Problems with a HP 204c Sine Oscillator - Page 1
Forum section: electronics / beginners

author: requim
author_rank: Regular Contributor
timestamp: December 31, 2011, 01:57:19 am
content: I've been playing around with my sine oscillator a bit since I realized what I had been doing wrong (see previous post on the subject). However I have since realized that the air variable capacitor on the unit was bad (the fins on the second half of it were bent and rubbing on the neighboring fins. Apparently they're not supposed to do that.). Thankfully I have a spare unit that I had planned to restore so I swapped the good one into the unit.Last night and today I was doing more testing and realized two more problems exist:First - It doesn't start producing a sine wave until it has been on for about 10 minutes or so. Is there something I can do about that?Second - The way the dials are setup it has a knob to set the frequency range: x5 x10 x100 x1k x10k x100k and another dial that ranges from 1 - 12. Everything works fine on ranges x100 - 100k. However on ranges x5 and x10 it works fine from 1 - 5.5 or so but everything > 5.5 generates a straight line.. Any ideas on what the issue could be?

author: amspire
author_rank: Super Contributor
timestamp: December 31, 2011, 02:25:47 am
content: It may be possible to straighten the variable capacitor fins, if you are careful. You may need some good long nose pliers.Anyway, it looks like something is not 100% right with the AGC (automatic gain control). To get a very low distortion sinewave, the AGC has to adjust the oscillators gain to be exactly unity. Over unity and you have a square wave. Below unity and you have nothing - a straight line.Can you do what I suggested several times previously? Capture a copy of the schematic, measure DC voltages around the circuit, and add the voltages to the schematic. Particularly voltages of the AGC - the FET and the circuit connected to the FET's gate. The problem is that it looks like the loop gain of the oscillator is a bit low.Richard.

author: requim
author_rank: Regular Contributor
timestamp: December 31, 2011, 03:16:22 am
content: Absolutely. I'll try to take the measurements tonight.

author: requim
author_rank: Regular Contributor
timestamp: December 31, 2011, 09:07:30 am
content: So I tried to take some measurements but I kept getting non-repeatable results. I set the range to x5 the freq dial to 12 and the amplitude to max. I have attached images of the schematic and the board layout. Can you please give me specific places to test? You mentioned the AGC, fet and the circuit connected to it's gate, but I'll be honest I can't identify what is you want me to measure. There are multiple things on the schematic related to AGC spread all over the place and I can't figure out which ones I need.

author: amspire
author_rank: Super Contributor
timestamp: December 31, 2011, 11:35:44 am
content: See how the schematic has DC voltages on it? Measure the same voltages with respect to ground. If a voltage is varying, tell us the variation. Whenever you see voltages on a schematic, they are important.If you want to measure anything else, that is good. The more, the better. Use a graphics editor to put them on the schematic in red or something.If we have actual numbers, we can start to help.Richard.

author: requim
author_rank: Regular Contributor
timestamp: January 02, 2012, 08:47:40 pm
content: Well I experienced a bit of a setback a few days ago on the project. I shorted something out when I was probing due to the awkward angles I had to probe and now nothing works. If I'm lucky it's just a single transistor and possibly a resistor, but I haven't had a chance to check on it yet. I'll post when I have more information.