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Old 11th April 2013, 10:27 PM   #21
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Mooly, sorry it is Dr Mooly I must say, you're about to do open heart surgery over the internet !
I think you would have been a good dentist , I will be following this ! Hope you guy,s get it sorted , looks like a great amp !
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Old 11th April 2013, 11:19 PM   #22
rsavas is offline rsavas  Canada
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Transformer sounds as if it is fine.
Try some 5 minute epoxy!!
Once you have the unit apart many people suggest to replace the old electrolytic caps with new ones.Also the trimmer R can cause troubles as well.
As for the big ecaps, C3-4. well at least look for any leakage or bulging.
Looks as if someones been at Q19(2SA810) by the mess of thermal compound.
I see some trany's pkgs, that I have never seen before.
See if you can get hold of "markthefixer" at audiokarma.org, to see if he knows of this model.
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Old 13th April 2013, 06:42 PM   #23
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Default Pcb update

I finished disassemble the amp, in order to identify any problem I spent some times cleaning the pcb and polish the silver connections.
I found 3 problems (marked in red in the next picture) 2 components are not original and there is an extra resistor. Any help on that?
At an eye inspection all the components and the pcd are fine but I will check with a mutlimeter all the restistors and diodes tomorrow.
As you guys can see I removed all the caps and I started mounting some new ones (elna, nichicon) as suggested. Should I also emove the caps like the one market in green in the picture?

I found most of the caps but I have a couple of problems:
- I need to substitue two radial caps 100V 470uf but I can't find something similar, max dimensions 25mm x 50mm. (could be snap in without problems)
The only thing close that I found is 680uf 100V from RDE, can I use those?
- I can't test the 33000uf 80V caps, but after 40 years I don't expect more than 30/40% out of them. I found two substitutes 1 , 2 they are perfectly similar in dimensions to mine but unfortunately they will be in stock in 17 weeks
If someone know where I can find a couple of those caps please let me know
Click the image to open in full size.
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Old 13th April 2013, 06:51 PM   #24
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Default Transformer

I finished repairing the transformer. The borders are not all perfectly aligned but is surely better than the beginning. I couldn't repair the upper left corner, for don't damaging the transformer I didn't use an hammer or direct hit but just a vise. I also reinstalled the nuts, the result is not really nice to see but it's perfectly working.
I will do the rewirng as soon as I finish substitute all caps in the pcb

Click the image to open in full size.
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Old 13th April 2013, 07:06 PM   #25
Mooly is offline Mooly  United Kingdom
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Hmmm.... I hope you made notes of how it all re-connects back together

If I'm honest, its not the way I would have approached this but what's done is done. I would have preferred (at least initially) in seeing what the original problems were before dismantling it so completely. The opportunities for error in reassembly and parts replacement are very real.

The extra resistor. You will have to identify the component reference number in question from the PCB layouts in the manual.

680uf in place of 470uf is probably fine, again, giving the component reference so we can see its function in the circuit would help.

The transistors mounted on wires off the PCB are probably the vbe generator (bias) for the output stage.

Unless proved faulty I wouldn't discount the 33000 uf caps as being a problem at this stage.
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Old 13th April 2013, 07:24 PM   #26
Mooly is offline Mooly  United Kingdom
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I think looking for suitable replacement transistors will be next on the list and that probably means using different package outlines to those on the PCB as they will be well and truly obsolete.

You also need to check all the low value resistors (particularly those wirewounds)

All that is for another day
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Old 13th April 2013, 07:52 PM   #27
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Quote:
Originally Posted by Mooly View Post
Hmmm.... I hope you made notes of how it all re-connects back together

If I'm honest, its not the way I would have approached this but what's done is done. I would have preferred (at least initially) in seeing what the original problems were before dismantling it so completely. The opportunities for error in reassembly and parts replacement are very real.

The extra resistor. You will have to identify the component reference number in question from the PCB layouts in the manual.

680uf in place of 470uf is probably fine, again, giving the component reference so we can see its function in the circuit would help.

The transistors mounted on wires off the PCB are probably the vbe generator (bias) for the output stage.

Unless proved faulty I wouldn't discount the 33000 uf caps as being a problem at this stage.
Two of the wires needed for the transformer were hidden in the front part of the amp. In order to access this zone I had to disassemble since I couldn't connect the transformer without doing so.
In order to reduce future problems I took many pictures and I numbered all the cables with the corresponding connection on the pcb plus I divided the screws by zones, I hope that will help.
My next step will be to substitute all the small caps and reistall the power supply section with the transformer


- The 100V 470uf are C11 and C12 in the power supply pcb
- The extra resistor is mouted at R31 2.2K on the power amp pcb
- The original transistor is an hitachi 2sc281, the one in the picture has no print, they only thing I know is that it's a little bit smaller than the hitachi
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Old 13th April 2013, 08:13 PM   #28
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Quote:
Originally Posted by Mooly View Post
I think looking for suitable replacement transistors will be next on the list and that probably means using different package outlines to those on the PCB as they will be well and truly obsolete.

You also need to check all the low value resistors (particularly those wirewounds)

All that is for another day
with "those on the PCB" do you mean just Q7-8-10-18-19 or all the transtistors?
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Old 13th April 2013, 10:07 PM   #29
rsavas is offline rsavas  Canada
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You originally showed a pics of one of the heatsinks, with sub transistors, was this matched with the pcb as shown, that had the work done on it (messy transistor)?
If so, when you should re-assemble by starting with the PA pcb and outputs that look as if they were not touched.
I strongly suggest to re-assemble the PS first, (especially since the transformer has been messed with) test, before adding any of the other pcb's. Start with just the transformer, big filter ecaps, no power supply pcb, test the transformer/main ecap DC is okay, then the power supply pcb, test. If PS okay proceed with the buffer,protect, meter pcb's more test. Then finally start with only one PA channel first, the one that looks as if it was original, step by step. I really ? doing a blanket trany sub at this point in time. Could use only one set of o/p trany (old/original spec'd ones if you can find a good set), just to check biasing first. You DO NOT want to do a complete re-assembly with out some intermediate steps or you can be wasting a lot of money on new parts. (Magic White Smoke)
Good luck
Rick

Last edited by rsavas; 13th April 2013 at 10:12 PM.
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Old 13th April 2013, 10:20 PM   #30
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For output transitors, take a look at pioneer sa-9900. 2sc1079 transistor [Archive] - AudioKarma.org Home Audio Stereo Discussion Forums
Motorola make a good replacement.
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