A-700 Restore

zaibatsu said:
You can use this for your 22Ω (fusible, flameproof). It's 1/3W instead of 1/4W but close enough.
Everything else sounds good.

On your tester:
B = beta = hFE (gain)
Uf = Vf (forward voltage, same as you would read on your meter in diode test mode)

Be careful with those output transistors, they can't be replaced in exact spec anymore.

Also, to echo what OMGCat said earlier, be careful with those bias transistors that were mounted on the heatsink. The legs are usually brittle with heat cycling and any bending can crack them and take out the output stage from thermal runaway. It's probably worth adding 2x BD135-16S to replace them in future, just leave them for now if they are intact, but for the future if you start poking around in there to recap etc.
Click to expand...

Sounds good I’ll get that order submitted tonight and that will give me a couple days to keep cleaning up the solder joints, They are bad. Being careful not to cause any bridging.
Thanks again for your advise.
 
Scottfer55 said:
will give me a couple days to keep cleaning up the solder joints
Click to expand...
Be careful if soldering any polystyrene capacitors. They won't tolerate much heat from your solder tip. I'm not familiar with the A-700. But I would imagine there are a few.
 
Got my shipment notification for a 3/13 delivery date. In the meantime I hope I’ll have the main PCB resoldered, heat sinks reinstalled and dim bulb tester assembled. I’m slow but that’s ok for me, I start getting double vision after about 30 minutes on the iron. I took avionics advise and identified the 6 poly caps and will be careful with the iron on those. Started to remove the buss bar and polish but will wait till later. Also I found 2 more caps on a smaller board that had the glue, but no corrosion... I scraped it all off but won’t get to deep into that section yet. I’d like to get the relay to click and see bright led’s then start to talk about recapping.
I’ve never used the phono section of this amp that I can remember, but probably will if it’s decent. Does anyone have an opinion about it? Any success at a mod to improve it?
 
I do the same as you are for solder work, go until I feel fatigued and then lay off. If you keep pushing that's just asking for mistakes.

I wouldn't worry about being slow. To do this job properly takes a lot of time and if you rush you will just have to go back in there anyway.
I'm going through one of my A-1000s right now and have about 8 hours into it and I haven't even removed all of the old parts yet. So far it's been cleaning, removing glue and retouching joints.
I've been unlucky as far as glue on the film caps though. Two of them are pretty much a fuzzy mess with one having had the solder turn into some sort of dust that was a giant pain to remove.

The phono stage is really nice in this amp. I'm not running any crazy setup but with my Signet Tk5e/AT120lc combo it sounds very nice.
To my ears it's pretty comparable to the stage in the C-80 though you don't have any loading options. I haven't tried to do any improvements on it as the best improvement for me would probably be to get a nicer table and cart.
 
OMGCat! said:
I do the same as you are for solder work, go until I feel fatigued and then lay off. If you keep pushing that's just asking for mistakes.

I wouldn't worry about being slow. To do this job properly takes a lot of time and if you rush you will just have to go back in there anyway.
I'm going through one of my A-1000s right now and have about 8 hours into it and I haven't even removed all of the old parts yet. So far it's been cleaning, removing glue and retouching joints.
I've been unlucky as far as glue on the film caps though. Two of them are pretty much a fuzzy mess with one having had the solder turn into some sort of dust that was a giant pain to remove.

The phono stage is really nice in this amp. I'm not running any crazy setup but with my Signet Tk5e/AT120lc combo it sounds very nice.
To my ears it's pretty comparable to the stage in the C-80 though you don't have any loading options. I haven't tried to do any improvements on it as the best improvement for me would probably be to get a nicer table and cart.
Click to expand...

I’m a firm believer in “haste makes waste”

I just got an old pioneer pl-514 for cheap and cleaned it up, it came with a shure m95ed and I ordered a 15$ stylus from needle doctor in Canada. It sounds ok with my onkyo a9711 but Just ok. I’m just getting my feet wet and will probably upgrade the cart and stylus. I’m gonna see how it sounds on the a700 first.
 
I used my Onkyo TX-8211 as a stand in a while ago and the phono stage on that was about the same. Adequate as in it played records but definitely nothing inspiring. The stage on the A-700 is worlds better though your Integra is quite a ways up the ladder from my little bedroom receiver.
 
progress update
Completed the resoldering of main PCB
Replaced all corroded parts and jumpers
Transferred one set of transistors to heat sink

Leads me to ask do the 2 smaller transistors need heat sink compound? There wasn’t any when I disassembled it.

Dim bulb tester has become a high priority
 
Sounds like you're making some great progress. Super great idea with the DBT. I wouldn't want to attempt a power up from a major repair without one if I could at all help it.

They didn't put any on the driver transistors but some of the bigger amps used the same devices and did come with compound so I always put some on there when I'm servicing one. I figure it can't hurt.
 
Success! The relay energized and all led’s are bright! Thank you to @avionic, @zaibatsu, and @OMGCat!
Now to set up my source and speakers to test it out.
Recap? I think so, this will be my first recap job. The amp is 34 years old I’m sure it will help. Any suggestions for new caps would be greatly appreciated.
What other components should I replace if any?
Nice way to start the day again thanks guys for your help
 
Excellent work.
Let us know how it sounds.
Did you end up building a DBT?

For the recap, my suggestions are below for key locations.

Power amp board:
100uF x4: Nichicon KZ 100uF 25V (C179-182)
330uF x4: Nichicon KZ 330uF 50V (out of stock, but digikey has them)
1000uF x2: Nichicon KZ 1000uF 50V
680uF x2: Chemicon GPD 750uF 80V or Chemicon GVD 820uF 80V

For the phono board:
47uF x2: Nichicon ES (bi-polar) 47uF 50V
1000uF x2: Nichicon KZ 1000uF 25V

Notes:
  • Both Nichicon and Chemicon 680uFs always measure way below spec, hence recommending 750uF or 820uF for replacement (even the 750s measure ~690).
  • I've gone up a voltage rating where possible, so that the caps match the stock footprint. I recommend this as it brings ripple current closer to factory intention.
  • For remaining caps, doesn't really matter what you use - my recommendation would be Nichicon PW / HE (both 105* rated and relatively cheap). If there are any 6.3V ones, bump those to 10V or 25V (if diameter allows): will have lower ESR and last longer.
  • If you want to spend more, you can use Wima MKS2 film caps for smaller values, just make sure the voltage is high enough per parts list (50-63V is fine for most, but there's 4x 2.2uF 100V near the front, C227-C230, electrolytics are easiest for those) and that physical fit is OK. There's 4x B.P. 2.2uF (2x 25V, 2x 50V) you can use these MKS2 for.
  • Most people replace the trimmer resistors on a rebuild (up to you), Bourns single turns or multiturns both work fine. Multiturns are nice for the idle adjustment. @OMGCat! might be able to tell you the part number as he did this task more recently than I, there's a version with longer legs that is easier to mount.
  • If you didn't order them already, to replace the two bias transistors (2SD-400), I use 2x BD135-16S. I can post a pic of how I mounted them, care is needed with polarity / orientation.
 
Last edited:
Great info as usual
Yes I did build the DBT, I was nervous at first seemed like the 8-10 seconds before the relay clicked was forever lol. The light came back on when I pressed auto-class A but slowly dimmed to near off. I’m hoping that is normal.
I’ll get the speakers hooked up soon and let you know what it does.
 
Which wattage bulb are you using?
It should go bright when you first power on, then dim, then relay click.
Auto A should be switched off, but if you turn it on, the bulb should go bright and the relay should unclick (it draws enough current that the DBT shows signs of a short). Depends on bulb wattage though.

The first thing to probably do once you take it off the DBT is to check the bias on both channels, you can use the minigrabber leads on your multimeter (test points are the tops of those two white standing resistors, check the SM for procedure). Auto A should be off. Best to clip the grabbers on with the amp off (to one channel's resistors), set your meter to mV DC, then turn amp on and watch it (should be around 15mV). If it looks good, turn it off, and repeat for the other channel. FYI if you don't use the grabbers it's pretty easy to slip and short stuff so be careful.

It's not a bad idea to check this before playing music, just to make sure those values are stable and not running away.
I wouldn't use Auto A (at all) until you're sure everything is stable and working.
 
It’s a 100 watt bulb. The bulb does just as you say except when pressing class a now it stays bright but doesn’t de energize the relay. And never goes back dim. I haven’t tried class a off the DBT.
As per SM the following is for idling adjustment
The voltage at left channel R307 both sides was 13.8 adjusted to 14.9
Right channel R308 adjusted from 9.8 to 15.5
This was without an 8 ohm load on the speaker outputs.
The only other adjustment in the manual is for auto class a switching, I don’t do anything with that one yet.
I need to clean these pots they were very erratic with the slightest tweak.
I will clean it all up before making sound.
 
That's fine, all sounds normal.
If those values stay stable around ~15mV while you watch for a few minutes, you're good to test with some speakers.

It's normal for those pots to be very sensitive. Doesn't hurt to clean them and re-set in my experience, but they do only have a tiny useful range (partly why the multiturns are nice to add if you plan to keep the amp for a long time, that and being sealed).

No rush on the switching adjustment, my advice would be to not clean or touch that pot until you're ready to make the adjustment (need some 8Ω load resistors). No harm in using the amp without messing with that, if you haven't touched it already.
 
That's awesome you got it going! I bet you'll enjoy it after all this time and even more now that you've repaired it yourself. Great job.

I'd say if the pots don't bug you and you can get them into adjustment (which it seems like you've had no problem) then just leave them be. I used these multiturns https://www.digikey.com/product-detail/en/bourns-inc/3296W-1-502LF/3296W-502LF-ND/1088058
The only issue is you have to make a little extension leg or they won't reach the center hole. Not a big deal but not really needed if the originals are working ok.
As zaibatsu says there are longer leg versions but I can't figure out/remember what the stupid part numbers are so I got those last time.
 
Im back! Havent touched my amp since the last post, been on vacation among other things, grandbabies etc... so ill be posting again soon once i review this thread and see where i was at lol. Glad you guys are still here
 
zaibatsu said:
Excellent work.
Let us know how it sounds.
Did you end up building a DBT?

For the recap, my suggestions are below for key locations.

Power amp board:
100uF x4: Nichicon KZ 100uF 25V (C179-182)
330uF x4: Nichicon KZ 330uF 50V (out of stock, but digikey has them)
1000uF x2: Nichicon KZ 1000uF 50V
680uF x2: Chemicon GPD 750uF 80V or Chemicon GVD 820uF 80V

For the phono board:
47uF x2: Nichicon ES (bi-polar) 47uF 50V
1000uF x2: Nichicon KZ 1000uF 25V

Notes:
  • Both Nichicon and Chemicon 680uFs always measure way below spec, hence recommending 750uF or 820uF for replacement (even the 750s measure ~690).
  • I've gone up a voltage rating where possible, so that the caps match the stock footprint. I recommend this as it brings ripple current closer to factory intention.
  • For remaining caps, doesn't really matter what you use - my recommendation would be Nichicon PW / HE (both 105* rated and relatively cheap). If there are any 6.3V ones, bump those to 10V or 25V (if diameter allows): will have lower ESR and last longer.
  • If you want to spend more, you can use Wima MKS2 film caps for smaller values, just make sure the voltage is high enough per parts list (50-63V is fine for most, but there's 4x 2.2uF 100V near the front, C227-C230, electrolytics are easiest for those) and that physical fit is OK. There's 4x B.P. 2.2uF (2x 25V, 2x 50V) you can use these MKS2 for.
  • Most people replace the trimmer resistors on a rebuild (up to you), Bourns single turns or multiturns both work fine. Multiturns are nice for the idle adjustment. @OMGCat! might be able to tell you the part number as he did this task more recently than I, there's a version with longer legs that is easier to mount.
  • If you didn't order them already, to replace the two bias transistors (2SD-400), I use 2x BD135-16S. I can post a pic of how I mounted them, care is needed with polarity / orientation.
Click to expand...
So I’m beginning to put together an order for caps. Shewee what a task... anyway the information you provided is absolutely valuable and thank you!! A few questions to start.
• the 330uF for the phono section that you suggest is 25v, should I up that to 50v? Same as C211,212 also the temp ratings are 85 and I know you also said use 105*, probably a reason for that as I’m thinking the higher temp= bigger can?
• is there any reason I want to spend more on the wima film caps? I’m thinking keep it simple but don’t neglect an obvious improvement.
• you suggest nichicon pw/he, I understand nichicon as manufacturer what does the pw/he mean? How do I ensure that’s what I order.

The amp sounded good with a cheap CD player and some old hk-20 speakers. I wanted to take it into my listening room and replace that onkyo but I’m waiting to do the recap first.
 
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