Electronics > Repair
Wavetek 2520a 0.2-2,200MHz RF sig gen repair
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dazz1:
Hi
Here are some outputs showing the harmonics generated by the 2520a at different selected frequency outputs.    They are consistent across different frequencies.

Dazz
dazz1:
Hi
I found another fault. 
The AM/FM internal modulation isn't working.  I quickly traced the problem to the op-amp pair that converts the square wave digital signal to a sine wave. 

There are two things that helped narrow down the fault.
1. Binary division of the signal chain.
2. Parallel operation of the 400Hz and 1000Hz signal chain.

There is a long chain of components that process the 400Hz and 1000Hz signals that modulate the RF output.  Rather than following the signal path from one end to the other, I started in the middle of both.  This just happened to be at the junction between the digital and analog signals.    If I had found that the digital signal was faulty, I would have jumped half-way toward the input/origin of the modulation signals.    I did find the signals were OK so I jumped half way towards the end of the signal path. Repetition of this binary division along the signal path quickly isolated the fault to IC107 (Fig 4.2), a dual 5534AN low noise op amp.  I have already replaced another of the same op-amp type elsewhere.

I also compared the 400Hz and 1000Hz signal paths because they are the same except for frequency.

The signal path is spread across two circuit diagrams, not including the control signals originating from the 6805.    Following the signals from end to end would have taken ages and been very inefficient.   In contrast, binary division to isolate the fault was quick. 
Comparing the 1000Hz and 400Hz signals confirmed the diagnosis beyond doubt.

Both filter inputs include 1uF tantalum Caps (C171 and C172).  These are DC blockers and I checked these for shorts.  The same component type has failed on the 18V rail, but none have failed on the 15V supply.  They are OK.

I think this brings the fault count to about 6. 

I am aware that AM/FM modulation isn't used much these days but it would annoy me if I didn't fix something that I can fix so easily.  The Wavetek 2520A includes an external modulation input which opens the possibility of other modulation techniques being applied to the RF output.

Dazz



tautech:
Nice.
You wouldn't expect problems from tants in a coupling role whereas across a rail decoupling is another matter.
dazz1:

--- Quote from: tautech on June 21, 2021, 07:53:23 am ---Nice.
You wouldn't expect problems from tants in a coupling role whereas across a rail decoupling is another matter.

--- End quote ---

Hi

But it only took a few seconds to check that they were OK.  If one was faulty and not checked, I would have dived down a rabbit hole.

Dazz
dazz1:
Hi
Although I found a fault, it wasn't the right one.  Attached are images of the 400Hz and 1000Hz modulation signals from different dual op-amps.   Both signal paths show a DC shift but the 1kHz signal looks better than the 400Hz.  I thought the problem was the dual opamp in the 400Hz signal path but this turned out to be wrong.   

Although the 1kHz signal was there, I did not see any AM modulation on the spectrum analyzer when I had it connected up.  Although I found a fault, I looked further down the signal path to see if there was another one.  I looked halfway along the down stream side of the signal path and did not see a signal.    It didn't take long to figure out that all of the analog devices were not working correctly. 

It turns out that by choosing the meeting point  of the digital and the analog signal I had landed right on top of the first symptom of a fault. What I failed to do was re-check the +/-15V voltage regulator output to the analog devices.  When I did check, I found that the +/-18V supply (that I had just fixed again) was all good, but the +15V regulator output was about 0.1V.     There a few 15V regulators spread across the PCBs so the 2520a mostly works OK.  It is just that the modulation function isn't working.

 It is almost certain that I have at least one shorted Tant cap on the +15V supply.    That is a first for this 2520A.  It is not that I have never checked the 15V supplies.    I have previously checked  the 15V regulators and they were all cold with good outputs.  Now one of the +15V regulator is running hot.  This is a fault that has developed since I started fault finding.

Binary division doesn't work for finding one faulty cap in a whole lot of parallel connected caps.    It looks like several designers have been responsible for the main board.  Each designer has added 15V and 5V regulators to power their circuits.  This reduces the number of capacitors I will need to lift and test to find the faulty one.


Dazz



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