Flip side of the chassis. Besides regulated +-15V rails there's also some electronics that's connected to the external control connection
Plugged it in, line filter didn't blow up. Filter caps are fine, <25mV RMS ripple on -15V, <5mV RMS on +15V. There is no RF output from the upper unit, and only a CW at around 11 Mhz from the lower. No sweep. Adjusting the sweep width adjusts the frequency a bit. Time to trouble shoot with no schematics
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I've been procrastinating on posting what I've been up to. Let me try to recall where I was at with this repair
I reverse engineered the sweep generation circuit.
It gets its clock from the line voltage. There is a tap on the transformer for that. Starting in the upper left corner. The signal gets limited with a diode, then squared off with those two transistors. A 14538 dual mono stable vibrator is used to set the duty cycles.
One of those two signals then gets buffered, fed into an integrating capacitor, into a Schmitt trigger and eventually into an integrator to produce the ramp voltage needed for the sweep.
It gets its clock from the line voltage. There is a tap on the transformer for that. Starting in the upper left corner. The signal gets limited with a diode, then squared off with those two transistors. A 14538 dual mono stable vibrator is used to set the duty cycles.
One of those two signals then gets buffered, fed into an integrating capacitor, into a Schmitt trigger and eventually into an integrator to produce the ramp voltage needed for the sweep.
There are five problems here.
1. I have no documentation for this unit. It has an external control connector, here labeled as J103, which would be cool to know the purpose of.
2. Knowing that would help me understand the purpose of the second signal generated by U101B and the MOSFET section.
3. The dual NAND gate 40107 is bad. It's got open drain outputs. I don't have a drop in replacement on hand. Still available on Reichelt though.
4. The value of C111 is quite large, making the charge curve incredibly flat and the adjustments very touchy.
5. The whole circuit is ass. You cannot control the sweep frequency, you have no external trigger. It's fixed at line frequency
1. I have no documentation for this unit. It has an external control connector, here labeled as J103, which would be cool to know the purpose of.
2. Knowing that would help me understand the purpose of the second signal generated by U101B and the MOSFET section.
3. The dual NAND gate 40107 is bad. It's got open drain outputs. I don't have a drop in replacement on hand. Still available on Reichelt though.
4. The value of C111 is quite large, making the charge curve incredibly flat and the adjustments very touchy.
5. The whole circuit is ass. You cannot control the sweep frequency, you have no external trigger. It's fixed at line frequency
I got it to sweep again but I was not happy because of the inflexibility of the whole unit
So I started to consider rolling my own circuit to get more out of it.
But is this really worth the effort? And how much effort do I want to put in there? Am I chasing some autistic obsession of the day down a rabbit hole with little to no gain?
As much as I was tempted to go full autismo on that and put a microcontroller in to control the sweep...
I want to eventually get a modern AWG, making this whole box obsolete
But is this really worth the effort? And how much effort do I want to put in there? Am I chasing some autistic obsession of the day down a rabbit hole with little to no gain?
As much as I was tempted to go full autismo on that and put a microcontroller in to control the sweep...
I want to eventually get a modern AWG, making this whole box obsolete
Little side project: messing with the TBA120 IF amplifier and FM demodulator, introduced in the 60s
Blue is the modulation signal, yellow the demodulated output.
This thing does 60dB amplification and the perfboard is not being kind to it
Blue is the modulation signal, yellow the demodulated output.
This thing does 60dB amplification and the perfboard is not being kind to it
Bun's Lab
So I started to consider rolling my own circuit to get more out of it. But is this really worth the effort? And how much effort do I want to put in there? Am I chasing some autistic obsession of the day down a rabbit hole with little to no gain? As much…
What do? Not that I have the time for that until next year
Anonymous Poll
28%
Fix it up as it was intended. Leave it be
12%
Add a simple op amp based sweep generator circuit
24%
Computerize the thing, go full autismo
36%
Show results