Bush TR82C.

[Robert Darwent]

Use a diode tester

Not always conclusive. I've tested TR82 detector diodes with the diode tester on a multimeter before and they often appear fine, but adding an additional one beneath the IFT has cured the problem.

It may be due to the way the diode fails in this particular circuit or perhaps the bias voltage on most multimeters is set up for silicon rather than germanium, not sure either way?

Regards

[pwdrive]

Weather or not to remove a faulty detector diode or bridge it is down to the individual but it's a gamble.
It's good service practice to remove it in the context of electrical terms, bridging certain faulty components is ok as a temporary measure to prove a fault ie: if the original component has gone open circuit or high resistance.
If the original diode has developed a reverse resistance fault and left in circuit with a new one connected across it then that combination isn't going to work as a fully functioning detector as the original detector was designed to do.
Good luck whichever way.

Lawrence.

[Robert Darwent]

I understand your concerns Lawrence, but in practice the fix seems ro work very well.

Its also the way I tend to approach the hobby - practical fixes to faults rather than being too concerned about what theory would suggest or predict in such situations. I find theory is only useful up to a point.

I can't claim originality for this fix either, I first became aware of it in several threads on other forums, all seem to report success with it so I tried it too and can only report the same - good enough for me.

Others need not take my advice and can approach their restorations in anyway they see fit - I only offer the solutions I have found to certain problems for consideration.

Regards

[Refugee]

If the old detector is left in circuit and it fails short circuit it is inconvenient but not fatal. It is a compromise to me and I would allow 15 minutes to remove and refit the IF can personally.
If the can is taken off can we have some photos so that we can work out how long it takes for ourselves?

[pwdrive]

No worries Robert, it's my own personal view but either approach can be taken, just pointing out the pitfalls of permanent bridging as there is no guarantee as to what the original component will do in the future if subject to electrical, heat or mechanical stresses, been caught out with that years ago with TV mains droppers where a section had gone open circuit and then bridged with a replacement dropper section only to find that the original section had remade it's contact which resulted in over run valves and CRT.
Theory and practice is an age old debate, all I would say is that a fundamental knowledge of circuit theory will stand good when fault finding.

Lawrence.

[Robert Darwent]

mains droppers where a section had gone open circuit and then bridged with a replacement dropper section only to find that the original section had remade it's contact

Yes if it were a mains dropper Lawrence, then safety becomes the prime concern and you would remove the faulty section without question - but it is a small signal diode we are talking about here.

If the old detector is left in circuit and it fails short circuit it is inconvenient but not fatal. It is a compromise to me

Let's consider the two failure extremes with this detector diode.

If it's failed open-circuit, then it has already taken itself out of the signal pathway and soldering a replacement underneath or inside the IFT is the same either way and you have saved yourself the hassle of removing the IFT can.

If it's failed short-circuit, then soldering a replacement underneath is going to make no difference to the existing fault condition as the new diode will be bypassed by the short. But at least you would now know the IFT can needs removing for definite.

If the detecor diode has failed in some other intermediate fashion, then adding a replacement underneath will either solve the problem or it won't. If it does solve it - again you have saved yourself the hassle of removing the IFT can.

There is no compromise I can see by adding a diode underneath in the first instance. It will either solve the problem or tell you for definite you need to remove the IFT can.

It's a bit of a faff to remove IFT3 as you have a few connections to disturb... You also need to exercise care when replacing the diode that you don't break any of the fine wires of the IFT windings.

As pointed out by David earlier in this thread, you risk damaging the fine wires inside the IFT by opening it. Remember, Laurie who will be carrying out this work is a self-confessed beginner and may not feel confident to safely open a delicate IFT as the rest of us might.

Regards

[Joe]

Its also the way I tend to approach the hobby - practical fixes to faults rather than being too concerned about what theory would suggest or predict in such situations. I find theory is only useful up to a point.

Well said, that man!

- Joe

[pwdrive]

The mains dropper example I quoted was to highlight the some times long term intermittent nature of component failure, it can happen with signal diodes as well as most other components.

Lawrence.

[pwdrive]

With fault finding It's understanding that leads to the quickest practical fixes, that's how it works, anything else is just wishing and hoping.

Lawrence.

[Refugee]

mains droppers where a section had gone open circuit and then bridged with a replacement dropper section only to find that the original section had remade it's contact

If the old detector is left in circuit and it fails short circuit it is inconvenient but not fatal. It is a compromise to me

There is no compromise I can see by adding a diode underneath in the first instance. It will either solve the problem or tell you for definite you need to remove the IFT can.

Regards

There is a compromise here.
Glass packaged germanium diodes are sealed in a controlled dry environment and if they "go to air" they may well slowly go short circuit or resistive this may show up as a poor signal and get worse over time so the added diode could end up being a short term solution and will only be a permanent solution for a clean open circuit fault.