AVO VCM MKIV Capacitor Questions - UK Vintage Radio Repair and Restoration Discussion Forum

Ricardo · 14th Apr 2020, 8:40 pm

I am just starting to check out my AVO MK4 VCM and have some questions about the wiring and circuit identification numbers for the two 8uF capacitors.

There is one 8uF capacitor which has clearly been replaced with a modern one mounted within the circuit selector switch assembly which is connected across the meter. The component table for the circuit selector switch in the AVO service manual identifies this capacitor as C1, but on the circuit diagram it is shown as C3? I am happy that this capacitor is connected correctly, just slightly confused by the identification number differences.

I am more concerned about the other capacitor in the diode test circuit however. In my VCM MK4 it is mounted with a clip and a rubber strap onto the board behind the meter. It is a polarised electrolytic and appears to be connected in reverse according to the circuit diagram. It is an old Hunt electrolytic capacitor and has red and black wires connected to its positive and negative terminals respectively. The red wire in particular does not look to be original. The negative side of this capacitor, black wire, is connected to R37 (nominally 500R) and the red wire goes to the relevant wiper connection on the electrode selector switch.

I would be grateful if someone with an AVO VCM can tell me where the capacitor identified as C1 on the circuit diagram is fitted in their unit and how it is wired (what wire colours are used, given that it may originally have been a non-polarised capacitor), etc.

Ricardo · 15th Apr 2020, 8:17 pm

Here is a photo which shows the capacitor which I believe is connected with the reverse polarity to the way in which it is shown on the circuit diagram in the service manual.

I could simply reverse the wires at their destinations. However that would then result in the red (positive) wire from this capacitor joining R37 together with an existing black wire at the switch which seems a bit odd.

Dekatron · 15th Apr 2020, 9:09 pm

I checked one of my Mk IVs and found that the negative connection on C1 goes to the Electrode Selector and that the positive electrode on C1 goes to the Meter Switch, see the photos I've included.

Ricardo · 16th Apr 2020, 9:14 pm

Thanks Martin,

Your photos have confirmed my suspicions that C1 in my unit is wired in reverse.

When mentioning R37 in my previous post I should have said that the wiring first goes via a connection on the Meter switch.

I have tried to take photos of the same points on my Mk IV using a USB snake camera. They are nowhere as clear as yours, but should be good enough to see by the wire colours that the connections from C1 are reversed.

Regards,

Richard

14th Apr 2020, 8:40 pm	#1
Ricardo Pentode Join Date: Jul 2006 Location: Bedfordshire, UK. Posts: 211	AVO VCM MKIV Capacitor Questions I am just starting to check out my AVO MK4 VCM and have some questions about the wiring and circuit identification numbers for the two 8uF capacitors. There is one 8uF capacitor which has clearly been replaced with a modern one mounted within the circuit selector switch assembly which is connected across the meter. The component table for the circuit selector switch in the AVO service manual identifies this capacitor as C1, but on the circuit diagram it is shown as C3? I am happy that this capacitor is connected correctly, just slightly confused by the identification number differences. I am more concerned about the other capacitor in the diode test circuit however. In my VCM MK4 it is mounted with a clip and a rubber strap onto the board behind the meter. It is a polarised electrolytic and appears to be connected in reverse according to the circuit diagram. It is an old Hunt electrolytic capacitor and has red and black wires connected to its positive and negative terminals respectively. The red wire in particular does not look to be original. The negative side of this capacitor, black wire, is connected to R37 (nominally 500R) and the red wire goes to the relevant wiper connection on the electrode selector switch. I would be grateful if someone with an AVO VCM can tell me where the capacitor identified as C1 on the circuit diagram is fitted in their unit and how it is wired (what wire colours are used, given that it may originally have been a non-polarised capacitor), etc.

15th Apr 2020, 8:17 pm	#2
Ricardo Pentode Join Date: Jul 2006 Location: Bedfordshire, UK. Posts: 211	Re: AVO VCM MKIV Capacitor Questions Here is a photo which shows the capacitor which I believe is connected with the reverse polarity to the way in which it is shown on the circuit diagram in the service manual. I could simply reverse the wires at their destinations. However that would then result in the red (positive) wire from this capacitor joining R37 together with an existing black wire at the switch which seems a bit odd. Attached Thumbnails

15th Apr 2020, 9:09 pm	#3
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO VCM MKIV Capacitor Questions I checked one of my Mk IVs and found that the negative connection on C1 goes to the Electrode Selector and that the positive electrode on C1 goes to the Meter Switch, see the photos I've included. Attached Thumbnails __________________ Martin, Sweden Last edited by Dekatron; 15th Apr 2020 at 9:10 pm. Reason: Forgot Photos...... :)

16th Apr 2020, 9:14 pm	#4
Ricardo Pentode Join Date: Jul 2006 Location: Bedfordshire, UK. Posts: 211	Re: AVO VCM MKIV Capacitor Questions Thanks Martin, Your photos have confirmed my suspicions that C1 in my unit is wired in reverse. When mentioning R37 in my previous post I should have said that the wiring first goes via a connection on the Meter switch. I have tried to take photos of the same points on my Mk IV using a USB snake camera. They are nowhere as clear as yours, but should be good enough to see by the wire colours that the connections from C1 are reversed. Regards, Richard Attached Thumbnails