PM5324-HF Oscillator schematic/PCB anomaly. - UK Vintage Radio Repair and Restoration Discussion Forum

djsbriscoe · 19th Jan 2020, 5:03 pm

I have been trying to fix the instability in the HF Oscillator section of my PM5324 RF Generator. I have come across a diode where there should be a resistor.
The first anomaly is that Resistor 621 is 5K1 on the schematic but is listed as 24k (under unit 2 heading on page 47 of the service manual, its called unit 1 on the schematic) in the parts list. Also, diode 406 is shown as 400 on the layout drawing. And the polarity of the diodes is shown reversed (diode 407 and 407 ARE the right way around on the actual PCB though and the voltage drops are correct at 0.76v).
The second anomaly is that there is a diode in place of the resistor(621) on the actual PCB. The diodes cathode is facing the cathode of diode 407 and its anode is facing resistor 615. This anomalous diode has a voltage drop of 3.71v and appears to be reverse biased. The above diode voltages are measured with a diode tester on a multimeter with the unit switched off.
I am wondering why this diode is here at all as it's a completely different device than a resistor.
Is this a modification introduced by Phillips and not shown on the schematic? Or is it that someone has wrongly interpreted the schematic at build time or in an attempted repair? I have attached a photo (part has a black triangle mark next to it on the PCB). Also attached is the schematic showing which part I am referring to. Could someone please have a look at their unit to confirm that there should be a RESISTOR (and it's value please) and NOT a diode in this position. I have checked at least 3 times to make sure I am not imagining it. Thanks.

David.

djsbriscoe · 19th Jan 2020, 11:33 pm

Here is a screenshot of the board layout. The resistor is highlighted in yellow.

djsbriscoe · 25th Jan 2020, 4:05 pm

The 5K1resistor has been replaced a BZX75C 2V8 "Stabistor" which is probably now known as a zener diode. This was a very specific mod and probably done at the factory to aid stability. It also has a better spec than most. Problem is a replacement goes for £10 on that auction site.
I am going to replace it with a BZX55C2V7 if it turns out to be faulty otherwise I'll leave well enough alone. I've put the zener on the reverse of the pcb for the time being so I can swap it out more easily. More later.

djsbriscoe · 25th Jan 2020, 8:44 pm

I've been doing a bit of reading up on the Stabistor. It appears (and you can actually see this through the glass of the diode) to be a diode that has 4 stacked PN junctions inside. So it is NOT the same as a zener diode as I thought it was. When you try to test it with a diode tester it does not conduct (unless it's knackered). I'll have to experiment with it.
In the meantime i've bought a few BZV87-C2V6 Stabilising Resistor Diodes to try out as well.

Chris55000 · 26th Jan 2020, 3:13 am

Hi!

Unlike a conventional zener–diode, a "Stabistor" is operated in the forward direction, so if you tried a (selected!) zener you would have to fit an ordinary zener the reverse way round to the "Stabistor" PCB marking!

Please note also that a "Stabistor" has a very limited reverse voltage rating – NXP quote it as "10V max", any more than this and you risk destroying it!

Chris Williams

19th Jan 2020, 5:03 pm	#1
djsbriscoe Pentode Join Date: May 2015 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 133	PM5324-HF Oscillator schematic/PCB anomaly. I have been trying to fix the instability in the HF Oscillator section of my PM5324 RF Generator. I have come across a diode where there should be a resistor. The first anomaly is that Resistor 621 is 5K1 on the schematic but is listed as 24k (under unit 2 heading on page 47 of the service manual, its called unit 1 on the schematic) in the parts list. Also, diode 406 is shown as 400 on the layout drawing. And the polarity of the diodes is shown reversed (diode 407 and 407 ARE the right way around on the actual PCB though and the voltage drops are correct at 0.76v). The second anomaly is that there is a diode in place of the resistor(621) on the actual PCB. The diodes cathode is facing the cathode of diode 407 and its anode is facing resistor 615. This anomalous diode has a voltage drop of 3.71v and appears to be reverse biased. The above diode voltages are measured with a diode tester on a multimeter with the unit switched off. I am wondering why this diode is here at all as it's a completely different device than a resistor. Is this a modification introduced by Phillips and not shown on the schematic? Or is it that someone has wrongly interpreted the schematic at build time or in an attempted repair? I have attached a photo (part has a black triangle mark next to it on the PCB). Also attached is the schematic showing which part I am referring to. Could someone please have a look at their unit to confirm that there should be a RESISTOR (and it's value please) and NOT a diode in this position. I have checked at least 3 times to make sure I am not imagining it. Thanks. David. Attached Thumbnails

19th Jan 2020, 11:33 pm	#2
djsbriscoe Pentode Join Date: May 2015 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 133	Re: PM5324-HF Oscillator schematic/PCB anomaly. Here is a screenshot of the board layout. The resistor is highlighted in yellow. Attached Thumbnails

25th Jan 2020, 4:05 pm	#3
djsbriscoe Pentode Join Date: May 2015 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 133	Re: PM5324-HF Oscillator schematic/PCB anomaly. The 5K1resistor has been replaced a BZX75C 2V8 "Stabistor" which is probably now known as a zener diode. This was a very specific mod and probably done at the factory to aid stability. It also has a better spec than most. Problem is a replacement goes for £10 on that auction site. I am going to replace it with a BZX55C2V7 if it turns out to be faulty otherwise I'll leave well enough alone. I've put the zener on the reverse of the pcb for the time being so I can swap it out more easily. More later.

25th Jan 2020, 8:44 pm	#4
djsbriscoe Pentode Join Date: May 2015 Location: Hemel Hempstead, Hertfordshire, UK. Posts: 133	Re: PM5324-HF Oscillator schematic/PCB anomaly. I've been doing a bit of reading up on the Stabistor. It appears (and you can actually see this through the glass of the diode) to be a diode that has 4 stacked PN junctions inside. So it is NOT the same as a zener diode as I thought it was. When you try to test it with a diode tester it does not conduct (unless it's knackered). I'll have to experiment with it. In the meantime i've bought a few BZV87-C2V6 Stabilising Resistor Diodes to try out as well.

26th Jan 2020, 3:13 am	#5
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,866	Re: PM5324-HF Oscillator schematic/PCB anomaly. Hi! Unlike a conventional zener–diode, a "Stabistor" is operated in the forward direction, so if you tried a (selected!) zener you would have to fit an ordinary zener the reverse way round to the "Stabistor" PCB marking! Please note also that a "Stabistor" has a very limited reverse voltage rating – NXP quote it as "10V max", any more than this and you risk destroying it! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!