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.

 

[Tubetec]

Ive had the HP4378 multimeter powered up for a month solid now .
gradually the amount of drift with room temperature variation came down to around 1/10th of an ohm on the 1kohms range , with a 600 ohm resistor .
I didnt change a single component , nor did I attempt to touch the calibration settings ,
The readings Im getting now are in close agreement with my fluke handhelds on both DC and ohms ,
at this stage theres no point in me touching the cal unless I get a more precise reference .

I didnt do much in the way of AC testing yet but its performance in terms of noise and bandwidth is far beyond any multimeter Ive owned before .
My advice is if you can find one of these that passes power up tests for a good price ,get it ,
Even though the design dates back to the mid 80's the straightforward user interface with no menus makes it a pleasure to use .

 

[Brian Roth]

I can only find HP 3478 online...hmmm.

Bri

 

[Tubetec]

Your right Brian ,
3478 it is .

 

[Brian Roth]

I'll take a look at that model, for sure!

Bri

 

[Tubetec]

A few surprises turned up in the yard today ,
an Altec lansing graphic eq , I imagine made sometime is the late 70s
I took a look inside and it was all gyrator based (no inductors), balanced in and out but transformerless ,
I didnt have any use for it so I took the mains transformers off to keep for project usage ,
One is your typical low voltage EI made by Peerless , the other is a quite nice Avel Lindberg heavily sheilded 15va toroid , 2x15v .

The other thing I found ,which presumably came from the same person was an old 70's style live mixer , ITAM London 10/4
I removed the end panel to see what was inside and low and behold , 10X Beyerdynamic 'peanut' mic input transformers , TR/BV351001061
Also another two of the same Avel Lindberg Toroids , 2x12v and 2x20v models .
I might return tomorrow and remove the limiters and maybe a few channel PCB's , but I cant take the frame .


https://reverb.com/uk/item/70453634...e-studio-mixer-mixing-desk-console-stand-loom

I feel a bit sad now about to butcher the thing , maybe I should give Ted Fletcher shout and find out if its of any historical interest to him ,
It has 4 band eq and by the looks a direct out ,pre fader and mixbus returns , The PCB layout looks good , thick traces on FRP boards so easy to repair .

What I dont take tomorrow will surely be sent off to the crusher ,
The picture below is not the actual unit . Its almost identical to the Alice 828 .

 

[ruffrecords]

What a waste.

Cheers

Ian

 

[rock soderstrom]

A few surprises turned up in the yard today

Very nice! The mixer is a real beauty, it would be really sad if this will end in the crusher!

 

[Tubetec]

I was able to confirm the Beyer input transformers are 1:1 1200 ohm ,
they top out at 300mV max @50hz .

I can probably take out a few extra channel modules if anyone wants to do a trade , Pm me if you do .

Oh I forgot to say , last week I picked up a working Zoom Q3HD that someone threw away .
I also found a non working H2 recorder .
There was an announcement in the media today about the size of the electronics waste pile growing , no surprise there .

 

[13engrsapper]

I love trash finds.

I've got a 64 Blackface Fender Bassman head, the last of the Pre CBS, it was a garbage pile find while driving home locally. It needs some love, but it was all there.

 

[KA-Electonics.com]

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

tubetech linked me to this thread and Gold's and your post warranted a late reply.

Some of the HP400's capacitors, in the solid state versions, are wet slug tantalum and are "special." Also super expensive. Not sure what the requirements are that made HP go with such esoteric caps but the last time I looked a single cap was around $35. That was prior to 2020 and runaway inflation. Perhaps it was a super low leakage requirement. IIRC they are in the meter bridge/feedback circuit.

I have several HP400s and the best advice I can give is do not replace them but leave the unit powered continuously so they can reform and stay reformed. On first power up, and for several days afterwards, the meter will appear noisy with a bouncing pointer. Eventually they settle down as mine did. I have two on the bench that have been (almost) continually powered for years. When we have power failures they go bouncy-bouncy for a few minutes/hours after recovery. What I've said of course applies to wet slug tantalums that haven't leaked. None of my 400's have done that.

Other capacitors in the unit, PSU for example, can be replaced with modern electrolytics as you suggest.

 

[Cqwet Dbdfte]

Which version of the Hp400?
What positions are those wet slugs in?
Many of the caps used were paper, unless hermetic in oil, they should be chucked. Most old electrolytes were junk by todays standards.

 

[KA-Electonics.com]

Which version of the Hp400?
What positions are those wet slugs in?
Many of the caps used were paper, unless hermetic in oil, they should be chucked. Most old electrolytes were junk by todays standards.

My recollection for the HP400F and HP400FL meters is that they are in the bridge and elsewhere. On the schematic I have, if I recall correctly they are C38 and C39.

Had a quick look at the parts list and it appears that C32, C33, C34, C38, C39 and C40 are all tantalums. I would have to open it up and confirm that the smaller 4.7 uF isn't a bead tant. Now that I think about there are definitely more than two. Once I saw the astronomic replacement cost I decided it was better to keep the originals formed by continuous power on. I recapped the PSU with conventional axial lead electrolytics.

There are paper caps or paper/oil in the older vacuum tube models I'm sure. I have a couple of those in the garage.

The HP400FL has a "log" (the L in the suffix) linear-in-dB 12 dB display range. If you look for a log converter in the meter you will not find one. It's in the actual movement and is somehow mechanically implemented perhaps by multiple springs.

 

[Cqwet Dbdfte]

If there is room PP caps could provide very low leakage current.