Dansette major rectifier - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Station X · 20th Sep 2018, 6:27 pm

That's OK then. I'd be inclined to measure the DC voltage across it. You'll soon see if the parallel electrolytic cap is leaking electrically.

leslie5555 · 20th Sep 2018, 6:30 pm

I will do that what DC value should it be?

Station X · 20th Sep 2018, 6:36 pm

I'll hazard a guess at 9V. It depends on the HT voltage and state of the valve.

leslie5555 · 20th Sep 2018, 6:51 pm

Ok I got 5.3 v a bit low do you think

Station X · 20th Sep 2018, 7:00 pm

Check it again with one end of the cap disconnected. If the voltage rises the cap is leaking. Problem is we have no idea whether the HT is correct.

leslie5555 · 20th Sep 2018, 8:40 pm

I take it you are referring to the 25uf cap

Station X · 20th Sep 2018, 9:00 pm

If it's the cathode bypass capacitor connected in parallel with the 120R cathode resistor then yes.

If it's electrically leaky it'll lower the effective resistance in the cathode circuit reducing the cathode voltage for a given combined anode and screen current.

--------------

As for the HT voltage, the unmarked end of the diode is the anode and should go to the mains transformer or motor tap. It'll have an AC voltage on it wrt chassis. The marked end is the cathode and should go to the smoothing capacitor. It'll have a DC voltage on it wrt chassis.

Boater Sam · 20th Sep 2018, 9:12 pm

Quote:

Originally Posted by Station X

The unmarked end is the cathode and should go to the smoothing capacitor. It'll have a DC voltage on it wrt chassis.

Error?

Station X · 20th Sep 2018, 9:13 pm

Thanks Sam. I'll edit it.

leslie5555 · 20th Sep 2018, 9:46 pm

Thanks for your help Graham readings had been taken on both sides of the diode and the results were posted in an earlier post.
I have removed the diode and reconnected the rectifier
If I'm going to disconnect the cap I'm thinking I may as well replace all 3 caps with new ones.

Station X · 20th Sep 2018, 10:09 pm

Quote:

Originally Posted by leslie5555

readings had been taken on both sides of the diode and the results were posted in an earlier post.

Yes, but apparently you had more voltage coming out of the transformer than was going into the rectifier! The attached circuit extract is representative only and may not reflect the exact circuit of your player.

Quote:

Originally Posted by leslie5555

I have removed the diode and reconnected the rectifier

Why? You said that fitting a diode improved the HT by 25V. A series resistor would drop that a bit, but not by much.

Quote:

Originally Posted by leslie5555

If I'm going to disconnect the cap I'm thinking I may as well replace all 3 caps with new ones.

If you must, but that defeats the object of doing any testing.

leslie5555 · 21st Sep 2018, 6:33 am

The diode was only temporally fitted to see if it made any difference, I will refit it properly later.
I am reluctant to remove the can as it is connected via a pcb and may cause damage to the pcb on removal, is there another way I can check the cap without removing it?

Station X · 21st Sep 2018, 9:23 am

While you have the diode or rectifier removed, check the off load AC voltage of the mains transformer's HT secondary winding (the point where the anode was disconnected from). This may help determine if something in the circuit is pulling down the DC HT voltage.

Do this first and then we can think about the caps in the can.

leslie5555 · 21st Sep 2018, 12:57 pm

Ok done that and got 147V

ms660 · 21st Sep 2018, 1:20 pm

If you want to figure out if the cathode bias voltage is within reason measure the resistance of the output transformer primary, the HT voltage on the HT side of the transformer the voltage drop across the transformer, the voltage on the screen grid, the voltage on the control grid and the cathode voltage.

Lawrence.

leslie5555 · 21st Sep 2018, 1:35 pm

Thanks for you help Lawrence I am willing to check all of the above but need a lot of help on exactly where I would connect the dmm leads and is it ac or dc etc .
Les

ms660 · 21st Sep 2018, 2:12 pm

Meter set to DC for the voltage measurements.

Meter set to Ohms/resistance for the resistance measurement (power off) expect a resistance reading of less than 1k ohm so set range accordingly.

Voltage measurements:

The HT side of output transformer's primary winding would normally be connected to the cathode of the rectifier, the other side of the primary winding will be connected to the valve's anode, the valve's electrode connections (pin numbers) are as viewed from underneath (eg: the track side of the printed circuit board) as shown in the UL84's valve data sheets here:

http://www.r-type.org/pdfs/ul84.pdf

Anode (a) = pin 7
Screen grid (g2) = pin 9
Control grid (g1) = pin 2
Cathode (k) = pin3

Black meter probe to chassis/HT-ve.

Select DC voltage range to suit....The HT, anode, and screen grid voltages won't be above 250 volts, the DC voltage drop across the output transformers primary winding shouldn't be too high but should read OK on a range setting that was selected for the HT, anode or screen grid measurements, the control grid measurement may or may not give a reading, if it does it should be very low so select a very low DC voltage range for that and observe the polarity of any reading given, the cathode shouldn't be more than 10 volts so again select a voltage range to suit.

The resistance measurement is self explanatory.

Lawrence.

leslie5555 · 21st Sep 2018, 2:57 pm

Thank you for your time in doing this Lawrence, I will give it a go and get back later with the results.
Les

leslie5555 · 22nd Sep 2018, 7:03 am

Measurements are...
Pin 7 = 112v
Pin 9 = 104v
Pin 3 = 005v
Pin 2 = +0.11v

Resistance says 0.250 on my cheapo dmm set at 2k

Do these look ok?

These readings were taken with the rectifier removed and a 1N4007 with a 100ohm resistor fitted in its place the HT across the diode now measures 137V

'LIVEWIRE?' · 22nd Sep 2018, 8:25 am

That HT seems too low to me. Surely it should be over 200v, maybe higher. If so, the readings at the valve would also be proportionally higher.

20th Sep 2018, 6:27 pm	#21
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier That's OK then. I'd be inclined to measure the DC voltage across it. You'll soon see if the parallel electrolytic cap is leaking electrically. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2018, 6:30 pm	#22
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier I will do that what DC value should it be?

20th Sep 2018, 6:36 pm	#23
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier I'll hazard a guess at 9V. It depends on the HT voltage and state of the valve. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2018, 6:51 pm	#24
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Ok I got 5.3 v a bit low do you think

20th Sep 2018, 7:00 pm	#25
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier Check it again with one end of the cap disconnected. If the voltage rises the cap is leaking. Problem is we have no idea whether the HT is correct. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2018, 8:40 pm	#26
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier I take it you are referring to the 25uf cap

20th Sep 2018, 9:00 pm	#27
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier If it's the cathode bypass capacitor connected in parallel with the 120R cathode resistor then yes. If it's electrically leaky it'll lower the effective resistance in the cathode circuit reducing the cathode voltage for a given combined anode and screen current. -------------- As for the HT voltage, the unmarked end of the diode is the anode and should go to the mains transformer or motor tap. It'll have an AC voltage on it wrt chassis. The marked end is the cathode and should go to the smoothing capacitor. It'll have a DC voltage on it wrt chassis. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2018, 9:13 pm	#29
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier Thanks Sam. I'll edit it. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

20th Sep 2018, 9:46 pm	#30
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Thanks for your help Graham readings had been taken on both sides of the diode and the results were posted in an earlier post. I have removed the diode and reconnected the rectifier If I'm going to disconnect the cap I'm thinking I may as well replace all 3 caps with new ones.

21st Sep 2018, 6:33 am	#32
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier The diode was only temporally fitted to see if it made any difference, I will refit it properly later. I am reluctant to remove the can as it is connected via a pcb and may cause damage to the pcb on removal, is there another way I can check the cap without removing it?

21st Sep 2018, 9:23 am	#33
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Dansette major rectifier While you have the diode or rectifier removed, check the off load AC voltage of the mains transformer's HT secondary winding (the point where the anode was disconnected from). This may help determine if something in the circuit is pulling down the DC HT voltage. Do this first and then we can think about the caps in the can. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

21st Sep 2018, 12:57 pm	#34
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Ok done that and got 147V

21st Sep 2018, 1:20 pm	#35
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dansette major rectifier If you want to figure out if the cathode bias voltage is within reason measure the resistance of the output transformer primary, the HT voltage on the HT side of the transformer the voltage drop across the transformer, the voltage on the screen grid, the voltage on the control grid and the cathode voltage. Lawrence. Last edited by ms660; 21st Sep 2018 at 1:28 pm. Reason: addition

21st Sep 2018, 1:35 pm	#36
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Thanks for you help Lawrence I am willing to check all of the above but need a lot of help on exactly where I would connect the dmm leads and is it ac or dc etc . Les

21st Sep 2018, 2:12 pm	#37
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Dansette major rectifier Meter set to DC for the voltage measurements. Meter set to Ohms/resistance for the resistance measurement (power off) expect a resistance reading of less than 1k ohm so set range accordingly. Voltage measurements: The HT side of output transformer's primary winding would normally be connected to the cathode of the rectifier, the other side of the primary winding will be connected to the valve's anode, the valve's electrode connections (pin numbers) are as viewed from underneath (eg: the track side of the printed circuit board) as shown in the UL84's valve data sheets here: http://www.r-type.org/pdfs/ul84.pdf Anode (a) = pin 7 Screen grid (g2) = pin 9 Control grid (g1) = pin 2 Cathode (k) = pin3 Black meter probe to chassis/HT-ve. Select DC voltage range to suit....The HT, anode, and screen grid voltages won't be above 250 volts, the DC voltage drop across the output transformers primary winding shouldn't be too high but should read OK on a range setting that was selected for the HT, anode or screen grid measurements, the control grid measurement may or may not give a reading, if it does it should be very low so select a very low DC voltage range for that and observe the polarity of any reading given, the cathode shouldn't be more than 10 volts so again select a voltage range to suit. The resistance measurement is self explanatory. Lawrence. Last edited by ms660; 21st Sep 2018 at 2:26 pm.

21st Sep 2018, 2:57 pm	#38
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Thank you for your time in doing this Lawrence, I will give it a go and get back later with the results. Les

22nd Sep 2018, 7:03 am	#39
leslie5555 Heptode Join Date: May 2015 Location: Reading, Berkshire, UK. Posts: 692	Re: Dansette major rectifier Measurements are... Pin 7 = 112v Pin 9 = 104v Pin 3 = 005v Pin 2 = +0.11v Resistance says 0.250 on my cheapo dmm set at 2k Do these look ok? These readings were taken with the rectifier removed and a 1N4007 with a 100ohm resistor fitted in its place the HT across the diode now measures 137V Last edited by leslie5555; 22nd Sep 2018 at 7:08 am. Reason: More info

22nd Sep 2018, 8:25 am	#40
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Dansette major rectifier That HT seems too low to me. Surely it should be over 200v, maybe higher. If so, the readings at the valve would also be proportionally higher.