Hitachi HA-4700 Repair and Restoration

[ubergeeknz]

Hi All,

Fairly new here 😁 thought I'd share here some of my story repairing and restoring a "parts" HA-4700 from TradeMe. If anyone's interested.

When I got it, the lights were on but there was no output at all.

What I found was one channel with the output emitters sitting around 8v, the other at the + rail more or less. Dead output on that one I thought. But it turned out that every single 1/4w "fusible" resistor in the thing was either high Z or open. This was the same problem as was found in the excellent YouTube video by JoulesperCoulomb which was what prompted me to start checking them.

Replacing them all with standard metal film resistors ("fusible" resistors in those values proved hard to come by and normal metal film resistors should work well enough at failing open if need be) brought it to life except for a several megahertz oscillation (another problem also found in the video! These must be reasonably common issues) which was due to some of the (weird) axial caps gone bad. Replaced those with mlcc type and that sorted it! Adjusted the bias and DC offset and was away. A few dry joints fixed along the way and of course new thermal paste for the power trannies.

Knowing it was serviceable, I moved onto cosmetics, restoration and tidy up. So far
* Replaced 8200uF bulk caps with 10000uF CDE caps. They actually tested fine at over 7000uF apiece but 8200 seemed kind of small for a 50W amp anyway.
* Replaced dead "TAPE" bulb with yellow LED
* Replaced incorrect 1N4004 diodes on indicator/selector board with the correct 10v zener
* Removed case bottom for corrosion and paint repair

Still to do:

* Replace other indicators with LED
* replace a switch I broke 😭 which is going to be tricky
* polish up the chromed buttons and knobs best I can, they're a bit corroded (green spots)
* Clean up rust on case and repaint
* Clean up rust on transformers and paint
* Receive new volume knob (was missing)
* Reassemble!

If people are interested I have loads of photos so... Let me know if you wanna see them.

Here's some pics of the state I got it in... Fairly rough:

"Parts" Hitachi HA-4700 Original State - Google Photos

[Osvaldo de Banfield]

Is the thing working in its original state, after replacement of faulty parts?

[ubergeeknz]

Originally Posted by Osvaldo de Banfield

Is the thing working in its original state, after replacement of faulty parts?

Yes it is working after replacing just the 1/4w resistors and a couple of capacitors (the Miller caps near the inputs of the output stage) and touching up a few dry solder joints. I suspect there is a tendency in these to build up heat between the heatsinks in the output amp stage, especially if something else is sat on top, the case has a very low profile and there aren't many other places for the heat to escape. This unit was assembled here in NZ, at least partially, this will be due to tough import levies in place at the time.

[Osvaldo de Banfield]

I work repairing inductrial equipment here in Argentina, and it is very common problem the solder joints fatigued and broken, with intermitent contact.

The question arises because I was to suggest that previously to start modding, was better to put it working as is, and once the thing jobs OK, then start modding. In any case, try one at time, so it is easily to go back if something goes wrong.

[ubergeeknz]

Originally Posted by Osvaldo de Banfield

I work repairing inductrial equipment here in Argentina, and it is very common problem the solder joints fatigued and broken, with intermitent contact.

The question arises because I was to suggest that previously to start modding, was better to put it working as is, and once the thing jobs OK, then start modding. In any case, try one at time, so it is easily to go back if something goes wrong.

Exactly my approach

[ubergeeknz]

Here it is working after the minimum repairs, playing some DSOTM at "hi neighbour" levels in my garage

Shared album - Dan Capper - Google Photos

[ubergeeknz]

Did some rust stripping tonight... Must have been dripped on there's even a hole. I've worked on cars with less rust

Shared album - Dan Capper - Google Photos

[ubergeeknz]

Between much difficulty in replacing the switch and trying to figure the best way to tackle the very rusted out top cover, I decided I would polish the buttons. First some ultra fine steel wool to take off the corrosion spots, then a car polish, then turtle wax scratch and swirl remover which is a fine polish and wax finish. Turned out not bad. Still need to do the smaller ones and tone controls when I get time, but should be easy.IMG_20190601_140237.jpgIMG_20190601_141354.jpg

[ubergeeknz]

Today, as I wait for new switches since the small ones I ordered aren't going to work, I replaced the remainder of the input selection indicators with LEDs. Turned out fairly neat I think. I also gave the front panel a light polish and wax. Looks like one side of the plastic trim needs retrobrite, I may attempt it tomorrow if the weather stays good and I can find what I need.IMG_20190602_154846.jpgIMG_20190602_164313_126.jpg

[ubergeeknz]

Since most of the other photos are just Google links I'll post them a few here for completeness...

1. Small buttons mostly polished
2. Transformers stripped and painted
3. Whole unit after changing the bulk caps
4. Old busted fusible resistors in the power amp
5. One channel with new resistors installed
IMG_20190601_210903.jpgIMG_20190523_205955.jpgIMG_20190522_225001.jpgIMG_20190409_220958.jpgIMG_20190511_144730.jpg