Author Topic: Wavetek 2520a 0.2-2,200MHz RF sig gen repair  (Read 3729 times)

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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #25 on: May 31, 2021, 05:45:35 am »
Hi

I may have found the source of the next fault.
The built in diagnostics routine for testing the DACs helped confirm that the DAC/OpAmp combination IC506 DAC1020 and IC513A  5532 shown on the lower part of the schematic are not responding to the digital inputs.    The VCO drive voltage should range from 2VDC to 12VDC.  I am seeing a constant 1.2 DCV.

I am reading 0.7V DC across pins 2,3 of the Op Amp, which points to that being the failed part.  I am going to look for a suitable op amp in my parts bin.  That is the simplest, cheapest, fastest way forward right now. 

I have considered using the SSA as a sig gen but my main effort has been towards the digital and low freq (<10MHz) circuits.  I haven't even measured the outputs of the 4x VCOs to see if they are working.

The main reason I have been working on fixing the Wavetek 2520a is to gently exercise the brain as part of my recovery from a major road accident in Feb 21 that nearly killed me.    That is also why it has taken so long to get to where I am fixing the 2520a.   Doing diagnostics on some old test gear has been therapeutic.   

Dazz
« Last Edit: July 10, 2021, 10:10:38 am by dazz1 »
 
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #26 on: June 02, 2021, 09:49:11 pm »
Hi
Achieved a major step forward today.
I replaced a suspect op-amp that is between a 12bit DAC and the VCOs.  The DAC sets the coarse control frequency in the main control loop.  The op-amp outputs the VCO frequency control voltage.
Another DDS loop sets the fine frequency.

I also replaced a number of tant caps.  The same type and voltage that have already failed.  That should improve future reliability.

After replacing the op-amp, the AutoCal routine No.19 for tuning the VCO frequencies passed.  This proves that the Sig Gen is functioning. This does not eliminate a fault somewhere degrading the RF output.

Next step is to do performance testing and calibration to prove full operation.

Dazz
« Last Edit: July 10, 2021, 10:10:53 am by dazz1 »
 

Offline tautech

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #27 on: June 02, 2021, 09:51:36 pm »
Cool Dazz, your therapy is working !  :)
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #28 on: June 02, 2021, 10:25:23 pm »
Cool Dazz, your therapy is working !  :)

Hi
I know I am not operating within spec, but the Wavetek 2520a just might be.

Dazz
« Last Edit: July 10, 2021, 10:11:09 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #29 on: June 03, 2021, 08:02:23 am »
Hi
I connected up the Spec Analyzer and got a good output signal during AutoCal.  I left the 2520a to run for a couple of hours to stabilize and ran AutoCal again.
While I was running AutoCal, the output RF went to zero and the AutoCal routine failed.

I have another fault but at least I know that it was working.  The symptoms are identical to the previous fault so there may be a deeper issue.
First thing I will look at is the power rails.  It is possible that one of the many remaining Tants has released their magic smoke.

Dazz
« Last Edit: July 10, 2021, 10:11:23 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #30 on: June 07, 2021, 08:14:51 am »
Hi
I removed and isolated the main board.  That takes about half an hour.   I applied power and the positive input power is shorted.  None of the +/-15V voltage regulators or caps are warm.  It is most likely a shorted cap on the input side of the voltage regulators.  I have already replaced 4 but there are plenty more to fail.

I have decided to go through and replace all tant caps on the +/-18 volt supply rail.    This includes replacing the no-name-brand new tant caps I sourced from the local hobby electronics shop. 
You may be wondering why I didn't do this from the start.

When I started, I didn't know if the 2520a was fixable.  It is old enough to include obsolete irreplaceable parts.  The eprom code and LCD displays cannot be replaced.
Now that I have had the 2520a running for at least a couple of hours before faulting again, I know that it is fixable. 

I also plan to replace the battery backed ram.  At 32 years of age, it should not still be working.
I am trying to find someone somewhere who can burn the eproms.   That process would give me a backup of the eprom contents. 

It is going to take me a while to get the parts and complete the install so this topic will probably be silent for a while.

Dazz
« Last Edit: July 10, 2021, 10:11:40 am by dazz1 »
 

Offline TheDefpom

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #31 on: June 10, 2021, 09:43:37 am »
How many EPROM's are there? I could only see one from the photos and diagrams you posted, and that was a 27C64.
Cheers Scott

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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #32 on: June 10, 2021, 10:46:10 am »
How many EPROM's are there? I could only see one from the photos and diagrams you posted, and that was a 27C64.
Hi
There are five.  1x 64 and 4x 128.
I have not been able to find an accessible eprom programmer.  Even if I did, I suspect find a PC that can run the code would be a problem.

Dazz
« Last Edit: July 10, 2021, 10:12:02 am by dazz1 »
 

Offline tautech

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #33 on: June 10, 2021, 08:27:26 pm »
How many EPROM's are there? I could only see one from the photos and diagrams you posted, and that was a 27C64.
Hi
There are five.  1x 64 and 4x 128.
I have not been able to find an accessible eprom programmer.  Even if I did, I suspect find a PC that can run the code would be a problem.

Dazz
Do you have a W10 PC ?
If so Defpom and I can make it happen.
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #34 on: June 11, 2021, 12:30:28 am »
Hi
W 10PC = Windows 10 PC ??   Yes I have several. 
Code that I wrote for 8086 PCs in the 1980's and 1990's  won't run on Win10 without some retro conversion software.

Dazz
« Last Edit: July 10, 2021, 10:12:19 am by dazz1 »
 

Offline tautech

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #35 on: June 11, 2021, 01:29:12 am »
Yep Windows 10.
Sent you an email you might like to study.  ;)
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #36 on: June 15, 2021, 09:53:29 pm »
Hi
I found the next fault.  Another shorted Tant cap.
The Wavetek seemingly has hundreds of them (only slight exaggeration).   I narrowed down to the main board (there are about 10pcbs)
I used the finger test to conclude that none of the voltage regulators were supplying a dead short.
This left just 5 candidates, one had already failed and been replaced.
After lifting the 3rd cap, I found the faulty one.

All of the failed caps have been on the +18V supply. I have now replaced all of the Tant caps on the +18V DC supply.  These were all rated 25V.  The new caps are 35V.

The power supply includes a soft start feature.  I am going to see if this is working properly.    If it is I will look at modifying it to make it a little softer to reduce the surge current seen by the Tant Caps.  Surge current is a known killer of Tant caps.

The 2520A has just passed the autocal process.  This is a good indicator that the 2520A is functional. 

The next steps, in no particular order, are to:

  • make a copy of the eproms holding the firmware.

    erase and re-burn the eproms

    replace the NVRam with a DS1225AD modern version

    investigate the soft start, and modify if required

    do a full calibration

If there is any interest, I am thinking of doing a video on the repair.

Dazz
« Last Edit: July 10, 2021, 10:12:30 am by dazz1 »
 

Offline wn1fju

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #37 on: June 15, 2021, 10:24:48 pm »
Almost sounds like Wavetek got a marginal batch of tantalums when they assembled your unit many years ago.  I've had a few (non-Wavetek) pieces where there were an excessive number of bad tantalums, all of which suspiciously were the same uF value.  It was much more than an isolated bad tantalum that one might expect.  Who knows?

As I said in my earlier post, I had a big problem with the connector between the main transformer and the rest of the power supply.  There clearly was enough arcing going on to burn the contacts.  You might inspect your unit just to be sure. 

So far, I've not had any tantalum failures in my 2510A.  But then again, I've probably only turned it on five times in the last year.
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #38 on: June 16, 2021, 03:14:32 am »
Hi
I suspect that 18V was just enough to kill the 32 year old Tant caps.   The same make/value Tant caps on 15V have survived.    The only difference is +3V and the 7815 regulators in the path of the surge current.  The same tant caps on -18V are OK.
There are 2x +18V supplies, and the failed Tants are connected to the same +18V supply.    I am going to test the hypothesis that the soft start is only fully active on one of the +18V supplies.

Dazz
« Last Edit: July 10, 2021, 10:12:45 am by dazz1 »
 

Offline HandWave25

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #39 on: June 16, 2021, 07:01:46 am »
I am going balder by the minute with a Wavetek 2505 (.1-550MHz) Sig Gen. I am guessing it is similar to the 200MHz one. I have a "tripped" text on the "Level" display and I get no RF out. Can it be reset somehow? or where do I look to fix? Can't find a manual anywhere on-line. :-(      rodg
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #40 on: June 16, 2021, 08:06:53 am »
I am going balder by the minute with a Wavetek 2505 (.1-550MHz) Sig Gen. I am guessing it is similar to the 200MHz one. I have a "tripped" text on the "Level" display and I get no RF out. Can it be reset somehow? or where do I look to fix? Can't find a manual anywhere on-line. :-(      rodg

Hi
I don't have any info on a 2505 but if the circuit is similar, then the attached may help.
The reverse power relay should be reset by cycling the power.

Wavetek give away an on-line book with all the Operator manuals   https://www.wavetek.org/.    Probably only of some value to you. 

 
Dazz
« Last Edit: July 10, 2021, 10:13:02 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #41 on: June 17, 2021, 10:33:17 pm »
...    I am going to test the hypothesis that the soft start is only fully active on one of the +18V supplies.

Dazz

Hi
Testing shows that both soft start circuits are working properly.  The RC time constant is 39ms, which is slow enough to make any surge current more like a crawl in electrical terms.
I have looked at the Wavetek 2520a output on the spectrum analyser and an initial look indicates the 2520a is functional.

Dazz
« Last Edit: July 10, 2021, 10:13:12 am by dazz1 »
 

Offline tautech

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #42 on: June 17, 2021, 10:38:38 pm »
Cool.
All that remains is to backup the EPROMs then so will see you next weekend to help with that.  :)
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #43 on: June 18, 2021, 09:39:57 am »
Hi
I have been looking at the output of the 2520a to see if it is working OK.  This is really the first time I have used the spectrum analyser connected to the Wavetek.  All of the faults found have been well within the freq range of my ancient oscilloscope.
The output looks like it is well within factory specs.  In all cases looked at so far, it exceeds specs.  In all cases, there is a 10dB attennuator between the 2520a and my spectrum analyser. 

I have found another fault.  The nvram does not retain memory.  The one good thing about this is that the failed nvram has not bricked the 2520a.  It appears that the calibration programs generate everything stored in the nvram.  Without nvram, there is no point in doing a full calibration.  The settings would be lost on power down.

So the next step is to buy new eproms and a new nvram.

Dazz
« Last Edit: July 10, 2021, 10:13:27 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #44 on: June 18, 2021, 10:08:27 am »
Hi
One of the calibration routines steps the control voltage on the 4x VCOs.  The nvram then stores a lookup table.  When the user selects an output frequency, the control system does a look up to get the rough frequency, selects the right VCO and sets the control voltage.  This is open look control.  The DDS then uses closed loop control to fine tune the output frequency.

The spectrum monitor output shows the No.19 routine stepping through the control voltages to generate the lookup table.

Dazz
« Last Edit: July 10, 2021, 10:13:39 am by dazz1 »
 
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #45 on: June 18, 2021, 10:12:31 am »
Hi
Here are some outputs showing the harmonics generated by the 2520a at different selected frequency outputs.    They are consistent across different frequencies.

Dazz
« Last Edit: July 10, 2021, 10:13:52 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #46 on: June 21, 2021, 07:10:19 am »
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



« Last Edit: July 10, 2021, 10:14:03 am by dazz1 »
 

Offline tautech

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #47 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.
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #48 on: June 21, 2021, 09:28:23 am »
Nice.
You wouldn't expect problems from tants in a coupling role whereas across a rail decoupling is another matter.

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
« Last Edit: July 10, 2021, 10:14:20 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #49 on: June 22, 2021, 04:31:33 am »
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



« Last Edit: July 10, 2021, 10:14:32 am by dazz1 »
 


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