HP 3312A small repair

 No big troubleshooting here, the HP3312A function generator had been developing small fault over time, the last time I used it was not outputting signal or with the wrong shape, so decided to see what could be done.

This was the "sinusoidal" output:

 

Not good! Also the frequency was not in line with the bezel markings.

Here's the offending component on top board near the front panel.

I just guessed it was the one causing the issue, the crack was easy to spot.

The reference marked in the capacitor, if needed:

TRW 8508
HEW-331
.5MF +/- 10%
50 VDC

Better now after replacement, I didn't had 0.5MF as originally so by mistake placed a 2.2uF, got better but still some distortion seen:

 

 I then replaced by a 1uF one and now it's perfect, in the future I will probably try to find the right value (0.5uF) and replace.

Have a nice day!

EIP-371 Source Locking Microwave Counter repair

 I've done a previous repair on this device because I got it without being working on the 18Ghz range. I suspect this will not be the last one given it's age.

Anyhow, one of this days I turned it on to check the output frequency of a FVC99 module and it was displaying all entrance selector LED's on (should be one at a time) and no change on the input by pressing the band selector. Also there was no activity on the display for the source locking.

I suspect the usual bad contacts (it's slot based construction) or power supply, more to the power supply side since the equipment hadn't been moved (it's a bit sensitive on moving/vibrations and that had been the cause of the first repair).

 First thing I did was removing and inserting all the boards one by one trying to find the one responsible for the selector LED's, I have the manual but didn't feel like to read it. In the end, had no luck. Then I realized there's another board that does not slide, instead is attached to the side panel. So I moved that one to the side and checked all the connections. 

 Image, now, after connecting back, started working, I can now select the input and have the digits lit:

I can measure again but I'm preparing a home made counter in case this one fails again in the future.

Have a nice day!

Wavetek 3001 signal generator repair - II

 Following the first repair of the Wavetek 3001, I knew that there was another issue with the 400Hz modulation oscillator, it was doing some "FM" oscillations so in practice I could only use the 1Khz tone.

Troubleshooting and comparing with the 1Khz oscillator, it was visible the waveform at C6/CR2 junction oscillating/FM'ing a little. Interesting enough by probing with the scope at the drain of Q2 it disappeared. I tried first to swap C6 to no effect, then swapped CR2 and it stopped but trying with the diode in and inverted polarization it would continue the same so it was either wrong assembled at the factory or it's for the moment just masking any other situation, probably Q2.
Before the previous testing I replace IC2 with IC1 (for the 1Khz) to no good effect so decided the issues was not the opamp.

I left like that, with the diode replaced (used a 1n4148 for the moment, didn't  even checked the exact spec in the manual)

After demodulation on an envelop detector it's now fine:

..envelope detector circuit and input signal (-25dBm) without the 56 ohm input load resistor connected on the circuit:

..I might start an AM receiver from here...

Since it's a bit of a pain to take and access the modulator/modulation board I opted to solder the diode from the pcb track side and leave it for the moment, also left the original diode in place with one leg open:

Since some times the unleveled light still comes on I suspect further recapping at the internal modules might be needed in the future.

Have a great day!

Wavetek 3001 signal generator repair

 Got this Wavetek 3001 signal generator just for testing the FM deviation meter since I haven't here my other signal generator and it was the cheaper unit that could generate a signal changing the deviation I could find in the usual online flea market.

Was expecting it to be fully working but unfortunately the listing  didn't matched the reality. Instead of sending it back I decided to try an fix it.
It was not giving any output and the indication meter for the output signal was not moving, also touching the back right panel where the power supply dissipation is, it was getting alarmingly hot after some seconds of power on. Testing the power lines for the + and negative 18V it was showing close to 0V... so either the supply was dead or there was a short. Because it was the two supply lines I suspected initially the output transistors of the supply.

 Looking at the service manual and some web resources was easy to see the 18V power supply lines in short or not working could be due the usual short electrolytic capacitors.

Two great resources I could find with a lot more information than this post are the following ones:

http://k5jxh.com/wave-tek/3006.html from K5JXH for the repair of a similar unit, a Wavetek 3006

and:

http://users.monash.edu.au/~ralphk/wavetek.html by Ralph Llimek on a repair of also a similar unit, Wavetek 3000

In my repair the only hard part was to find which module is the one at fault, if not multiple. That was my approach because looking at the power supply I could not find any fault (top right part vertical PCB).

 

 

My approach was to removed 5 random ones and see. I choose the first 5 on the bottom from left to right. What I did also was to put labels on all the cables for future reference although after unbolting them and turning the box upside down they started to fall with the cabling attached, saving me some time. I was lucky the first time, with 5 modules removed it was back to life, that is, there was no longer a short on the power supply and the output metter moved, then I installed one after the other until find the culprit, it was module M29-2, see bellow.



The modules are easy to remove from the chassis, just unbolt the bottom screen and pull them from the top side.

Inside the module I could see a previous repair had been done on one of the supply lines capacitor because of the two different type, a more modern black on the right and the original one on the left. You can see also the small choke in series to the supply lines, positive and negative.



After measuring, it was confirmed as a plain short, so it was removed and replaced (not the same value but works anyhow):

 In the mean time I bough some more spares for future replacement.

I suspect it's also developing and issue on the 400Hz audio oscillator part, 1Khz is good but that will be for another day. Here's the two oscillators schematic in question:


Have a good day!