Heathkit AO-1U - UK Vintage Radio Repair and Restoration Discussion Forum

Refugee · 19th Dec 2012, 1:07 am

This is the other item distributed to me from Markie's bits with the help of Greenstar.

Getting a circuit diagram was a bit of an odd experience as the AO-1 is no problem the "U" version has an extra band and keeps coming up with the three band circuit when a search is done.
When I opened it up I was rewarded with the fruits of a reverse engineering job.
There is an error on the grid coupling capacitor on pin 7 of the oscillator valve but all the same it is a good effort.
On the cap reformer it went and the three Hunts caps began to pull the grids up.
I left it on for a while for the two 8uf electrolytic ones to fully reform. I replaced the three black Hunts ones two of them being under the electrolytic 8uf ones.
I then showed it some power and it went dead at the lower end of all the bands

It was the tuning capacitor.
A little tweak with a trimming tool sorted it out after the end float test failed to reveal a problem. It was the vane furthest from the control.
A tweak of the padder got the scale to within about 5% over almost all of the ranges.
I didn't expect the square wave to me much good at the top end of the highest range as this was added as an afterthought.

greenstar · 19th Dec 2012, 6:33 pm

Good on getting this going - I think I would have tried everything before finding the error, if I did. Virtues of tracing the circuit. is this the first thing you usually do? had any luck with the Avo?
Tony

Refugee · 19th Dec 2012, 11:41 pm

There is only one moving part when adjusting the frequency and that is the cap.
I examined it under a magnifying lamp and looked at the gaps with the cap closed in order to work out what side it was most likely to be shorting on.
Once it was at the point where the output goes dead I very gently nudged each vane until I found the one that was causing it and then flexed it a tiny bit first and progressively more each time until it was fixed.
It did not take too long as it is quite a simple instrument.
I have started a thread for the AVO in the workshop test gear section as it is fighting the repairs.

andywal · 16th Jun 2015, 11:38 am

I have the original assembly manual for it and can copy it if it's of use to you. Regards Andy.

Refugee · 16th Jun 2015, 1:04 pm

It has been working fine since I changed several tar coated capacitors. I have a home drawn reverse engineered circuit diagram that has one or two mistakes but they are nothing for me to worry about.

19th Dec 2012, 1:07 am	#1
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Heathkit AO-1U This is the other item distributed to me from Markie's bits with the help of Greenstar. Getting a circuit diagram was a bit of an odd experience as the AO-1 is no problem the "U" version has an extra band and keeps coming up with the three band circuit when a search is done. When I opened it up I was rewarded with the fruits of a reverse engineering job. There is an error on the grid coupling capacitor on pin 7 of the oscillator valve but all the same it is a good effort. On the cap reformer it went and the three Hunts caps began to pull the grids up. I left it on for a while for the two 8uf electrolytic ones to fully reform. I replaced the three black Hunts ones two of them being under the electrolytic 8uf ones. I then showed it some power and it went dead at the lower end of all the bands It was the tuning capacitor. A little tweak with a trimming tool sorted it out after the end float test failed to reveal a problem. It was the vane furthest from the control. A tweak of the padder got the scale to within about 5% over almost all of the ranges. I didn't expect the square wave to me much good at the top end of the highest range as this was added as an afterthought. Attached Thumbnails

19th Dec 2012, 6:33 pm	#2
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,685	Re: Heathkit AO-1U Good on getting this going - I think I would have tried everything before finding the error, if I did. Virtues of tracing the circuit. is this the first thing you usually do? had any luck with the Avo? Tony

19th Dec 2012, 11:41 pm	#3
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Heathkit AO-1U There is only one moving part when adjusting the frequency and that is the cap. I examined it under a magnifying lamp and looked at the gaps with the cap closed in order to work out what side it was most likely to be shorting on. Once it was at the point where the output goes dead I very gently nudged each vane until I found the one that was causing it and then flexed it a tiny bit first and progressively more each time until it was fixed. It did not take too long as it is quite a simple instrument. I have started a thread for the AVO in the workshop test gear section as it is fighting the repairs. Last edited by Refugee; 19th Dec 2012 at 11:44 pm. Reason: Note about the AVO forgotten

16th Jun 2015, 11:38 am	#4
andywal Retired Dormant Member Join Date: Jun 2015 Location: Macclesfield, Cheshire, UK. Posts: 1	Re: Heathkit AO-1U I have the original assembly manual for it and can copy it if it's of use to you. Regards Andy.

16th Jun 2015, 1:04 pm	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Heathkit AO-1U It has been working fine since I changed several tar coated capacitors. I have a home drawn reverse engineered circuit diagram that has one or two mistakes but they are nothing for me to worry about.