Kenwood TS-430 Oscillator repair

[swooshcmk]

Hello everyone!  Ok, quick back story - I'm trying to learn electronics better because they interest and intrigue me.  I've been watching Dave's videos for years, as well as Mr Carlson's lab, Radio Repair Shop and others on YouTube.  I picked up an amateur radio license for VHF/UHF, but didn't find a whole lot of use in my area, so I picked up a general class license to get on the HF airwaves.  My main goal for doing that was to better learn electronics, so what better way to force myself to learn than to pick up a radio that needed some work!  (Yes, I'm a sucker for self imposed punishment lol)  So, I have on the healing bench a Kenwood TS-430.  I went with this radio as it's 80's tech using lots of easy to work on through-hole components, relatively simple circuits, easy to find service manual/schematics, and lacked the corrosive goop that other models had utilized.

So, The radio powers up in that it has static on the speaker, you can hear the relays click up and down the band, switch modes, but there's no display for frequency output, and no receive.  So my first guess is something's wrong with the PLL circuit..
But that's not where we start, so first things first - check voltages.  They're all checking in range, so I move to step 2 in the service manual, which is checking the frequency of the primary oscillator.  This is supposed to read 36Mhz.  I hook up my frequency counter using a BNC to aligator clip lead, ground the negative and clip on the positive - nothing.   So I switch my frequency count out for my signal generator which can also count, and I get the same deal, so I assume there's an issue with the oscillator circuit, but since I've got the gear, I figured I'd hook the oscilloscope up, and I got the same reading grounding the probe ground to chassis and reading the frequency.  So this seems like a fairly simple solution, likely a bad crystal perhaps... but I'm also a noob at this, so maybe I'm trying to measure this oscillator wrong    I did notice that if I left the ground from the BNC to alligator clips off, or the oscilloscope probe ground off, I'd get a measurement varying from 82 - 111 Hz, but I am suspect that in this instance the lead is acting like an antenna picking up some other interference perhaps, but it was rather consistent amplitude, which doesn't sound like noise lol

So, I tried swapping the crystal out for a new 36Mhz crystal.  No change.  So my next though was "Well perhaps another part of this circuit is dragging down the output", so I figured the next approach would be to try to isolate the crystal oscillator to itself from the rest of the circuits, so I pulled Q6 out, as well as C38 (though maybe this is needed for the oscillator to work?  I'm doing this to learn oscillators and RF stuff better.  The voltages on Q6 were pretty close to indicated).  There's 6v feeding R24, which along with R25 forms a voltage divider that feeds just over 4v to the left side of the crystal, and those resistors are both within spec.  So my understanding is that with the voltage applied to the crystal it'll self oscillate, and the reference to ground being through the parallel inductor/resistor through C36 and the trimmer capacitor that allows you to adjust the frequency (capacitive coupled to ground??).   At any rate, testing with the oscilloscope on either side of the crystal shows 0 oscillation with the scope's probe grounded with Q6 and C38 removed.  Did I remove components vital to the oscillator working?  Should I be testing a bit further downstream by removing Q7?  I'm I not measuring for oscillation correctly?  Is this indicative of the 1uhenry inductor being bad?   

I sort of get the feeling that Q6 and C38 form part of the needed "feedback" for the oscillator to work, so perhaps I should pull C40 and then measure for frequency at the Q6 emitter?  This is where I start to lose understanding of how things are working and it starts to turn into "black magic" haha    I assume Q7 and Q9 are essentially acting as "amplifiers" to drive down stream circuits.  This primary oscillator is used to drive many other parts of the radio, including the PLL circuit that drives the display and controls tuning etc..  The official test point for measuring for 36Mhz is at connector 1 at the bottom of the supplied schematic image.

Thanks for any help or understanding anyone can provide on this!

Chris

[swooshcmk]

Ok, looking at this some more I see this as a colpitts oscillator, so I do need the capacitor back in there.  Is the transistor *required* too?  Or is it just to amplify output for use?  I didn't catch it as a colpitts at first and assumed the crystal portion would self oscillate similar to an RLC oscillator.  Perhaps that is an incorrect assumption?

[bd139]

You may actually be stopping the oscillator if you're using a BNC to alligator clip. They have a huge parasitic capacitance which alters the circuit performance pretty badly. You need to use a proper 10X scope probe for this job and probe at a relatively low impedance point to ensure the oscillator is not affected by the probe.

In this anything around Q6 is quite fragile so I would not probe there. My diagnostic flow would be:

1. Check the DC voltages with a DMM as per the schematic.
2. Get a 10X probe on the emitter of Q7 and check for signal there. That's part of the emitter follower buffer and is much lower impedance and has less side effects probing it there.

Proper probing technique is quite difficult here as the ground strap of the average probe is actually a pretty hefty inductor as well which has some effect at 36MHz. Best to use a ground spring attachment and probe across R30. If you don't have one, connect the probe strap to the ground on the oscillator board rather than the chassis. You might get a wonky looking output but it'll at least confirm it's working.

As for function....

Q6 is a colpitts oscillator
Q7 is a common emitter amplifier to uncouple later stages from the oscillator so they don't change the oscillator frequency due to loading.
Q8 is a common emitter amplifier to drive whatever that IC is to the left.
Q9 is a common emitter amplifier to drive the clock transmission line to wherever that goes

Edit: nice radio BTW. Shouldn't be too horrible to get working and lots of spares available 

Edit: also be very careful. Never remove components unless you have identified they are dead. This can cause more problems or change the problems you have. Start with measuring DC voltages always.

[swooshcmk]

Thanks bd139.   I did verify voltage prior to removing components to test, and I was getting the same results on the scope with the 10x probe as I was getting on the frequency counter and signal generator with the bnc aligator clips.   I'll put Q6 and the capacitor back in the circuit and test in the area you indicated.  All the voltages have checked out so far so I am trying to figure out what's bringing the oscillator output down (or causing it to not oscillate)  I guess I just tried to get too close to the oscillator right out of the gate, and I was starting to suspect my probing technique was killing my oscillator results - part of the learning process!.  My first response was to remove the connector for the downstream gear to make sure that wasn't pulling the signal down, so I know it's within the region of the schematic I posted - just need to find the culprit now.

Thanks!  I'll post back with my findings later tonight.

[swooshcmk]

Ok, so I've checked for any signs of life from the oscillator at the Q7 emitter, however it's still a flat line (scope probe 10x, ac coupling.  If I switch to DC coupled I of course see the voltage present, but still a flat line).

For completeness, I'll include the measured voltages for each the Q6 and Q7 transistors.

Q6 (2SC460):  E= 3.252v,  C=6.75v, B=3.504v
Q7 (2SC1815): E=2.373v, C=6.76v, B=3.062v

Q6's base is a tad higher than the specified 3.0v at a measured 3.504, but I don't think it's alarmingly out of range?  But perhaps it tells others something I don't yet see.  Looking at R24 and R25 as a voltage divider with the 6.75v supply the transistors are seeing I calculate an estimated 3.7v (not accounting for oscillator resistance seen in parallel with R25), and I attribute the difference primarily to the fact that they're being supplied 6.75v instead of a flat 6v.  While not exactly what is specified, I don't think it'll cause the oscillator to completely stop working (and the service manual doesn't indicate checking a 6v rail prior to the oscillator, and I assume it may be higher voltage if other parts of the radio aren't firing up due to no oscillator etc and the circuit not being fully loaded).

But, where does this then leave me?  The resistors forming the oscillator circuit and the circuit up to Q7 don't show any signs of damage or overheating etc, and passives like resistors tend to not fail too often.  The capacitors utilized are ceramic disc capacitors, and those almost never fail either, and I don't see any of them with visible signs of damage.  I tried swapping the crystal with no change (figuring the crystal was the most likely culprit from the get go).  The inductor is also in a plastic case, and I don't imagine a plastic case enclosed inductor is all too likely to fail.   I don't want to pull out the parts cannon and blast in new components until it works or just rebuild the whole oscillator circuit, but I'm personally at a loss of what to check next short of pulling out each component to test them individually 

Thanks for any tips or guidance you can give me!

[JKKDev]

I would hazard a guess that your Q6 3V at the base being out of spec is directly connected to your 6V rail being out by 10%. Maybe figure out why the 6V rail is so out. It probably won't solve the problem but it might tell you what happened to the oscillator circuit. Power rail shouldn't just be 10% out of spec for no reason. Maybe I'm wrong but I feel it's worth checking out.

[swooshcmk]

It appears the 6v rail is created via R23 drop resistor from the 8v supply rail.  The spec in the manual for the 8v supply rail is 7.4v to 8.4v, with mine coming in at 7.92v
 
The only items between the 8v supply rail and the "6.0v" supply for the transistors is a 100uH inductor and the 100 ohm R23 resistor just before the transistors.   There are several other circuits fed from this 8v supply line, but as as what's between the 8v supply and those transistors, it's only those 2 devices.