High Voltages and Poking Around with a Signal Tracer - UK Vintage Radio Repair and Restoration Discussion Forum

JayBee66 · 22nd Dec 2016, 12:09 am

I'm getting further with my Philips B5S12A. Having replaced a melted down capacitor and resistor I now have output from the left speaker but it is a little quiet and there are some crackles and pops.

I'd like to probe around with a signal tracer to pinpoint what else was damaged during the meltdown. However, as this model uses 800 ohm speakers with output valves I am a little worried about the high voltages present.

The anodes of the two EL86 valves (which have been swapped around so they are not the problem) are at 180V AC and there are other points in the audio output stage where there are high voltages.

Not having worked on a radio such as this before, what should I do and not do when probing for audio signals?

Thanks for any help.

Graham G3ZVT · 22nd Dec 2016, 1:03 am

Firstly, if you really have 180v AC on the EL84 anodes you have a real problem. Perhaps you meant DC, or perhaps you are getting a fluke reading (see what I did there?) on an AC range.

In order to advice you on the signal tracer's suitability I would need to know more about it. I would expect the probe to have a small value capacitor, say 1000pF of at least 1000v working, to be suitable for valve work.
Also a "signal tracer" is probably a signal injector (tone generator) but I suppose it could also be a little amp with its own little speaker. Not that that makes any difference to the probe capacitor. If in any doubt, make your own probe with a suitable cap.
Then you can probe around with impunity.

At least that chassis uses a transformer for heaters and HT rather than the "live chassis" technique used in many other sets of the period.

David G4EBT · 22nd Dec 2016, 9:46 am

Signal tracers (basically a small amplifier with a probe) are used to trace RF/IF/AF signals at various points in a radio or amplifier. They invariably have an isolating capacitor rated at > 400V which allows signals to pass to the input, but block high DC voltage to protect the tracer. But they need to be used in a methodical way by reference to the circuit of the equipment, looking for signals where you'd expect to find them in a working radio. Some of those points will be at high DC potential - others not so. I've attached a pic of the output stage of a DAC90A showing the audio coupling capacitor (C18). The input to the capacitor is from the anode of the preceding stage, so will be at the anode DC potential of that valve. The output of the capacitor will be at very low/zero DC potential (assuming it's a good capacitor), so it's best to check for signals on that side of the cap. If you hear tunable signals there, all of the preceding stages are working.

If you here nothing from the speaker, personally, I'd inject a signal from a signal injector or audio generator of say 500Hz - 1KHz and if nothing is heard from the speaker, start looking into why that might be. In the case of a DAC90A, the usual suspect would be a burnt out output transformer, a known weakness in that set often caused by the output stage being overdriven due to the audio coupling cap passing DC. Other reasons of course - speaker open circuit, output valve dud etc.

The point to bear in mind is that a signal tracer isn't a magic bullet - as with a multi-meter, it's a useful adjunct to proper diagnosis by first checking voltages and comparing them to the service data, checking resistors to see if they've gone high in value, substituting valves if you have any known good ones to hand, checking the 'switchery' for open circuits and so forth. Then having done those checks, looking for signals where you'd expect to find them. EG, if you apply a tracer probe to the slider of a volume control and hear tunable signals, that means the preceding RF/IF stages are working, and if nothing is heard from the speaker, the problem lies in the audio stages.

Just randomly poking around in a haphazard way rarely produces a successful outcome.

This thread might be of interest:

https://www.vintage-radio.net/forum/...Signal+Tracing

Hope that's helpful.

Station X · 22nd Dec 2016, 12:23 pm

Two off topic posts deleted.

JayBee66 · 22nd Dec 2016, 2:47 pm

Yes, my earlier reference to AC was a speed-typed mistake. I meant DC.

I got a few over-priced components from Maplins today (1200V caps and 2W resistors) and bread-boarded a tracer. Works great.

Thanks for the clarifications.

Refugee · 22nd Dec 2016, 3:00 pm

You just need an audio amplifier with a couple of 1N4148s in inverse parallel across the input.
The amplifier needs a high input impedance so that you can add a 100K resistor in series with the probe to eliminate ant pulses from touching the probe on an HT rail with a discharged capacitor when checking mains ripple. The capacitor would be something like a yellow 0.1uf 630V and the resistor something like 22K in a probe that is rugged enough to handle the resistor burning out. The probe can be switched to the output of the amplifier to inject a signal to give a 500mv protected signal with a low value resistor to protect the diodes if the amplifier is powerful enough to overload them.

22nd Dec 2016, 12:09 am	#1
JayBee66 No Longer a Member Join Date: Nov 2015 Location: Kilkenny, Ireland Posts: 138	High Voltages and Poking Around with a Signal Tracer I'm getting further with my Philips B5S12A. Having replaced a melted down capacitor and resistor I now have output from the left speaker but it is a little quiet and there are some crackles and pops. I'd like to probe around with a signal tracer to pinpoint what else was damaged during the meltdown. However, as this model uses 800 ohm speakers with output valves I am a little worried about the high voltages present. The anodes of the two EL86 valves (which have been swapped around so they are not the problem) are at 180V AC and there are other points in the audio output stage where there are high voltages. Not having worked on a radio such as this before, what should I do and not do when probing for audio signals? Thanks for any help.

22nd Dec 2016, 1:03 am	#2
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,710	Re: High Voltages and Poking Around with a Signal Tracer Firstly, if you really have 180v AC on the EL84 anodes you have a real problem. Perhaps you meant DC, or perhaps you are getting a fluke reading (see what I did there?) on an AC range. In order to advice you on the signal tracer's suitability I would need to know more about it. I would expect the probe to have a small value capacitor, say 1000pF of at least 1000v working, to be suitable for valve work. Also a "signal tracer" is probably a signal injector (tone generator) but I suppose it could also be a little amp with its own little speaker. Not that that makes any difference to the probe capacitor. If in any doubt, make your own probe with a suitable cap. Then you can probe around with impunity. At least that chassis uses a transformer for heaters and HT rather than the "live chassis" technique used in many other sets of the period.

22nd Dec 2016, 9:46 am	#3
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,760	Re: High Voltages and Poking Around with a Signal Tracer Signal tracers (basically a small amplifier with a probe) are used to trace RF/IF/AF signals at various points in a radio or amplifier. They invariably have an isolating capacitor rated at > 400V which allows signals to pass to the input, but block high DC voltage to protect the tracer. But they need to be used in a methodical way by reference to the circuit of the equipment, looking for signals where you'd expect to find them in a working radio. Some of those points will be at high DC potential - others not so. I've attached a pic of the output stage of a DAC90A showing the audio coupling capacitor (C18). The input to the capacitor is from the anode of the preceding stage, so will be at the anode DC potential of that valve. The output of the capacitor will be at very low/zero DC potential (assuming it's a good capacitor), so it's best to check for signals on that side of the cap. If you hear tunable signals there, all of the preceding stages are working. If you here nothing from the speaker, personally, I'd inject a signal from a signal injector or audio generator of say 500Hz - 1KHz and if nothing is heard from the speaker, start looking into why that might be. In the case of a DAC90A, the usual suspect would be a burnt out output transformer, a known weakness in that set often caused by the output stage being overdriven due to the audio coupling cap passing DC. Other reasons of course - speaker open circuit, output valve dud etc. The point to bear in mind is that a signal tracer isn't a magic bullet - as with a multi-meter, it's a useful adjunct to proper diagnosis by first checking voltages and comparing them to the service data, checking resistors to see if they've gone high in value, substituting valves if you have any known good ones to hand, checking the 'switchery' for open circuits and so forth. Then having done those checks, looking for signals where you'd expect to find them. EG, if you apply a tracer probe to the slider of a volume control and hear tunable signals, that means the preceding RF/IF stages are working, and if nothing is heard from the speaker, the problem lies in the audio stages. Just randomly poking around in a haphazard way rarely produces a successful outcome. This thread might be of interest: https://www.vintage-radio.net/forum/...Signal+Tracing Hope that's helpful. Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.

22nd Dec 2016, 12:23 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: High Voltages and Poking Around with a Signal Tracer Two off topic posts deleted. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2016, 2:47 pm	#5
JayBee66 No Longer a Member Join Date: Nov 2015 Location: Kilkenny, Ireland Posts: 138	Re: High Voltages and Poking Around with a Signal Tracer Yes, my earlier reference to AC was a speed-typed mistake. I meant DC. I got a few over-priced components from Maplins today (1200V caps and 2W resistors) and bread-boarded a tracer. Works great. Thanks for the clarifications.

22nd Dec 2016, 3:00 pm	#6
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: High Voltages and Poking Around with a Signal Tracer You just need an audio amplifier with a couple of 1N4148s in inverse parallel across the input. The amplifier needs a high input impedance so that you can add a 100K resistor in series with the probe to eliminate ant pulses from touching the probe on an HT rail with a discharged capacitor when checking mains ripple. The capacitor would be something like a yellow 0.1uf 630V and the resistor something like 22K in a probe that is rugged enough to handle the resistor burning out. The probe can be switched to the output of the amplifier to inject a signal to give a 500mv protected signal with a low value resistor to protect the diodes if the amplifier is powerful enough to overload them.