Help getting AD65 (Actually an AD75) up and running - UK Vintage Radio Repair and Restoration Discussion Forum

Dick Glennon · 3rd May 2023, 8:14 pm

Hi again, the output transformer on my AD 75 has gone O/C on its primary. It should read primary 255ohms and secondary 0.2 ohms. How critical are those measurements in replacing with another op transformer. Thanks again, Dick.

Ed_Dinning · 3rd May 2023, 8:34 pm

Hi Dick quite a common fault but the pri could read up to about 400R and still be OK.
Note that DVM's can give bad readings on inductive parts

Ed

Sideband · 3rd May 2023, 9:20 pm

It's not the DC resistance that is important. That doesn't give any indication of the ratio (impedance matching) of the transformer. The DC resistances given in service information are only a guide. Exact matching is not so critical (at least not on a domestic radio) but it should be somewhere near correct. You need to look up the data on the output valve and find out the anode impedance. From that you can work out the transformer ratio required for good matching to the speaker.

Before you change the output transformer, it's a good idea to make sure nothing else is wrong that might have caused the failure.

According to the valve data for CBL31 the anode load (Rl) is 4,500 ohms, divide that by 3 (3 ohm speaker) gives 1500. Take the square root of that and you get 38.7. Ignore anything after the decimal point so the optimum ratio is 38:1. In practice 40:1 or even 50:1 will be OK so you need a transformer in that sort of ball-park. The anode current is quite high at 45mA so the primary should be able to handle about 50mA or more.

stevehertz · 3rd May 2023, 9:32 pm

You could get it rewound.

Dick Glennon · 22nd May 2023, 8:44 pm

Hi again, how critical is the resistance of the ballast resistor (R19) in the AD75? It says 815 ohms on the trader sheet but I only have 735 ohms actual. 631 ohms (615) 103 ohms (100) and 1 ohm (100) Would this put extra voltage on the valves or would it be OK?

Regards, Dick.

G6Tanuki · 22nd May 2023, 8:59 pm

I'd first check the sort of mains-voltage you have at your location, and use this to inform your choice.

Here, I have my own dedicated 11Kv-to-240V transformer in a substation a couple of hundred yards away, and as a result the mains spends most of its time a smidge above 250V. So when dealing with droppers I always err on the higher-resistance side of any calculations.

In your case I would go for the highest resistance. Is your resistor a tapped one with the various resistances you've measured being on the taps? If so maybe you could wire an extra resistor in series to control the heater-current?

In practice, valves can happily tolerate something like +/- 6% of the stated voltages/currents.

Station X · 22nd May 2023, 9:22 pm

The OP states that one of the 100R sections of the resistor only measures 1R. There's clearly something wrong there!!

Is the section being shorted by the voltage selector?

peter_sol · 23rd May 2023, 5:59 am

Maybe the shorting link is across the first one in line.

Dick Glennon · 23rd May 2023, 8:17 pm

Hi again,Yes the voltage selector was on the 220 volt spur was knocking that part of the resistor out. When I took off the fly lead and measured the total resistance it shot up to 863 ohms, (815) . Should I Leave it on the high ohms resistance so as to accommodate the higher voltage now (240volts) Thanks again Dick.

Station X · 23rd May 2023, 8:55 pm

How are the taps labelled? The Trader Sheet doesn't show this.

Silicon · 24th May 2023, 9:16 am

The heater current is set by the choice of ballast resistor and the local mains voltage.

If you measure the AC voltage across the 100 Ohm ballast resistor, you can calculate the actual heater current.

I would prefer to slightly 'under-run' the heaters (rather than over-run) with the hope of getting a longer life from the valves.

You don't have to stick rigidly to the resistor values that Ekco originally installed.

stevehertz · 24th May 2023, 1:03 pm

The fact remains you need to look into that 100 ohm section that only measures one ohm. Clear/clarify that issue first, then go from there.

Cobaltblue · 24th May 2023, 1:06 pm

Quote:

Originally Posted by stevehertz

The fact remains you need to look into that 100 ohm section that only measures one ohm. Clear/clarify that issue first, then go from there.

Looks like that was sorted in post #5 the 220V tap shorted it.

Cheers

Mike T

stevehertz · 24th May 2023, 1:51 pm

Quote:

Originally Posted by Cobaltblue

Quote:

Originally Posted by stevehertz

The fact remains you need to look into that 100 ohm section that only measures one ohm. Clear/clarify that issue first, then go from there.

Looks like that was sorted in post #5 the 220V tap shorted it.

Cheers

Mike T

Ah right, thanks Mike.

Dick Glennon · 29th May 2023, 9:09 pm

Hi again,I have discovered that a diode was inserted in the circuit of the rectifier between pin 5 and pin 8. I now have a good rectifier CY31 so I was hoping to put it back into circuit. Does this mean that I can take away the diode and let the valve do the work. I tried powering up the set with the diode still in and the lamp limiter lit fairly bright .I switched it off immediately. The resistance at the live and neutral pins with the lead plugged out and the set switched on is 826 ohms. Any advice would be very much appreciated Thanks again, Dick.

Mr 1936 · 30th May 2023, 8:12 am

Hi

On some versions of this set a double rectifier is used with the two halves wired in parallel. As far as I can work out, pins 5 and 8 are the two anodes of the rectifier, so a diode in this position is rather odd. Bear in mind that side contact valveholders are numbered counterclockwise.

It's not unusual for previous owners to fit semiconductor rectifiers when they can't get hold of the proper valve. If you have a suitable valve and want to use it, then the diode should be removed. I suggest you take a careful look for any other modifications, with the aid of the service information that is readily available. Don't assume the previous owner knew what they were doing !

The cold resistance measured at the mains input, with a valve rectifier (which won't be conducting), will be that of the heater chain including the wirewound ballast resistor. The latter is 760 ohms if all sections are in circuit, as they normally would be when set to modern mains voltage of 240 volts or thereabouts. What you are measuring sounds about right, but it would be useful to also check the resistance of just the valve heater string. This string will have a lower resistance than when operating, as the heaters increase in resistance by a considerable amount when hot.

Station X · 30th May 2023, 9:10 am

You'll probably find that the smoothing capacitors, which are housed in a cardboard container, need replacing as they're unlikely to reform.

mister valve · 30th May 2023, 11:54 am

AS Graham (Station X) says, it will almost certainly be the reservoir & smoothing Capacitors that caused your lamp limiter to glow brightly, as a semiconductor diode conducts straight away. BTW the "CY31" has an international OCTAL base fitted.... The valve specified for your set is "UR2" which has a 'side-contact' base fitted. Also it has a 30 Volt heater, whereas the "CY31" (and "CY1") have 20 volt heaters. If the base fitted in your chassis is still the side-contact base, you could use a "CY1" with extra heater ballast resistance connected, or a direct replacement "CY2" valve which has a 30 Volt heater like the "UR2".

But I do recommend you replace the Reservoir and Smoothing caps and all other 'paper' and 'Electrolytic' type capacitors.... They will all be electrically leaky by now, some may even be dead shorted.

Dick Glennon · 30th May 2023, 3:10 pm

Hi again, I have already replaced the electrolytic caps and all other caps (paper). In the trader sheet 748 the rectifier is stated as CY31 in the Ekco AD75. Hope this information helps.

Dick

Dick Glennon · 30th May 2023, 3:14 pm

Hi , I have just seen a big mistake . I should have said Help getting an AD75 up and running. Sorry for my mistake . Dick.

3rd May 2023, 8:14 pm	#1
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	Output transformer gone open circuit on AD 75. Hi again, the output transformer on my AD 75 has gone O/C on its primary. It should read primary 255ohms and secondary 0.2 ohms. How critical are those measurements in replacing with another op transformer. Thanks again, Dick.

3rd May 2023, 8:34 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Output transformer gone open circuit on AD 75. Hi Dick quite a common fault but the pri could read up to about 400R and still be OK. Note that DVM's can give bad readings on inductive parts Ed

3rd May 2023, 9:20 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,566	Re: Output transformer gone open circuit on AD 75. It's not the DC resistance that is important. That doesn't give any indication of the ratio (impedance matching) of the transformer. The DC resistances given in service information are only a guide. Exact matching is not so critical (at least not on a domestic radio) but it should be somewhere near correct. You need to look up the data on the output valve and find out the anode impedance. From that you can work out the transformer ratio required for good matching to the speaker. Before you change the output transformer, it's a good idea to make sure nothing else is wrong that might have caused the failure. According to the valve data for CBL31 the anode load (Rl) is 4,500 ohms, divide that by 3 (3 ohm speaker) gives 1500. Take the square root of that and you get 38.7. Ignore anything after the decimal point so the optimum ratio is 38:1. In practice 40:1 or even 50:1 will be OK so you need a transformer in that sort of ball-park. The anode current is quite high at 45mA so the primary should be able to handle about 50mA or more. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 3rd May 2023 at 9:34 pm.

3rd May 2023, 9:32 pm	#4
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,828	Re: Output transformer gone open circuit on AD 75. You could get it rewound. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

22nd May 2023, 8:59 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: AD 75 Ekco heater ballast resistor I'd first check the sort of mains-voltage you have at your location, and use this to inform your choice. Here, I have my own dedicated 11Kv-to-240V transformer in a substation a couple of hundred yards away, and as a result the mains spends most of its time a smidge above 250V. So when dealing with droppers I always err on the higher-resistance side of any calculations. In your case I would go for the highest resistance. Is your resistor a tapped one with the various resistances you've measured being on the taps? If so maybe you could wire an extra resistor in series to control the heater-current? In practice, valves can happily tolerate something like +/- 6% of the stated voltages/currents. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

22nd May 2023, 8:44 pm	#5
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	AD 75 Ekco heater ballast resistor Hi again, how critical is the resistance of the ballast resistor (R19) in the AD75? It says 815 ohms on the trader sheet but I only have 735 ohms actual. 631 ohms (615) 103 ohms (100) and 1 ohm (100) Would this put extra voltage on the valves or would it be OK? Regards, Dick.

22nd May 2023, 9:22 pm	#7
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: AD 75 Ekco heater ballast resistor The OP states that one of the 100R sections of the resistor only measures 1R. There's clearly something wrong there!! Is the section being shorted by the voltage selector? Attached Thumbnails __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

23rd May 2023, 5:59 am	#8
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: AD 75 Ekco heater ballast resistor Maybe the shorting link is across the first one in line.

23rd May 2023, 8:17 pm	#9
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	Re: AD 75 Ekco heater ballast resistor Hi again,Yes the voltage selector was on the 220 volt spur was knocking that part of the resistor out. When I took off the fly lead and measured the total resistance it shot up to 863 ohms, (815) . Should I Leave it on the high ohms resistance so as to accommodate the higher voltage now (240volts) Thanks again Dick.

23rd May 2023, 8:55 pm	#10
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: AD 75 Ekco heater ballast resistor How are the taps labelled? The Trader Sheet doesn't show this. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

24th May 2023, 9:16 am	#11
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,162	Re: AD 75 Ekco heater ballast resistor The heater current is set by the choice of ballast resistor and the local mains voltage. If you measure the AC voltage across the 100 Ohm ballast resistor, you can calculate the actual heater current. I would prefer to slightly 'under-run' the heaters (rather than over-run) with the hope of getting a longer life from the valves. You don't have to stick rigidly to the resistor values that Ekco originally installed.

24th May 2023, 1:03 pm	#12
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,828	Re: AD 75 Ekco heater ballast resistor The fact remains you need to look into that 100 ohm section that only measures one ohm. Clear/clarify that issue first, then go from there. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

29th May 2023, 9:09 pm	#15
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	Help getting AD65 (Actually an AD75) up and running Hi again,I have discovered that a diode was inserted in the circuit of the rectifier between pin 5 and pin 8. I now have a good rectifier CY31 so I was hoping to put it back into circuit. Does this mean that I can take away the diode and let the valve do the work. I tried powering up the set with the diode still in and the lamp limiter lit fairly bright .I switched it off immediately. The resistance at the live and neutral pins with the lead plugged out and the set switched on is 826 ohms. Any advice would be very much appreciated Thanks again, Dick.

30th May 2023, 8:12 am	#16
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 521	Re: Help getting AD 65 up and running Hi On some versions of this set a double rectifier is used with the two halves wired in parallel. As far as I can work out, pins 5 and 8 are the two anodes of the rectifier, so a diode in this position is rather odd. Bear in mind that side contact valveholders are numbered counterclockwise. It's not unusual for previous owners to fit semiconductor rectifiers when they can't get hold of the proper valve. If you have a suitable valve and want to use it, then the diode should be removed. I suggest you take a careful look for any other modifications, with the aid of the service information that is readily available. Don't assume the previous owner knew what they were doing ! The cold resistance measured at the mains input, with a valve rectifier (which won't be conducting), will be that of the heater chain including the wirewound ballast resistor. The latter is 760 ohms if all sections are in circuit, as they normally would be when set to modern mains voltage of 240 volts or thereabouts. What you are measuring sounds about right, but it would be useful to also check the resistance of just the valve heater string. This string will have a lower resistance than when operating, as the heaters increase in resistance by a considerable amount when hot.

30th May 2023, 9:10 am	#17
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Help getting AD 65 up and running You'll probably find that the smoothing capacitors, which are housed in a cardboard container, need replacing as they're unlikely to reform. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

30th May 2023, 11:54 am	#18
mister valve Hexode Join Date: Aug 2009 Location: Blackpool, Lancashire, UK. Posts: 311	Re: Help getting AD 65 up and running AS Graham (Station X) says, it will almost certainly be the reservoir & smoothing Capacitors that caused your lamp limiter to glow brightly, as a semiconductor diode conducts straight away. BTW the "CY31" has an international OCTAL base fitted.... The valve specified for your set is "UR2" which has a 'side-contact' base fitted. Also it has a 30 Volt heater, whereas the "CY31" (and "CY1") have 20 volt heaters. If the base fitted in your chassis is still the side-contact base, you could use a "CY1" with extra heater ballast resistance connected, or a direct replacement "CY2" valve which has a 30 Volt heater like the "UR2". But I do recommend you replace the Reservoir and Smoothing caps and all other 'paper' and 'Electrolytic' type capacitors.... They will all be electrically leaky by now, some may even be dead shorted. __________________ If you live, the chassis was dead; if you're dead, the chassis was live !!!!

30th May 2023, 3:10 pm	#19
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	Re: Help getting AD 65 up and running Hi again, I have already replaced the electrolytic caps and all other caps (paper). In the trader sheet 748 the rectifier is stated as CY31 in the Ekco AD75. Hope this information helps. Dick

30th May 2023, 3:14 pm	#20
Dick Glennon Hexode Join Date: Oct 2006 Location: Edenderry, Republic of Ireland. Posts: 428	Re: Help getting AD 65 up and running Hi , I have just seen a big mistake . I should have said Help getting an AD75 up and running. Sorry for my mistake . Dick.