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.
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.
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
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
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.
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
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
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.
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.
Hi Sy,
Coupling caps may as well be done now too. That way there will be something to check out.
-Chris
Coupling caps may as well be done now too. That way there will be something to check out.
-Chris
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.
Again, the amp is mono.(I've got two)
I found an online shop, Take Five Audio,
http://www.takefiveaudio.com/purchase_caig_products_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)
http://www.takefiveaudio.com/purchase_caig_products_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)
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
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
I've bought cheapo "Contact 2000" at my local store. If I play too much with the control before the product dries, it turns worse than before.
So, now, instead of a squee, I get 1.5Hz pulses with the tape head
input.
I also changed 1/3 of the resistors 2 months ago. When I will order from Mouser(Will it happen...) I will get the ones I've not changed and new caps. I also found a tweaking guide in the EICO yahoo group.
So, now, instead of a squee, I get 1.5Hz pulses with the tape head
input.
I also changed 1/3 of the resistors 2 months ago. When I will order from Mouser(Will it happen...) I will get the ones I've not changed and new caps. I also found a tweaking guide in the EICO yahoo group.
Hi DragonMaster,
Cheap cleaner = wrong tool. The caps may change things a fair bit.
Be careful about tweaking, sometimes well meaning changes do more harm than good. Try to get it repaired first in stock condition. Then try the changes. Go back if the change makes it sound worse. Do not oversize coupling caps whatever you do. The old engineers who designed your amp did a good job with the parts they had. New "engineers" need to learn the art.
-Chris
Cheap cleaner = wrong tool. The caps may change things a fair bit.
Be careful about tweaking, sometimes well meaning changes do more harm than good. Try to get it repaired first in stock condition. Then try the changes. Go back if the change makes it sound worse. Do not oversize coupling caps whatever you do. The old engineers who designed your amp did a good job with the parts they had. New "engineers" need to learn the art.
-Chris
In the EICO group, about everything these people do is it tweak and restore their amps, and some of the people there might be "old engineers", they have been the first owners of the amps.
The guide in question have been tried by several people(The writer also), and is telling to add a grounded cord, removing the cap from chassis to hot or neutral, changing some caps in the audio signal path and that's about all. The group is worth suscribing to it.
Hi DragonMaster,
I think I just found it. I am waiting for approval. Can you post a link so I'm looking at the same thing?
I was concerned about increasing the size of the coupling caps. Often they are sized so you don't overdrive the output transformer at low frequencies. I've seen to much of that in the past.
-Chris
I think I just found it. I am waiting for approval. Can you post a link so I'm looking at the same thing?
I was concerned about increasing the size of the coupling caps. Often they are sized so you don't overdrive the output transformer at low frequencies. I've seen to much of that in the past.
-Chris
Here it is:
http://groups.yahoo.com/group/eico/message/1497
Someone also talked about a mod which was to give an R28 and a C15 to each output tubes instead of making them share these. He also told to double the R28 value I think.
scheme here:
http://users.rcn.com/fiddler.interport/SCHEM.JPG
http://groups.yahoo.com/group/eico/message/1497
Someone also talked about a mod which was to give an R28 and a C15 to each output tubes instead of making them share these. He also told to double the R28 value I think.
scheme here:
http://users.rcn.com/fiddler.interport/SCHEM.JPG
Hi DragonMaster,
I've not seen a commercial amp like that. Ever. Doesn't mean one does not exist.
I'd like to hear what PRR has to say on this.
-Chris
I've not seen a commercial amp like that. Ever. Doesn't mean one does not exist.
I'd like to hear what PRR has to say on this.
-Chris
Attached is the summary of what's going to be done for now. I'm sure there are other "mods" that can be done but that's all I could find on the EICO Yahoo group. Also, there are some items with question marks, I need to know if it's possible or not. The link to the scheme is in one of the previous topics. I can't get any answers on EICO group for these.
Hi DragonMaster,
My advice to you would be the same. Rebuild the amp first, repairing the problems. Then you can try different mods and properly assess the changes.
I've seen too many "hacked" amplifiers over the years. Many times the road to happiness was to return the unit to stock circuitry.
-Chris
My advice to you would be the same. Rebuild the amp first, repairing the problems. Then you can try different mods and properly assess the changes.
I've seen too many "hacked" amplifiers over the years. Many times the road to happiness was to return the unit to stock circuitry.
-Chris
... with 15$ of shipping each times? I don't really like this option. I should look for other user's reports before...
Finally, I'm going to restore the amps as much as possible. Only the chassis, pots, transformers, tubes, PEC and possibly the sockets are going to stay original.
In fact, some mods are done both for tweaking and the fact that the parts can't be found anymore. The multicap and the 0.025µFs give me some problems if I use the original values. People report good results for 0.047µF instead so I'm going to switch to this but I think that's all.
I don't remember where I found that but I think someone told to keep carbon resistors for the grid resistors to prevent problems with inductive metal film resistors. Can I put metal films everywhere else?
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