I recently got my hands on a Sony STR-7055 with the usual issues: crackling potentiometers and background noise. I managed to clean the pots using Deoxit and significantly reduced the noise with the materials I had on hand. Since noise is often caused by failing transistors, I decided to replace several components and ended up nearly rebuilding the amp-power board.
Here’s what I replaced:
I’m no tech and unsure where I went wrong. Could I have missed something during the component replacement, or is there another issue causing these symptoms? Any suggestions on where to start troubleshooting would be greatly appreciated!
Here’s what I replaced:
- All capacitors (electrolytics and films)
- All transistors
- Blob diodes and zeners
- The tuner no longer works.
- The 220Ω resistor at R826 burned out, and a new one overheats quickly.
- The bias on one channel is unadjustable, but the other channel adjusts fine.
- The voltage on the main capacitors is only 26V instead of the expected 50V.
- The orange wire rails on the amplifier board also show 26V.
- The control board is receiving 12V instead of 17V.
I’m no tech and unsure where I went wrong. Could I have missed something during the component replacement, or is there another issue causing these symptoms? Any suggestions on where to start troubleshooting would be greatly appreciated!
- I don't know where you got the idea that crackles and hiss came from transistors.It certainly was not here. Bad solder joints, broken lands, bad welds in resistors and capacitors are just as likely as bad welds in transistors and diodes. Sixties Seventies transistors sometimes had hiss from dirt in the processing area, but by 1980 most fabs had gone to clean rooms in filtered air with tyvek bunny suits for the few maintenance people required to go in there.
- Amateurs make about 1/3 bad solder joints if not worse. While I do advocate replacing all rubber caps in units I intend to keep, they are usually the cause of hums, buzzes, low volume, imbalance, and too many or teo few highs or lows. They do not cause crackles typically. In a working unit I test the unit after each one or two replaced parts to prove that I made the unit better, or did not damage it at least. I use trash speakers and a battery radio input in case I caused large voltage to come out either end. If the unit goes worse, I back up and figure out what I did wrong before proceeding. I usually use a scope or sound probe to find crackles and hiss, the exact part causing that symptom, before I replace anything else.
- Furthermore, if you have been using the exact number matching capability of online markets like ebay, facebook marketplace, gumtree, aliexpress or similar, you have injected a lot of beautifully labeled trash into your unit. Very few leaded transistors are made these days, and finding a match for most old numbers is a game for experts. Plenty of people will label trash they bought with exactly the number you are looking for. I buy transistors from authorized distributors like Farnell, RS of UK, Reichelt of GER.
- Film capacitors almost never fail in class AB amplifier service. The exception is units killed by shorted speaker wiring or speaker drivers. After the output transistors short, high rail voltage rampages out the base line and blows up many other components, including tantalum and film caps that are not rated for the rail voltage. This short on speaker terminals is the cause of most failed transistors resistors and diodes, starting at the back. In one shorted unit I replaced 140 parts including one film cap in the VI limiter and one shorted tantalum cap filtering the op amp 15 v supply.
- This unit now has so many problems I suggest you scrap it and buy some modern trash that will give you 4 to 6 years of okay service. Modern consumer electronics are designed to fail, just not during the warranty period.
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He's right. Your best option is probably to put back all the original components and start from there with some decent measuring equipment. And the knowledge to use it.
I suggest you post decent photos of what you've done, both sides of the boards, so we can see what is going on.
So I recapped and re-tested almost all the boards. The used parts were purchased from a trusted supplier, Mouser, and some local sources.
Substitutes were carefully selected to be as close as possible to the originals.
After support from AK-ers, I concluded that something is drawing too much current.
Testing through DBT:
Should I check all the resistors as well? I tested a bunch of them, and they were all within spec.
I also pulled out the output transistors and observed the same voltage.
Ignore markings on schematics...
Thank you in advance for some suggestions. It truly means A LOT and is deeply appreciated.
Substitutes were carefully selected to be as close as possible to the originals.
After support from AK-ers, I concluded that something is drawing too much current.
Testing through DBT:
- Rail voltage is ±26V.
- Bias is non-adjustable.
- R826 is smoking.
- If I pull out R827, the voltage increases to 32V (38V without DBT), and the smoking stops.
Should I check all the resistors as well? I tested a bunch of them, and they were all within spec.
I also pulled out the output transistors and observed the same voltage.
Ignore markings on schematics...
Thank you in advance for some suggestions. It truly means A LOT and is deeply appreciated.
Test on diode scale of DVM of transistors & diodes at 2 volts or less can sometimes prove them bad. 0 v forwards instead of 500-750 mv. Lower than 9999 or ---- backwards.
It does not prove that the junction can withstand 10-60 volts of actual conditions.
Iceo tests at 12 v found that some transistors that were fine on the diode scale, were trash at 12 v. 12 v supply series 47 k resistor series 2000 uA scale of dvm. Base open. Result <10 uA, good. Result 255 uA, trash. + to c of npn, - to c of pnp.
Amps are usually powered up with a 60 w tungsten bulb series the AC input. Lamp flickers and goes out, proceed to voltage checks. Lamp stays on, serious problems. Called the DBT procedure. Slows the self-destruction of parts while tech probes the amp for problems.
To avoid killing yourself, never use 2 hands to test an amp with the AC on. Use alligator clip to connect black lead of DVM to analog ground. Probe with 1 hand, write down answers, and use subtraction to determine voltage across any component.
Wear no Jewelry or metal on hands wrists or neck. 1 v at high current can burn your flesh to charcoal.
Wear safety glasses. Parts can explode, solder can splash in the eyes. Especially desoldering.
I see a lot of bad joints, broken tracks, and its obvious this board needs to be screwed down to complete the circuit.
Your not just barking up the wrong tree, your not even in the right forest.
Your not just barking up the wrong tree, your not even in the right forest.
Thank you both for your answers.
I am very careful at my work, just on this Sony I was too much confident since I successful replace a lot parts in Marantz units which I am a little more familiar with.
I will take a high resolution photo to inspect the board. I guess my old eyes are not what they use to be.
thanks again.
BR
I am very careful at my work, just on this Sony I was too much confident since I successful replace a lot parts in Marantz units which I am a little more familiar with.
I will take a high resolution photo to inspect the board. I guess my old eyes are not what they use to be.
thanks again.
BR
Illuminated bench mount magnifying glass.
You can then rotate and move the work piece to the best angle needed, as well as being able to use both hands as you work.
The poor joints are not purely yours, but also a lot of factory joints look bad.
You can then rotate and move the work piece to the best angle needed, as well as being able to use both hands as you work.
The poor joints are not purely yours, but also a lot of factory joints look bad.
It is likely that some of them are missing or half because I have pulled out pins that feed another boards.
Thx anyway.
Thx anyway.
TO-3 are in sony 7065. Boards are the same I think.
I am hitting a wall here.
I have ordered new diodes, rectifiers, big W resistors, and main caps. Material is a little more than 10€ so I think I could try.
I am hitting a wall here.
I have ordered new diodes, rectifiers, big W resistors, and main caps. Material is a little more than 10€ so I think I could try.
They are on chassis heatsink. Output transistors on amp-power board were original sonys and I think that nobody messes with the receiver.
OK you found the outputs. Did you measure correct +/- 50V rail voltages before starting to fiddle with the board? What is the mains where you are: 125V or 230V? Check that the voltage setting is correct in the back of your amp.
I checked voltage selector. It is ok. 230V position.
At first rail voltage was 50V and bias adjustable. All in all working unit with hum...
At first rail voltage was 50V and bias adjustable. All in all working unit with hum...
Guys, I am on to something. D701 and 751 were blob diodes VD1221. I replace them with 1N4148.
So I googled a bit more. For replacement there should be two 1N4148 in series on D701 and D702. I solder them together and .....
Voltage on rails drop at 17V. DBT almost bright. Resistor R826 Is not smoking anymore. So there is some kind of a progress. Even in backwards.
Now I think I just have to find proper replacement or figure it out where is the short coming.
So I googled a bit more. For replacement there should be two 1N4148 in series on D701 and D702. I solder them together and .....
Voltage on rails drop at 17V. DBT almost bright. Resistor R826 Is not smoking anymore. So there is some kind of a progress. Even in backwards.
Now I think I just have to find proper replacement or figure it out where is the short coming.
I really feel your pain. I started a hobby of repairing a Sony STR-6045 and went through similar experiences. Changing transistors and capacitors can create more problems if you are not careful. Are you sure you don't have one transistor installed with the wrong pin out?
You can check this thread for ideas on how to troubleshoot: https://www.diyaudio.com/community/threads/sony-str-6045-i-did-it-again.406979/
In my case, it was a tiny scratch that was cutting off a trace on the board.
If you have dim bulb tester, this is safer to take measurements while the power is on for a short period of time. Do you have a good working channel? If so, write the voltages value on the schematic to compare the non working with the good one. Users of this forum are most likely to help if you give them these readings directly on the schematic diagram.
You can check this thread for ideas on how to troubleshoot: https://www.diyaudio.com/community/threads/sony-str-6045-i-did-it-again.406979/
In my case, it was a tiny scratch that was cutting off a trace on the board.
If you have dim bulb tester, this is safer to take measurements while the power is on for a short period of time. Do you have a good working channel? If so, write the voltages value on the schematic to compare the non working with the good one. Users of this forum are most likely to help if you give them these readings directly on the schematic diagram.