A little help with AU-717

[PF Sound]

Just got done restoring this using a lot of Leestereo tips......but have an issue of low volume on the left channel....... no static/distortion, just a lower volume.
The unit was pretty clean & all original, complete with glue, lol. I've seriously sprayed & worked all the pots/switches (except volume pot), it has new ON Semi outputs/drivers on both boards, fully recapped, new relay, glue removed, glue-damaged components replaced etc.
I got it in a "no power" condition..........someone had been in there, as 3/4 fuses were blown & the 4th was missing, but no components were touched. I found that one channel had shorted outputs/drivers, & the other had 1 shorted driver but the outputs were good.....odd.
Anyhow, it now looks good on paper.......55.2 volts/-55.2 volts at main caps.......DC offset is 1-3mV & bias stable at 20-21mV.
I'm using a tablet as a source & have found: Swapping L/R inputs of "AUX" , TUNER" , "Tape 1" had no effect......still low on left. Using "amp in" produced a balanced, solid sound. So can I eliminate 2721 as the problem? I'm trying to learn more. Per the schematic, I would "anticipate" the problem to then be on the tone board 2720.......... am I in the ballpark?

 

[Common Beat]

I had a similar problem with a AU-517 and the solution was to pull out the board that the filter switches are on, remove the switches, disassemble and manually clean each one internally. This cured a left channel low level/distortion issue I was having.

 

[Robisme]

Not sure if the 2SA726 transistors would cause a low channel. Usually noise. Were they replaced?

Pre/main switch cleaned?

You may have to trace the signal to see where it degrades.

Rob

 

[PF Sound]

Not sure if the 2SA726 transistors would cause a low channel. Usually noise. Were they replaced?

Pre/main switch cleaned?

You may have to trace the signal to see where it degrades.

Rob

No, I didn't replace them. I did a quick check on all the transistors on the driver boards & main power board because of glue damage & faulty drivers/outputs.......just lookin for shorts mainly.
I just ran a 1kHz signal through the unit (very low volume) via AUX input. At pins 27-30 (near the relay), I was getting 320mVac on one side & 117mVac on the other, so it's real & not imagined. It's steady though.......no distortion/static anywhere. Not getting any noise when activating switches/pots, including the volume pot.......& yeah, I sprayed/worked the pre/main switch pretty good after reading about issues with them.

CB.....thanks for the input. I sprayed/worked em but I've read where sometimes that aint enough.

 

[EastPoint]

The 2SA726s are garbage and should be replaced, along with pretty much all the other transistors in that unit.

 

[bzb882]

Its recommended that the VD1212 diodes in there should be replaced. Two 1N4148 in series is the recommended replacement. Worked for me, I've read that they can test good and still be bad.

 

[PF Sound]

The 2SA726s are garbage and should be replaced, along with pretty much all the other transistors in that unit.

I'll follow up on that......thank you.

bzb....... As per the "Leestereo restoration" thread that has been done......thanks

 

[Robisme]

Just a thought. Is the balance knob on correctly? Just trying to think of everything.

Rob

 

[blhagstrom]

You located and replaced all the fusistors, right?

Your soldering skills are good, right?

You confirmed the orientation and values of replaced parts, right?

I built one from a basket case pile of parts once and F’d up a resistor value and had a low output. You may want to verify the resistors in the tone area and pre-amp area.

 

[GrisPato]

it has new ON Semi outputs/drivers on both boards, fully recapped, new relay, glue removed, glue-damaged components replaced etc.

No mention of fuse ( fusable ) resistors. They were not in there or . .
When they are and you did not replace them that is the first thing to do. Other wise start with verifying you regulated powersupply output voltage. Than you continue to check / replace components like resistors and / or diodes. I just did an AU 417 with the same problem. Was a combination of the fusable resistors and the volumepot. Had to clean it more than 5 times. Don't forget to finish of pots with some approved lube spray.

 

[Hyperion]

Regarding fuse resistors and replacement, it wouldn't be the first time someone had omitted part of a textbook restore (and failed to tell us!) 'because they looked ok so I left them alone' I mention 'fuse resistors' particularly as they would be my first choice for what is causing the problem.

 

[PF Sound]

Just a thought. Is the balance knob on correctly? Just trying to think of everything.

Rob

Yeah I thought about that too. But it's one of those that "locks" at the mid point so it was easy to line up.

The 82 ohm & 150 ohm fuse resistors were replaced with 1/4w metal film resistors of the same value. It's always possible that I made a mistake, especially when replacing so many components, so I'll go back & verify. Due to lack of noise/static, it's probable I did mix in a wrong value somewhere.

 

[PF Sound]

Resistors check out.........I've checked the orientation of all the VD1212's I replaced & I didn't screw those up........need to finish my a.m. coffee & go over my checklist one by one.

question........the dc voltages appear to be consistent throughout......so after I go through my checklist of items replaced, & should I not find any discrepancies, should I be trying to track the signal? I have a scope but Im pretty green as far as it's use so far.

 

[PF Sound]

I've got it narrowed down............to the volume pot. I've got a balanced signal going in on leads 5&6 (right on the pot leads) but on leads 7/8 & 9/10 I lose it.
When powered down, on lead 7 & 9 (common) I have 16mVAC.......lead 8= 48mVAC..........lead 10= nada or very little.
Noticed on lead 8 that the volume pot doesn't move the values until right at half volume. They go up proportionately. Nothing changes on 10 at all.
The signal going in F2720/leads 1&2 is 150mVAC on both sides (& the same on volume pot leads 5/6). With the unit powered up, on leads 17/18 (exiting the board back to volume pot) I'm seeing a 3 to 1 ratio of signal difference, & of course the same leaving on leads 3/4.
I'm no rocket surgeon, but it looks like garbage in/garbage out of the volume pot. I verified continuity of the leads......wish it had just been a broken wire but no such luck.

Guess I'm gonna get a look inside this infamous pot.

 

[PF Sound]

I found the thread on disassembling this pot......but before I do......are there still knock-offs available to replace this unit? Most of the threads concerning this are several years old, & one mentioned ALPS clones but I'm not seeing any. Only option is to fix this one???
I'm not afraid to delve into it, my only concern is that this 717 isn't mine.
Are there any other options, other than buying a used one?

 

[blhagstrom]

Wait.

to the volume pot. I've got a balanced signal going in on leads 5&6 (right on the pot leads) but on leads 7/8 & 9/10 I lose it.
When powered down, on lead 7 & 9 (common) I have 16mVAC.......lead 8= 48mVAC..........lead 10= nada or very little.
Noticed on lead 8 that the volume pot doesn't move the values until right at half volume. They go up proportionately. Nothing changes on 10 at all.
The signal going in F2720/leads 1&2 is 150mVAC on both sides (& the same on volume pot leads 5/6). With the unit powered up, on leads 17/18 (exiting the board back to volume pot) I'm seeing a 3 to 1 ratio of signal difference, & of course the same leaving on leads 3/4.
I'm no rocket surgeon, but it looks like garbage in/garbage out of the volume pot. I verified continuity of the leads.

That's clear as mud.

Seeing any voltage when powered down is confusing.
Either your meter is just messing with you or there is something weird going on.
Maybe stray AC leaking on the chassis. mV AC is perhaps just noise.

Signal at volume pot 5/6 are good. Matching board pins 1/2. Should be, nothing in the way.
Volume 9/10 must be the muting pins. Dirty mute switch could mess with that but when muting off, the switch is out of circuit, so make sure its off.
Volume 11/12 should be ground, you won't find any voltage signal there. That's how the pots "kill" the signal to turn volume down.
Volume 7/8 feed the signal through to F2720.

Are you checking for voltage with the volume pot turned down?

You should do continuity checks on the pot. Ohms, powered down. to verify the wipers and resistance surface is OK or not.

Time to be a rocket surgeon.

Pins 5-11 and 6-12 should be 150K ohms
Pins 5-7 and 6-8 should vary from 0 to 150K ohms when you turn the control..

Now, moving over to the output side of F2720.
Volume 17-21 and 18-21 should be 5K ohms.
Volume 17-19 and 18-20 should vary from 0 to 5K as you turn the control.

Volume 7/8 should have equal signal
Volume 17/18 should have equal signal
If 7/8 are equal and 17/18 are not, the problem is on F2720, not the volume pot.

FWIW, the loudness tap should feed off 2 leads on the volume pot, mid way (half point).
Loudness is only available on lower volume and doesn't work above 1/2 volume on many (most volume pots).

 

[PF Sound]

Wait.



That's clear as mud.

Seeing any voltage when powered down is confusing.
Either your meter is just messing with you or there is something weird going on.
Maybe stray AC leaking on the chassis. mV AC is perhaps just noise.

Signal at volume pot 5/6 are good. Matching board pins 1/2. Should be, nothing in the way.
Volume 9/10 must be the muting pins. Dirty mute switch could mess with that but when muting off, the switch is out of circuit, so make sure its off.
Volume 11/12 should be ground, you won't find any voltage signal there. That's how the pots "kill" the signal to turn volume down.
Volume 7/8 feed the signal through to F2720.

Are you checking for voltage with the volume pot turned down?

You should do continuity checks on the pot. Ohms, powered down. to verify the wipers and resistance surface is OK or not.

Time to be a rocket surgeon.

Pins 5-11 and 6-12 should be 150K ohms
Pins 5-7 and 6-8 should vary from 0 to 150K ohms when you turn the control..

Now, moving over to the output side of F2720.
Volume 17-21 and 18-21 should be 5K ohms.
Volume 17-19 and 18-20 should vary from 0 to 5K as you turn the control.

Volume 7/8 should have equal signal
Volume 17/18 should have equal signal
If 7/8 are equal and 17/18 are not, the problem is on F2720, not the volume pot.

FWIW, the loudness tap should feed off 2 leads on the volume pot, mid way (half point).
Loudness is only available on lower volume and doesn't work above 1/2 volume on many (most volume pots).

Thru the AUX input it's a straight shot to the balance & volume pots. I had no problem picking up the signal there with power off. You saw the readings I got, & 7/8 were uneven.
Since I was already taking this out, I checked the measurements as you suggest. Before unsoldering leads I did a few checks & found oddities. On 7/8 I was seeing an uneven short to ground.......one had about 11k & the other 46k. I wish I would've written it down right then. Anyhow, by the time I got the signal leads removed, that discrepancy was gone. 12-8 & 11-7 have 46k ohm resistance between them, so that's where the ground came from.
The only obvious fault is that the lead 10 point is open in the pot.........6-10, 8-10, & 12-10 all test open, whereas 5-9 shows 12 ohms, 7-9 shows 12 ohms, & 11-9 shows 46k ohms........ 5-11 & 6-12 show 142k ohms..........17-21 & 18-21 only measures 3.5k ohms but at least they're within a few ohms of each other. 5-7 & 6-8 measure 95k ohms & drop to 1 ohm with volume advance. As you suggest, no signal to lead 10 shouldn't really be a big deal, but perhaps the lack of resistance is causing the imbalance.
I'm debating insulating the setup as it is & running a signal through leads 7/8 to see what it looks like at 17/18......should be able to that safely.
Thanks for your input on this.

 

[blhagstrom]

Thru the AUX input it's a straight shot to the balance & volume pots. I had no problem picking up the signal there with power off. You saw the readings I got, & 7/8 were uneven.

Ah, that's what the AC when powered off was. A supplied signal.

Since I was already taking this out, I checked the measurements as you suggest. Before unsoldering leads I did a few checks & found oddities. On 7/8 I was seeing an uneven short to ground.......one had about 11k & the other 46k. I wish I would've written it down right then. Anyhow, by the time I got the signal leads removed, that discrepancy was gone.

That would point to R09/R10, they are supposed to be 680K. But it may be r11/r123/tr1, r12/r14/tr02 issue, or c07/c08 weirdness.

12-8 & 11-7 have 46k ohm resistance between them,

Those should be variable when you turn the control. You did a static test?

so that's where the ground came from.

????, explain.

The only obvious fault is that the lead 10 point is open in the pot.........6-10, 8-10, & 12-10 all test open, whereas 5-9 shows 12 ohms, 7-9 shows 12 ohms, & 11-9 shows 46k ohms.

As I said, I think 9/10 is the muting (attenuation) switch. If that switch is OFF, it should not affect anything. If 10 is open, attenuation won't work. SO far, we have not verified that all this poking and prodding is done with that switch OFF. Confirm that it is OFF.

17-21 & 18-21 only measures 3.5k ohms but at least they're within a few ohms of each other.

Should be 5K, but it may depend on where the pot is set because 19/20 tap off that and may skew the readings.

5-7 & 6-8 measure 95k ohms & drop to 1 ohm with volume advance

Close enough, What does 17-19, 18-20 read and do when you turn the control?

 

[PF Sound]

Ah, that's what the AC when powered off was. A supplied signal.



That would point to R09/R10, they are supposed to be 680K. But it may be r11/r123/tr1, r12/r14/tr02 issue, or c07/c08 weirdness.



Those should be variable when you turn the control. You did a static test?
Yes, value doesn't change til about 50% volume, then they go to 142k.



????, explain.
I initially had one probe to ground......11/12 are ground.



As I said, I think 9/10 is the muting (attenuation) switch. If that switch is OFF, it should not affect anything. If 10 is open, attenuation won't work. SO far, we have not verified that all this poking and prodding is done with that switch OFF. Confirm that it is OFF.
Yes, all controls are off/defeated.



Should be 5K, but it may depend on where the pot is set because 19/20 tap off that and may skew the readings.



Close enough, What does 17-19, 18-20 read and do when you turn the control?
work as expected......zero out at max volume.

Again, I appreciate your help here.
After checking all this, I went ahead & ran a signal through the unit, bypassing the pot (bridged 5/7 & 6/8). The result was very balanced..........242mVAC & 232mVAC on leads 17/18 at one point. The signal generator on my tablet (may be tablet itself or RCA cable) has about a 4-5% imbalance in its outputs, & I saw this output to be consistent when changing amplitude on the tablet.
One other thing....... leads 9/10 actually go to the "loudness" switch (S02), not the "mute" switch. But again, all switches are off/defeated for testing.
So I'm not sure where to go from here. Trust me, I'm in no hurry to tear that thing apart, but the issue seems to be pointed towards it.

 

[PF Sound]

It's starting to make sense. The problem isn't one side getting dragged down, it's that one side ISN'T getting attenuated when it should be. Per the schematic, leads 7/9 & 8/10 are essentially common on the volume pot, with leads 9/10 being attached to a 15k resistor when the "loudness" switch is off, & that resistor being bypassed when "loudness" is on. If lead 10, as in this case, is open then the signal isn't getting attenuated as it should be with the "loudness" button in the "off" position.

Unfortunately, this still means the pot needs repair.