Jump to content

Vintage Amplifier with No Sound OutPut


Recommended Posts

I got a vintage amp (Realistic STA-76) that has not been used many years and just powered on and there is no sound output through speaker but only minor low frequency hum.

The small hum does not change whether volume is up or down. 

 

+ Checked if there are any cold solder or bad solder on circuit boards but all solders look good.

+ Visual checked on all of capacitors and resistors and they seem okay. (Have not checked with multimeters yet though).

+ Cleaned volume knob and all the other knobs with Deoxit but it does not help on bringing sound.   

+ Fuses are all seem okay and they do not go pop when power on and off. so it is a good thing at least. 

 

My guess is that Mosfet output transistors are gone bad hence no output of sound.  Is output transistor the common cause of no sound output for vintage amplifier?    

When I measure resistances on those 4 transistors with multimeter, they seem all over the place. I see reading when positive and negative probes on emitter and collectors of transistors.

 

In order to test if they are the cause, I am trying to replace those MOSFET and I cannot find any alternative ones other than used one taken out and sold on eBay.

 

The Mosfet output transistors are 4 x Sanyo 2SD313D - 7648

Does anyone know where to get replacement of 2SD313D transistors?

 

Here are a couple of shots of inside amp.

 

Any input or suggestions would be greatly appreciated. Thank you very much...

 

IMG_6261.JPG

IMG_6260.JPG

Edited by Spider27
Link to post
Share on other sites

2SD313D does not appear to be a MOSFET, just a common BJT. Resist the urge to buy NOS(or whatever) off ebay, plenty of modern day equivalents that should do the job (OnSemi?). I didn't see where you listed the receivers make/model, would be helpful.

 

Best to test transistors using the diode function on your multimeter (google diode test...) need to work through 6 combinations for each transistor, looks like you can use the sets of 3 posts in front of the transistor. Looks like an early cap coupled amp, they tend to sound "valve like", yeah worth spending a little time on it.

 

Suggest,

make/model?

carefully measure dc voltages at each pin (4 groups of 3 pins) with black probe connected to chassis/ground. Use minigrabbers or similiar to clip to the wire wrap posts.

 

EDIT: Most common cause in vintage for failed output is dirty switches. Since both channels affected the power supply would be a strong candidate, either a failed transistor or cap. Output transistors are probably ok, would expect some noise otherwise. Hum maybe "normal" or pointing to tired main cap.

Edited by mbz
cause
Link to post
Share on other sites


1 hour ago, miyouzaic said:

Make sure the tape monitor switch is in the correct position...

Show us a picture of the front and rear panel.

Yep, that was the first thing that I checked. I tried both on and off and does not make any difference. 

Link to post
Share on other sites
13 minutes ago, mbz said:

2SD313D does not appear to be a MOSFET, just a common BJT. Resist the urge to buy NOS(or whatever) off ebay, plenty of modern day equivalents that should do the job (OnSemi?). I didn't see where you listed the receivers make/model, would be helpful.

 

Best to test transistors using the diode function on your multimeter (google diode test...) need to work through 6 combinations for each transistor, looks like you can use the sets of 3 posts in front of the transistor. Looks like an early cap coupled amp, they tend to sound "valve like", yeah worth spending a little time on it.

 

Suggest,

make/model?

carefully measure dc voltages at each pin (4 groups of 3 pins) with black probe connected to chassis/ground. Use minigrabbers or similiar to clip to the wire wrap posts.

 

Thank you for your feedback. 

 

The model is Realistic STA-76 and HifiEngine has manual with schematics. The attached part is main amp section with highlight where those transistors are. :) 

 

 

Screen Shot 2020-09-19 at 7.13.27 pm.png

Link to post
Share on other sites

Above schematic has guiding values, if you are comfortable doing the measurements. I would be betting on a power supply issue.

Link to post
Share on other sites
2 hours ago, mbz said:

Above schematic has guiding values, if you are comfortable doing the measurements. I would be betting on a power supply issue.

 

I found that these are NPN bipolar transistors and just did diode test and below are the measurement.

 

1st transistor from left (TR511) - B-C 0.175v, B-E 0.174v, Reverse should not have continuity but I got reading exactly same on opposite so C-B 0.175v, E-B 0.174v

2nd transistor (TR509) - B-C 0.045v, B-E 0.099v. Reverse are same C-B 0.045v, E-B 0.099v

3rd transistor (TR510) - B-C 0.505v, B-E 0.124v, Reverse are same C-B 0.505v, E-B 0.124v

4th transistor (TR512) - B-C 0.492v, B-E 0.121v, Revsere this time a bit different C-B 0.639v, E-B 0.124v

 

If I understood correctly, reverse should not have reading and no continuity but both way have diode drop so it means that they are faulty? 

 

Link to post
Share on other sites


When I check the DC voltage output through speaker out is about 0.45v for both channels. I am not sure how much DC voltage out should be but seem very low. 

Link to post
Share on other sites

Looks like you did the diode test with transistors "in circuit", this can be misleading, eg, TR509eb has an alternate path through R541,R533 etc... There is also the question whether your multimeter puts out enough "amps" to test power transistors. Voltages at the mentioned pins would help.

 

I would have expected a higher "DC-offset" assuming no load/spkrs connected, possibly indicates failed output/driver transistor.

 

Problem could be anything, you can start replacing parts or you can try and work out what parts need replacing.

Link to post
Share on other sites

DC bias aim is not more than 10mV - 20mV MAX. Your amp needs adjustment and re-biasing. Also probably new adjustment pots - replace them with a multi-turn cermet version. But start from the basics - PSU first everything else later. 

Link to post
Share on other sites
9 hours ago, Spider27 said:

When I check the DC voltage output through speaker out is about 0.45v for both channels. I am not sure how much DC voltage out should be but seem very low. 

That is actually too high. 50 millivolt would be the max you should tolerate IMO. remember DC kills voice coils. Just a multimeter won't cut it to figure out where's the error. you need oscilloscope to test the signal at input gain stage first and see if that's all good then test the output stage. Secondly please don't test this through a wall socket, take the power supply out and test the circuit with current limited DC power supply. you also need to have load resistor while testing. But my suggestion find a friend who knows what their doing to teach you. I assume you're doing this as more of a learning things and doing it alone is not a good idea. this can go wrong really easy if you're just guessing what to do. I learned that the hard way.

Link to post
Share on other sites


1 hour ago, mbz said:

Looks like you did the diode test with transistors "in circuit", this can be misleading, eg, TR509eb has an alternate path through R541,R533 etc... There is also the question whether your multimeter puts out enough "amps" to test power transistors. Voltages at the mentioned pins would help.

 

I would have expected a higher "DC-offset" assuming no load/spkrs connected, possibly indicates failed output/driver transistor.

 

Problem could be anything, you can start replacing parts or you can try and work out what parts need replacing.

Yes, I was testing in circuit.. (I must be too lazy

 

I completely took them out and measured again and here are the figures.

 

1st transistor from left (TR511) - B-C 0.308v, B-E 0.308v, Reverse C-B 0.319v, E-B 0.319v

2nd transistor (TR509) - B-C 0.048v, B-E  O.L. Reverse C-B 0.048v, E-B  O.L

3rd transistor (TR510) - B-C 0.529v, B-E  O.L, Reverse C-B  O.L, E-B  O.L

4th transistor (TR512) - B-C 0.514v, B-E 0.536v, Revsere C-B O.L, E-B O.L

 

If I understood meaning of these figures correctly, 4th transistor is ok but the rest of 3 are all bad.

 

I am planning to replace all caps and transistors since they are all original and over 40 years old. 

 

One question. may I ask where I can get these modern alternative version of 2SD313D in Australia? 

 

I usually order caps from RS online but does not seem to have these BJT transistors. 

Link to post
Share on other sites
1 hour ago, Decky said:

DC bias aim is not more than 10mV - 20mV MAX. Your amp needs adjustment and re-biasing. Also probably new adjustment pots - replace them with a multi-turn cermet version. But start from the basics - PSU first everything else later. 

Thank you. Yes, that is the plan of attack. I might eventually replace most of parts including adjustment pots and will start from BJT transistor which seem damaged and also all those power caps. 

Link to post
Share on other sites
14 minutes ago, mloutfie said:

That is actually too high. 50 millivolt would be the max you should tolerate IMO. remember DC kills voice coils. Just a multimeter won't cut it to figure out where's the error. you need oscilloscope to test the signal at input gain stage first and see if that's all good then test the output stage. Secondly please don't test this through a wall socket, take the power supply out and test the circuit with current limited DC power supply. you also need to have load resistor while testing. But my suggestion find a friend who knows what their doing to teach you. I assume you're doing this as more of a learning things and doing it alone is not a good idea. this can go wrong really easy if you're just guessing what to do. I learned that the hard way.

Thank you very much. Yes, this is for learning a thing or two. I wish I have a friend who can guide me but I have no friend who knows about this stuff :(  hence asking here.

 

I might get a Variac for safety and testing purpose. This might be handy for this and future use I guess. 

Link to post
Share on other sites

@Spider27 if you don't have anyone you know next best thing is youtube video and watch loads of them. Reading I find helps but nothing beat learning it visually in regards with electronics. And always safety first.

Link to post
Share on other sites


This is an old school cap coupled amp, dc offset should be checked with either speakers connected or 8ohm "dummy" load. Schematic show a mid point voltage of 23Vdc, there will be some leakage through the cap.

image.png.6d9acef506765fe85addad6d62824f68.png

Link to post
Share on other sites
3 hours ago, Spider27 said:

One question. may I ask where I can get these modern alternative version of 2SD313D in Australia? 

MJE15032G is a little overkill but I'd go for it over TIP41C or others. The MJE is a common transistor, buy from a reliable source.

Link to post
Share on other sites
1 hour ago, mbz said:

MJE15032G is a little overkill but I'd go for it over TIP41C or others. The MJE is a common transistor, buy from a reliable source.

Thank you very much. I just checked spec between the two and MJE15032G has higher tolerance in pretty much all categories. Would this be okay?

 

 

Transistor Comparison.jpg

Link to post
Share on other sites
45 minutes ago, Spider27 said:

I just checked spec between the two and MJE15032G has higher tolerance in pretty much all categories. Would this be okay?

Yep, that's why I mentioned it was overkill. It will do the job.

If you want a closer "match", KSD526, though a little harder to find

 

Regarding MJE15032G, forgot to mention that the outer case/flange that the screw goes through is live (=collector) so will need a "silpad" or mica for insulation. Also, looking at the pix again, the thermal paste on the transistors is not original, someones been there before, maybe to renew the paste but more likely to replace the transistors with ebay specials. Point is other (driver) transistors may also be damaged. Suggest you build a dim bulb tester, a simple current limiting device required for first power up after parts replacement, google for plans...

Link to post
Share on other sites
2 hours ago, mbz said:

Yep, that's why I mentioned it was overkill. It will do the job.

If you want a closer "match", KSD526, though a little harder to find

 

Regarding MJE15032G, forgot to mention that the outer case/flange that the screw goes through is live (=collector) so will need a "silpad" or mica for insulation. Also, looking at the pix again, the thermal paste on the transistors is not original, someones been there before, maybe to renew the paste but more likely to replace the transistors with ebay specials. Point is other (driver) transistors may also be damaged. Suggest you build a dim bulb tester, a simple current limiting device required for first power up after parts replacement, google for plans...

Thank you.... Understood . I will order together with other caps and will give them a go. Hope I can pull this out and make this amp working and use it in my secondary system. :) 

Link to post
Share on other sites
1 hour ago, Spider27 said:

Hope I can pull this out and make this amp working and use it in my secondary system.

It's only 12W so will need semi-efficient spkrs, however it's the sound of such amps that may make you smile. I'm tinkering with a Sansui 5050, it's not cap coupled but has that warm sound, gobsmacked at times, still plenty of bargins in vintage.

Link to post
Share on other sites

Have gone through checking all of 47 caps with capacitance meter and @mbz is right. Someone has worked on this and replaced caps already because all of caps seem fairly new and capacitance value is still strong. 

 

There are a couple of odd things noticed.

 

1) C257 capacitor is meant to be 16V 4.7uf but 10V 10uf cap is installed so it is out of spec. Lower voltage value is not good if I understood correctly so will replace this to correct value.

 

2) The schematic says 0.5 (2) for all 4 (R541, R543, R542, R544) resistors. I guess that it means 0.5ohm 2W.

However, R541 & R543 have 2W 5% 0.47ohm resistors and R542 and R544 have 2W 5% 0.51 ohm so both channel has slightly different value. Not sure if it is important but will replace blackened 0.47ohm resistors to 0.51ohm resistors to match with the other channel.

 

Edited by Spider27
Link to post
Share on other sites

They all look like original caps in those pics, to me anyway. They are not modern caps as far as I can see.

Careful going by service manuals or schismatics as often products were revised and alterations were made, very evident is some SM's with some Sansui amps.

 

Edit:  @Spider27 sorry, the blue sleeve caps may be newer as comparing pics of the net most seem to have all the gray sleeve types.

Edited by muon*
Link to post
Share on other sites
5 minutes ago, muon* said:

They all look like original caps in those pics, to me anyway. They are not modern caps as far as I can see.

Careful going by service manuals or schismatics as often products were revised and alterations were made, very evident is some SM's with some Sansui amps.

 

Edit:  @Spider27 sorry, the blue sleeve caps may be newer as comparing pics of the net most seem to have all the gray sleeve types.

Thank you..  Yes, those TK caps seem replacement of originals. Grey ones are Unicon caps.

 

Since those capacitance values are still strong, not sure if I should recap those yet. I would like to find out what caused shorting 2 sets of BJ transistors at TR511 and TR509 then recap if I can fix the issue first.

 

What is confusing about schematics is some of them does not show unit of measurement. For instance R541 shows 0.5 (2). My guess is that it means 0.5ohm at 2w but not 100% sure if it is the other way around. 😅

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.




×
×
  • Create New...