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AU-717 Rebuild General Questions

C

Crumbs

Member
Slowly working through a AU-717 rebuild. As far as I know the amp died while in storage/last use due to glue corrosion. I know the history but I haven't heard it play in years. I've read a bunch of posts by @Leestereo @Super Noob and others.

  1. Thermal Paste Part 1: Should the thermal paste be non-conductive or conductive type?
  2. Thermal Paste Part 2: Thoughts on shelf life for unopened Type Z9 10-8109 Si Heatsink Compound still pliable but of unknown age in old-school pink tube. The newer stuff indicates 5 years but I don't know if they changed the compound, and am partially asking to understand how you know when it is bad.
  3. Thermal Paste Part 3: Should all thermal paste parts be removed from their heat sinks, the heat sinks cleaned and the thermal paste be replaced?
  4. F-2663 power board: What is the rule for when to replace the relay, surrounding diode D608, and TR603? I've seen reference to replacing these with and without clear failure. Note: My relay contacts are still bright copper and the amp comes from a non-smoking environment.
  5. Setting Bias: Curious if someone has a good bias/offset setting how-to write up they could point me to.
Thanks in advance.
 
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1. Troubleshoot and repair the problem before rebuilding.
2. Don't replace the heat sink grease, it's a waste of time.
3. always replace the speaker relay. It's 40 years old and cleaning or burnishing the contacts will only work for a short time. No need to replace the relay drive transistor if the relay is working.
4. Replace all the black flag caps on all boards. They are black mica caps.
5. When you work on the boards, desolder the flat flex cable on one end first or it will break.
6. Be very careful of the wafer switch. The wafers are very fragile. When you move the board, keep your hand well away from the switch.
 
dr*audio said:
1. Troubleshoot and repair the problem before rebuilding.
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Given that the core boards, power and both channel drivers, had the infamous corrosion issue I'm rebuilding those boards to get them to work enough that I can troubleshoot. I'm asking the questions I asked now to decide how much to do on the power board given how many wires had to be desoldered to get it out for cleaning/repair.

dr*audio said:
2. Don't replace the heat sink grease, it's a waste of time.
Click to expand...
I already encountered thermal paste on the driver board, not for the main power transistor, which I unfortunately cleaned off because I expected to replace it and didn't want to make a mess. So I need to replace at least some and trying to figure out if the compound I have will do.

dr*audio said:
6. Be very careful of the wafer switch. The wafers are very fragile. When you move the board, keep your hand well away from the switch.
Click to expand...

Is the wafer switch on a selector board for source or something else? I haven't touched those yet.
 
Crumbs said:
Slowly working through a AU-717 rebuild. As far as I know the amp died while in storage/last use due to glue corrosion. I know the history but I haven't heard it play in years. I've read a bunch of posts by @Leestereo @Super Noob and others.
  1. Setting Bias: Curious if someone has a good bias/offset setting how-to write up they could point me to.
Click to expand...

Hi Crumbs,

Welcome to Audiokarma

The Bias and DC Offset adjustments can be found on page two of the service manual.

Regarding heatsink grease, replace it. Keeping transistors cool is the name of the game, any improved thermal coupling is a win. Although using heatsink grease/micas is a bit of an antique they still do their job very well. Many use thermal pads instead of grease/micas - that can be saved for another day. You can re-use the Mica if it is in good shape, or replace it. Super Noobs suggestion is a very good choice. Similarly I use what was easily available from Amazon, Dow Corning 340. Just do not use computer thermal paste (although I have) as it may have questionable longevity. Absolutely do not use conductive or compounds with adhesive properties. https://www.amazon.com/dp/B00D0HVYHI

Get on a dim bulb tester if you do not have one. Work on a board and test, too many horror stories of people going through an entire unit without testing only to be defeated by endless troubleshooting.
 
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For some reason when I'd looked at the manual for adjustment I figured, there has to be more to it than this:
MIJ_Pioneer said:
The Bias and DC Offset adjustments can be found on page two of the service manual.
Click to expand...

Should have given you a shout out as I read your and Hyperion's post in preparation. When I'm done I plan to put the comparison parts tables on git with an overview document that cross-references the useful posts, what people ordered, what I did. I'd do it now but it needs to be cleaned up a bit and unfortunately I haven't had as much time to work on the amp lately as I'd like.

My path so far:
  1. Realize it doesn't work and read many posts to define a path forward
  2. Figure out what parts were used and if they are still available (cross referencing Leestereo, Supernoob, jwjarch and daviddi922)
  3. Purchased sub-set of damaged driver board parts and put them on as a minimal viable effort test
  4. Built a dodgy but safely working dim-bulb tester from the parts bin
  5. Figured out I had near 0 power at the driver boards
  6. Evaluated if I could get replacements for the unobtainable OEM parts before I took the full plunge, ordering all the power board parts and big capacitors
  7. Cleaned, and repopulated the power board replacing damaged and suggested parts. Mostly followed Leestereo w/ some Super Noob guidance thrown in.
Next steps:
  1. Probably order replacement thermal grease; I really appreciate the suggestions and the confirmation that PC paste bad, conductive pastes bad.
  2. Replace the relay per guidance
  3. Replace the 4 power caps and filters + resolder the power board
  4. Dim-bulb test with driver boards disconnected to measure voltage supplied to boards
  5. Dim-bulb test with driver boards connected
  6. Direct plug test to verify low audio operation with crappy speakers
  7. Replace remaining driver board parts that were not originally damaged.
  8. Dim-bulb test
  9. Offset adjustment
Then I'll have questions about how to clean/lube the switches, but I'll wait because I haven't read those posts yet. Appreciate the help reviving this heirloom.
 
Find and replace all the fusistors.
Besides the glue problem, they are the biggest problem with those amps.

Do yourself a favor and recap it while it’s all apart. All e-caps.

If you plan on using a TT, there are trouble maker transistors in the eq amp. Replace them while it’s apart.
 
After a period of not working on, I returned to this project. The amp boards are rebuilt and now give clean sound. I'm moving on to the preamp and tone boards next as I know there are issues there.

I've noticed some seem to replace the 2SA750 transistors and others do not. Can anyone point me to a comparison or argument for why to replace them (or any other of the transistors for that matter)? So far I've replaced TR03 on the driver boards but that was due to corrosion on 1 board and deciding to match the channels from a 'sound' perspective. Thanks for the help.
 
leesonic said:
Not at all. It can and does dry out, making it much less effective. Do the job right.

Lee.
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I plan to; still need to order it. I've been waiting to see what else I need to order. I also need to read the posts on switch cleaning and disassembly prior to the preamp build.
 
Crumbs said:
I plan to; still need to order it. I've been waiting to see what else I need to order. I also need to read the posts on switch cleaning and disassembly prior to the preamp build.
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In my last 2 rebuilds I used keratherm 86/82. https://www.amazon.com/Keratherm-Thermal-Pad-100x100x0-25mm-Conductivity/dp/B08DDH16SS. You will obviously need to make pad yourself. I used existing mica pad (first cleaned of grease of cause) as a template and leather puncher to punch holes. The advantage is very good thermal conductivity and they do not dry out ;). Disadvantage you have to make TO3 pad yourself and the price. But if you are carefull then out of 100mm by 100 mm you can make 6 TO3 pads
 
I think ^^this^^ is too fussy for the average rebuilder, but then you are much more than an 'average' rebuilder TF :)
 
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If I go with thermal pad I can toss the mica and presumably I can use the same stuff for all the transistor heatsinks not just the TO3 pads.
 
MIJ_Pioneer said:
Hi Crumbs,The Bias and DC Offset adjustments can be found on page two of the service manual.
Click to expand...

With a DMM do you need to use a dummy 8 ohm load and measure the amp output during the offset measurement across it, or direct across the output without a dummy load?
 
TupolevFan said:
In my last 2 rebuilds I used keratherm 86/82. https://www.amazon.com/Keratherm-Thermal-Pad-100x100x0-25mm-Conductivity/dp/B08DDH16SS. You will obviously need to make pad yourself. I used existing mica pad (first cleaned of grease of cause) as a template and leather puncher to punch holes. The advantage is very good thermal conductivity and they do not dry out ;). Disadvantage you have to make TO3 pad yourself and the price. But if you are carefull then out of 100mm by 100 mm you can make 6 TO3 pads
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Shouldn't we be careful using this. Although the thermal conductivity is comparable to mica this value counts 1 m thickness. An average mica is one fifth thickness of this pad ( 0.05 mm to 0.01 inch( approx. 0.25 mm)) so 5 times worse energie transfer! Or I am a fool!? Ofcourse you have to count the thermal paste with the mica.
 
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GrisPato said:
Shouldn't we be careful using this. Although the thermal conductivity is comparable to mica this value counts 1 m thickness. An average mica is one fifth thickness of this pad ( 0.05 mm to 0.01 inch( approx. 0.25 mm)) so 5 times worse energie transfer! Or I am a fool!? Ofcourse you have to count the thermal paste with the mica.
Click to expand...

I decided to go with the Dow Corning 340 @MIJ_Pioneer recommended after reading this post on the comparison. So thermal paste has shelf-life, is messy, but fills in voids/non-smooth shapes better, creates a good thin bond and was what was used before. That and it requires less effort to apply than cutting out the pads.
 
Crumbs said:
With a DMM do you need to use a dummy 8 ohm load and measure the amp output during the offset measurement across it, or direct across the output without a dummy load?
Click to expand...

Good question

You do not need to put the amp under load for offset, as a matter of fact you want it to have no load at all. No dummy load needed. No input signal, no speakers connected, volume at zero, aux selected. Insert your pos / neg probes accordingly into the terminal and set it for DC. Turn on the speaker selector switch which you have connected your probes to and begin adjusting. You want to get it as close to 0mv DC as possible, anything within +/- 10mv is excellent - a bit of drifting in the reading is totally normal.

upload_2021-6-23_9-30-17.png
 
GrisPato said:
Shouldn't we be careful using this. Although the thermal conductivity is comparable to mica this value counts 1 m thickness. An average mica is one fifth thickness of this pad ( 0.05 mm to 0.01 inch( approx. 0.25 mm)) so 5 times worse energie transfer! Or I am a fool!? Ofcourse you have to count the thermal paste with the mica.
Click to expand...
Like famous philosopher said "there are no foolish questions. There are only foolish answers" :))). You are absolutely right that mica is 5 times thinner in your example. however thermal conductivity of mica is 0.71W/(m K). and Keratherm 86/82 is 6.5 W/(m K) which is 9 times better thermal conductivity ;) .
 
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