Hum in 1950s Truetone Record Player - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Boater Sam · 5th May 2019, 10:56 am

Does this help?
Miniature 7-pin base
The B7G (or "small-button" or "heptal") seven-pin miniature tubes are smaller than Noval, with seven pins arranged at 45-degree spacing in a 9.53 mm (3/8th inch) diameter arc, the "missing" pin position being used to position the tube in its socket (unlike octal, loctal and rimlock sockets). Examples include the 6AQ5/EL90 and 6BE6/EK90.

bennie8 · 5th May 2019, 4:46 pm

Quote:

Originally Posted by 'LIVEWIRE?'

One caveat, Sam-the terms B7G, B9A, etc., may not be in common use in the States-9pin valves & sockets there are called 'noval', IIRC, but I don't know what term is used to describe the 7-pin ones.

Thanks again Boater Sam and Livewire. I ordered 7 pin ceramic sockets from an eBay seller yesterday.

bennie8 · 5th May 2019, 8:37 pm

Quote:

Originally Posted by TonyDuell

Have you checked the resistance values given in the table on the schematic?

You do this with the player unplugged from the mains and with the valves fitted. Volume control at minimum. For pins 1 to 5 of each valve you connect one meter lead to the chassis and the other lead to each valve pin in turn. For pins 6 and 7 of each valve, one meter lead goes to the cathode of the rectifier M1 (connected to the red tag of the electrolytic capacitor can C1) and the other lead to the valve pin.

With the player still unplugged, can you measure the resistance between the 'outside' 2 tags of each section of the volume control.

Hello Tony,
These resistance numbers still don't look right. The values I get for the volume pots are correct, 1 meg ohm. The values I get for both valves pins 6 and 7 match what the chart indicates they should be, meter lead to the rectifier diode cathode other lead to valve pin.

However the resistance readings for pins 1 through 5 of both valves are all in the 12 to 13 M ohm range, one meter lead clipped to the chassis and the other lead on the valve pins.

What am I missing?

ms660 · 5th May 2019, 8:41 pm

Chassis is not B-, measure with reference to B- (HT -ve) There should be no resistance reading between chassis and B-, if there is suspect the isolating capacitor C4 or some other leakage path.

Lawrence.

bennie8 · 5th May 2019, 9:00 pm

Thank you for the input but I do not know where the meter lead goes. Can you give me a specific circuit location for the meter lead?

Thank you.

ms660 · 6th May 2019, 11:05 am

Quote:

Originally Posted by bennie8

Thank you for the input but I do not know where the meter lead goes. Can you give me a specific circuit location for the meter lead?

You can connect the meter lead to any circuit point that shows the B- sign in the schematic, eg: The -ve connection of C1.

Don't forget when working on your record player with power applied that your record player is a live -ve rail design and that only one side of the mains supply is switched....

....which means that one side of the mains is connected directly to the HT's -ve rail (B-) when the record player is switched on and that the other side of the mains is connected to that same rail via the valves (tubes) heater circuit when the record player is switched off.....

The phono input ground is connected to the chassis, the B- rail is connected to the chassis for signal and safety isolation purposes via the isolating capacitor C4.

Lawrence.

ms660 · 6th May 2019, 12:42 pm

Quote:

Originally Posted by ms660

....which means that one side of the mains is connected directly to the HT's -ve rail (B-) when the record player is switched on and that the other side of the mains is connected to that same rail via the valves (tubes) heater circuit when the record player is switched off.....

My apologies, please read as:

....which means that one side of the mains is connected directly to the HT's -ve rail (B-) when the record player is switched on and that the other side of the mains is connected to that same rail via the valves (tubes) heater circuit, motor and rectifier circuits etc. when the record player is switched off.

Lawrence.

bennie8 · 6th May 2019, 11:31 pm

Thank you for the help. I cut the original plug from the cord and installed a polarized two prong plug. Glad you clarified that for me. Attached is a chart comparing the manufactures original resistance values and my measured values. I'm not knowledgeable enough to comment on it except that it looks like the manufactures values for pin 4 of V1 and V2 are reversed, typo?

TonyDuell · 7th May 2019, 5:16 am

I wouldn't worry about the pin 3 and pin 4 differences, looks like the valve heaters (which are in series), are wired in the other order.

The differences on pin 2 can be explained by the circuit diagram showing 4700 ohm grid resistors and you having 47000 ohm ones fitted.

I am a little concerned by pin 1 (cathode). Is R6 (82 Ohm) correct?

bennie8 · 7th May 2019, 3:55 pm

Quote:

Originally Posted by TonyDuell

I wouldn't worry about the pin 3 and pin 4 differences, looks like the valve heaters (which are in series), are wired in the other order.

The differences on pin 2 can be explained by the circuit diagram showing 4700 ohm grid resistors and you having 47000 ohm ones fitted.

I am a little concerned by pin 1 (cathode). Is R6 (82 Ohm) correct?

Tony, I am not sure of your question. On pin one of both valves the manufactures resistance value is 82 ohm. I measured 185 ohm and 190 ohm respectfully. I just measured the resistance of R6 again and it reads 194 ohm.

Bernie

Station X · 7th May 2019, 4:07 pm

What's the colour coding of R6?

82R should be Grey, Red, Black.

EDIT. Has the cathode voltage which should be around 4.5VDC ever been measured?

bennie8 · 7th May 2019, 8:43 pm

Quote:

Originally Posted by Station X

What's the colour coding of R6?

82R should be Grey, Red, Black.

EDIT. Has the cathode voltage which should be around 4.5VDC ever been measured?

I believe the color bands are yellow-black-red and silver or yellow-black orange and silver. Picture of R6 is attached.

Don't recall that cathode voltage has been read. Is that the heater voltage, valve pins 3 and 4?

Station X · 7th May 2019, 9:35 pm

Very difficult to tell, but maybe originally it was Grey, Red, Black? Measure the resistance directly across it.

If you're looking at the correct resistor it will be connected to pin 1 of both valveholders.

The easiest way to measure the cathode voltage is to connect your meter on volts range across R6.

bennie8 · 7th May 2019, 10:22 pm

Quote:

Originally Posted by Station X

Very difficult to tell, but maybe originally it was Grey, Red, Black? Measure the resistance directly across it.

If you're looking at the correct resistor it will be connected to pin 1 of both valveholders.

The easiest way to measure the cathode voltage is to connect your meter on volts range across R6.

I measured directly across R6 and the resistance is 194 ohm, the voltage drop across R6 is 6.22 volts DC. And indeed R6 is connected to pin one of both valve mounts.

Thanks again.

Bernie

Station X · 7th May 2019, 10:36 pm

I'd say that R6 has overheated at some point and changed value. I'd replace it.

It probably won't do anything for the hum problem though.

5th May 2019, 10:56 am	#41
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Hum in 1950s Truetone Record Player Does this help? Miniature 7-pin base The B7G (or "small-button" or "heptal") seven-pin miniature tubes are smaller than Noval, with seven pins arranged at 45-degree spacing in a 9.53 mm (3/8th inch) diameter arc, the "missing" pin position being used to position the tube in its socket (unlike octal, loctal and rimlock sockets). Examples include the 6AQ5/EL90 and 6BE6/EK90.

5th May 2019, 8:41 pm	#44
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Hum in 1950s Truetone Record Player Chassis is not B-, measure with reference to B- (HT -ve) There should be no resistance reading between chassis and B-, if there is suspect the isolating capacitor C4 or some other leakage path. Lawrence.

5th May 2019, 9:00 pm	#45
bennie8 Retired Dormant Member Join Date: Apr 2019 Location: Lee's Summit, Missouri, USA. Posts: 22	Re: Hum in 1950s Truetone Record Player Thank you for the input but I do not know where the meter lead goes. Can you give me a specific circuit location for the meter lead? Thank you.

7th May 2019, 5:16 am	#49
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,222	Re: Hum in 1950s Truetone Record Player I wouldn't worry about the pin 3 and pin 4 differences, looks like the valve heaters (which are in series), are wired in the other order. The differences on pin 2 can be explained by the circuit diagram showing 4700 ohm grid resistors and you having 47000 ohm ones fitted. I am a little concerned by pin 1 (cathode). Is R6 (82 Ohm) correct?

7th May 2019, 4:07 pm	#51
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Hum in 1950s Truetone Record Player What's the colour coding of R6? 82R should be Grey, Red, Black. EDIT. Has the cathode voltage which should be around 4.5VDC ever been measured? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

7th May 2019, 9:35 pm	#53
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Hum in 1950s Truetone Record Player Very difficult to tell, but maybe originally it was Grey, Red, Black? Measure the resistance directly across it. If you're looking at the correct resistor it will be connected to pin 1 of both valveholders. The easiest way to measure the cathode voltage is to connect your meter on volts range across R6. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

7th May 2019, 10:36 pm	#55
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Hum in 1950s Truetone Record Player I'd say that R6 has overheated at some point and changed value. I'd replace it. It probably won't do anything for the hum problem though. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.