2275 Amp Distortion

[Jamie Starr]

Guys: I am restoring a 2275 and have distortion issues in the left channel. I have narrowed it down to the left power amp.

The distortion problem was there when I obtained the receiver. I initially thought it was just really dirty pots. I then thought it may be the tone board. But I have definitely ruled those out. (I have a working 2275 and swapped out the amps. No distortion when I do that.)

The distortion begins to happen after about four minutes of running the amplifier. That’s what kind of threw me off. I thought I had the problem solved but did not.

Any suggestions on where to look? Output transistors? Cold solder joint? The fact that it doesn’t show up for four minutes is some kind of clue. Thanks for your help. I have already recap the board.

 

[TMaster]

From your pic you may have a few cold solder joints. I see a couple that look like it. Top left area, and the lower middle. Reflowing them all shouldn't take long, 10-15 min...

A component could be heating up and leaking or getting thrown out of tolerance. This first thing I would do is Probe each transistor with the diode checks. Write down the voltage drops for each one. Example, NPN: B to C = .601V and B to E .591V. PNP will be E to B and C to B. You don't have to take them out of the circuit, just get the 2 voltages per transistor. These voltages should be close, if you have one that is way out, like .450V and .601V then I would replace it. This goes for all the transistors. Personally I would just replace them all and the diodes. Oh and Deoxit the bias and DC offset trimmers.

I have the BOM parts list for the P700 amp boards if you need it.

 

[EastPoint]

If you reflow the solder joints and it's still acting up, go to the bad channel and hit the transistors with freeze spray, one by one. Don't forget to try the outputs. If the noise clears up or gets substantially worse when you hit a transistor, that's the culprit.

 

[Jamie Starr]

From your pic you may have a few cold solder joints. I see a couple that look like it. Top left area, and the lower middle. Reflowing them all shouldn't take long, 10-15 min...

A component could be heating up and leaking or getting thrown out of tolerance. This first thing I would do is Probe each transistor with the diode checks. Write down the voltage drops for each one. Example, NPN: B to C = .601V and B to E .591V. PNP will be E to B and C to B. You don't have to take them out of the circuit, just get the 2 voltages per transistor. These voltages should be close, if you have one that is way out, like .450V and .601V then I would replace it. This goes for all the transistors. Personally I would just replace them all and the diodes. Oh and Deoxit the bias and DC offset trimmers.

I have the BOM parts list for the P700 amp boards if you need it.

Thanks for great information. I reflowed all the solder joints that looked suspect. Didn’t help. I will look again. I also changed out the output transistors. That didn’t do it either. I will check voltages and see what happens. I would appreciate your BOS list.

I replaced caps C701-704, and C707-708. I also replaced H703,704, H715-718, H720-721. I’ll keep looking. Frustrating, but I want to find it. Thanks again.

 

[Jamie Starr]

If you reflow the solder joints and it's still acting up, go to the bad channel and hit the transistors with freeze spray, one by one. Don't forget to try the outputs. If the noise clears up or gets substantially worse when you hit a transistor, that's the culprit.

I have never used the freeze spray technique. Can I find that stuff locally or online only. So do I power up the receiver and literally spray each transistor on the board? What should happen if I spray the bad transistor? Thanks for your help.

 

[Perkinsman]

EastPoint said, "go to the bad channel and hit the transistors with freeze spray, one by one. Don't forget to try the outputs. If the noise clears up or gets substantially worse when you hit a transistor, that's the culprit." I'm sure you can find some at your local electronics store or online.

 

[EastPoint]

Mouser has it.

 

[dr*audio]

Do you have an oscilloscope? Did you check the bias current on that channel? What does the bias current do when the distortion starts? You have to check with no signal.

 

[Jamie Starr]

Do you have an oscilloscope? Did you check the bias current on that channel? What does the bias current do when the distortion starts? You have to check with no signal.

I don’t. Will a DMM work?

 

[Jamie Starr]

From your pic you may have a few cold solder joints. I see a couple that look like it. Top left area, and the lower middle. Reflowing them all shouldn't take long, 10-15 min...

A component could be heating up and leaking or getting thrown out of tolerance. This first thing I would do is Probe each transistor with the diode checks. Write down the voltage drops for each one. Example, NPN: B to C = .601V and B to E .591V. PNP will be E to B and C to B. You don't have to take them out of the circuit, just get the 2 voltages per transistor. These voltages should be close, if you have one that is way out, like .450V and .601V then I would replace it. This goes for all the transistors. Personally I would just replace them all and the diodes. Oh and Deoxit the bias and DC offset trimmers.

I have the BOM parts list for the P700 amp boards if you need it.

Just checked voltages on all transistors. Most showed acceptable voltage differences. 729/718, 636/630 ect.
I thought I might have found something on H705 and 706. C=630 E=061; C=624 E=058. But when I checked those same transistors on the working right channel board, the values were the same, so I doubt that is the problem. I didn’t find any dead shorts on the transistors, so I am scratching my head as to whether those are the problem or not.

 

[JJCalvillo]

Just checked voltages on all transistors. Most showed acceptable voltage differences. 729/718, 636/630 ect.
I thought I might have found something on H705 and 706. C=630 E=061; C=624 E=058. But when I checked those same transistors on the working right channel board, the values were the same, so I doubt that is the problem. I didn’t find any dead shorts on the transistors, so I am scratching my head as to whether those are the problem or not.

705 & 706 sound fishy to me.

And when you hit the transistors with the spray, give each one a few seconds to react to the freeze. You're looking for the distortion to get worse or better. I have a Denon amp where I know the problem is in one channel's transistors, but can't get it to play long enough to use the spray. So you are in pretty good shape.

 

[dr*audio]

I don’t. Will a DMM work?

You can use the DMM to set the bias current, per the service manual. A scope would allow you to see how the waveform is distorted and give you a clue as to the part of the circuit causing it. If it clips on the top of the waveform the problem is probably in the positive drive portion of the circuit, if it clips on the bottom, the problem is in the negative drive.
Check the bias first and see what it is doing when it fails. Get some mini - hook clip leads so you can leave them in and monitor the current.

 

[TMaster]

Since there is a delay before the problem arises, think outside the box - a cheap test before you go and spend more money on freeze spray (when you could pay for all new transistors), blow the hair dryer on the transistors one at a time see if the problem starts right away. Tape a funnel to the hair dryer to get a more precise blow.

Seriously, these receivers are 40+ years old, just replacing an affected transistor is just patching the problem. The others will fail eventually.

Marantz 2275 P700 list Transistors and diodes:
H701 2SC1327 - KSC1845
H702 2SC1327 - KSC1845
H703 2SA721 - KSA992
H704 2SC1327 - KSC1845
H705 2SC945 - KSC2383
H706 2SA733 - KSA992
H707 2S945 - KSC2383
H708 2SA733 - KSA992
H709 2SC496 - 2N4923G
H710 S3016R - 1N4148 diode
H711 2SA794 - KSA1381
H712 2SC1567 - KSC3503
H713 2SD382 - KSC2073
H714 2SB537 - KSA940

​

H715 WZ-150 - BZX85B15-TR
H716 WZ-150 - BZX85B15-TR
H717 1S2470 - 1N4148
H718 1S2471 - 1N4148
H719 1S2471 - 1N4148
H720 VD1122 - 1N4148
H721 VD1122 - 1N4148
R741 - 2K Trimmer 3386F-1-202LF
R742 - 1K Trimmer 3386F-1-102LF

​

 

[Jamie Starr]

Since there is a delay before the problem arises, think outside the box - a cheap test before you go and spend more money on freeze spray (when you could pay for all new transistors), blow the hair dryer on the transistors one at a time see if the problem starts right away. Tape a funnel to the hair dryer to get a more precise blow.

Seriously, these receivers are 40+ years old, just replacing an affected transistor is just patching the problem. The others will fail eventually.

Marantz 2275 P700 list Transistors and diodes:
H701 2SC1327 - KSC1845
H702 2SC1327 - KSC1845
H703 2SA721 - KSA992
H704 2SC1327 - KSC1845
H705 2SC945 - KSC2383
H706 2SA733 - KSA992
H707 2S945 - KSC2383
H708 2SA733 - KSA992
H709 2SC496 - 2N4923G
H710 S3016R - 1N4148 diode
H711 2SA794 - KSA1381
H712 2SC1567 - KSC3503
H713 2SD382 - KSC2073
H714 2SB537 - KSA940

​

H715 WZ-150 - BZX85B15-TR
H716 WZ-150 - BZX85B15-TR
H717 1S2470 - 1N4148
H718 1S2471 - 1N4148
H719 1S2471 - 1N4148
H720 VD1122 - 1N4148
H721 VD1122 - 1N4148
R741 - 2K Trimmer 3386F-1-202LF
R742 - 1K Trimmer 3386F-1-102LF

​

Thanks so much for this. My soldering skills are way better than my diagnostic skills.
Do H701-702 have to be “matched”?

 

[Jamie Starr]

You can use the DMM to set the bias current, per the service manual. A scope would allow you to see how the waveform is distorted and give you a clue as to the part of the circuit causing it. If it clips on the top of the waveform the problem is probably in the positive drive portion of the circuit, if it clips on the bottom, the problem is in the negative drive.
Check the bias first and see what it is doing when it fails. Get some mini - hook clip leads so you can leave them in and monitor the current.

Will do. I’ll report back. Thanks for your help.

 

[TMaster]

Matching them is better, order 10... You can get a transistor tester on amazon pretty cheap. Make sure the BCE are right when placing the transistors. Some of the old ones might be reversed, I can't remember, don't go by how they look. Also build a dim bulb tester (DBT), there are how to plans on the web and here. I would not power it up directly plugged into the wall after changing multiple components out at a time.

 

[Jamie Starr]

A few things to report. I changed out the 2 pots at 741 and 742 since I had the parts on hand. The idle current pot at 742 will not adjust. Holds steady at .006 mv's. I don't know if it adjusted before because I didn't bother biasing once I heard the distortion. The removed pot works and installed pot is brand new.

I decided to hit the transistors I could reach with freeze spray (compressed air can upside down). The distortion was gone when I sprayed H714! At least now I have an idea where to look.

Does anyone know if H714 would affect idle current pot not adjusting? Thanks for all of your suggestions.

 

[catrafter]

Since there is a delay before the problem arises, think outside the box - a cheap test before you go and spend more money on freeze spray (when you could pay for all new transistors), blow the hair dryer on the transistors one at a time see if the problem starts right away. Tape a funnel to the hair dryer to get a more precise blow.

Seriously, these receivers are 40+ years old, just replacing an affected transistor is just patching the problem. The others will fail eventually.

I don't agree with wholesale semiconductor replacement, I think you should have a good reason and some kind of 'test' to back it up. Yes to the replacement of problematic parts such as VD and other 'gob' diodes and known problematic transistors such as 2SC458 types. Yes to overly temperature sensitive parts. Yes to some glue and corrosion damaged parts. Yes to parts with some types of physical damage.
Semiconductors don't tend to wear out, they generally fail for a reason, such as loss of a hermetic seal, overheating or a manufacturing defect.
The MTBF of many semiconductors is in the 100's of thousands of years.

YMMV.
Tom

 

[EastPoint]

Replace H714 and go from there.

 

[TMaster]

"The idle current pot at 742 will not adjust. Holds steady at .006 mv's"

Do you have the "Main" speaker button pushed in?