Decca DRM88 Tingle from chassis and hum thru speaker - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

ms660 · 17th Jan 2017, 1:41 pm

Quote:

Originally Posted by johnny english

I measured the leakage current on my dvm ( thanks julie m for the how to) ma-ac range =12ma and ac voltage range = 116 though after 5 minutes or so this rises by 2volts. Now I dont know whether this indicates if I have a problem or not. Ed.

12mA through a 1meg resistor? Or have I got that wrong.

Lawrence.

johnny english · 17th Jan 2017, 3:15 pm

Yes it was across a 1 meg. Ed.

ms660 · 17th Jan 2017, 3:20 pm

12mA flowing through a 1meg resistor equates to a voltage of 12,000 volts across it.....

Lawrence.

julie_m · 17th Jan 2017, 3:29 pm

If you really measured 12 mA with the meter on a current range between chassis and mains earth, then something is very wrong and you need to investigate.

The fact that you measured 116V across a 1MΩ resistor could be interesting. (There obviously is not enough voltage to push 12mA through 1MΩ). That figure is remarkably close to both half the supply voltage, and the lowest tapping on the primary.

Check the mains input wiring very, very carefully, and report back.

ukcol · 17th Jan 2017, 5:03 pm

These latest readings are very different to that in post #1 and I quote " There's a voltage of 13.5 volts on the chassis".

So what has changed? Presumably this first reading was between chassis and earth without a 1meg resistor which suggests the leakage was very low when the first reading was taken.

Have you fitted a 4.7uF in place of C61 instead of a 4.7nF in error perhaps?

johnny english · 17th Jan 2017, 5:03 pm

Will do.

johnny english · 17th Jan 2017, 5:11 pm

Hi. Colin you advised me to remove c61 and provide an earth to the mains which worked on stopping the tingling. I cant quite remember how I got that reading I know I had the live meter probe on the chassis and I was checking the connections on the speaker and when I brushed my hand against the speaker frame and felt a tingle I took a quick glance at the meter and it read 13.5 the negative probe was,nt being used.

ukcol · 17th Jan 2017, 5:49 pm

Quote:

Originally Posted by johnny english

the negative probe was,nt being used.

Ah. it makes sense now.

Herald1360 · 17th Jan 2017, 7:00 pm

The standard circuit for measuring earth leakage is basically a 1K resistor from chassis or other accessible conductive part to earth across which the voltage is measured. (There's also a simple low pass filter in the pukka box to stop any stray RF from upsetting the readings, but that isn't needed for a quick'n'dirty check).

1K is around the typical resistance of a person with wet or broken skin at the point of contact.

Your 12mA wouldn't trip a typical 30mA RCD unless the rest of the installation was leaking more than about 20mA already.

More than 50mA for any length of time "may have possible irreversible effects"- an interesting turn of phrase, that

johnny english · 18th Jan 2017, 12:43 am

Hi.
I did a simple continuity test (cold) from the mains input to the on-off/ tone control pot in the (off position) the mains L&N totally isolated between terminals. In the (on position) continuity thru switch though L&N are`nt isolated between each other. On to the mains transformer when looking underneath the chassis the negative terminal is on the left and live on the right there is continuity across these terminals. Testing the same terminals individually to chassis there is no continuity. I will post some photo,s tomorrow showing the the internals etc. Thanks Ed.

johnny english · 21st Jan 2017, 6:09 pm

Hi.
I just have,nt got anywhere with this radio at all. First, l will explain the initial problem "loud hum on all wavebands" so I suspected the smoothing can and duly ordered and replaced 2x32uf caps with 33s I also noticed that the original can got warm but put it down to the ez81 rectifier valve radiating its heat onto the can body as the valve sits below it. There are 3 32uf smoothing caps the third one was separately fixed to the chassis but connected to the other caps. Switched the set on it still hummed and the smoother with the 1kohm 6w and 2.1kohm 2w resistors wired in series to pin 3 of the ez81 started to get very warm, so I switched off and in a desperate attempt I replaced the third smoothing cap still the same problem. I am now beginning to suspect the mains transformer which gets warm but does,nt overheat and I would like to know what others think as I have never met this problem before and would like to tackle it. I have also replaced the hunts mouldseals and since those photo,s were taken I have replaced the 3rd smoothing cap and reinstalled c61. Thanks ED.

ms660 · 21st Jan 2017, 7:07 pm

If possible, do some voltage measurements, with respect to chassis, rectifiers anode voltages (AC) rectifiers cathode voltage (DC) output valves anode, screen grid, cathode and control grid voltages (DC)

It might give an indication as to the general health of things.

I don't have a schematic.

Lawrence.

johnny english · 21st Jan 2017, 7:41 pm

Thanks for the info Lawrence. The process will be I think valves out power on meter negative to chassis and the positive to required valve contacts. Am I correct. Ed.

ms660 · 21st Jan 2017, 7:58 pm

Valves in if possible eg: nothing seriously cooking.

Lawrence.

julie_m · 21st Jan 2017, 8:44 pm

It might be instructive to measure the voltages on each of the EZ81 anodes (make sure to select the highest AC range), both with the valve fitted (and warmed up to operating temperature) and removed. If there is a big difference, then that would indicate something drawing excessive current. You will have to do some DC checks to discover what; preferrably as quickly as possible, so as to minimise the likelihood of further damage. As a first test, try removing the EL84 (which is a suspect anyway).

johnny english · 22nd Jan 2017, 1:52 am

here,s the results..
el84 control grid pin 2 valve out = 0dcv valve in =7dcv
cathode pin 3 ............. = 0dcv ........... = 4dcv
anode pin 7 ............. = 355dcv ........ = 228dcv
screen grid pin 9 ............. = 355dcv ........ =254dcv

ez81 anode pin 1 = 343acv
anode pin 7 =343acv
cathode pin 3 = 419dcv

The el84 valve in readings kept fluctuating up and down the figures posted are the highest recorded on test. Ed.

johnny english · 22nd Jan 2017, 6:08 am

Sorry about this my post above is wrong. The following figures are correct.

EL84 valve in socket
pin 2 = 0dcv
pin 3 = 6dcv
pin 7 = 263dcv
pin 9 = 253dcv

EZ81 valve in socket
pin 1 = 334acv
pin 7 = 334acv
pin 3 = 347dcv
Ed.

17th Jan 2017, 3:15 pm	#22
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Yes it was across a 1 meg. Ed.

17th Jan 2017, 3:20 pm	#23
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Decca DRM88 Tingle from chassis and hum thru speaker 12mA flowing through a 1meg resistor equates to a voltage of 12,000 volts across it..... Lawrence.

17th Jan 2017, 3:29 pm	#24
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Decca DRM88 Tingle from chassis and hum thru speaker If you really measured 12 mA with the meter on a current range between chassis and mains earth, then something is very wrong and you need to investigate. The fact that you measured 116V across a 1MΩ resistor could be interesting. (There obviously is not enough voltage to push 12mA through 1MΩ). That figure is remarkably close to both half the supply voltage, and the lowest tapping on the primary. Check the mains input wiring very, very carefully, and report back. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

17th Jan 2017, 5:03 pm	#25
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Decca DRM88 Tingle from chassis and hum thru speaker These latest readings are very different to that in post #1 and I quote " There's a voltage of 13.5 volts on the chassis". So what has changed? Presumably this first reading was between chassis and earth without a 1meg resistor which suggests the leakage was very low when the first reading was taken. Have you fitted a 4.7uF in place of C61 instead of a 4.7nF in error perhaps?

17th Jan 2017, 5:03 pm	#26
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Will do.

17th Jan 2017, 5:11 pm	#27
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Hi. Colin you advised me to remove c61 and provide an earth to the mains which worked on stopping the tingling. I cant quite remember how I got that reading I know I had the live meter probe on the chassis and I was checking the connections on the speaker and when I brushed my hand against the speaker frame and felt a tingle I took a quick glance at the meter and it read 13.5 the negative probe was,nt being used. Last edited by johnny english; 17th Jan 2017 at 5:31 pm. Reason: additional information

17th Jan 2017, 7:00 pm	#29
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Decca DRM88 Tingle from chassis and hum thru speaker The standard circuit for measuring earth leakage is basically a 1K resistor from chassis or other accessible conductive part to earth across which the voltage is measured. (There's also a simple low pass filter in the pukka box to stop any stray RF from upsetting the readings, but that isn't needed for a quick'n'dirty check). 1K is around the typical resistance of a person with wet or broken skin at the point of contact. Your 12mA wouldn't trip a typical 30mA RCD unless the rest of the installation was leaking more than about 20mA already. More than 50mA for any length of time "may have possible irreversible effects"- an interesting turn of phrase, that __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O Last edited by Herald1360; 17th Jan 2017 at 7:06 pm.

18th Jan 2017, 12:43 am	#30
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Hi. I did a simple continuity test (cold) from the mains input to the on-off/ tone control pot in the (off position) the mains L&N totally isolated between terminals. In the (on position) continuity thru switch though L&N are`nt isolated between each other. On to the mains transformer when looking underneath the chassis the negative terminal is on the left and live on the right there is continuity across these terminals. Testing the same terminals individually to chassis there is no continuity. I will post some photo,s tomorrow showing the the internals etc. Thanks Ed.

21st Jan 2017, 6:09 pm	#31
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Hi. I just have,nt got anywhere with this radio at all. First, l will explain the initial problem "loud hum on all wavebands" so I suspected the smoothing can and duly ordered and replaced 2x32uf caps with 33s I also noticed that the original can got warm but put it down to the ez81 rectifier valve radiating its heat onto the can body as the valve sits below it. There are 3 32uf smoothing caps the third one was separately fixed to the chassis but connected to the other caps. Switched the set on it still hummed and the smoother with the 1kohm 6w and 2.1kohm 2w resistors wired in series to pin 3 of the ez81 started to get very warm, so I switched off and in a desperate attempt I replaced the third smoothing cap still the same problem. I am now beginning to suspect the mains transformer which gets warm but does,nt overheat and I would like to know what others think as I have never met this problem before and would like to tackle it. I have also replaced the hunts mouldseals and since those photo,s were taken I have replaced the 3rd smoothing cap and reinstalled c61. Thanks ED. Attached Thumbnails

21st Jan 2017, 7:07 pm	#32
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Decca DRM88 Tingle from chassis and hum thru speaker If possible, do some voltage measurements, with respect to chassis, rectifiers anode voltages (AC) rectifiers cathode voltage (DC) output valves anode, screen grid, cathode and control grid voltages (DC) It might give an indication as to the general health of things. I don't have a schematic. Lawrence.

21st Jan 2017, 7:41 pm	#33
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Thanks for the info Lawrence. The process will be I think valves out power on meter negative to chassis and the positive to required valve contacts. Am I correct. Ed.

21st Jan 2017, 7:58 pm	#34
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Decca DRM88 Tingle from chassis and hum thru speaker Valves in if possible eg: nothing seriously cooking. Lawrence.

21st Jan 2017, 8:44 pm	#35
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Decca DRM88 Tingle from chassis and hum thru speaker It might be instructive to measure the voltages on each of the EZ81 anodes (make sure to select the highest AC range), both with the valve fitted (and warmed up to operating temperature) and removed. If there is a big difference, then that would indicate something drawing excessive current. You will have to do some DC checks to discover what; preferrably as quickly as possible, so as to minimise the likelihood of further damage. As a first test, try removing the EL84 (which is a suspect anyway). __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

22nd Jan 2017, 1:52 am	#36
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker here,s the results.. el84 control grid pin 2 valve out = 0dcv valve in =7dcv cathode pin 3 ............. = 0dcv ........... = 4dcv anode pin 7 ............. = 355dcv ........ = 228dcv screen grid pin 9 ............. = 355dcv ........ =254dcv ez81 anode pin 1 = 343acv anode pin 7 =343acv cathode pin 3 = 419dcv The el84 valve in readings kept fluctuating up and down the figures posted are the highest recorded on test. Ed.

22nd Jan 2017, 6:08 am	#37
johnny english Retired Dormant Member Join Date: Nov 2013 Location: Liverpool, Merseyside, UK. Posts: 160	Re: Decca DRM88 Tingle from chassis and hum thru speaker Sorry about this my post above is wrong. The following figures are correct. EL84 valve in socket pin 2 = 0dcv pin 3 = 6dcv pin 7 = 263dcv pin 9 = 253dcv EZ81 valve in socket pin 1 = 334acv pin 7 = 334acv pin 3 = 347dcv Ed.