Problems with EICO HF-12 amp.

[DragonMaster]

I've got problems with one of my HF-12 amps, so I decided to replace the caps finally, especially wax paper ones.

When opening the amp I found two wires short(The wires melt together) that were just before a wax paper cap that already leaked in the preamp circuit.

This cap is in the signal path right before the tape out / power amp in and the first preamp tube.

Also, when I'm selecting tape head, I hear a continuous squeeee, which is so loud I hear it pretty good at minimum volume.

When is was on the kitchen table to see why it had problems, this amp 's tranny compared to the other made a loud hum and vibrated a lot more.

When the volume is at minimum, I hear tons of noise and when on maximum, as much as the other, even if the audio is the lowest at minimum and highest at max.

I think I should replace all the preamp caps because I tested all the tubes and they're all OK.

Scheme is at http://users.rcn.com/fiddler.interport/HF12.HTM, click on schematic next to the model name.

Note, the way I wrote might be strange, I re-edited a reply I made in electronic parts.

[anatech]

Hi DragonMaster,
That sounds like a good start. The switches and controls sound like they need cleaning as well.
I rebuilt completely an HF-87a. Very rewarding. The most important thing is to make sure the components are placed exactly where they were. Do not increase the coupling caps sizes very much. It's not neccessary.
If you can, measure the resistors and change them if they are out of tolerance. Don't go nuts on metal film types. The Japanese ceramic types do fine. You can increse the wattage rating to get more voltage breakdown resistance on the new types. Match R22 and R23 as close as you can.
-Chris

[DragonMaster]

The switches and controls sound like they need cleaning as well.

Pots aren't scratching at all, for the volume pot I don't know what it is,

The Treble pot might be changed by a Honeywell Clarostat 53series (2" shaft) and the knob too in this case. One of the power switch pins is broken, so there's 3 wires twisted together to make it always on(It was like that before I opened it).

I choose the 53 series because they can accept a rotary switch and have a 2" shaft, but I don't see how the switch thing work.

[anatech]

Hi DragonMaster,
The input selector switch sounds like it is noisy. I figured that if it is noisy, others might be also. The "squeeee" noise may be the preamp oscillating because there is poor contact on the eq. section of the input selector. De-ox-id is my chemical tool of choice. Active may carry it.
Try using a decent stereo pot with another switch for power. The build up controls you are looking at do not track L-R very well.
-Chris

[SY]

Change out the power supply electrolytics, the volume controls, and the rectifier, then see if the transformer is still humming away.

All pots will probably need to be replaced, too, but some might be hard to source these days. Clean all switches thoroughly.

At that point, you can survey what you've got and see if it's worthwhile doing component upgrades.

[anatech]

Hi Sy,
Coupling caps may as well be done now too. That way there will be something to check out.
-Chris

[SY]

ana, d'accord. Especially those awful old paper caps.

[DragonMaster]

The build up controls you are looking at do not track L-R very well.

No problems with this, the amp is mono.

These seem to be the ONLY pots with 2" shafts.

Since I saw how shafts are supposed to be measured(It's 2", not 1.5"), I haven't searched for other pots than Clarostat. Now, I need to find some 500KOhm linear pots with an about 300W switch and 2" shaft.

I found Ohmite builds 2" too, but I still need to find some with a switch.

Try using a decent stereo pot with another switch for power.

Again, the amp is mono.(I've got two)

[DragonMaster]

I found an online shop, Take Five Audio,

http://www.takefiveaudio.com/purchas...cts_online.htm

but they sell 3 different bottles.

Any other places I can get it, and which one do I buy?(Spray, needle dispenser, brush)

[anatech]

Hi DragonMaster,
Buy the type you are more comfortable with. I use both the spray can and the bottle with the dripper needle. The dripper is sold seperately. I have not seen this stuff with a brush. Mind you, I'm talking about General Cement #10-1906 (spray) or #10-312 (liquid).
I forgot the unit you have was mono, sorry. The controls you are looking at will be fine. Make sure you get the switch with the control in case they discontinue the switch. Wouldn't be the first time.
-Chris