peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
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Post by peter on Sept 12, 2021 4:29:40 GMT 12
I've just acquired one of these off trade me and while it gives beautifully clear sine and square waves their amplitude pulses slowly up and down when seen on a scope. The meter also swings slowly up and down. I think it's probably the power supply section and perhaps a capacitor reforming - any thoughts on the subject/previous experience?? I think I'll leave it on power for say 20 mins to see if it stabilises or not. Heathkit sig gen.pdf (221.52 KB) |
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Post by Peter Walsham on Sept 12, 2021 8:54:14 GMT 12
Good morning Peter
I have a copy of the complete manual for this instrument along with a copy of some modifications to the original design, which address some design issues with the instrument. I would be more than willing to send copies of these to you - if you would like them?
Peter W
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
|
Post by peter on Oct 10, 2021 14:40:50 GMT 12
Good morning Peter I have a copy of the complete manual for this instrument along with a copy of some modifications to the original design, which address some design issues with the instrument. I would be more than willing to send copies of these to you - if you would like them? Peter W Many thanks Peter but I already have the manual and article on the 'greening'/improvements of this sig gen. |
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
|
Post by peter on Oct 14, 2021 0:23:43 GMT 12
All finally sorted - continuing weird signals were due to the Oscilloscope probe being intermittant/dicky - last thing I expected. Once I'd replaced that and recalibrated the unit again, lovely clear sine and square waves appear of the right frequency and consistent amplitude. Because of the dicky scope probe I don't know exactly when it was fixed but I replaced the filter caps in the power supply, the voltage regulating transistor and a 47 ohm resistor that had drifted to 80+ ohms. One the main board I replaced both 250uf 35 dc electrolytics as well with 220uf 35vdc as the 0.680uf 35 dc with a 50vw version. I suspect once the voltage regulating transistor and high resistor in the power supply were replaced, the problem was fixed as it was the former playing up that caused identical signal amplitude problems on a you tube video.
The scope probe was working fine at the outset so it's subsequent demise was just one of those fun things that can happen during servicing - what made me realise eventually was that the output signal voltage measured on an ac multimeter (part of the calibration proceedure) showed 9 volts ac (as did the output from a known good sig gen) but the scope showed barely any signal on the 10mv sensitivity scale. Quick ohms check on the probe showed it to be open circuit.
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Post by Peter Walsham on Oct 14, 2021 7:51:39 GMT 12
It's not easy when your test equipment lets you down. I have an RCA senior Voltohmyst VTVM that I use a lot. When I got it, it was doing really 'stupid' random & intermittent 'things', which I tracked down to leakage in the range & funtion selector switches - as well as the printed circuit board. I ultimately discovered the problem by removing all the components off the printed circuit board, and checking leakage across various PCB tracks using a 1000 Volt insulation tester. The leakage problem was solved by putting the PCB & the rotary switches in to an ultrasonic cleaner and using isopropyl alcohol as the solvent. Needless to say - I am guessing the leakage was probably caused by a leaky battery at some time in the VTVM's life - although there was no sign of any visible corrosion anywhere to be seen. Having said all of that I, more recently, had funny things happen with my old analogue 'bench basher' Sanwa multimeter. I was working on a valve radio with a dickey power transformer which I had substituted, what I thought, was a suitable replacement. The Sanwa was giving a different (higher) reading on the 1000 Volt AC range that I had expected it to, but the B+ voltage was reasonable. The Sanwa meter goes from a 250Volt range to a 1000 Volt range, and the voltage I was reading was over scale on the 250 Volt range (but not by much), yet the meter was giving me a reading of over 400 Volts on the 1000 Volt range. After checking multiplier resistors, the germanium rectifier diodes etc, and even suspecting the 'varistor' I 'discovered' that the voltage reading that the meter was displaying was different at times. When I 'huffed' on to the battery holder, the reading went 'mental. I removed the battery holder & cleaned this. Reassembled the meter - hey presto - no more stupid readings! I have even had recent problems with a Telequipment oscilloscope that I bought new in 1976 - and has given me terrific service - until it stopped working. Ideally, one needs a 'scope to fix a 'scope but, in the absence of another, I had to rely on passive testing. This, ultimately, revealed one of the timebase multivibrator transistors was leaky. Damn test gear!!
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
|
Post by peter on Oct 17, 2021 5:06:09 GMT 12
My classic 'hair tearing out moment' was with a nice old Avo8 mk 3 I'd got for $25 off an auction years ago. Started off fine but every now and again it gave me a weird reading. This eventually became so bad it was time to investigate. Nothing seemed to affect the reading systematically and all resistors etc checked out fine. I was coming to the reluctant conclusion that the meter movement itself had a problem when I noticed that face down on the bench it gave consistent readings in a mirror under the meter face.
As the only other bits it was resting on were the red and black terminals, a close look at them was in order. Sure enough the red socket was very, very slightly loose and the lead inside to its post had a tiny hairline fracture where it was soldered onto the post. 5 mins work with a soldering iron fixed it forever. The slightly loose socket allowed the hairline crack to open and give a high resistance when it was upright and the lead plugged into it moved but inverted and resting on the terminals closed the crack.
Most times finding/fixing a fault is ho hum but I felt really pleased with finding that particular little horror!
Another job was with a transistorised rf sig gen that some poor soul had owned and gone almost bonkers over - it came with pages of notes as well as test point measurements noted on the circuit and he'd clearly spent hours trying to figure out the problem. He sold it to me cheap with the problem described but unsolved - at the top end of the frequency ranges it's signal was fine but as you moved down it sharply dropped off to nothing. The first thing I did was an ohms test on the probe lead and the central single wire core was open circuit - it had broken off just beyond the solder point on the plug that went into the unit and presumably acted as a capacitor for the really hf. Repairing that fixed the problem and gave me a nice solid state unit. That quick diagnosis was just pure luck but it made up for some of the longer jobs.
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