What transistors are in the HP461A 10x 100x amplifier ? - Page 1
Forum section: electronics / repair

author: MathWizard
author_rank: Super Contributor
timestamp: January 09, 2024, 03:21:59 pm
content: I've found 1 or 2 copies of the manual and it's hard to make out the BJT model's. I only care about the input and output buffers, and the 5 amplifiers.I'm just trying to understand the biasing, and calculate a few likely currents. Some voltages are listed, if they are to be believed, the 1st buffer would have a V_EB of 0.35V, but then they also say the 2nd last amp would have a killer VBE of 9.4V, and the last buffer would be off w/ -1.5VBE.In LTSpice most of the collector voltages are around 4V, but I want to know the correct transistors to use in a model.I want to make a version with S9018's, I just got 100 legit SMD ones. In LTSpice I had it working more or less, but there was a lot of ringing past 50MHz, like +/-10dB for some inductor values

author: Hamelec
author_rank: Regular Contributor
timestamp: January 09, 2024, 03:34:27 pm
content: see pdf, page 27 and page 31 - change#1

author: wasedadoc
author_rank: Super Contributor
timestamp: January 09, 2024, 03:45:38 pm
content: That -9.1V at Q8 emitter is obviously an error. Like all of Q4 to Q7 it should be about -0.8V

author: Hamelec
author_rank: Regular Contributor
timestamp: January 09, 2024, 04:06:00 pm
content: just measured : Q8 emitter -1.07V

author: MathWizard
author_rank: Super Contributor
timestamp: January 10, 2024, 06:36:32 pm
content: Yeah thanks I missed a better pdf copy was posted on another thread. I'm surprised there's so little mention of the probe anywhere on this site, seeing how nice and simple the layout is, and it's all big old parts. Maybe there's lots of similar stuff.1 of my new hobbies should be looking up old scope accessories that aren't too hard or expensive to make.I see the main amplifier blockis supposed to have 8.4dB gain for each stage. And the 50 ohm matching makes the buffer's lose 2dB each. So best I can do it try to match something in LTSpice with my parts, and try a copy. Soon I'll have a bunch of 0805 LCR parts including lots of sub uH inductors. I have some very small tune-able inductor form's, I hope they will do for a few adjustable inductors.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 10, 2024, 08:35:19 pm
content: I don't think the votlages noted are all correct. With a current flowing into the base of the transistors the base voltage should be slightly negative and not +0.1 V. Some -1.1 V for the emitter makes absolutely sense.

author: factory
author_rank: Super Contributor
timestamp: January 10, 2024, 09:56:16 pm
content: We seem to have 3 different circuits, the complete diagram in the OP is for the oldest version, which has some Ge transistors, including Q3 and the PSU ones. And 2 later ones all Si, one of which has half the diagram missing & different voltages reading around Q9.David

author: MathWizard
author_rank: Super Contributor
timestamp: January 11, 2024, 10:15:40 pm
content: Ok thanks I never checked the other schematic's yet, I figured it was a better copy. For the voltages, some mix up at the printer's I guess.My SMD LCR parts won't be here for ages, so I'll probably try a version without 50 ohm's input/output, for now, and even if it only gets to 50Mhz, that's fine.