Issues with Kenwood Receiver R2000/Problems with the 36 MHz Reference Oscillator - Page 1
Forum section: electronics / repair

author: miette_Kruemelchen
author_rank: Contributor
timestamp: August 27, 2023, 07:20:48 am
content: Hello members of the EEVblog community,On my workbench stands the Kenwood R2000 receiver. Regrettably, repairing this device has posed several challenges, which is why I am now reaching out for your assistance.Fault description: Although the device can be powered on, there is an audible signal noise across all signal bands. Despite conducting a frequency scan, the field strength indicator remains persistently at zero.The following measures have already been undertaken on the device:Measurements and verification of operating voltages at the following points:Measurement of the operating voltage at connector 52 (13.95 VDC)IC 18 (4.98 VDC)IC 19 (4.98 VDC)IC 20 (9.00 VDC)Operating voltage at C 191 (-12.9 V DC)Operating voltage at C 192 (-6.06 V DC)Notably, the previous owner inexplicably desoldered diodes D35 and D36 on the cathode side.Actions taken for troubleshooting:Reinstallation of diodes D35 and D36 into the circuitReplacement of the memory battery due to aging defectsRenewal of the backlight for the field strength indicator due to a faulty light sourceHowever, the core issue persists the absence of the reference oscillator signal at test point TP 4. At this juncture, I face a challenge and find myself at an impasse. The 9 MHz quartz crystal, X1, is indeed present, yet exhibits pronounced distortions. Direct current values at the collector circuit Q1 do not align with specifications, suggesting an inappropriate operating point.The measured DC voltage relationships are as follows:A voltage of approximately 1.82 V is observed across R12 and R11. Consequently, the base of Q1 resides at this voltage level.The collector voltage stands at 4.98 V.The emitter of Q1 registers a voltage of 3.2 V, indicating a significant fault.The current flowing through R12 currently measures around 67 µA, and over R13, it amounts to approximately 3.9 mA.I have already replaced Q1, as well as resistors R12 and R11, without discernible alteration in the error. Attached is an oscillogram: the slightly distorted yellow sinusoidal waveform is at the base, while the erroneous blue oscillation is located at the emitter of transistor Q1. This oscillation can be traced up to transistor Q5, albeit with considerable attenuation and multiple superimposed oscillations. After C31 (0.5 pF), no further measurement is viable. Detaching the input from T2 still yields a measurable voltage.Could the issue point to a faulty 9 MHz quartz crystal, X1? The amplitude of the quartz signal is notably high.How might I further narrow down the fault? Presently, I have removed the PLL unit board and externally powered it for conducting measurements and refining error localization.Should anyone have insights into how I can pinpoint the fault more precisely, or if my assumption regarding the defective quartz is plausible – I am genuinely appreciative of any guidance. An unusual detail in my measurements is intermittent rapid frequency fluctuations in the kHz range. I regret to admit that I am uncertain if these stem from my measurements. Nonetheless, I am measuring a very high sinusoidal amplitude from the 9 MHz quartz. I have never encountered a quartz defect that exhibited such behavior before!I eagerly await your support and perhaps novel approaches.best regards,Sven

author: floobydust
author_rank: Super Contributor
timestamp: August 27, 2023, 06:02:05 pm
content: I think you're possibly going down the wrong rabbit hole.That the 9MHz xtal oscillator is running I think is good enough. It looks like the oscillator feeds PLL IC2 and divided down by IC1 4.5MHz, to feed PLL IC5. It's a square-wave to IC5 pin 3 I think. So a distorted sine doesn't matter I would say, unless amplitude is way low.Scope probe loading is a problem and can give a misleading waveform. Measuring DCV on a node with RF on it can often give readings that are off a bit.Q1-E 3.2V looks high, it should be centered about ~1.8V but again it can be test equipment and loading.I would look further downstream at TP3, TP5 for both PLL's inputs pin 3 1.2Vpp, 0.9Vpp and the output at TP13, TP7.There are many VCO's as well. I can't see all of them failing.Pick a band and stay on it, something simple say MW and see if the main PLL is working.If someone was in there attempting to do mods i.e. D35, D36 then I would expect oddball troubles. Perhaps trying to add some funky extra bands? Or it was a clown in there.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 27, 2023, 06:41:26 pm
content: That waveform at the emitter of Q1 does look suspect. Disconnect one end of C14 and see if the waveform inproves significantly, especially its amplitude.

author: floobydust
author_rank: Super Contributor
timestamp: August 27, 2023, 10:45:23 pm
content: It could be the load Q2 off of Q1 is non-linear a bit of rectification but you could tell by comparing waveforms across R16 to see Q2 base current.OP's scope could do an FFT as well, see what harmonic that is.The buffers Q2, Q3, Q4 are emitter-followers so no voltage gain (aside from the flip-flop).One scope channel on TP2 and another around Q1/Q2 you can notice loading effects when probing around if that is what we are seeing. I don't know the Rigol probes, that oscillator it's probably best looked at with an active probe.