Manson EP-925 PSU - Fan trigger fault - UK Vintage Radio Repair and Restoration Discussion Forum

3Phase615V · 21st Feb 2019, 1:13 am

Hi there,

I have partially repaired a Manson EP-925 PSU that was blowing its 3A mains fuse. The fault was the 35A bridge rectifier (3504) had gone short circuit. So, I replaced the BR with a 50A equivalent (5004) which restored the PSU to (almost) normal operation.

However, having loaded the PSU for roughly 30mins and got the heatsink beyond hand-touch hot, the fan failed to kick in. On disconnecting one side of the (Normally Closed) fan thermostat to test whether or not the fan would kick in, it did not. Testing the actual fan revealed it to be working fine on an external 12V supply. TR7 (TIP31C), D11 (1N4148) and D7 (1N4001) were also all tested and found to be okay.

My next test (perhaps overdue) was to test for any voltage on the LM339 side of the thermostat without the other side connected to ground, and it's zero.
Unfortunately, it looks like I'll have have to separate the front panel from the PCB as I wasn't able to test for a supply voltage on pin 3 of the LM339, but it was a very awkward measurement so needs to be retested. My question is: assuming power is reaching or is returned to the LM339, what voltage should I expect on the LM339 side of the thermostat - pins 2 and 7 of the LM339?

The schematic for Manson EP-925 can be found almost near the top of the following link: https://pa0fri.home.xs4all.nl/Divers...5/ep925eng.htm

Thanks

PETERg0rsq · 21st Feb 2019, 11:50 am

With the switch closed you would obviously get 0V

With switch open the volts at Q7 base will depend on the motor current, as the comparitors apear to be a current limiting, for fan overload, or maybe to limit fan speed!

The lower the fan load, the higher the voltage at Q7 base will be.

David G4EBT · 21st Feb 2019, 11:50 am

It would be wiser to arrange for the fan to be on all the time, which it mostly is because it won't go off till the heatsink cools to 40C, which it rarely will do, even if only 5 Amps or so is being drawn. When the fan is off, the thermostat is closed - when the fan comes on, the thermostat is open, so if I recall, just disconnecting the thermostat will leave the fan on permanently.

The EP925 has significant design shortcomings, some of which are highlighted at the PA0FRI link you provided, where extensive modifications are discussed.

When you say you've 'loaded it for 30 minutes', be very cautious about using a dummy load such as a car headlight bulb to draw more than maybe 5 Amps. The 925 in allegedly rated for 'continuous use' but that doesn't mean 25A at 13.8V full power (the equivalent of strapping it to a dummy load at 100% duty cycle), it means 'continuous' for the purpose for which it's intended. Generally, that is to power an FM or SSB transceiver, with fairly short 'overs' of several minutes on transmit, (modulated by speech), with an equal amount of time on receive, when the EP925 can take a rest. Arguably, it brings a new meaning to 'continuous' - 'continuously intermittent'.

The EP925 (which comes with several other names such as 'Palstar'), is neat, compact, attractive and budget priced, so it's easy to see why it's been so popular over time. However, it has some quite serious shortcomings, the most serious of which is no overvoltage protection should the transistors fail, as they do. If they fail open circuit, that's fine but if short circuit, they'll plonk maybe 25V or more across the equipment to which the PSU is connected.

The fan is optimistically described as 'forced air cooling' but the reality is that the (inadequately sized mains transformer) shields the heat sinks so no fanned are reaches them - the air just blows at the transformer. What seems to happen is that because the heat sinking is so inadequate (one heatsink stacked is above the other, and quite small), is that one transistor fails, further stressing the others, so they fail too in succession.

Back on the late 90s when I was active in amateur radio and the packet network existed, at many BBS stations, which tended to use ex PMR sets, the 925 was a popular choice of PSU. The 925s often failed, maybe because of the manner of their use. Only powering 10Watt 2M & 70cm transceivers, but not used for speech - for receiving and sending packets of tones, and in use 24/7. (At its peak, the local BBS, which closed down in 2003, had some 200 regular users, sending and receiving packet messages worldwide).

Mercifully, on the EP025s that I repaired, the 2N3055 transistors had failed open circuit, but if just one had failed short circuit, unless the equipment to which the EP925 is connected can tolerate 25V or so, it's a costly outcome.

As well as the PA0FRI link and others on internet, the EP925 has been covered in several threads on the forum, including my own experiences with them and the faults that I found and repairs that I carried out:

https://www.vintage-radio.net/forum/...t=Manson+ep925
https://www.vintage-radio.net/forum/...ghlight=Manson
https://www.vintage-radio.net/forum/...d.php?t=104212

Not perhaps the succinct answer you were looking for, but I hope it's of interest.

3Phase615V · 21st Feb 2019, 4:28 pm

Thanks for your replies and I had already read several previous posts on the EP-925, including your posts David (very useful - thank you) but none describe the fan failing to activate in the way mine is. At this point I just want the fan to activate in the way it was designed to and then look to add an inline resistor with the thermostat to have it running at a lower speed continuously.

Quote:

My next test (perhaps overdue) was to test for any voltage on the LM339 side of the thermostat without the other side connected to ground, and it's zero.

So by this I meant that despite the fan being connected and there being no connection to ground, there is no power being sent to the fan. As I mentioned, the fan is not faulty so I believe the LM339 is not triggering it for some reason. With there being no voltage at all on pins 2 and 7 I'm expecting to find either a power failure to the LM339 (R13 perhaps) or the LM339 is blown.

Thanks for your thoughts folks.

3Phase615V · 25th Feb 2019, 3:56 pm

Further to my last previous message, I've repaired the fan circuitry and added a 2.2K resistor in series with the thermostat to activate the fan at a medium speed from switch-on to improve overall cooling.

I was about to reassemble the PSU when remembered that although the voltage meter indication is accurate the notch in the voltage controller, at 12 o'clock, doesn't notch at 13.8V - I have to rotate the control to about 11 o'clock to achieve 13.8V (the notch reading is about 14.2V). I looked at the VRs in the circuit but I'm not certain about the function of all of them. VR1 and VR4 look like they are for calibrating the volt and amps meters respectively, but I'm not so sure about VR2 and VR3. Any ideas anyone? Can either of these used to set the middle position of the voltage control to 13.8V or would I need to, say, replace R11 with a 20K preset and set it from there?

Also, the image here: https://pa0fri.home.xs4all.nl/Divers...25/printf2.jpg, shows "VR5" in between the preset at the top-right corner and the voltage control VR. Is the voltage control actually VR5?

Thanks!

3Phase615V · 25th Feb 2019, 10:26 pm

Actually, I worked this out: VR5 is the preset missing from the schematic and controls the DC output to the voltage control. I've successfully set the correct voltage on the centre notch with it. I misread the VR2 line as being connected with the voltage output instead of the current. VR2 sets the threshold for the over-current protection I believe.

21st Feb 2019, 1:13 am	#1
3Phase615V Tetrode Join Date: Nov 2011 Location: London, UK. Posts: 90	Manson EP-925 PSU - Fan trigger fault Hi there, I have partially repaired a Manson EP-925 PSU that was blowing its 3A mains fuse. The fault was the 35A bridge rectifier (3504) had gone short circuit. So, I replaced the BR with a 50A equivalent (5004) which restored the PSU to (almost) normal operation. However, having loaded the PSU for roughly 30mins and got the heatsink beyond hand-touch hot, the fan failed to kick in. On disconnecting one side of the (Normally Closed) fan thermostat to test whether or not the fan would kick in, it did not. Testing the actual fan revealed it to be working fine on an external 12V supply. TR7 (TIP31C), D11 (1N4148) and D7 (1N4001) were also all tested and found to be okay. My next test (perhaps overdue) was to test for any voltage on the LM339 side of the thermostat without the other side connected to ground, and it's zero. Unfortunately, it looks like I'll have have to separate the front panel from the PCB as I wasn't able to test for a supply voltage on pin 3 of the LM339, but it was a very awkward measurement so needs to be retested. My question is: assuming power is reaching or is returned to the LM339, what voltage should I expect on the LM339 side of the thermostat - pins 2 and 7 of the LM339? The schematic for Manson EP-925 can be found almost near the top of the following link: https://pa0fri.home.xs4all.nl/Divers...5/ep925eng.htm Thanks

21st Feb 2019, 11:50 am	#2
PETERg0rsq Heptode Join Date: Jun 2009 Location: St Helens, Lancashire, UK. Posts: 641	Re: Manson EP-925 PSU - Fan trigger fault With the switch closed you would obviously get 0V With switch open the volts at Q7 base will depend on the motor current, as the comparitors apear to be a current limiting, for fan overload, or maybe to limit fan speed! The lower the fan load, the higher the voltage at Q7 base will be. __________________ SPECIALIST.....Knows everything about nothing EXPERT..........Knows nothing about everything

21st Feb 2019, 11:50 am	#3
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: Manson EP-925 PSU - Fan trigger fault It would be wiser to arrange for the fan to be on all the time, which it mostly is because it won't go off till the heatsink cools to 40C, which it rarely will do, even if only 5 Amps or so is being drawn. When the fan is off, the thermostat is closed - when the fan comes on, the thermostat is open, so if I recall, just disconnecting the thermostat will leave the fan on permanently. The EP925 has significant design shortcomings, some of which are highlighted at the PA0FRI link you provided, where extensive modifications are discussed. When you say you've 'loaded it for 30 minutes', be very cautious about using a dummy load such as a car headlight bulb to draw more than maybe 5 Amps. The 925 in allegedly rated for 'continuous use' but that doesn't mean 25A at 13.8V full power (the equivalent of strapping it to a dummy load at 100% duty cycle), it means 'continuous' for the purpose for which it's intended. Generally, that is to power an FM or SSB transceiver, with fairly short 'overs' of several minutes on transmit, (modulated by speech), with an equal amount of time on receive, when the EP925 can take a rest. Arguably, it brings a new meaning to 'continuous' - 'continuously intermittent'. The EP925 (which comes with several other names such as 'Palstar'), is neat, compact, attractive and budget priced, so it's easy to see why it's been so popular over time. However, it has some quite serious shortcomings, the most serious of which is no overvoltage protection should the transistors fail, as they do. If they fail open circuit, that's fine but if short circuit, they'll plonk maybe 25V or more across the equipment to which the PSU is connected. The fan is optimistically described as 'forced air cooling' but the reality is that the (inadequately sized mains transformer) shields the heat sinks so no fanned are reaches them - the air just blows at the transformer. What seems to happen is that because the heat sinking is so inadequate (one heatsink stacked is above the other, and quite small), is that one transistor fails, further stressing the others, so they fail too in succession. Back on the late 90s when I was active in amateur radio and the packet network existed, at many BBS stations, which tended to use ex PMR sets, the 925 was a popular choice of PSU. The 925s often failed, maybe because of the manner of their use. Only powering 10Watt 2M & 70cm transceivers, but not used for speech - for receiving and sending packets of tones, and in use 24/7. (At its peak, the local BBS, which closed down in 2003, had some 200 regular users, sending and receiving packet messages worldwide). Mercifully, on the EP025s that I repaired, the 2N3055 transistors had failed open circuit, but if just one had failed short circuit, unless the equipment to which the EP925 is connected can tolerate 25V or so, it's a costly outcome. As well as the PA0FRI link and others on internet, the EP925 has been covered in several threads on the forum, including my own experiences with them and the faults that I found and repairs that I carried out: https://www.vintage-radio.net/forum/...t=Manson+ep925 https://www.vintage-radio.net/forum/...ghlight=Manson https://www.vintage-radio.net/forum/...d.php?t=104212 Not perhaps the succinct answer you were looking for, but I hope it's of interest. __________________ David. BVWS Member. G-QRP Club member 1339.

25th Feb 2019, 3:56 pm	#5
3Phase615V Tetrode Join Date: Nov 2011 Location: London, UK. Posts: 90	Re: Manson EP-925 PSU - Fan trigger fault Further to my last previous message, I've repaired the fan circuitry and added a 2.2K resistor in series with the thermostat to activate the fan at a medium speed from switch-on to improve overall cooling. I was about to reassemble the PSU when remembered that although the voltage meter indication is accurate the notch in the voltage controller, at 12 o'clock, doesn't notch at 13.8V - I have to rotate the control to about 11 o'clock to achieve 13.8V (the notch reading is about 14.2V). I looked at the VRs in the circuit but I'm not certain about the function of all of them. VR1 and VR4 look like they are for calibrating the volt and amps meters respectively, but I'm not so sure about VR2 and VR3. Any ideas anyone? Can either of these used to set the middle position of the voltage control to 13.8V or would I need to, say, replace R11 with a 20K preset and set it from there? Also, the image here: https://pa0fri.home.xs4all.nl/Divers...25/printf2.jpg, shows "VR5" in between the preset at the top-right corner and the voltage control VR. Is the voltage control actually VR5? Thanks! Last edited by 3Phase615V; 25th Feb 2019 at 4:23 pm.

25th Feb 2019, 10:26 pm	#6
3Phase615V Tetrode Join Date: Nov 2011 Location: London, UK. Posts: 90	Re: Manson EP-925 PSU - Fan trigger fault Actually, I worked this out: VR5 is the preset missing from the schematic and controls the DC output to the voltage control. I've successfully set the correct voltage on the centre notch with it. I misread the VR2 line as being connected with the voltage output instead of the current. VR2 sets the threshold for the over-current protection I believe.