Avo MK4 Restoration - UK Vintage Radio Repair and Restoration Discussion Forum

mikko rintala · 30th Aug 2019, 3:57 pm

I am starting Avo Mk 4 for the restoration.First, the component board Assembly s 2M2 all resistor is way off so i change those. Now i ask is it an idea to replace those diodes with that board a more modern one or similar?

Best regaird Mikko from Finland

Ps. How i can add picture my text?

Craig Sawyers · 30th Aug 2019, 5:17 pm

Click "Go Advanced" just under the message window. Then you get an option "Manage Attachments" to attach files of different types. Note that there are file size limits.

Craig

mikko rintala · 30th Aug 2019, 10:51 pm

Here is the board. i think my Avo is December/1960

Best regards Mikko

Craig Sawyers · 31st Aug 2019, 7:15 am

OK - this is a MkIV valve tester, not a Mk4 valve voltmeter!

Those 2M2 resistors are not critical value. The diodes are used in pairs (because I assume the PIV of a single diode is not high enough), and those resistors simply ensure that the voltage is shared between them when they are in the "off" state. In other words if the leakage current of one diode of the pair is more than the other, one might exceed its PIV if the 2M2 resistors aren't there. Provided they are about the same value within a factor of 1.5 (see below), it really does not matter if they are 2M2 or 3M3 or whatever they have ended up being.

Since there is no tolerance band the resistors are +/-20% anyway!

If the unit works, or that part of the unit works, don't unnecessarily replace things.

Craig

30th Aug 2019, 3:57 pm	#1
mikko rintala Triode Join Date: Aug 2018 Location: Kauhava, Finland Posts: 37	Avo MK4 Restoration I am starting Avo Mk 4 for the restoration.First, the component board Assembly s 2M2 all resistor is way off so i change those. Now i ask is it an idea to replace those diodes with that board a more modern one or similar? Best regaird Mikko from Finland Ps. How i can add picture my text?

30th Aug 2019, 5:17 pm	#2
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,976	Re: Avo MK4 Restoration Click "Go Advanced" just under the message window. Then you get an option "Manage Attachments" to attach files of different types. Note that there are file size limits. Craig

30th Aug 2019, 10:51 pm	#3
mikko rintala Triode Join Date: Aug 2018 Location: Kauhava, Finland Posts: 37	Re: Avo MK4 Restoration Here is the board. i think my Avo is December/1960 Best regards Mikko Attached Thumbnails

31st Aug 2019, 7:15 am	#4
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,976	Re: Avo MK4 Restoration OK - this is a MkIV valve tester, not a Mk4 valve voltmeter! Those 2M2 resistors are not critical value. The diodes are used in pairs (because I assume the PIV of a single diode is not high enough), and those resistors simply ensure that the voltage is shared between them when they are in the "off" state. In other words if the leakage current of one diode of the pair is more than the other, one might exceed its PIV if the 2M2 resistors aren't there. Provided they are about the same value within a factor of 1.5 (see below), it really does not matter if they are 2M2 or 3M3 or whatever they have ended up being. Since there is no tolerance band the resistors are +/-20% anyway! If the unit works, or that part of the unit works, don't unnecessarily replace things. Craig Last edited by Craig Sawyers; 31st Aug 2019 at 7:20 am.