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There must be some instability in the circuit somewhere...

No, instability causes unwanted signals (such as audible squealing or loud hissing) to be created.
What you are looking at is low gain, somewhere in the oscillator circuit.

Guess i'll just work through it testing each component in turn...

Many components cannot be easily tested, you'll need to substitute or measure in many cases. For example:
- low value ceramics will be swamped by the capacitance of the meters test leads
- gain of transistors can only be measured by a meter designed to specifically do this

In addition, components can read OK on a multimeter but fail when in-circuit - they read OK on the lower voltage that's given out be a multimeter, and fail when the higher voltage in the circuit is applied. In particular, capacitor leakage (as in passing DC voltages, as opposed to goo coming out of them) - I got caught out with this a few times in the early days.

tried swapping crystals (lo for hi) and that didn't help

By this, I take it that the higher crystals still showed the fault?

Did you try changing C204 (39p) - say from 39p to 47p - to give the oscillator more drive?
Not so much as a permanent solution (as this will throw the radio off-frequency), but just to see if increasing the drive will reduce the problem?

Summary of fault: Tune and clarifier acting as a severe RF gain...The more the tune and clarifier controls are tuned to the right the less the signal gets.

When you turn to the right, it decreases the capacitance (and hence increases the frequency) and reduces the drive (positive feedback).
Reduced drive = decreased amplitude to the mixer = lower signal (on both receive and transmit).

You just need to increase the feedback to compensate - this could mean either a slight mod or finding a faulty component.
I'm not too much of a purist for keeping things 100% original, especially with 30+ year old equipment... at $75 per hour, most people that bring us radios for repair just want them returned to a working and reliable state, and don't care exactly how we do it. I always ask first though, just in case...

Cheers

Thanks for the lengthy reply Sue. From what you say, i'm definately concentrating on the right area to find the fault...around the Q4, T1 area and down to the crystals.

I plan to replace the capacitors as required, Starting from TP4 and working back to the crystals... I'll bet theres a bad capacitor somewhere.

And yes, if I was doing the radio for someone else I would get the job done with a workaround soloution...but as its my personal project i have plenty of time i'd like to find the cause of the fault if I can

Changing C204 brings the radio under frequency and so turning the clarifier fully clockwise only brings the crystal to center frequency and so the problem does not happen..

I'm determined to find the fault and then I'm gonna post it so everyone in the future will know how to fix this problem properly...

I'm begining to wonder if T1 has gone narrow bandwidth for some reason...maybe its shorting out or something...Guess i'll swap it out and find out...I know this is rare, but T1 must have been adjusted many times over the years and maybe its quacked...no wait it cant be... oh, i dont know...lol maybe i'll just swap it anyway.

Changing C204 brings the radio under frequency......so the problem does not happen

Then it's a low drive problem. This will be on the crystal side of the transformer.

A slightly technical, highly simplified explanation:

A capacitor's impedance (AC resistance) reduces with increasing frequency - in other words, it's open circuit at DC, it's a short circuit at high frequencies.
An inductor behaves in exactly the opposite manner - it's short circuit at DC, it's a open circuit at high frequencies.

Put the two together and they act as a pair, becoming either open or short circuit at only one frequency (what frequency this happens at depends on the component values). This is called their "resonant frequency". For this example, I'll choose an inductor and capacitor pair that resonate at 20 Mhz.

If we connect them in series, it's a short circuit at 20 Mhz, and open at other frequencies:


If we connect them in parallel, it's an open circuit at 20 Mhz, and short at other frequencies:


Therefore, by choosing our component values correctly, we can make a filter that lets us choose one wanted frequency and reject the other unwanted ones.

Part 2:

When an amplifier circuit is designed, the bias components are normally selected to keep distortion and harmonics (multiples of the input frequency) to a minimum.
If we actually want harmonics to be generated, we deliberately mess up on the bias circuit:


This is a simplified example of the T1 circuit. Note that there is an unconventional bias arrangement (this is called "class C").
This will generate lots of harmonics, but no worries, as we have a parallel tuned circuit at the collector which is open circuit at 20 Mhz....

On the +Vcc line, there will be lots of capacitors shorting Vcc to ground at anything other than DC. These are called "bypass capacitors".
At 20 Mhz, the C / L pair are open circuit, and the signal goes out to the mixer.
At any other frequency, the signal goes through the capacitor "C" and is bypassed (shorted to ground) by the bypass capacitors.

In reality, a small range of signals will be let through, but only one will be at the peak. Therefore, it's a compromise which one works best.

I'm begining to wonder if T1 has gone narrow bandwidth for some reason...maybe its shorting out or something...Guess i'll swap it out and find out.

In your case, the 10 Mhz signal has a problem, and the doubler is just doing its job. Therefore, you have to solve it on the stage before the doubler.
If T1 can be adjusted to a peak (in other words, resonance) then it is working OK.

This explanation was deliberately simplified by the way... it gets very mathematical, have a look at http://en.wikipedia.org/wiki/LC_circuit and you'll see what I mean.

Hope this helps.

Cheers

lbcomms wrote:This explanation was deliberately simplified by the way... it gets very mathematical...

Ok, so I totally understood the complicated maths on the Wiki page... ... I'm so glad you decided to simplify it for me

lbcomms wrote:If T1 can be adjusted to a peak (in other words, resonance) then it is working OK.

But now, after reading your last post, I'm wondering about the internal capacitor thats built into the transformer T1... if that internal capacitor has gone bad and is not performing correctly, then wouldn't that put the circuit out of balance?

Anyway, I count only about 7 components on the path between T1 and the Crystals, so if it comes to it I'll replace them all...

so I totally understood the complicated maths

I think your sarcasm meter just went right off the scale

Seriously though, it's fairly straightforward for those of us that work with this stuff for a living. That was only with 2 or 3 unmodulated carriers - if you want complicated, try digital information encoded on 16 or 64 separate carriers. Spent my last 18 months at UTS (University) on those

internal capacitor thats built into the transformer T1

That's just part of the resonant circuit - if you can "peak" T1, the internal capacitor will be OK.

I count only about 7 components on the path between T1 and the Crystals

It's not just these 7, you need to look at the whole oscillator circuit. There are lots of components there and a bit of electronic switching going on - a limitation of the era.
Like I said before, it only just made the grade when it was new. Signal reduction when the tune / slider clarifier is turned towards its upper limit was a common complaint with these not long after they first came out, way back in the late 1970's.

Cheers,
Sue

It's not just these 7, you need to look at the whole oscillator circuit. There are lots of components there and a bit of electronic switching going on

More work than expected then, but I'm up for the job I'll just gradually swap out every component in that part of the oscillator circuit until i find the fault.

I've started from TP4 working back towards the crystals, replacing components. I still have a couple of caps to change on that line then I'll move on to the other side of the crystals... Its gonna take time, as I cant work on the radio for too long at a time due to medical conditions, but I'll plug away at it a few components at a time and get there eventually (at least I hope so!!! )

UPDATE: Replaced R12 and R13 (PTOS006AOX) and the problem has improved, with the signal/white noise now only dropping out about 1/2 as much as before...Frequency at TP4 now stable and slides with the tune control as it should... So, I'm certainly on the right tracks to sorting this problem

I know that even when new the signal dropped out a little at maximum tune, so I'm not going for perfection here, but its still not right yet, so the work continues!!!

Well the Jumbo still isnt done yet! unfortunately i havent been able to work on it much these past few weeks...

The last thing I did was to replace C21 which made a noticible improvement in the problem.

Seeing that C21 made the problem improve, i'm wondering if I should maybe also be looking around the mixer and dc switch area (Q21, Q22) on the main PTBM059AOX board where C21 is going to? well thats my plan anyway eventually I'll get this radio done.


UPDATE 25/07/2011: My Jumbo is now fixed... I could not locate the problem on the PTOS006AOX oscillator board, so I opted to do a somewhat crazy soloution. I bought a Major M588 of ebay, took all the components out of its oscillator board (virtually same as jumbo), then desoldered all components from the jumbo osc board, cleaned up the jumbo board and then re-built it following the schematic using the Major's parts.

The Jumbo osc board is now working as it should. There is still a tiny bit of fade when tune and clarifier is turned, but thats normal.

UPDATE 14/05/1012: The true cause of the fault was found in the end... it was one of the AM IF amp transistors on the PTBM059COX board that was weak (either Q14, Q15, or Q16 i can't remember exactly now and cant be asked to look at the schematic at the moment but one of those three is AM IF amp and thats the one that was weak). replacing the weak transistor totally sorted it and now i get barely no fade on the tune/clarifier

I came across this thread while trying to sort a jumbo I had.

Mine lost tx and rx when the tune and clarifier was turned left.

It turned out someone had changed the 5.4 volt to a 6.8 giving too high a voltage on the pll and oscillator circuit.

Changed that and it's sorted

Dave

Hi Dave,

All the work i did to the oscillator board in the jumbo made the issue loads better, but it still was not 100%... turns out one of the AM IF amp transistors on the 059 board was weak (it was either Q14 15 or 16, I cant remember now) and that was the root of my 'clarifier-reverse-rf-gain' problem. The 'no-swing' problem was cured by me getting a decent swr/power meter that correctly mesures PEP, there was no fault with the jumbo :-/

Just a side note dave, please be aware that it is considered bad practice to re-open old posts that have been idle for a long time (over 18 months in this case) as in most cases the original poster is probably long gone. Its not a hard rule, its just considered 'forum etiquette' to let sleeping dogs lie, and it generally applies to pretty much all forums/boards.

Cheers,
Norm

Ah not heard of that on forums before.
Sorry

no worries dave, i've even been guilty of it myself in the past on various forums, until i was made aware of it a few years ago i didn't know it either...

Scott: I hope my last post about the forum etiquette was ok, its a habit for me now (being a moderator on ERC forum) and i forget myself sometimes... I dont wanna be stepping on RadioMods moderators toes

I have just known good President Lincoln swing test showed no swing, so it appears you are correct treasure may be working properly, SWR / Power meter is neither the most work.