/R2 and C4 the transistors flip and flop backwards and forwards.

Current therefore flows sequentially through the primary of the transformer - 3 to 1 then 3 to 2 and back and forth. Therefore a square wave voltage at some frequency (which I have not measured or found) comes out of the three secondaries. The top pair I guess are two windings in series and give out 28volts peak to peak and the other bigger winding 62volts peak to peak.

Then follows three pretty standard voltage regulators - not much current involved so no massive heatsinks. The diode Q1 conducts on the positive half cycle and charges the capacitor C7 to +30volts, the other two diodes D3 and D6 conduct on negative half cycles and charge capacitors C11 to -31volts and C16 to -10volts. The zener diodes at the base of all three transistors are fed through their resistors from the collector voltage and these zeners stabilise the base voltage of their transistors.

By normal emitter follower action, the emitter voltages are about 0.6volts less than the zener voltage. There are some magic bits of wire in there that have different voltages on one end than they do at the other, but we'll take that with a grain of salt!

So - the emitter of Q5 is 0.6v lower than it's base, -8.2v + 0.6v = -7.6volts, Q3 base 24volts, emitter 23.6 volts and a bit less through the R7 and a bit of filtering = 22.0v, and Q4 base -12v, emitter plus 0.6v = -11.4volts and another bit of filtering = -11.0volts.

The 9v comes in and goes out again without any circuitry in the way.

So there you are, treat it like four bits of separate circuit and you be able to find where the problem is.

Have fun - persevere!

 

[smithy959]

I will have a look at the schematic this weekend and will try to follow your instructions,thanks for all your effort I got a Yaesu FRG8800 going earlier this year but the Icom is a little more technical.
Thanks once again
Regards:-Ian