D718 & B688 Transistors - Substitutes?

[Nevillet]

Hi All,

I have been given a 40 year old Technics SU-C04 amplifier that blows fuses.

I desoldered the 4 power transistors and discovered the D718 (PNP) and B688 (NPN) were shorted.

These don't appear to readily available so I'm looking for substitute suggestions. I'd obviously replace all four so they would be matched pairs.

I've attached a picture of the transistors attached to their heatsink. Because of, what appears to be, overspill of heat compound, i'm guessing the transistors are not original.

I've also attached the service manual.

Thanks in anticipation.

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

njw1302g
njw3218g
should do it . are the drivers ok ?

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

Hi Pete

I've checked the drivers out of circuit and seem OK:

Q503 2SD415Q (NPN) OK
Q504 2SD515Q (NPN) OK
Q505 2SB549Q (PNP) OK
Q506 2SB549Q (PNP) OK

I've powered the amp without the output transistors and the current draw is 78mA at 240V.

Is there any thing else worth checking?

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

Sorry, Typo:

Q503 2SD415Q (NPN) OK
Q504 2SD415Q (NPN) OK
Q505 2SB549Q (PNP) OK
Q506 2SB549Q (PNP) OK

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

you could check the dc offsets and if looking good try some low vol music through headphones . dont crank it up or have any speaker loads or it might cook the drivers

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

Yes I have output via the headphone socket

I've got 0.0V DC at both test points but the main o/p transistors are missing at the moment (But on order from RS)

[redwire]

Re: D718 & B688 Transistors - Substitutes?

You have Q507/Q503 on the heatsink label but schematic says the output transistor pair is Q507/Q509. Q503/Q505 is the driver stage.

I would look for side effects from the O/P transistors shorting.
If R503 220R or emitter resistors Z501 or R702, R701 (common for both ch?) went open circuit then expect other parts to have failed. The amp has diodes on the pre-driver so I think any overload cannot work its way back and cause more damage to Q315/Q317.

With no output transistors there's nothing connected to the output... so you would get 0V there. It doesn't tell anything.

For what it's worth, there's only four electrolytic caps C703, C301, C305, C311 which I would replace. Again it depends on how things failed, with large DC offset C311 could get damaged.

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

i hadnt looked at schem before .. now i have i can see a couple of 1k resistors can be added in each channel instead of outputs . these connect between where base and emitter connect on the board .
this is just for testing purposes .

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

On the mother and daughter boards all the electrolytics have been replaced (they were all within uF & ESR spec) The resistors mentioned were also fine including Z501 & 2. All the diodes have been checked and are also fine.

I'm not sure were to place the 1K resistors. Could you add some details please.

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

go from b to e on the board where the outputs have been removed from . leave c empty .

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

I bridged the B-E with the 1K resistors one each of the O/P transistors and powered it up.

83mA at 240V. Still no voltage across the test points 1>2 & 3>4

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

you wont see bias current without the outputs installed .
how about the dc offsets now there is a current path ?

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

Measured at the speaker connections:
Left = 0.026mV DC
Right = 0.010mV DC

[R_J]

Re: D718 & B688 Transistors - Substitutes?

Measured at the speaker connections:
Left = 0.026mV DC
Right = 0.010mV DC

The only way you can do this is if the amp is working and the speaker protection relay engages. You need to check the offset voltage on L501/L502

[redwire]

Re: D718 & B688 Transistors - Substitutes?

I was going to say that seems suspiciously low offset V - is the protection relay clicking in? I would expect 5-50mV at the output and what do you measure on the other side of the 1k resistors?

You can check other voltages, given on the schematic. Around -2.0V and +2.0V at the pre-drivers or VBE multiplier Q501 C and E (don't slip with probe and short pins, ECB pinout, on heatsink).

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

The relay engages after power up.

I measured the DC voltage at L501 & L502 to the metal chassis:

L501 = 0.059mV
L502 = 0.060mV

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

On a complete tangent.. In Southampton, UK we are expecting -2C over the next few days and a light dusting of snow. What's the weather like in Alberta?

[petehall347]

Re: D718 & B688 Transistors - Substitutes?

is your meter set on mv or volts ? if volts then mv are 59 and 60 . ok for now . try with new outputs installed and then check the bias .

[Nevillet]

Re: D718 & B688 Transistors - Substitutes?

Sorry, my meter is set to volts and I should have written it as 0.059V & 0.060V

The new transistors will hopefully arrive Tues

[R_J]

Re: D718 & B688 Transistors - Substitutes?

I just check the temperature and it warmed up to -29C. -2c is t-shirt weather here.
So the dc offsets are likely ok and you just need new outputs.