Sunn(0))) Amp Beta Bass Repair

[nevetslab]

1N458....doesn't ring a bell, but in the circuit for CR6 & CR7, if you have any 1N4148, 1N914 or similar signal diodes, they're fine for that. For the limiter coupling diodes, 1N4004's would work just fine. To get you going and not having to wait three weeks for parts to arrive, I suggested the BD139/BD140 parts as a possible substitute to get the circuit working again. There's many other's that would suffice. The 2N3440 is a TO-5 part. Just about anything in the NPN/PNP that has a 100V or greater Vceo rating and good for 1-1.5A would suffice in the exercise.

On setting up the power supply to run the amp, The Common GND connection will have the (-) terminal of the Master output and the (+) terminal of the Slave output tied to GND on the ext supply, and that also connects to your circuit ground. You can leave the fuses out, so they're not attached...not knowing if they've been damaged. Then, just hook up the lab supply correctly (no reversing this time), and see if the circuit comes up. If you want, leave Q7 & Q16 out, leave Q13 out or jumper it's Collector to Emitter (or both collectors of Q5/Q14), then connect that to the output buss so there's feedback.
That makes this a voltage amplifier, sufficient to see if everything ahead of that stage sets up ok.

[wirelessliquid]

I've found the full parts list online and a little trouble shooting guide which is useful. it shows CR6 to be the 1n458A


I don't really have any spare signal diodes apart from the 1n4004 but the 1n458, BD139 and BD140 can all be found on RS and they have really quick delivery so only a few days and I should be able to try it out
I'll see if I can find anything lying around similar for the 2n3440 before the BD parts arrive

I'll leave Q7 & Q16 out for the mean time along with Q8 - Q10 and Q17 - Q19 and test it as a voltage amplifier in the mean time.

Yeah this is how I remember setting up the dual power supply, I'll give it a test on the meter to make sure the outputs are all god before wiring it in. I replaced F2 and F3 but I think its best to remove them with Q8 - Q10 Q17 - Q19 out for the testing.

So I attach Q13 CE together and then I attach this to the output buss, where is the output buss?

[nevetslab]

The output buss is the speaker output connection, ahead of the Inductor/resistor network L1/R52. It's where the emitter resistors of the output stage and driver stage all come together. There's a feedback resistor from there back to the inverting input of the amplifier circuit R7, with the shunt resistor of that network R31 going to ground. Those set up the gain for this non-inverting amp for 31.7dB gain. There's also IC1A set up as a long time constant DC offset integrator that feeds the input side of the amp. You might want to lift R22 up during your initial circuit investigation, so there's not any influence if IC1 took a hit and has failed.

I'd also disconnect the +/- 15V output from this power amp that feeds your preamp section. You could power that up with the lab supply, set for +/- 15VDC. That may be drawing around 100mA, based on what I see in the schematic showing that much current flowing thru R3.

When you do get past this voltage amp circuit working, and begin setting up the output stages, you'll then want to remove that short between bias xstr Q13, so it can set up the proper bias to run the Pre-driver, driver and output stages.

And, if you do use the BD139/BD140...their pin-out is such that you have to bend the leads a bit to fit the TO-5 pattern, since the base is on the right side, collector is the middle terminal, emitter is the left terminal, facing the top of the part. Data sheet will clarify that better.

[wirelessliquid]

So slow reply as I've been busy with the band and such but I think I'm reaady to test

checked all the semi conductors are there relative resistors and they all seem to be in good order now I have a question on CR5 and CR1 which are 1n4744 15V zener diode. I removed them to test and both have a 0.6V drop in forward and reverse current but as they are 15V zener diodes I thought that they should only have the reverse drop after 15V has been reached, does my Fluke 179 output 15V+ to test semiconductors? is this normal?

I replaced C6 for good measure as it was playing up in the end.

still waiting for the 2n3440 for the full test but I have the BD140 and BD139 ready for the mean time.

So testing on my power supply I've soldered some wire on Q13 between the CE to short it. I have removed the bit transistors Q8 - Q10, Q17 - Q19. I just slave up my power supply onto the +40V -40V pins looking for a 30mA draw but my power supply can only run upto +-30V but it should be all good. anything I'm doing wrong here or need to change before testing?

[Jazz P Bass]

" CR5 and CR1 which are 1n4744 15V zener diode. I removed them to test and both have a 0.6V drop in forward and reverse current "

That does not sound like a good reading to me.
They should conduct in one direction only, no?

[Tom Phillips]

...I have a question on CR5 and CR1 which are 1n4744 15V zener diode. I removed them to test and both have a 0.6V drop in forward and reverse current but as they are 15V zener diodes I thought that they should only have the reverse drop after 15V has been reached, does my Fluke 179 output 15V+ to test semiconductors? is this normal?...

I agree with Jazz P Bass. Your 15V zeners are bad per your test report.

As you suspected the Fluke 179's test voltage is too low to test a 15V zener. Unless, of course, the zener is bad as you found out. I measured the test voltage of a Fluke 79 and that particular meter put out 2.9V open circuit in diode test mode.

[nevetslab]

Since you DO have a lab supply, put a 1k resistor in series with the zeners you want to check, once you at least know the part appears to be good (measured forward drop around 0.6V, open in reverse connection). That's what I do when uncertain. I can't say I've ever seen one measure the same in both directions out-of-circuit.

+/- 30V from your lab supply is more than adequate to test/verify the front end is functional or not. Don't forget to connect the collectors of Q5 & Q14 to the feedback resistor/output buss, which completes making this behave as a voltage amplifier. I just noticed resistor R11 5.6k off of the collector of Q5. I'd lift one end of that in this test. If all IS functional, you should see similar relative voltages as shown on your schematic (should show similar currents flowing in each circuit branch, which is what we're looking for.

[wirelessliquid]

I only had a 10K resistor on hand so I used that, Looks like no matter the voltage or forward or reverse current there is a 0.3V drop over the zener, as you bring the voltage up higher it gets closer to 0.6V so I assume this 0.3V is a leaky zener so that will need replacing too. I went up to around 20V in both directions to account for the 10K resistor and V drop over the zener.

Now looking at the circuit it looks like Q1 & Q11 are being used as voltage regs along with the zener which is leaky, do Q1 & Q11 test as normal transistors or should I test them as voltage regulators on my power supply to look for leakage? If so how would I go about doing this?

[wirelessliquid]

As you can see Q1 & Q11 have this white gum stuff on them which I think is some sort of thermal gum? If I replace Q1 & Q11 if there bad whats this stuff called?

[g1]

That's not thermal paste. It's just silicone (RTV) used to secure the parts as they are relatively heavy and prone to breaking their solder connections due to vibration.

[nevetslab]

Now looking at the circuit it looks like Q1 & Q11 are being used as voltage regs along with the zener which is leaky, do Q1 & Q11 test as normal transistors or should I test them as voltage regulators on my power supply to look for leakage? If so how would I go about doing this?

Replace the zener's. And, as I had stated in post #12, Q1 & Q11 are the pass xstrs for the +/- 15V zener-regulated supplies to power the preamp. I'm thinking Q1 & Q11 survived the supply connection reversal. Check it without the preamp circuit attached, just to be safe.

[wirelessliquid]

Zeners are on order at the moment and it looks like Q1 & Q11 survived hopefully the preamp board is all good aswell then. Couple days and I should have the zeners in

[wirelessliquid]

That's not thermal paste. It's just silicone (RTV) used to secure the parts as they are relatively heavy and prone to breaking their solder connections due to vibration.

Pretty much this stuff?
https://uk.rs-online.com/web/p/produ...B&gclsrc=aw.ds

[g1]

I prefer when they mention electrical use, like this one: https://uk.rs-online.com/web/p/silic...lants/0555588/
Some silicone sealants give off gasses that are corrosive to solder joints etc., so when they mention electrical, you can be pretty sure it's safe.

[wirelessliquid]

Replaced those zeners, trying to test my power supply for a + and _ voltage before i plug it onto the board and my meter is reading +15V, +15V for each channel and +30V across, have a done somthing wrong or am I reading it wrong



from left to right my meter reads,
so negative meter pin on negative on power suppy and pos meter pin on ground on power supply +15V

negative meter pin to ground on power supply and positive meter pin on posative on posupply +15V

negative to negative and positive to positive +30V, what am I doing worng here