Philips Oszi, PM3295A power supply repair, Help needet - Page 1
Forum section: electronics / repair

author: Lenny
author_rank: Contributor
timestamp: November 08, 2020, 01:08:07 pm
content: Hello everyone, I have a defective PM3295a and can't get any further with the repair of the power supply.The power supply does not start.I found out that the Zener diode V3212 is defective and replaced it.The service manual says that the voltage at test point X30 should be set to 5V via R6318.This is my first problem. The voltage can only be regulated between 8 and 9.5V. I can't get to 5V.I checked all the components in the area. Unfortunately without success.Now I have simulated the area, and the 5v cannot be set there either. Somewhere I seem to be missing something.Does anyone have experience with the device and can help me?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 06:06:19 pm
content: I have already repaired this oscilloscope model, and even like that, I do not understand anything about your explanations- you only give a small portion of the diagram, where is the service manual?Personally, I have it in my archives, but think of the other forum participants.- The power supply for this oscilloscope is made up of two primary AC / DC power supplies of 25V each, placed in series to provide 50Vdc. and an inverter supplied with 50Vdc which supplies all the internal power supplies of the oscilloscope.Are both 25V AC / DC power supplies working?You have changed the Zener diode V3212, where is it on the diagram?Where are the point X30 and R6318.?OkQuoteIndeed, with the components we see on this partial diagram, the lowest adjustable voltage should be 8.57V....R6321 should be 100K and not 10K

author: Lenny
author_rank: Contributor
timestamp: November 08, 2020, 07:02:02 pm
content: Of course you are right. Thank you!The device consists of two primary power supply units, each delivering 25V output voltage, and a secondary power supply unit.The primary power supply units are OK and the input voltages for the secondary power supply unit are correct.The fault must be in the secondary power supply. I have attached the circuit diagram.The 12V Zener diode "V6312" was defective and was replaced.The voltages 50V, 27V and 12V are available.The test point X30 would have to be set to a voltage of 5V URef via the potentiometer R6318 (Please see 2nd attached document). And here is my problem. I can only regulate the voltage between 8v and 9.5V. Setting to 5V is not possible.Resistors R6318, R6319, R6321 and R6322 are OK and correspond to the values in the circuit diagram. I have already swapped the IC D6301 and D6302. Since there are no other affected components in the voltage regulation and I didn't know what to do now, I created the simulation described above.I found out that it is not possible to achieve the required 5v with the given components?!Do you have a suggestion how I should proceed?

author: TurboTom
author_rank: Super Contributor
timestamp: November 09, 2020, 12:53:54 am
content: If the 12V zener was defective and replaced, I'ld assume that it's not supposed to be a 12V zener. If the resistor values are as specified in the schematic, a 12V zener will not work. I'ld rather say a 6.8V zener should be approximately correct, maybe even a 6.2V version. It's well possible that the schematic is at fault.Edit: You should have googled "BZV12" -- that's not a 12V zener as the designation may suggest but a high stability/low tempco 6.2V reference!

author: Lenny
author_rank: Contributor
timestamp: November 09, 2020, 11:20:40 am
content: That's it. It was so obvious to me that I didn't check it out.Thank you so much!Now I'll look around for a replacement. BVZ12 seems obsolete.

author: 2N3055
author_rank: Super Contributor
timestamp: November 09, 2020, 11:30:59 am
content: You need 1N829 or similar. Those can be found even today.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 09, 2020, 12:19:41 pm
content: You said that the zener diode V6312 (BZV12) was faulty .... what fault did it have?In my opinion, you measured the voltage at its terminals and you found that around 6V was not correct because you were sure that it was a 12V zenerTypically, a faulty zener diode is either shorted or open.This shows that components should not be replaced hastily and that you must be 100% sure that they are defective before replacing them.The PM3295 I repaired had a shorted tantalum capacitor in a power rail.It was inaccessible and I remember I had to dismantle the oscilloscope almost entirely to replace it ...

author: Lenny
author_rank: Contributor
timestamp: November 09, 2020, 03:34:16 pm
content: V6312 was shorted and that was also the original fault.As a test, I just installed a 6.9V Zener. X30 can be set to 5v and the output voltages also match. Power supply works again.By the way, I swapped all of the electrolytic and the X2 capacitors.Now i will order the right component and hopefully enjoy the device for a long time.Many thanks for your help!