Q814 was acting strangely so I replaced it. It now reads,(B-1.6v with DMM, & sawtooth pattern with scope, just on Base) & clean, C 5.7v, E,0.00v.Having another go
Those collector volts sound OK although as most of these are used as regulators its actually the emitter volts (except for Q818 which does output on its collector) that are important. So Q806 and 807 are - and + 21 volt regulators (approx) and Q816 should be very near to 5.6 volts.
I suspect the collector volts on Q814 are probably correct. Worth checking though that there is a negative voltage on the base as shown. A scope check should also show a clean 5 volts here.
I really don't know what to suggest next. The DMM on diode range sounds as though it is turning on Q820 and firing the relays. We checked Q820 and its drive didn't we ? Should be a logic high (5 v) on R818 so I'm assuming thats not present as if it were the relays would operate. Was Q820 proved good by substitution ? 0.7 volts on the base (so 0.7 v across B and E) should turn it on.
uP pin 5,measures 0.73v at the pin & 0.72v on the left of R818 & 0.59v on the right side using DMM, its a flat clean line on the scope & the digital probe says its reading high? I'm wondering now why it is measuring 0.59v at the base of Q820? because that's not enough voltage to turn Q820 on is it? I have changed R818 & R816 with 1% tolerance resistors,but it made no difference.What if I wired it directly from uP pin 5 to the base of Q820,would that turn on Q820 do you think.Q820 is OK Ive checked it in & out of circuit with a proper in circuit tester.I replaced it even though the old one was alright. (just in case)
A clean 5.7v sounds good for Q814. I think either the manual is wrong or a 20K/ohm/volt meter was specified and used for the readings (which was the standard back then).
Pin 5 at 0.73 sounds odd. I would expect either zero or a logic high. Haven't got the circuit in front of me on this PC so will have to look at that later but its suspect. Wiring it direct won't fix the fault I suspect. Voltage readings like that might point to a problem with the IC but it really is the last thing to suspect. What makes me wonder is that no mater what problems may or may not be going on, the output port (pin 5) should always be at a defined logic level. Have you scoped it ?
Pin 5 at 0.73 sounds odd. I would expect either zero or a logic high. Haven't got the circuit in front of me on this PC so will have to look at that later but its suspect. Wiring it direct won't fix the fault I suspect. Voltage readings like that might point to a problem with the IC but it really is the last thing to suspect. What makes me wonder is that no mater what problems may or may not be going on, the output port (pin 5) should always be at a defined logic level. Have you scoped it ?
This has got to be the worst service manual Ive ever come across,its thrown that many red herrings, this time I noticed that R818 is 10k and in the parts section it is 10k but what I have installed is 4.7k ohms? So I changed it over to 10k and now uP pin 5 measures 0.84v & the same 0.59v as before at Q820 base.I will change it back to 4.7k, Pin 5 looks good on the scope. Pin 8 to pin 13 have a square wave in the middle of what otherwise is a good clean line, dropping to zero from 5v & back again to 5v.(I have the scope set to 1Ms. The SM says they should be low? Its only for the display but I thought I would try to give as much info as I can. I can see this receiver heading to silicon heaven soon!!! ( maybe if I threaten it a bit it may decide to work)LOL
What does pin 5 read if you leave out R818 ? Does it come up to 5 volts ?
In practice anything from around 1K to 22K or higher should work depending on the gain of the transistor. Its function is to limit base current (5-0.7)/R so 10K allows for around 0.4 ma. All your readings so far are (against all the odds) pointing to a possible problem with the uP.
In practice anything from around 1K to 22K or higher should work depending on the gain of the transistor. Its function is to limit base current (5-0.7)/R so 10K allows for around 0.4 ma. All your readings so far are (against all the odds) pointing to a possible problem with the uP.
Very strange...
I think you now have to try monitoring pin 5 voltage and dabbing a 10k from pin 5 to ground. If the voltage falls away it points to a problem with the uP not being able to supply any current from that pin. Maybe try a higher value too and see if the voltage falls but not as much.
If that were all that is wrong (and I doubt it) then that one issue can be overcome.
I think you now have to try monitoring pin 5 voltage and dabbing a 10k from pin 5 to ground. If the voltage falls away it points to a problem with the uP not being able to supply any current from that pin. Maybe try a higher value too and see if the voltage falls but not as much.
If that were all that is wrong (and I doubt it) then that one issue can be overcome.
Strange alright! Now uP pin 5 reads 4.77v,10k ohms to ground from pin 5, reads 0.16v,39k ohms from pin 5 to ground reads 0.51v. I'm totally lost now?
That really does sound like an issue internal to the uP. The output ports are normally like CMOS logic and should source/sink a couple of milliamps at least. Its very very rare to get problems like this. If this were all that was wrong you could replace Q820 with a Fet that would draw no current from the uP but would switch with the 4.7 volt voltage swing. I think there are more problems though. I know you said earlier that the unit had been worked on and modified so there are some unknowns to its history.
The other problem with the uP is pins 8 thru to pin 13 show error on my digital probe and a clean 5v on the scope except for what looks like a negative square wave drop to zero in the middle of the screen that is approx 3/4 of a graticule wide.The scope is set to Channel 1,5v, DC, Auto trig ,Source int,Time div 1 millisec. It doesn't seem to affect the radio playing at all? What type of FET should I use do you think? R818 would have to go because resistors cause current,Ive been led to believe.Sorry about the scope description,Ive never used a scope before.If the FET works OK I will leave it at that.I appreciate your patients & help Mooly and just hope your not too p....off with what has turned into an epic problem.
R818 
Its function is to limit base current into Q820 the base emitter junction of which behaves as a diode. This means that with no resistor (R818 shorted out) the base current would only be limited by the source driving it and would increase to destructive levels if enough current were available.
Unlike a transistor, a FET draws zero current into the gate terminal (think "base" equivalent). The only current that flows is that required to charge and discharge the junction capacitance.
You could try the common IRF240, better still perhaps the IRF2907 or even the small 2N7000. Ideally you want a FET (must be an N channel FET) with a low turn on voltage (Vgs) For B, C and E read Gate, Drain and Source. I would leave R818 fitted simply to "isolate" the uP from any transients that may appear through junction capacitance. Absolute value unimportant, anything from 1k to 10Meg would be fine with a FET.
Its function is to limit base current into Q820 the base emitter junction of which behaves as a diode. This means that with no resistor (R818 shorted out) the base current would only be limited by the source driving it and would increase to destructive levels if enough current were available.
Unlike a transistor, a FET draws zero current into the gate terminal (think "base" equivalent). The only current that flows is that required to charge and discharge the junction capacitance.
You could try the common IRF240, better still perhaps the IRF2907 or even the small 2N7000. Ideally you want a FET (must be an N channel FET) with a low turn on voltage (Vgs) For B, C and E read Gate, Drain and Source. I would leave R818 fitted simply to "isolate" the uP from any transients that may appear through junction capacitance. Absolute value unimportant, anything from 1k to 10Meg would be fine with a FET.
You have no idea how much I have learnt from you Mooly! I have read all the theory but its only here that the pennys have dropped when actually doing it hands on,your instructions are so clear & easy to understand that I suspect you have a teaching background,excellent! Because I live in a remote location,near the square killometer array in Western Australia Ive had to order the 2n7000 FET so that will take approx 2 weeks to get here. I will let you know how it goes,until then Adios Amigos.
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