Trying to repair a 403LA RF amplifier  - Page 1
Forum section: electronics / repair

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 30, 2015, 04:16:22 pm
content: I step on a vintage 403LA RF amplifier tried to make it work, but obviously something goes wrong.I dismounted it and realize that the very simple power supply :transformer, rectifiers, big cap.provides a voltage of 46 V to the main board.Is this normal ?This seems to me somewhat high compared to the rating at 50 V of the main electrolytic caps(which are OK) of the PSU and board.Thanks

author: Mr Simpleton
author_rank: Unknown
timestamp: May 30, 2015, 10:21:54 pm
content: I do have the bigger brother 603L (3w and up to 1GHz) and schematics... The voltage should be 22V and this is also used in the smaller brother 300L. So my bet is 22V on yours too!Search for ENI 603L and you will find user manual that contains schematics. Should be pretty similar to yours.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 31, 2015, 12:59:20 pm
content: Thanks a lot,This makes it much more clear. There was no schematics in the available 403LA docs I found. They were probably too recent. I suppose that in some oldest docs, the schematics will be there as well.I realized I made a mistake. I took the voltage after the rectifiers, but before the regulator.It was indeed 46 V, but only 22 V, as expected, after the regulator, with 2.5 mV rms ripple (on the voltmeter).Now I can go beyond the PSU and search for what is wrong in my unit.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 31, 2015, 04:03:32 pm
content: I have made some progress, but now I am stucked.Below is a picture with in red all the points where I made a oscilloscope measure of a sinusoidal signal that is input to the amplifier.The number refers to the picture of the scope.Things seems to be OK up to 26, but deteriorate at 27 and 28.The transistors are the CP2240 but I was not able to find any information on them.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 31, 2015, 04:32:50 pm
content: Is there a bias voltage at 28? What about the voltage at both sides of the 47 Ohm resistor at the top left?

author: Mr Simpleton
author_rank: Unknown
timestamp: May 31, 2015, 05:52:02 pm
content: As this amplifier runs class A you should verify that the DC voltages are correct.If a transistor fails it should show on voltages. Only rare that all DC voltages are OK and still fails at RF.Most likely it is a transistor, but of course a passive component may have bitten the dustGood Luck!

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 31, 2015, 05:58:07 pm
content: Quote from: Mr Simpleton on May 31, 2015, 05:52:02 pmYeah, that's basically what I posted

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 31, 2015, 09:09:26 pm
content: Thanks to both Mr Simpleton and PA0PBZ !As I am really new in this game, any advice is appreciated.I have checked the voltages and they look quite OK21 : 1.34 V22 : 14.9 V23 : 1.66 V24 : 15. 4 V25 : 2.79 V26 : 19.16 V27 : 2.35 V28 : 2. 64 Vand across the 47 Ohms resistor : 22 V and 21.26 VAs all the voltages, and additional measures I did were looking quite reasonable, I wonder if I had put the proper frequencyfor the input signal.Indeed, It is written 39Db from 0.15 to 300 Mhz, and I had put a 10 khz signal, with the habit that this should be OK for any amplifier. But I was wrong.When I increased the frequency in the nominal range, everything was much better, and in the nominal range.I could try up to 30 Mhz and on the lower level up to 10 khz, but on this side, it deteriorates rapidly below 15khz.So it seems that it is working finally. The good thing is that I have a much better idea of how it works now.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 31, 2015, 09:22:28 pm
content: I gathered the data of the measures I made into this TableSo the amplifier is OK (39 DB announced) in its nominal range (0.15- 300 Mhz). At least tested up to 30 Mhz.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: May 31, 2015, 10:37:34 pm
content: Another way to see the previous data

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: June 01, 2015, 07:38:33 am
content: An update of the previous plot.

author: Mr Simpleton
author_rank: Unknown
timestamp: June 01, 2015, 09:46:07 am
content: Glad it is working... yes as you noted it will drop fast if you go below therated lower frequency limit! interstage capacitors and ferrite cores do not like it <100 kHz