Resistive capacitors - UK Vintage Radio Repair and Restoration Discussion Forum

Boater Sam · 24th Jun 2018, 10:16 pm

Last week I put an HMV 1121 radio on the bench.
It's in good order with all the glasses intact and a set of good valves.
Initial tests on transformers and a good look around found no problems.

Some of the components are a bit difficult to see, the ones on the "tag" strips are not soldered to tags at all but have their wires through holes in paxolin strips and wrapped around. Some are a devil to remove neatly.

As it looked OK, gently powered it up on the lamp.
Rewarded with sound, on full power it worked well.
Then it started to cook a decoupling wax capacitor, so turned off and start replacing the leaky ones in critical places where they have a good few volts on them.

Now working very well, thought I would check the new audio coupling cap leakage, it less than 300mV but cathode voltage is a bit low.
Surprised to find the wire wound cathode resistor is open circuit. The capacitor has become a 14k ohm resistor!

Replaced both and got the correct voltage figures and bags of volume.

Is this unusual, that the set will work using the capacitor as a resistor? There was no obvious deterioration in the sound quality.

Sam.

Herald1360 · 24th Jun 2018, 10:35 pm

Providing the cap can pass enough current- 10mA or so without the cathode voltage rising high enough to drop the anode-cathode voltage below about 100v then yes, it will work. The leaky grid capacitor will provide high enough positive grid bias. Output power will be down, but a couple of hundred mW is plenty for a reasonable volume level. Most early trannies and late valve portables only managed that much.

turretslug · 24th Jun 2018, 10:50 pm

An excellent demonstration of how valve circuitry can shuffle its operating point and carry on! Also of how peripheral component values are often "optimal" or "compromise" as opposed to hard-and-fast unique. You often see statements along the lines, "the 470k had gone up to 502k, so I replaced it, but it made no difference" made by the unenlightened.

turretslug · 25th Jun 2018, 9:25 pm

I'm guessing that the original cathode capacitor was an electrolytic of relatively low voltage rating? Perhaps the sequence of events was that the wire-wound cathode resistor went O/C, the cathode voltage floated high (well above the capacitor rating), thus the oxide dielectric layer broke down at some point, allowing cathode current to flow again, bringing the voltage across the capacitor down so that a leaky equilibrium was established. A sort of self-mending! (to an extent).

Perhaps if this failure sequence happened in a TV field output as opposed to an audio output stage (i.e. a similar architecture, but always operating at high "volume" as opposed to the low third or so of the volume control that radio amplifiers tend to sit at), then some field cramping or non-linearity would have been obvious from operating with the 14k cathode "resistor".

brightsparkey · 26th Jun 2018, 4:01 pm

My Grundig had exactly that due to a previous replacement of the main decoupling electrolytics, resulting in the resistor being orphaned from ground.

The electrolytic just breaks down (at about 20V for the 16V cap) and the dissipation is relatively minor - must have been like that for 15-20 years judging by the 1980's vintage caps employed, although the set may have not had much use.

Eventually it went pop and further investigation showed that a track was actually cut to introduce the error - very odd. Resistor re-united to chassis and new caps etc and all's well.

Kevin.

24th Jun 2018, 10:16 pm	#1
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Resistive capacitors Last week I put an HMV 1121 radio on the bench. It's in good order with all the glasses intact and a set of good valves. Initial tests on transformers and a good look around found no problems. Some of the components are a bit difficult to see, the ones on the "tag" strips are not soldered to tags at all but have their wires through holes in paxolin strips and wrapped around. Some are a devil to remove neatly. As it looked OK, gently powered it up on the lamp. Rewarded with sound, on full power it worked well. Then it started to cook a decoupling wax capacitor, so turned off and start replacing the leaky ones in critical places where they have a good few volts on them. Now working very well, thought I would check the new audio coupling cap leakage, it less than 300mV but cathode voltage is a bit low. Surprised to find the wire wound cathode resistor is open circuit. The capacitor has become a 14k ohm resistor! Replaced both and got the correct voltage figures and bags of volume. Is this unusual, that the set will work using the capacitor as a resistor? There was no obvious deterioration in the sound quality. Sam. Last edited by Boater Sam; 24th Jun 2018 at 10:19 pm. Reason: spells

24th Jun 2018, 10:35 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Resistive capacitors Providing the cap can pass enough current- 10mA or so without the cathode voltage rising high enough to drop the anode-cathode voltage below about 100v then yes, it will work. The leaky grid capacitor will provide high enough positive grid bias. Output power will be down, but a couple of hundred mW is plenty for a reasonable volume level. Most early trannies and late valve portables only managed that much. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

24th Jun 2018, 10:50 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Resistive capacitors An excellent demonstration of how valve circuitry can shuffle its operating point and carry on! Also of how peripheral component values are often "optimal" or "compromise" as opposed to hard-and-fast unique. You often see statements along the lines, "the 470k had gone up to 502k, so I replaced it, but it made no difference" made by the unenlightened.

25th Jun 2018, 9:25 pm	#4
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Resistive capacitors I'm guessing that the original cathode capacitor was an electrolytic of relatively low voltage rating? Perhaps the sequence of events was that the wire-wound cathode resistor went O/C, the cathode voltage floated high (well above the capacitor rating), thus the oxide dielectric layer broke down at some point, allowing cathode current to flow again, bringing the voltage across the capacitor down so that a leaky equilibrium was established. A sort of self-mending! (to an extent). Perhaps if this failure sequence happened in a TV field output as opposed to an audio output stage (i.e. a similar architecture, but always operating at high "volume" as opposed to the low third or so of the volume control that radio amplifiers tend to sit at), then some field cramping or non-linearity would have been obvious from operating with the 14k cathode "resistor".

26th Jun 2018, 4:01 pm	#5
brightsparkey Pentode Join Date: Sep 2017 Location: Cambridge, Cambridgeshire, UK. Posts: 148	Re: Resistive capacitors My Grundig had exactly that due to a previous replacement of the main decoupling electrolytics, resulting in the resistor being orphaned from ground. The electrolytic just breaks down (at about 20V for the 16V cap) and the dissipation is relatively minor - must have been like that for 15-20 years judging by the 1980's vintage caps employed, although the set may have not had much use. Eventually it went pop and further investigation showed that a track was actually cut to introduce the error - very odd. Resistor re-united to chassis and new caps etc and all's well. Kevin.