Two great pulls from the scrap heap

[JohnRoberts]

I have an HP 400 that only works after it has been on for a couple of days. The electrolytic caps look like nothing else I've ever seen. Can they be replaced with normal high quality electrolytic caps?

I don't know what a HP 400 is but capacitors are still capacitors.

Modern capacitors are generally better, smaller, and lower ESR. I might avoid the extremely low ESR caps in some applications.

If you can read the values off the old parts, maybe measure them first to see how bad they are, and identify the weak sisters to replace first.

JR

 

[Tubetec]

Hi Paul ,
Id have a look at the HP400 manual ,
theres lot of good fault finding tips and spot checks that can help determine the cause of problems .

My HP4378 has settled down very nicely over the last few days being powered on ,
it reads very slightly high on ohms range compared to the fluke 187 ,
51.211 ohms vs 51.15
I read in the manual that HP cal is performed at 23 degrees C and ambient indoor temperature here now is only around 15 C , they speicfy a range of +/- 5 degrees C ,which Im obviously outside.
so anyway I'll need to wait until the weather warms up a bit before I taking a trustworthy reading .

One word of warning about precission test gear that uses dual or triple section caps in the PSU , they may be relying on the precise physical properties of that cap to meet the noise performance or other spec , the metal can and concentric wound elements can provide sheilding and screening that discrete modern components cant ,

The Mallory brand metal can multicaps seem like the modern equivalent to whats used in both the Tek and HP units I have , unfortunately most now come with fast-on terminals , not PCB mounting .

 

[RoadrunnerOZ]

Nagravox have recap kits for all the HP400 series (they also sell the meters) you click on your model then click on the condition tab and choose recap kit only (not exactly cheap though):
https://nagravox.com/collections/to...-calibration-kit-for-most-other-tape-machines

 

[Gold]

Nagravox have recap kits for all the HP400 series (they also sell the meters) you click on your model then click on the condition tab and choose recap kit only (not exactly cheap though):
https://nagravox.com/collections/to...-calibration-kit-for-most-other-tape-machines

It looks like the recap kit uses commonly available electrolytic caps. The ones in the original had a form factor I’ve never seen before. They looked expensive. I’m definitely not paying $200 for a recap kit. I probably have everything I need already. Now all I need is time and will power.

 

[Cqwet Dbdfte]

I went thru one of those beasts a few months ago that I had sitting in my garage for 30 years. Some film caps were OK but I changed all caps, film and electrolytics. I also replaced the rectifier tube with 4007 and 360 ohm chassis mounted resistors, and used 280uF main +B cap.
One tube was bad, the oscillator pentode, had an equivalent spare.
Not happy with the amplitude drift I also added a silicon +B voltage reg.
It still drifts but much less. After about 3 hours it is stable. The tempco on those old carbon comp resistors are not stellar and may need replacing.
The tubes are original HP labeled with 1957 dates. The manual brags about 5 - 10 years of great electrolytic capacitor life.

 

[Gold]

I went thru one of those beasts a few months ago that I had sitting in my garage for 30 years. Some film caps were OK but I changed all caps, film and electrolytics. I also replaced the rectifier tube with 4007 and 360 ohm chassis mounted resistors, and used 280uF main +B cap.
One tube was bad, the oscillator pentode, had an equivalent spare.

Mine is a solid state version. I have a tube Waveforms 520A which I like better than the HP400 I have. It's more compact and also has a buffered output to feed a scope on the front panel.