Cheers Bruce. I just treated nyself to a £200 thermal camera - seems good enough to do the job.

Exactly!

Forgotten Gear Restorations on KZfaq just posted a video about using MOV's instead of diodes, wired from transformer primary B+ to output tube plate(s), to protect the output transformer from an inductive spike in case of a no-load situation. The MOV's tend to be self-healing and are unlikely to short circuit, which means that you probably won't have to take the amp in for repair if you accidentally try to run it without a speaker load. PS, Stuart apparently plugged the primary-side power connector of the output transformer back into the board with the unit turned on and powered up, which isn't a good idea, especially if the chassis is out of the amp and the speaker is disconnected. Perhaps not as bad if output tubes aren't installed, but still something to be avoided.

Very useful info thanks.

Many thanks!

Thanks John

Cheers Bob

Cheers Dirk. Just off to dry-clean my cape....

Thanks Peter

Thanks!

Thanks Mike. Good luck fixing yours.

Thanks Ian. Ever so 'umble I am, Guv...

Thanks!

Yes I spent ages building that but it has proved to be invaluable.

Thanks Dennis

Thanks I'm glad you enjoyed it.

Yes it's all down to cost of course and they don't vare about repairability

That's the way to go. It's all down the cheapness of course. People want to pay $600 not $1600.

Yes they're not bad.

Nice one! Wow you would never have found that if you hadn;t seen those streaks!

I'll struggle on!

Thanks yes I thought it was something like that to kill high flyback voltages.

Thanks!

That's an oscillation issue and they can be quite tricky to diagnose.

I just bought one and it will definitely make life easier. You can pinpoint the exact component at a glance.

@@stuartukguitarampguy5830 Many, many years ago I worked for a semiconductor company determining why broken chips failed, which involved microscopic examination, etc, to find the failing component. We used liquid crystal in a syringe to find hot spots (as you know, that's almost always an indication of the failure site), but alcohol cleverly does the same thing on a board level.

How very interesting!Thanks Jim.

I got a nice one for £200!

@@stuartukguitarampguy5830 Great! Hope to see it in use on your channel.

No idea I'm afraid!

H wayne. Those diodes protect the output transformer against very high voltage transients. Although tbh I'm not sure under what conditions those voltage transients arise. Obviously you don;t want to be slopping alcohol all over the board if turned on, but you'll notice I was very precise and just put a small drop onto the component.

@@stuartukguitarampguy5830 Yes I noticed it but its Risky with an amplifier its high voltage +400vdc to +600vdc put high current so that is why I'm scared of the risk to short out something use Alcohol because it has water.

'what conditions those voltage transients arise' - when some d-head doesn't plug in a speaker

Hi Wayve Yes you've got to know what you're doing, like anything really. I guess one option would be to unplug the amo and then do the alcohol if you were ubsure. The faulty component would still be hot.

@@stuartukguitarampguy5830 what is the AMO? You mean take out all the output tubes and what else?

Thanks. I doubt you'd hear the difference thiugh as they don;t do anything unless there's a fault condition.

​@@stuartukguitarampguy5830, Forgotten Gear Restorations recently posted a video about using MOV'S instead of diodes to protect the transformer; the MOV'S are usually self-healing and can shunt the spike without shorting out. (Circuit placement would be slightly different compared to using diodes: from transformer B+ to output tube plates).

Ok thanks. Under what circumstances does the o/t experience these high reverse spikes?

Thanks for your reply - I have been an amp repairer/technician for 50+ years. High end manufacturers started to fit these fly back diodes post mid 80’s. They were fitted mainly to high powered valve amps 100w + to protect the valves from transient voltages produced at various frequencies by the audio signal. In my experience I had many failures due to shorting even when some manufacturers had put 3 diodes in series. The amount of valves that failed due to these transients was a lot less and if the HT fuse was always of the correct value no serious damage would occur. This is what I have experienced as a repair tech. Hope this helps - regards Mark

@@stuartukguitarampguy5830 , the transformer needs to see the reflected speaker load on the secondary winding. Trying to play through the amp without a speaker load, due to a speaker that is completely blown or accidentally disconnected, can blow the output tranny or cause arcing inbetween the tube pins or tube socket terminals. The higher the voltage and the more powerful the amp, the more likely that just one power chord without a speaker load could cause major damage. (Dust and grunge on the sockets or between the tube pins can make arcing more likely to occur, which is another reason why oily or greasy cleaner residue on the tube sockets should be avoided).

Probably!!

@@stuartukguitarampguy5830 It's a sad thing that partly because of an deluge of Chinese fake components in Brazil, I'm closing my service at the end of the month (my repairs are no longer reliable). I will be returning to the UK in June and eventually set up a service centre again there, but focused on Reel to reel tape machines, Vintage Roland equipment and other stuff.

Thanks!

No you need to be able to scan the whole board in one go an spot the hot component.

Hi Philip. Sorry I don;t think you're right on this one. They're reverse biased diodes there to protect against very high voltage transients which can occur ubder certain circumstances. I'm unclear what those circumstances are!

Thanks Pete, yes it's all down to cost as I expect you know.