Being challenged by my 7070

[toymanmsp]

Hi all, this is my first technical post, and am in need of some help and direction. I don't have an electronics background, I'm just learning as I go along. I've run into several issues with my 7070, some of which I've worked out some I haven't. So here goes...

1. I've redone the driver board replaced, the caps, transistors, and fuse resistors reinstalled the board and it was good to go. All inputs worked correctly until I started to work on the phono board.

2. Phono board debacle - Replaced the caps and transistors, started with no sound in either channel with the exception of a very loud whoosing noise coming from the right channel (when the volume was turned up, the right channel meter would jump). No whoosing sounds coming from the left channel. Also, when connected to speakers, the woofer on the right channel speaker practically shot out of the cabinet. Nothing on the right speaker. No movement of the left channel meter.

I found an incorrectly installed transistor and a backward cap which I corrected, along with finding a loose solder on a different cap. After I found and made the changes, I turned it on and found that the output volume level is extremely low. The output level was normal (for FM, Aux, didn't test the tape inputs) until these changes were made. I've checked the DC offset and I've reset the bias again per the service manual. Still no change in the low output volume on both channels. The good bit of news is that after these changes, I did get the left channel to work on the phono stage, albeit at a very low volume. (so I'll call that a bit of win)

Other details to note:

- The unit does come out of protection mode (click is heard about 5-8 seconds after powered on)
- The meters are no longer working
- I have not replaced the relay, not sure this should be done as it clicks on. I do have a new one that I can install if recommended.
- All controls have been treated with DeOxit but will plan to give another pass-thru.
- All lamps have been changed to LED's needed to reflow a few solder joints to get the lamps to stay on.
- Installed new banana plug speaker terminals. Both A&B speaker output work.

Planned next steps:

I will likely plan to pull all the transistors to make sure that the pinning orientation is correct and the correct transistors are in the right location. I've already done this with the caps.

I'll look for anything else that is loose on the board.

I've read that volume pot could potentially have a crack and this would cause a decreased volume, could this be part of the issue? The phono board is difficult to access, so I will plan to check the solders on the volume pot as well.

Questions:

There is a resistor on the underside of the phono board, that looks to be soldered to the base pin on a transistor. Is this correct or should it be a different pin? I've checked my photos and I'm pretty sure that it should be the lower pin which should be the base?

Should the relay be replaced?

What else should I be considering?

Any input, comments, or direction would be appreciated.

Thanks! Adam

 

[Sglaw461]

Do you have a transistor checker and a capacitor esr meter? I would highly recommended you go back to the phono board and check components. Incorrectly installed transistors can have a cascade affect and take out a lot of other components. If it's possible, disconnect the phono board from the main board and see if the problem goes away.

 

[birchoak]

Hi Adam--I'll offer some general advice from the viewpoint of someone who is completely self-taught as has made many mistakes!

1. Try to work on one board at a time, testing your work frequently. Do you have a Dim Bulb Tester? If not, EngineerNate has an excellent post about how to make one--follow the directions exactly or it will not work correctly. It's a crude but effective tool to safely detect shorts in your gear.

2. I try to desolder and replace ONE cap at a time. I pull the old cap out and stand it up in front of me, then I get the new cap out and double check the values and make sure it's not bipolar. I also make a small drawing of the board and note where the negative terminals of each cap are.

3. Before thinking of more complicated possible problems (cracked volume pot--have never encountered this personally), always think of the simplest things it could be first. Did you blow the fuses (if equipped)? Are the relay (if equipped) contacts clean? Some amps will still play sound with blown speaker fuses but at a very low sound level,

4. I would NOT mess with the transistors yet, if possible. Do the recap first and see how things go. Simplest stuff first.

5. Do not underestimate the importance of thorough pot and switch cleaning--I've had several great receivers that gave up a channel or sounded terrible and they needed repeated pot and switch cleaning--and then the missing channel came back!

6. If it was working before you started (this is a tough pill to swallow), and it's not working right now, you probably did something wrong. I am not putting you down; this has been my own experience and that of many others here--nothing to be ashamed of but rather a trial-and-error thing. The 7070 is a nice machine; I tried to learn on gear that was already broken or didn't cost a lot--that way I didn't feel so bad when I messed up. If you're like me, you do your best learning by actual doing, by making mistakes in real time and learning from them.

7. Cleaning: I've found that Deoxit can take a while to work. I use CRC Electronics cleaner (red can, get at Home Depot, etc.) and it works fast. I follow it up with Faderlube to lube the pots. Sometimes you think you've cleaned a pot or switch but the cleaner didn't make it all the way in or not at all. I had a Sansui 800 whose power switch had to be completely taken apart to get it to conduct pixies--Other culprits are often: TAPE MON switch, SPEAKER SELECTOR switch, PRE/MAIN switch (if equipped--usually RCAs with jumpers).

That's a nice machine and pretty well-known; others can give you specific advice for what's actually going on, but I think these general practices will help you--they helped me!

P.S. Yes, I've successfully recapped many amps and receivers, so the mistakes do lead to success.

 

[Tom B]

my suggestion is to touch nothing else, other than the phono equalizer board. You didn't have these problems until you messed with the equalizer board. Verify all the parts you replaced and the orientation of those parts. But truthfully messing up things on this equalizer board should only affect phono, as the other functions (FM, AM, AUX etc) don't go through the equalizer board. But incorrect transistor or capacitor installation could have affected the regulator power supply. On the F2626 board check pin 2, 3, and pin 4 for DC volts ~40V from each pin to ground. You could have fried a transistor or caused a resistor to open like the 22 ohm (R04 resistor). If your 40v is missing, you will then need to fix that board.. If the voltage is missing, verify ohm reading of pin 2 pin 3 and pin 4 to ground (Especially suspect pin 3) to see if you created a short somewhere.
The above is assuming that you worked on the driver board and everything was fine after that, and then you worked on the phono equalizer board ONLY and developed problems. If you also worked other areas, then I would not know where to start to help you. As others have said, always work only one board/one area at a time. Working any more than that before retesting sets you up for disaster.

Another suggestion is to refrain from changing transistors unless they are bad, or known to cause problems such as 2SA726, 2SA750, 2SC458 etc. Replacing all transistors doesn't buy you anything, and can set you up for problems, as replacement transistors may have different pinouts, causing problems such as you have.

 

[toymanmsp]

Do you have a transistor checker and a capacitor esr meter? I would highly recommended you go back to the phono board and check components. Incorrectly installed transistors can have a cascade affect and take out a lot of other components. If it's possible, disconnect the phono board from the main board and see if the problem goes away.

Thanks for the reply. Yes, I have a transistor tester and a DMM. Very helpful. I will look at the transistors pinning orientation again on the phono board. I've already gone thru the caps.

 

[toymanmsp]

Hi Adam--I'll offer some general advice from the viewpoint of someone who is completely self-taught as has made many mistakes!

1. Try to work on one board at a time, testing your work frequently. Do you have a Dim Bulb Tester? If not, EngineerNate has an excellent post about how to make one--follow the directions exactly or it will not work correctly. It's a crude but effective tool to safely detect shorts in your gear.

I do go one board at a time. I'm replacing the caps first, install a 2-3, and power up to test. I do have a DBT, invaluable.

2. I try to desolder and replace ONE cap at a time. I pull the old cap out and stand it up in front of me, then I get the new cap out and double check the values and make sure it's not bipolar. I also make a small drawing of the board and note where the negative terminals of each cap are.

I do go one cap at a time as mentioned above. I will use both the service manual and schematics, provided they are available. I will take your suggestion of drawing out the board for notation. I have been doing this with the transistors, but not for the caps.

3. Before thinking of more complicated possible problems (cracked volume pot--have never encountered this personally), always think of the simplest things it could be first. Did you blow the fuses (if equipped)? Are the relay (if equipped) contacts clean? Some amps will still play sound with blown speaker fuses but at a very low sound level,

I do try to keep it simple, however, the forums just add ideas. I don't believe that I blew any fuses at least from what I could see, but did install new ones. My relay does click on, but not sure if the contacts are clean. I do have a new relay, which I haven't installed.

4. I would NOT mess with the transistors yet, if possible. Do the recap first and see how things go. Simplest stuff first.

Recap is done, transistors have been installed. I'm rechecking these. The board is a bit tight, so it makes it difficult to see the board clearly.

5. Do not underestimate the importance of thorough pot and switch cleaning--I've had several great receivers that gave up a channel or sounded terrible and they needed repeated pot and switch cleaning--and then the missing channel came back!

I'll give her another bath, nothing to loose there.

6. If it was working before you started (this is a tough pill to swallow), and it's not working right now, you probably did something wrong. I am not putting you down; this has been my own experience and that of many others here--nothing to be ashamed of but rather a trial-and-error thing. The 7070 is a nice machine; I tried to learn on gear that was already broken or didn't cost a lot--that way I didn't feel so bad when I messed up. If you're like me, you do your best learning by actual doing, by making mistakes in real time and learning from them.

You're spot on as I learn best from doing and thru making mistakes. It is a tough pill to swallow, as I'm sure that the issue is likely due to my error. That said, I knew the risks going in and will work my way thru it. The 7070 is a very nice machine and more challenging than the other amp and receivers that I've recapped. I've successfully recapped an AU-101, 2000X (driver board, replaced the problematic fuse resistors), 2 Pioneer SX-535s, and partway thru a SX-636. I'm working my way thru my receivers and amplifiers, 3 additional Sansui's to get to.


7. Cleaning: I've found that Deoxit can take a while to work. I use CRC Electronics cleaner (red can, get at Home Depot, etc.) and it works fast. I follow it up with Faderlube to lube the pots. Sometimes you think you've cleaned a pot or switch but the cleaner didn't make it all the way in or not at all. I had a Sansui 800 whose power switch had to be completely taken apart to get it to conduct pixies--Other culprits are often: TAPE MON switch, SPEAKER SELECTOR switch, PRE/MAIN switch (if equipped--usually RCAs with jumpers).

I do have CRC electronics cleaner, Deoxit is what I use primarily, but will give the CRC a try. Certainly doesn't hurt to give this another rinse.

That's a nice machine and pretty well-known; others can give you specific advice for what's actually going on, but I think these general practices will help you--they helped me!

Thanks for your comments, it's reassuring to know that I'm not alone in wanting to get the best sound out of my equipment. Regardless, of the issues that I'm running into with the 7070, I'm having fun working on my gear. It's very satisfying when things do go right! I'll get there.

Adam

P.S. Yes, I've successfully recapped many amps and receivers, so the mistakes do lead to success.

 

[toymanmsp]

my suggestion is to touch nothing else, other than the phono equalizer board. You didn't have these problems until you messed with the equalizer board. Verify all the parts you replaced and the orientation of those parts. But truthfully messing up things on this equalizer board should only affect phono, as the other functions (FM, AM, AUX etc) don't go through the equalizer board. But incorrect transistor or capacitor installation could have affected the regulator power supply. On the F2626 board check pin 2, 3, and pin 4 for DC volts ~40V from each pin to ground. You could have fried a transistor or caused a resistor to open like the 22 ohm (R04 resistor). If your 40v is missing, you will then need to fix that board.. If the voltage is missing, verify ohm reading of pin 2 pin 3 and pin 4 to ground (Especially suspect pin 3) to see if you created a short somewhere.
The above is assuming that you worked on the driver board and everything was fine after that, and then you worked on the phono equalizer board ONLY and developed problems. If you also worked other areas, then I would not know where to start to help you. As others have said, always work only one board/one area at a time. Working any more than that before retesting sets you up for disaster.

Another suggestion is to refrain from changing transistors unless they are bad, or known to cause problems such as 2SA726, 2SA750, 2SC458 etc. Replacing all transistors doesn't buy you anything, and can set you up for problems, as replacement transistors may have different pinouts, causing problems such as you have.

Thanks for your input, I will look at the F2626 board today and check the pin for DC measurements. Your assumption is correct that I've already done the driver board and everything was fine until I moved to the equalizer board. I do go one board at a time, along with removing a couple of caps, then testing with my DBT, remove a few more caps, etc. I am only replacing the known bad transistors as you've indicated 2SA726, 2SA758, etc. and not replacing all of them on the board. I do have a transistor tester which I am using (it's extremely helpful) I test after each pull, along with testing the new transistor as well. I'm still learning best practices when it comes to these projects. I have recapped other pieces of my gear previously, this one is more complicated than the others that I've done. Partly, due to the difficulty in accessing the board, it's not the easiest board to work on.

I will say that while this is challenging, I'm enjoying it. Something very satisfying about a successful recap.

I'll update when I have more to share. Thanks.