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Old 10th Mar 2019, 7:16 am   #1
hiro, JJ1FXF
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Default 9R-59D - No Oscillation below 8.1MHz on C Band

Hi, all.

Can anyone give me some hints or advice to fix my problem, no oscillation below 8.1MHz on C band?

A frequency counter which installed to the cathode (pin#3) of OSC valve (V3a, 6AQ8) shows A, B, D bands oscillate well from the high to low frequencies, and also from 14.5MHz to 8.1MHz on C band is OK, but when the counter comes down around 8.0MHz, the oscillation suddenly stops and the frequency counter shows ZERO.

I suspect the padding capacitor, PC3, at C band rotary switch. However, if this is the cause, why 14.5-8.1MHz oscillates precisely?

Maybe somewhere else is wrong that affects oscillation below 8MHz on C band.

Any info or comments are appreciated.
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Old 10th Mar 2019, 8:08 am   #2
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

It could be any one of many things in the oscillator circuit.

1) The cathode of this oscillator circuit is an important node. Connecting it to a frequency counter will load it, and you may have resonance effects due to cable length. Remove the counter connection and see if it improves. It's good practice to have a buffer amplifier to stop the counter loading the oscillator.

2) An oscillator which is low on loop gain will often cease oscillation over a range of frequencies, and that range will widen as the problem gets worse. Has the ECC85 gone low on emission or Gm? Does it get the right HT voltage on its anode (Measure on a band where it is oscillating) Has the grid leak resistor or the anode feed resistor gone high? Losses in decoupling capacitors can reduce loop gain.

David
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Old 11th Mar 2019, 7:04 pm   #3
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

The coils on these receivers were impregnated with a wax that does not 'age' well - it degrades and causes increased RF loss.

Eventually the loss stops the oscillator.

Warm the oscillator-coils with a warm-air-gun and use a cotton-bud to take away as much of the old wax as possible.
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Old 13th Mar 2019, 10:29 pm   #4
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

Thanks for your reply, David.

The frequency counter was just a temporarily installed one. I installed it to know which exact frequencies the oscillation was stopped.

However, I am planning to add a well-known buffer amp soon to the OSC and the BFO in order to maintain my 9R-59D better.

Anyway, I observe that the oscillation stops under 8.1MHz on C Band even though the frequency counter is installed or not.

I haven’t check the voltage of each pin of 6AQ8/ECC85, yet. I only checked if the NOS valve of it would change the result and that wasn’t observed and so I just thought the OSC valve was OK. (I don’t have a valve tester.)

And my question is, if the decoupling capacitor is the cause, does the loss of loop gain may cause no oscillation on a certain range of frequencies, such as 8MHz to 14.5MHz on C band but it oscillations at lower and higher frequencies are OK?

Anyway, I will check the voltages of valve pins this weekend.

Many thanks, David.
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Old 13th Mar 2019, 10:45 pm   #5
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

G6Tanuki,

Year, I have read about the coil wax, removing it and dipping into melted candle.

My 9R-59D oscillated well a few weeks ago but one day stopped oscillating under 8.1MHz on C band suddenly.

Do you think this wax issue may affect to only certain frequencies?

Anyway, I’ll keep in my mind that there is a possibility that I need to have a kind of big surgery on the coils of my 9R-59D soon or later.

Thanks for your advice, G6Tanuki.
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Old 14th Mar 2019, 5:59 am   #6
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

Anything reducing the gain will first show up with oscillation stopping over a limited frequency range.

A bad decoupling capacitor is one possible cause.

Wrong bias due to something like an HT dropping resistor or screen resistor going high is another possibility.

Fixed capacitors in the oscillator circuit are another possibility.

G6Tanuki's covered coil losses.

Resistors can be checked with a multimeter. From then on component replacement may be the best way forwards. Debugging a non-oscillating oscillator is remarkably difficult. In the day-job I have to debug new designs, so I don't even have the comfort of knowing that it once worked. I've used a spectrum analyser to look for noise peaks, then added a little injection from a tracking generator. An oscillator below the gain threshold of oscillation becomes a circuit known as a Q-Multiplier and should work like a sharply-tuned amplifier. Seen this way, I could investigate causes of why the gain was lower than it was intended to be. For just repairing a single radio (rather than evaluating a new design) component swapping may be a lot easier!

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Old 24th Mar 2019, 2:42 pm   #7
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

Many thanks, David.

Your advice made me some progresses.

I once checked the plate voltages of 6AQ8/EXC85 on different bands, but they are all the same.

But I found that my 9R-59D had an OA2/VR150MT, a voltage stabilizer.

And so I removed it and measured once more, then found that the plate voltage dropped down rapidly on C band unlike other bands.

It goes down from 187V to 175V or less when it comes to around 8MHz.

I still haven’t found what the root cause of it, yet. But such a rapid voltage change occurs only on C band.

However, you mentioned that the decoupling capacitors, dropper registors and other parts which are commonly used in different bands may cause this low voltage on only C band.

So I will replace easier parts at first and see what they affect.

Again, thanks for you reply, David.
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Old 14th Apr 2019, 8:05 am   #8
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Default Re: 9R-59D - No Oscillation below 8.1MHz on C Band

Hi Hiro,

The plate or anode voltage is probably changing simply due to the oscillator “stalling”.

I have a 9R59DS which had a very similar problem, although it affected the upper two bands. I tried the remedy suggested by G6Tanuki and it cured the problem.

Remember to remove the valves in the vicinity of where you apply the heat - I forgot to do this, and one cracked

Good luck!
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