Sony TA-1055 repair (problems)

[Teodor Emanuel]

Hello guys,
I have a vintage sony TA-1055 amplifier (paid 10 quids for this one along with Sony ST-5055L tuner, so was a bargain).The tuner works perfectly, but I have one problem or more with the amplifier.
After deep investigations I found out that the pre-final drivers, Sony 2SA706/2SC1124 transistors are faulty, along with two rectifier diodes, two 470ohm resistances linked between pre-final driver collector and final transistor emitter (if I well remember), and of course the final transistors.
So, those 2SA706/2SC1124 are hard to find, and I need a quick fix.I found them in China but I can't wait 2 months for them to come, I really need this amplifier for next weeks.
I thought about BD139/140 and already ordered them, but than I found that transition frequency is 50MHz and I need something like minimum 60MHz, and this is so hard to find.Do you think it would work 50MHz without making problems in the circuit?

 

[davenn]

thought about BD139/140 and already ordered them, but than I found that transition frequency is 50MHz and I need something like minimum 60MHz, and this is so hard to find.Do you think it would work 50MHz without making problems in the circuit?

its an audio amplifier at audio freq's ~ 300 Hz to ~ 20 kHz .... 50MHz is far above that

 

[Teodor Emanuel]

its an audio amplifier at audio freq's ~ 300 Hz to ~ 20 kHz .... 50MHz is far above that

So you think is ok and fine to replace 2SA706/2SC1124 with BD139/140?
Thank you for your answer

 

[davenn]

just ensure the other ratings are adequate, collector current etc

 

[Teodor Emanuel]

I think it is just fine:

2SA706
Material of transistor: Si
Polarity: PNP
Maximum collector power dissipation (Pc), W: 7.9
Maximum collector-base voltage |Ucb|, V: 60
Maximum collector-emitter voltage |Uce|, V: 60
Maximum emitter-base voltage |Ueb|, V: 6
Maximum collector current |Ic max|, A: 1
Maksimalna temperatura (Tj), °C: 150
Transition frequency (ft), MHz: 60
Collector capacitance (Cc), pF: 20
Forward current transfer ratio (hFE), min: 70

BD139
Material of transistor: Si
Polarity: NPN
Maximum collector power dissipation (Pc), W: 12
Maximum collector-base voltage |Ucb|, V: 80
Maximum collector-emitter voltage |Uce|, V: 80
Maximum emitter-base voltage |Ueb|, V: 5
Maximum collector current |Ic max|, A: 1
Maksimalna temperatura (Tj), °C: 150
Transition frequency (ft), MHz: 50
Collector capacitance (Cc), pF:
Forward current transfer ratio (hFE), min: 40

 

[davenn]

You have posted opposite types there

the 2SA xxx is a PNP

the BD139 is a NPN

I assume you realised that ?

 

[Teodor Emanuel]

Yes, no maters.I will need pairs anyway, PNP+NPN, so BD139 + BD140.

 

[Teodor Emanuel]

I replaced all the final (2sc1061) and pre-final drivers(2SA706/2SC1124) transistors with TIP41A(final) and BD139/BD140 prefinal.Also found bad Q104 and Q204 transistors (2SA678) and replaced them with 2SA1027 (the most appropriate equivalent I could find.Any other components seemed to be fine.Also I have to switch base-colector pins from BD139/140 since they are ECB not EBC as original 2SA706/2SC1124.
All being set-up, double checked all the connections and passive components that appears to be fine, I fire it up and surprise, the two 470 Ohms resistors from both channels (R143 and R242 rounded with red in the picture) were burned along with all the drivers and final transistors.I have no idea now about what should I do next and what was the problem.

 

[KrisBlueNZ]

OK, here's the schematic of the left channel output stage with R142 circled in red.



It's clear that it shouldn't be possible for any significant voltage to appear across it, because it's connected across the base-emitter junction of Q109. Either Q109 isn't connected correctly, or Q109 has failed.

I suggest you start by putting some 47Ω fusible resistors between the power supply and the output stage, to protect the transistors until everything has been fixed up. I described how to build fusible resistors onto blown fuses in another thread where I'm helping someone else with an amp output stage. See https://www.electronicspoint.com/threads/stage-line-500w-amp-repair-project.270979/#post-1628698.

Your unit doesn't have fuses in the power supplies, so you'll probably have to cut tracks, or at least find the appropriate jumpers. You need to connect the resistors in both the positive and negative supplies, in between the smoothing capacitors and the output stage (Q108 collector and Q109 emitter). The whole amplifier will be protected.

Once you've figured out what's wrong with the connections to Q109 and added the fusible resistors, you can start measuring some voltages to figure out what's wrong. Unfortunately you can't just leave the output transistors out during testing, because this is a quasi-complementary output stage, and the DC condition will be all wrong if Q109 isn't there. But we should be able to figure out what's going on.

 

[Teodor Emanuel]

Thank you very much sir, I really appreciate your help.
I have another spare pair of TIP41A transistors, but only a single pair for a single channel, so can I just leave the other channel with transistors out?Also I would like a simple advice about getting equivalent transistor for 2SA678

 

[KrisBlueNZ]

Thank you very much sir, I really appreciate your help.
I have another spare pair of TIP41A transistors, but only a single pair for a single channel, so can I just leave the other channel with transistors out?

Yes you can remove the output transistors to disable the output stage, but you'll need to remove Q207 as well, otherwise R242 will go up in smoke again.

I think it's unlikely that the new TIP41A transistors are damaged. More likely something is not connected properly.

Also I would like a simple advice about getting equivalent transistor for 2SA678

According to http://www.audiokarma.org/forums/archive/index.php/t-408487.html it can be replaced with 2SA1015.
Specifications are listed at http://www.americanmicrosemi.com/information/spec/?ss_pn=2SA678
PNP BJT; 50V; 200 mA; h_FE(min) 250; f_T 140 MHz; TO-92 variant.
You'll have to work out the pinout by matching the pins on the board to the schematic. I expect it will be the Japanese pinout, ECB looking at the front side with the leads pointing downwards, but it might not, and you'd better check.

Digi-Key seem to be getting out of through-hole transistors and there aren't many to choose from any more. One that looks suitable (apart from the different pinout) is the PN4250A http://www.digikey.com/product-detail/en/PN4250A/PN4250ACS-ND/4806940. Its f_T isn't specified. Alternatively the Panasonic 2SA0720ARA http://www.digikey.com/product-detail/en/2SA0720ARA/2SA0720ARACT-ND/972377 looks good but it's not a stocked item at Digi-Key. You could contact them and ask whether they can get them. It probably has the Japanese pinout so would probably drop straight in.

 

[Teodor Emanuel]

Thank you again.
Digikey is fine but not worth to pay 13£ delivery for a 3£ order.I have to find another provider, but since I am new to UK, I don't know and couldn't fine one with big components stock to choose from.Till now I ordered from eBay but is so damn expensive, and if parts will continue to burn like they do till now, I am afraid that it will not worth to repair this amplifier anymore.

 

[KrisBlueNZ]

Digi-Key probably have the widest range but in the UK you can try Mouser and Element 14 aka Farnell (http://uk.farnell.com). Don't bother with Maplin. I don't know of anyone else with a wide range.

Element 14 / Farnell have the ON Semi MPS2907: http://uk.farnell.com/on-semiconductor/mps2907ag/transistor-bjt-pnp-60v-0-6a-to/dp/1611212

 

[KPS]

It's a few years after the fact but I can tell you exactly why the amp started smoking after Teodore Emanuel's diligent repair efforts...the screen printing on the PCB for the far right driver transistor is WRONG. Apart from the fact you need to twist all 'base' connections of any replacement transistors so that they sit in the middle - EBC or CBE as opposed to the usual ECB or BCE, the furthest right of the 4 driver transistors (looking towards the rear of the unit) is marked the wrong way around. The E symbol is shown on the right whereas it should be on the left. If you don't pay attention and follow that PCB mistake then you'll be starting your repairs again from the beginning. Hope that helps someone. P. S. For what it is, it's a superb little amp.