HP 204C sine generator out of spec.  Does it matter - Page 1
Forum section: electronics / repair

author: gooseEL34
author_rank: Regular Contributor
timestamp: May 23, 2016, 06:45:01 pm
content: Hey all.I have an old HP 204C that I mainly use for troubleshooting guitar amps and pedals (inject a 1 volt 1k sine into the front end of an amp or pedal and figure out where the signal stops).The problem is that the 204C has this problem:When the dial and factor switch are set to a particular frequency (say 1 on the dial, x1k on the selector) the frequency seems much slower. Now, I may be testing this wrong, but I took a Fluke with frequency counting and put it with banana plugs right into the 600Ohm outs on the front.I got the following:Lowest selector (x5), dial at 1 = 14.09 HzDial at 1 and selector at x1k = 2.76 kHzWhere I use it (1volt at 1k) is achieved by setting the selector at x100 and the dial at 3.8 (which should be giving a 380 Hz wave)Now, I am not testing with a scope and maybe when you just output the sine directly into a Fluke you affect the reading, but does this seem right?I have not opened it up. I am handy with a soldering iron, can read a schematic, and know my way around Mains voltages (and 400+ vDC tube amp B+) so feel free to suggest. I downloaded the service manual, but to service it, you need a whole bunch of test equipment I don't have.I could just leave it as is and know where I need to set it to trick it into giving me the wave I desire, but if it is just a few resistors that have floated due to age and heat, I wouldn't mind making it close to correct as possible.Thanks guysI am not an EE but have learned a bunch on this forum)

author: Andy Watson
author_rank: Super Contributor
timestamp: May 23, 2016, 09:17:00 pm
content: What test equipment do you have?If the Fluke has a frequency counter it should be good for the audio range and somewhat beyond. You should be able to test all the frequency ranges up to at least 100kHz. If this doesn't throw-up anything obvious the next step would be to start probing the power supply voltages and then various voltages around the transistors - especially the output stage - and report back.

author: dacman
author_rank: Frequent Contributor
timestamp: May 24, 2016, 02:00:38 am
content: I'd take a look at the tuning capacitor to see if it looks OK. The 204C does have some electrolytic capacitors, one is in the back. I'd want a decent LCR meter to check the capacitors, and a DMM (with a diode function) to check the resistors and transistors and diodes.

author: edavid
author_rank: Super Contributor
timestamp: May 24, 2016, 02:13:25 am
content: It shouldn't be that far off unless it's broken. You should borrow a scope and see what it's actually generating.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 24, 2016, 11:15:46 am
content: The DMM frequency readings are not especially reliable, if the waveform is nor a simple sine or square. So it might be a good idea to get a kind of scope. Even a addition to use a sound card would be enough.I won't expect a power supply problem, as this is a Wien - bridge type generator. So it is relatively insensitive to voltage variations. Bad caps in the supply might cause an extra hum, but not a frequency way off.There are not that many parts that could go wrong to make the frequency to be off by about a factor of 3 over much of the ranges.One thing might be a broken gain / amplitude control that causes way to much gain. Is the Amplitude still in specs and the output still a sine ?

author: vk6zgo
author_rank: Super Contributor
timestamp: May 27, 2016, 09:55:26 am
content: This is not much help at the bottom range where you seem to be having problems,but if there is a mechanical misalignment it may show up on the top range which coincides with the MW AM Broadcast band.Using an AM radio,set it to 1MHz,& tune the oscillator across till you hear a drop in the background noise---this should be at IMHz on the radio dial.For a more accurate check,tune the radio to a station close to 1MHz,then tune the oscillator until you hear a "beat" note interfering with the program.Keep tuning the osc till the beat note drops in frequency to the point where it becomes a low "growl",then becomes inaudible----the oscillator is now tuned to the same frequency as the radio station.Check the dial on the osc to see if it is reading the correct frequency.