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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 10th Nov 2019, 6:40 pm   #1
Electronpusher0
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Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
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Default Wayne Kerr RA200 Frequency Response Analyser repair

This post describes my repairs on a Wayne Kerr Frequency Response Analyser I acquired through the forum from Gonzini.

I will start with the most important finding for those who do not want to read the entire article.
The all the CRT voltages are referenced to a point referred to in the service manual as “COMMON” (pins 107,108,109 on the power supply board). The following are connected to this “COMMON” point:
• The DAG coating of the CRT
• The spring clip to the DAG mounted on one of the CRT retaining bolts (by insulating post)
• The green wire from the DAG connection to pin 5 of the CRT socket (no pin on the CRT itself)
• The 0v of the EHT module
THIS “COMMON” POINT IS NOT 0V! It is connected to -21V via R133, 47ohm, on the power supply board.
Connecting any of the above items to 0V or chassis, say by connecting an earthed ‘scope, WILL cause R133 to burn up and char the pcb.
To make matters worse the “COMMON” point described above is connected to an insulated standoff connector on the underside of the chassis by a pink wire. Also attached to this standoff is a decoupling capacitor.
The problem is that the standoff is only a few mm away from the bottom panel of the case which is uninsulated. If the unit is placed onto anything that puts pressure onto the centre of the bottom panel the panel can touch the standoff and short the capacitor to ground, again burning R133.
In my unit I have glued a piece of insulating board to the base of the case to prevent this happening.

To return to my report on repairs.
I purchased the unit as not working, visual inspection showed an area of burn on the PSU board, the HT regulator section. The culprit was a R133, as described above. The burn was so severe that the pcb was completely carbonised under the resistor.

I have cut out the burn and glued in a piece of fibreglass board to mount the components on. R129 and the EHT adjustment pot R130 were also burnt by R133 and had to be replaced.
I ran the EHT module from an external 24V supply and it generated the 300v rail and the feedback voltage seemed in the right order so fingers crossed.

The missing / burnt components on the EHT regulator were fitted to the repaired pcb and the unit run up slowly on a variac.
Unfortunately the magic smoke was let out on the CRT drive Amplifier, this was traced (sit and watch the smoke) to R468 on the X drive amplifier.
This resistor is in the emitter circuit of the drive transistor. Further testing showed that the drive transistor, TR463, 2N2905 was short E-C and that one of the output transistors, TR467, TIP30, was short B-C – see attached schematic.
I did not have a TIP30 to hand but did have TIP32 and TIP31 handy, so I replaced both TR466 and TR467 with the uprated types.
The board was refitted and the unit run up slowly again, no smoke this time.

Returning to the EHT section of the power supply board all the voltages were correct, including the EHT feedback, voltage and adjusting the pot varied the +24V feed to the EHT module so all seems well.
I then followed the operation manual to run some basic functional tests on it.
However I was unable to get a trace.
Testing the output of the unit with a scope showed that it is generating a swept frequency sine wave as expected.

The grid of the CRT is fixed at the “Common” voltage (-21V, see above) and the Cathode voltage is set by the brightness control and modulated to effect blanking. After probing the Cathode drive circuit I eventually traced the lack of trace to the rear switch marked CRT Drive INT/EXT, the contacts were very tarnished.
After stripping and cleaning the switch I managed to get a trace on the screen, however this was not horizontal across the screen but at quite an angle.
Examining the scan coils showed that they were rotated and away from the tube body by some 10mm. I checked the manual which confirmed in the calibration procedure that the coils should be tight up against the tube body. I suspect that the unit has been dropped on its back at some point.

I have decided that the only proper way to proceed now is to follow the complete calibration from the beginning. This requires inserting signals and dc voltages into the external control socket on the rear.
Unfortunately this is a 26 way Centronics type socket and the plugs are unobtainium. Since not all the pins are used I decided to change this for a 24 way Centronics socket for which plugs can be obtained. There is a danger that this could be confused with a GPIB / HPIB socket so I will label the socket with a suitable warning.
As I have posted in the “Sets, Parts and Service Information Offered” section I have the manual and have scanned it. If anyone wants a copy PM me.

Peter
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Old 10th Nov 2019, 6:42 pm   #2
Electronpusher0
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Default Re: Wayne Kerr RA200 Frequency Response Analyser repair

Another view of the "Common" (-21V) stud that can be shorted and the X drive schematic

Peter
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Old 10th Nov 2019, 9:16 pm   #3
Gonzini
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Default Re: Wayne Kerr RA200 Frequency Response Analyser repair

That's great Peter, glad you have had the time to get stuck in with it.
Interesting about the stud that could touch the metal base plate, bit of a design over sight there i guess.
No idea what happened to it originally, i wondered if the EHT module had caused it but luckily by the sounds of it its good.
The service manual came along some time after i got hold it, things had moved along in that time and i was using a PC with spectrum analyser software / interface by then so the interest / time required in getting it going turned it into a shelved project.
I think it was back in 2004 / 2005 when it originally landed!
Will be following the progress as and when.
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