Hi all,
I’m picking up my oscilloscope this weekend and would appreciate any guidance on how I can confirm the correct frequencies are being passed through the IFs to 6AD8 and 6AV6
Cheers
Scott

Being scared of the voltage is not good. Being respectful is more appropriate. Terror leads to mistakes & rash decisions, that's when the accidents happen. If you learn to understand the nature of electricity and not the paranoia of the media, you will be at an advantage.

The object is to identify what (like the mains) is where. The plates & screens are the ones that bight & rarely kill if the unit has a transformer.
The mains should have an RCD on it in this day & age and they can be bought as a stand alone item.

The Meter has to be able to withstand the voltages.
Probes should have the least amount of metal showing to avoid shorting something.
The main idea is also to keep a free hand, well away from the works.
DC is potentially more dangerous than AC as it grabs, by causing muscle contractions one way.

There is literature on the better forums on how to go about testing.

Marc

Thanks Marc,
I’m not scared by any means, just want to make sure I’m prodding where I should be.
Cheers
Scott

Hi Scott,

I am always reluctant to provide advice as I am usually unaware of a persons knowledge, experience, familiarity with their test equipment, knowledge of simple pitfalls etc etc etc etc .... it is a large topic to cover (properly) and sometimes, the downside of a mistake could be significant.

However, as general advice .... I use a variety of approaches depending, for example, on whether I am doing a "Tip rescue" or chasing a newly developed issue. If chasing a low volume / distortion issue on a familiar radio, one that I knew all resistances and voltages were approx correct ... I would start with a 400 to 1kHz sine wave injected into the Grid of the 6V6 and listen to the audio output and probe the audio signal at the Speaker connections ..... and this is where all my concerns/qualifiers would start ....

Input to the 6V6 via an isolating capacitor (630V rated cap just in case, as part of test cables) ... Hook connectors so I am no where near the radio when I turn it on) ... Volume pot low .... Injected signal is low to start with ... Tuning is away from any stations ... AVC off if possible ... At the audio end .... Hook connectors to Scope .... x10 probes ... Scopes settings at minimum sensitivity etc etc .... No doubt I have missed something. When I find the signal, I am looking for amplification through the audio output stage and shape of sine wave for distortion, as a first 'pass'.

Beware of the higher voltages if you decide to probe around the 6AQ5 .... there is a lot going on between the 6V6 and Audio output, so I would focus on getting that running properly before moving backwards to the IF.

Logical procedure In my book, starts with examination rather than the often dangerous & destructive turn it on to see if it goes.

Example The Philips 3003 PSU that followed me home Saturday has been tested for earth leakage etc. Has not been powered. I do not consider the capacitors viable.

Before applying power, (including battery) it must be assessed as being safe to do so: Rare! Most times its check coils & tampering, then when that's sorted you move onto the audio.

My shortcut there is to set the signal generator for alignment and do that if its working.

Do note the input voltage on the oscilloscope One of mine is 400Vp, of the other two, one has to check the book for the BWD and the Input caps of the Heathkit OS-1 are now 2KV DC Do note that Voltage "Peak to Peak" is not the same as RMS voltage.

When things are in awkward places, I will often get a marker and using different colours mark Plates Red, Screens Blue, Cathodes black & perhaps control grids green if not top caps, Red still applies to Top caps which are on some: Plates.

That also helps with meter settings if its not auto ranging. Note that some modern meters have a rudimentary frequency counter.

Marc

Thanks Marc and Ian,
I’m picking up my oscilloscope this weekend and want to get to the bottom of the issues with the CPP in the next week. In terms of testing etc, I’m comfortable with working on the chassis and the voltages don’t worry me, I just have a healthy respect for these things. I don’t have a signal generator and don’t really want to buy one for just this project, are you able to advise on how I can generate (a tone I presume) at 455Hz to calibrate the coil into the 6AD8 and I assume the 6AV6 too.
Cheers
Scott

Hi guys,
I’ve got my oscilloscope now, Robbert, you were really helpful with this chassis (as were everyone else) I’d like to test the frequency inputs into the 6AD8 and 6AV6, some advice would be great. I have new valves too that I pickup up over the weekend.
Cheers
Scott