SX-880 with protection issues

[merlynski]

I guess a lot of questions from noobies are annoying to senior more experienced members. I only hope that members will be understanding of this. I'm not trying to annoy anyone.

@mgl99 I was/am not annoyed, just confused, questions are good!!
Manufacturers will often use different transistors in a particular location (i.e. Q3) if they have a stock shortage or an engineering change, but a transistor designated as "Q3" on a particular board, in a particular unit, will stay labeled as Q3. Its designation is tied to its function / location during circuit analysis so just referring to it as a "2SA850" does not tell those of us trying to help which 2SA850 you mean (there may be many, or none in the documentation). I am sure you understood what you meant, but we cannot see what you see, and may never have seen or had hands on the unit you are working on. In my case I work from Service Manuals and Schematics, rather than a background of 'hands-on' lots of units, like some here. I have lots of training, background and experience in electronics, but only have hands-on experience with Vintage Audio equipment I own. I have learned an immense amount from the others on this site, both experienced and 'noob'. I try as hard as I can to help if I can.

 

[mgl99]

I am aware that the manufacturer tends to start renumbering components for each individual board, which is why I had stated the board number AWR-156. The transistor comp IDs that I was discussing are on this board; not some other board, this board. The Q1 - Q6 I have been speaking of are on the AWR-156 board; which is why I had stated the board number which apparently confused some.

As it turns out, the culprit appears to have been Q6, 2SC945. I pulled it and it tests defective. I replaced it with a KSC1845. Q2 and Q3 tested okay but I replaced them with the correct replacements which I also had in stock. It appears to have corrected the problem. I want to hook up the same set of speakers as before and verify this. Results to be posted here, as no one has told me what thread I should be posting on. Sorry I don't understand the protocol of the forum.

 

[merlynski]

The Original Poster that created this thread also created the title, which indicates this thread is about his SX-880 and its problem with Protection issues.
The normal protocol is that the focus of the thread is on the subject that it was originally created for, rather than a general 'Protection Issues' subject line.
The AWR-156 is part of a US SX-980, a different unit, so it would normally be discussed in a thread about the SX-980. I was focusing on the SX-880 Protection issue, which is why I was confused by the reference to the AWR-156 power supply board. I will be happy to help you with your SX-980, lets just move to a thread for that purpose. Create one and/or mention @merlynski in one you are already working on, I will show up. If I am causing you grief just let me know and I will stay away, your choice entirely.
Unfortunately this thread wandered off track about post #100. That is partly my fault because I answered a question only somewhat related to the original thread.

 

[mark long]

I am aware that the manufacturer tends to start renumbering components for each individual board, which is why I had stated the board number AWR-156. The transistor comp IDs that I was discussing are on this board; not some other board, this board. The Q1 - Q6 I have been speaking of are on the AWR-156 board; which is why I had stated the board number which apparently confused some.

As it turns out, the culprit appears to have been Q6, 2SC945. I pulled it and it tests defective. I replaced it with a KSC1845. Q2 and Q3 tested okay but I replaced them with the correct replacements which I also had in stock. It appears to have corrected the problem. I want to hook up the same set of speakers as before and verify this. Results to be posted here, as no one has told me what thread I should be posting on. Sorry I don't understand the protocol of the forum.

Now that you've done that, if you want to work on the heat issues, follow the R1 suggestions. Don't replace it with a higher value resistor until you've calculated the voltage drop. Also, try to find a larger/better heatsink than the stock one.

 

[mgl99]

Final post on this SX-980. The SX-880 is running great after switching out C418 as described above. I will be pursing the R1 issue when I receive the parts required.

The SX-980 was exhibiting very similar problems in that it was tripping out the protection relay at approx. 45% volume control level. The difference between the power boards in the two units is substantial, also the fact that the 980 has a step attenuator for volume, the 880 has a standard wound pot. As it turns out, the failure of Q6 on the AWR-156 board, essentially caused the full spread of power from the volume reaching maximum at approx. 45% of volume. More than likely the protection relay was seeing imbalance that caused it to trip; basically doing what it is supposed to do. Replacing Q6 returned the volume span to normal (as it was designed). After no trip occurred without spkrs being driven, I then hooked up to the speakers, and again, no trip (until near maximum volume). I'm satisfied that this was the cause of the issue. I'm sure others will dispute my findings., but regardless, I'm happy. Thank you to those who have helped out.

 

[mark long]

Final post on this SX-980. The SX-880 is running great after switching out C418 as described above. I will be pursing the R1 issue when I receive the parts required.

The SX-980 was exhibiting very similar problems in that it was tripping out the protection relay at approx. 45% volume control level. The difference between the power boards in the two units is substantial, also the fact that the 980 has a step attenuator for volume, the 880 has a standard wound pot. As it turns out, the failure of Q6 on the AWR-156 board, essentially caused the full spread of power from the volume reaching maximum at approx. 45% of volume. More than likely the protection relay was seeing imbalance that caused it to trip; basically doing what it is supposed to do. Replacing Q6 returned the volume span to normal (as it was designed). After no trip occurred without spkrs being driven, I then hooked up to the speakers, and again, no trip (until near maximum volume). I'm satisfied that this was the cause of the issue. I'm sure others will dispute my findings., but regardless, I'm happy. Thank you to those who have helped out.

if it's working I see no reason to dispute it, good work and welcome

 

[dustmaker]

I last posted two months ago on this sx-880. It wasn’t working, then it was, then it wasn’t…. I got frustrated and went on to other projects. The last thing I did before it stopped working and i set it aside was to put Q37 in backwards. Now I am back to it and here is the status:

I corrected Q37 and it’s now working again. But it still has an issue. It has very low power while in AUX or Tape1 or Tape 2. I am only measuring about 10 watts of power with both a DMM and the power meters in the unit. However, when on FM, it seems to have full power. While on FM, I can only turn it up <1/4 volume and it is as loud as I can tolerate. When I use AUX or either tape input, I can rotate the volume knob all the way (using an iPad as source of music with volume turn up to 80%). I also noticed this when using a 1K 150mV sine wave, I only measure 10 watts.

i looked at the schematics but I can’t distinguish a difference in the path of FM from other inputs.

as a reminder so that you don’t have to search the thread, I was testing previously and I was using 2 - 4 input volts (instead of 150mV) as an input for my sine wave, so I was way over driving the input stage. I suspect I damaged something with that voltage

any suggestions?

 

[dustmaker]

Additional information for anyone following this thread.

the PCB does have incorrect orientation shown for C418, the + and - are reversed but the schematics are correct.

I did try using a 220 μF/35V non-polar capacitor for C418 as suggested in another post and mentioned above (and I believe my unit originally had a 220 μF here when I received it) and the unit did work EVEN WITHOUT Q35 INSTALLED. The relay would pull in immediately without Q35 installed. I suspect a lot of folks use the 220 as a hack when they have a damaged Q35

I took out the 220μF capacitor (and replaced with a 100μF) because I suspect this is somehow bypassing the protection circuit. I put my Q35 back in. Should I leave in the 100μF or go back to the original 47μF?

 

[mgl99]

I didn't say anything about using a non-polar cap for C418. I used a regular polarized cap (220 uf @ 35v) and put it in with the correct +/- orientation (as shown on the schematic). It solved the problem. I never mentioned using a bi-polar type cap. I guess I don't understand the reason why you used a "non-polarized" cap for C418. Why would it compromise the protection relay? With speakers being driven, mine still trips the protection relay, but at a level that I would expect it to; not at 45% volume control.

 

[dustmaker]

I last posted two months ago on this sx-880. It wasn’t working, then it was, then it wasn’t…. I got frustrated and went on to other projects. The last thing I did before it stopped working and i set it aside was to put Q37 in backwards. Now I am back to it and here is the status:

I corrected Q37 and it’s now working again. But it still has an issue. It has very low power while in AUX or Tape1 or Tape 2. I am only measuring about 10 watts of power with both a DMM and the power meters in the unit. However, when on FM, it seems to have full power. While on FM, I can only turn it up <1/4 volume and it is as loud as I can tolerate. When I use AUX or either tape input, I can rotate the volume knob all the way (using an iPad as source of music with volume turn up to 80%). I also noticed this when using a 1K 150mV sine wave, I only measure 10 watts.

i looked at the schematics but I can’t distinguish a difference in the path of FM from other inputs.

as a reminder so that you don’t have to search the thread, I was testing previously and I was using 2 - 4 input volts (instead of 150mV) as an input for my sine wave, so I was way over driving the input stage. I suspect I damaged something with that voltage

any suggestions?

At this point I suspect D18is bad since I had other diodes that were bad. When I look at the schematics, it show MZ-061 which is a 6.1v zener but it also shows 13.5 volts (pic attached)
I’m confused here. What is the recommended replacement part?

 

[mark long]

At this point I suspect D18is bad since I had other diodes that were bad. When I look at the schematics, it show MZ-061 which is a 6.1v zener but it also shows 13.5 volts (pic attached)
I’m confused here. What is the recommended replacement part?View attachment 2464682

https://www.digikey.com/en/products/detail/nte-electronics-inc/NTE5013A/11647770
https://www.mouser.com/ProductDetail/onsemi-Fairchild/1N5233B?qs=SSucg2PyLi7cQocVn5ubiw==

verify datasheet

That is not forward voltage

 

[mgl99]

"However, when on FM, it seems to have full power. While on FM, I can only turn it up <1/4 volume and it is as loud as I can tolerate."

This was my first indication on the 880, that something was wrong. This was totally corrected by the change out of C418.

 

[shawnmgray]

I have an SX 880 that had this protection issue. I did not do the resistor mod. I installed a 220 uF @ 50 volts cap, opposite the board markings. Protection issue is gone. Sounds great!

 

[ring]

Can confirm meaningful improvement with 220uF non-polar C418 installed.

Problem was very frequency sensitive, and only showed up using inductive load - problem can't be observed using resistive load.

Before modding 110Hz was fine into a 4 Ohm speaker, but at 40Hz it would cut out before even 30W. After cap swap it can go beyond 60W at 40Hz.