KB KR019

[Refugee]

A whisker between collector and screen will short out the coil and make no difference to the DC voltages. I also have some NOS with NKT numbers and whiskers.

[Crackle]

hi Ref
Yes tin whiskers are a possibility but would that account for a 10.7mhz AM signal passing through better than an FM one, or is that normal.
Some of these transistors are impossible to get at and I am resisting the temptation to dismantle the whole board assembly and tuning cord to get at them.

The Regentone TR419 is not working on FM either, or at least it can just pickup one station very quietly. I am sure it was working OK when I put it on the shelf about a year ago.

What frequency range would I be expected to pickup on a SW radio if the FM oscillator is working.

thanks
Mike

[Crackle]

Just tried sweeping the Signal gen through on 87MHz and I can pick up Gateway FM station on 97.8MHz.
So it looks like a problem with the oscillator circuit around TX7.
Luckily TX7 is accessible to unsolder, so that is the next job.

Mike

[pwdrive]

I would replace the discriminator electrolytic as well while you're at it.

Lawrence.

[Crackle]

TX7 had a short from collector to screen, well done Ref you were correct.
I have replaced it with a AF125 and the radio worked for the first time. But there is another fault, it fades to silent after a while, and returns to working when switched on after a while.

I have replaced C27 10uF, C50 5uF, & C42 30uF.

Mike

[Terry]

What frequency range would I be expected to pickup on a SW radio if the FM oscillator is working.

As I think you've already discovered, you won't!

The local oscillator of any receiver is always going to be Fs ± Fif where Fs is the signal frequency and Fif is the IF frequency.

There is an Airband immediately above Band II, so thousands of badly screened local oscillators interfering with aircraft communications would not please the authorities so, as you've found, the local oscillator operates 10.7MHz  below the signal frequency.

Note that the local oscillator for any frequency above 87.5 + 10.7 = 98.2MHz will actually be in Band II - your signal generator test only just misses the bottom end of Band II.

You might find it useful to experiment a bit - you may find it useful in the future. If you've got any far eastern sets with entirely unscreened front ends, one of those should make an excellent signal source - unmodulated, of course!

Many years ago I was involved in experiments to see if it was viable to use the Labgear Televerter to upconvert the entire VHF spectrum - 22 adjacent channels - to UHF and found two strong sources of interference.

One turned out to be a production fault in the ITT VC200 chassis: a ferrite bead on the base of the first IF amplifier to suppress spurious oscillation turned out to be a white ceramic bead such as used to insulate the leadout wires of heating elements! *

After much head scratching I found that the second one was local oscillator radiation from the UHF tuner in  a 12" Loewe Opta which was running on the opposite side of our large workshop - in fact, as the two receivers were both more or less in corners, the distance between them - about 20 feet - couldn't have been greater!

* When I reported my findings to ITT (it wasn't confined to one set) I was told that the sets were built in Portugal and as Portugal, at that time, didn't use UHF for broadcasting, they would probably never have found out!

[Crackle]

So I would need a good scanner to pick up the oscillator on an FM radio.
If I tuned the radio to 87.7Mhz then I should pickup the local oscillator on 77MHz.
Anyway, Ref is correct these Toshiba transistors do suffer from the same tin whisker problems as the AF11x ones.
I have taken the plunge and removed the printed board from the chassis in order to unsolder the transistors.
I am replacing the inaccessible transistors with new AF125's and where they can be got at in future I am replacing the Toshiba ones.
I have already had a couple of narrow escapes with the nylon tuning cord and the iron.

This is the chassis as it was!!! lots of components connections are hidden from view by the chassis.
   
Here I have freed the board enough to be able to move it around slightly to get to the transistor solder connections.
   

I have put bluetack on the tuning drive idler gear and the spindle to help keep the cord in place to make it easier to resemble.

It is still not easy as the board is still attached by about 25 wires which go to the wave change switch.

Mike
 

[Refugee]

Would it not be easier to undo the pots and wave change switch and the other boards and put the chassis to one side.
It is certainly not an easy set to work on.

[Crackle]

I am reluctant to remove all the wiring, and doing so would make it impossible to test each change I make.
I should correct a mistake I have made in previous posts, these are Matsushita transistors, not Toshiba that I mentioned before.


Mike

[Terry]

So I would need a good scanner to pick up the oscillator on an FM radio.
If I tuned the radio to 87.7Mhz then I should pickup the local oscillator on 77MHz.  

Yes and no, Mike.

The point I was making that any working FM set can be used to monitor the local oscillator in another set at all signal frequencies above 98.2MHz, so would even work with early sets that only covered up to 100MHz. All you wanted to know about the receiver under discussion was if the local oscillator was working - yes or know.

Another set would have told you that there was no activity from 98.2 to 108MHz, which would have told you what you wanted to know.