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HP339a Modification Summary

Introduction

The HP339a was a state of the art distortion analyzer in the 1980s.
It was and is a full featured all-in-one product including a volt meter,
low distortion oscillator, analyzer, AM detector, and VU meter.

There was an in-depth analysis of low distortion oscillators
on DIYAudio on the following link:

<Low-disortion Audio-range Oscillator>
This was followed by a specific HP339a thread, linked here:

<HP339a>

This is a summary of those threads along with the links, conversations, and
posts, about the research and development that went into the suggested modifications to the venerable HP 339A.

Outline

Why HP 339A?

Cheapest quality instrument for DIY mods

From Post #1146:
The 339A is about the cheapest instrument that DIY'ers will ever be able to do simple mods to (parts, value changes and IC replacements) and get excellent results. However, next up the ladder of affordability might be the popular HP 8903_ or others? If the full potential can be realized in that one or other readily available used gear, what rewards will they bring? This little 339A exercise shows us there is a lot that can be had in refurbishing them for improved performance and that the fundamental design is sound.

Wide range of frequencies and input voltages

From Post #1577:
-- just change the IC's and a few caps and trimmers and you are done with the mods. Then just use its' own analyzer to tune the thd with the trimmers for a null. That's about it. The nice thing about the 339A is that it has a wide range of frequencies and it can be used as an rms voltmeter with db scales. The analyzer will accept a wide range of voltages on its input. It will give you a fast indication of the thd+n with a minimum of fuss and cost less than any other options.

We here got carried away with the oscillator and have been curious as to how low can we make it can go... far lower than the THD meter/analyzer part... but an indication well under .001 (-100dB) should show that you have a pretty good amp/preamp circuit.

Accepts up to 300 Vrms on attenuated input

Similar to Dick Moore's project

From Post #1554:
For all those thinking of using an HP339A and want to improve it:
I just looked at a similar project by Dick Moore. He copied the 339A oscillator circuit and put it into another existing chassis. Didn't use the 339A analyzer section. If you go to his site, you will get a well written, detailed summary of what to do on the oscillator. -- RNMarsh
Our colleague, Dick Moore (aka RichEEM), and his mod pages w/FFT plots:
http://www.moorepage.net/IG-18-3.html

Base line performance.

From Post #980: Low Distortion Analyzer

Stock 339A Performance Numbers -


Being able to have a wide range of test freq is a nice convenience.

So far the complete HP339A system -- reading its own oscillator at almost stock (oscillator + analyzer distortion) - with just a tune up and changing the two null trimmers (using a 3580A spectrum analyzer on the output monitor port of the 339A) -- gives a noise floor of -145. Harmonics, H2 is -105 and H3 -130. Its a good beginning and its getting better! :-) Thx-RNMarsh



Intro to the 339A boards. Unmodded
1. Pic
2. Pic
3. Pic
4. Pic
5. Pic

Modified HP339A Performance Numbers

FFT Plots w/Test Equip

From Post #2156:
The 339A is a thd+N meter. The osc and analyzer spec is .0018% or better. That is a tick below the 0.2 mark on the 0-1 scale (-80dB =1).

After opamp and parts changes and upgrades, the thd+n is near the 0 mark -- well below -100dB. This is THD+N.

Using the 339A monitor output shows the two dominant harmonics, 2H at -106 dB and 3H at -110 dB. The top is -100 dB and 10 dB/div. and measured with shibasoku 725D analyzer ported to an HP 3580A Spectrum Analyzer to measure individual harmonic and noise levels. This is a compromise tuning for a wide range of freqs; For minimum thd+n at only one freq, it can be much lower:



Disassembly and Cleaning

Things to be careful about

Careful when making adjustments that you don't short the case of the Jfet.
It's very close to one of the trim pots and if it shorts it's toast.

Stability Build Before Modding:
1. Testing and Replacing power supply electrolytic capacitors.
2. Other Platform stability building.

Modifications

Most bang for the buck.

Davada - Explains the HP339a configuration
and the limitations of the mods:

Output Level Pot

Davada - Post #48:
Davada found the output level pot to be a source of distortion. You can
find the voltage level output pot for under $10 at Mouser or other suppliers.
The pot is a Bourns cermet type, p/n 51SADU25A15L.

The output level pot is easy to change. Loosen the screws on the shaft extender and spin the nuts off. 20 min tops. The pot shaft is longer than needed so you
have to cut it to proper length about 8 to10 mm.

Oscillator and Pot change

From Post #45, #47 (HP339a):

If you wanted to do the minimum, just changing the oscillator opamp
(AD797 etc) and replace its control pot with a new multi-turn and add
another trim for the fixed resistor that is feedback on the control jFET (d-g)
with a multi-turn (for 2H only).

But, a must do is replace the output level pot. That pot introduces a lot of harmonic distortion. -- RNMarsh

Most used oscillator change

From Post #75 HP339a:
The bottom of the board is completely exposed once the analyzer is opened up.
It's a bit tight from the top of the board but all your doing is inserting
components. Just remember to remove all the compensation and clamping
components around the op amp for the oscillator.

If you use an LT1468 the compensation is wrong and not needed.
There's is not much to go wrong. Just be careful when making adjustments
that you don't short the case of the Jfet. It's very close to one of the pots and if it shorts it's toast. --Davada

FFT with These Mods

From Post #83:
FFT at this point:

A1 Board Oscillator Schematic

Davada - Post #88 Schematic

A1 Board Specific Mods

From Post #86:

The jFET feedback R51/2K.

Replace with 28 turn pot/trimmer. It is in series with a 60mfd cap (C30)..... together they go from jFET gate to its' drain. Replace C30 with 100mfd 105c bipolar.

The Oscillator Amplitude Adjust pot (R30/2K)

Replace, as it's
old, has contact issues, and makes the adjustment difficult and 'touchy'.

The output pot is R3 (10K). Replace.

Replace:
Bourns cermet, p/n 51SADU25A15L

HP ID: A1U1

Replace Oscillator with Linear Technology OpAmp LT1468. These simple mods are all you need to do to have the level of THD shown.

Notes and errata

Service/Test Notes

PhotoCell LED Measurement

Dick Moore wrote this up to help us
test the Photocells and see if they are
still good or not. Thank you Dick, R.I.P.
Link to Post#39

Basic Mumbo Jumbo
Hocus Pokus Dominatrix

Read up to about post #2300 in the low distortion analyzer thread.
Start: Photos before mods example
End: Photo after mods exemplary example
How to, that is what to do.
Parts
Resources
Work arounds
To do
To not do

Brought to you by these fine folks at DIY Audio:

Davada

Richiem, R.I.P.

RNMarsh

1Audio

et al

and a cast of 10s

on thousands of written pages

Boiled down for the average Joe by SyncTronX.

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