hi all friends of the SS amplifiers forum. Sony 1150 one channel after replacing the output transistors.....burning again. I noticed that the faulty channel suddenly changes after connecting the open 4 ohm bias and the pnp output transistor burns.. I marked the changes on the circuit. Thanks for any help.
Hi hooman,
I don't think you should connect any test load until bias current is correct and offset is acceptable.
I'm not clear about the present condition. Are the four output transistors present or removed? The Vbe on the PNP looks reasonable, the NPN is only 0.2V, so may measure low because of circuit conditions, or perhaps failed. Can't tell yet. Were they measured out-of-circuit or under power?
You've noted 0.4V the collector of VT403, but the Sony's noted voltage assumes the output offset is 0V; you haven't told us what you measured.
We need more details. Thanks.
I don't think you should connect any test load until bias current is correct and offset is acceptable.
I'm not clear about the present condition. Are the four output transistors present or removed? The Vbe on the PNP looks reasonable, the NPN is only 0.2V, so may measure low because of circuit conditions, or perhaps failed. Can't tell yet. Were they measured out-of-circuit or under power?
You've noted 0.4V the collector of VT403, but the Sony's noted voltage assumes the output offset is 0V; you haven't told us what you measured.
We need more details. Thanks.
Thank you for the answer. ,....The first time when the volume was turned off and there was no sound, the speaker wires were shorted for a moment and the fuses burned!!!!It was found that the power pnp transistor is defective.... There is no offset... Without connecting the load resistance, there is no significant difference in the voltages of the two channels. All the transistors have been replaced many times, but there is no change. The main problem is why when a 4 ohm load is connected and there is no signal.... Voltage. Collector..of..vt403 from the 1.25 vgo to the 0.4 v ...and the bias of the negative power transistor increases and eventually becomes defective.. this does not happen in the other channel? I am not inexperienced and I have been repairing amplifiers all my life. Friends who have experience, please help me. Searching the internet, all the problems are found... capacitor failure and simple things! But in my case. The parts were tested.
Always be aware that there can be errors in the circuit diagram, even by Sony.
Notice the voltage at the emitter of VT404 (the bias transistor) which reads "-1.1V', but contradicts the reading of "-1.2V"at the emitter of VT411.
As a "1" looks like a "7", the proper reading must be "-1.7V", which correspond within the bias range: 2.5 (lowest) - 3.2 (mid point) - 4.5 (highest).
This extra PNP VT411 is somewhat surprising, but needed for the C408/R407/R406 bootstrap circuit.
But most important is the odd C406 (C-B VT404 bias) which has the wrong polarity: the collector is higher then the base.
If it fails, the bias fails. 2B exchanged properly (both channels).
Notice the voltage at the emitter of VT404 (the bias transistor) which reads "-1.1V', but contradicts the reading of "-1.2V"at the emitter of VT411.
As a "1" looks like a "7", the proper reading must be "-1.7V", which correspond within the bias range: 2.5 (lowest) - 3.2 (mid point) - 4.5 (highest).
This extra PNP VT411 is somewhat surprising, but needed for the C408/R407/R406 bootstrap circuit.
But most important is the odd C406 (C-B VT404 bias) which has the wrong polarity: the collector is higher then the base.
If it fails, the bias fails. 2B exchanged properly (both channels).
Attachments
Excellent analysis by Citizen!
As Citizen advises, be sure to examine actual installed polarization of C406 (and its brother in the other channel); the cap's plus terminal(s) must be on the collector of the transistor(s). Good catch!
Your labels of "v eb" seem to imply base-emitter voltage, but I'm suspecting that these are voltages with respect to ground. With no load, would you make fresh measurements re ground of:
Amp output
VT403 collector
VT405 base
VT407 base
VT408 base
VT406 base
VT411 base
VT404 emitter
VT404 base
Thanks!
As Citizen advises, be sure to examine actual installed polarization of C406 (and its brother in the other channel); the cap's plus terminal(s) must be on the collector of the transistor(s). Good catch!
Your labels of "v eb" seem to imply base-emitter voltage, but I'm suspecting that these are voltages with respect to ground. With no load, would you make fresh measurements re ground of:
Amp output
VT403 collector
VT405 base
VT407 base
VT408 base
VT406 base
VT411 base
VT404 emitter
VT404 base
Thanks!
This schematic does look correct. But most important is to confirm the physical part is installed with the correct polarization.
The most important question for me is this...why a 4 ohm DC resistor in the output...or a short circuit in the output...without a signal...causes a decrease in the DC voltage in the collector of the transistor. 403 😱 ..please Don't forget, I did everything. I changed everything. The voltages of the corresponding points of both channels are the same. But when the load comes to the output... one channel's bias changes a lot at once. Why? It seems that something is still wrong. I don't know much about the transistor amplifier made forty years ago. thanks for helpe.
I agree thsee are baffling questions and we’ve not forgotten. We’re trying to locate the defect without destroying parts.
Without a load connected, does the output bias to 0V and what is the VT403 collector voltage? Another experiment is to try a 1k resistor as the load (or 100, 50. etc), the idea being to provoke the odd behavior without causing damage.
I still advocate taking the measurements noted in post 6.
Without a load connected, does the output bias to 0V and what is the VT403 collector voltage? Another experiment is to try a 1k resistor as the load (or 100, 50. etc), the idea being to provoke the odd behavior without causing damage.
I still advocate taking the measurements noted in post 6.
With no offset there can be no change in the DC conditions with and without a load.
This is a very old design and one of my first suspects would be that modern faster parts have caused instability. The other very real issue is that modern parts have a slightly higher Vbe due to differences in modern doping and manufacturing processes and this can often put the bias adjustment out of range.
Use a DBT and put a scope on the output.
Better not short the output...
The drop of the voltage at the collector of VT403 is caused by the correction mechanism of the feedback loop. Those parts are working as expected!
The sensing node for this behaviour is VT402, which draws a small base current from the output through R420//R421 (54kΩ).
Given a negative rail of approx -33V, the tail current is 1mA and VT401 and VT402 are set at 0.5mA when in balance.
The base current of VT402 is 0.5mA / beta (120) is 4µA, so the output offset with this design is -0.015V (VT401) + 4µA * 54kΩ = -15mV + 216mV = +201mV.
A 4Ω load will draw 50mA from VT407/VT409 to maintain the balance. Their base currents are supplied by VT405, which draws its base current from VT403.
When the collector voltage of VT403 drops, VT403 (running at 5mA approx) is not able to supply enough current to maintain the loop stability (it should go up a bit). Something is drawing current from this node unexpectedly and much more then the calculated base current of VT405 (say 50µA?)
Main suspect: VT405 (beta fail), less possible VT403 (not able to increase from 5.00mA to 5.05mA?), but also the protection network starting with VD555, VD557 and VT552 (replaced?), not to mention to keep a keen eye on C411 (quality issue).
I removed the all power transistors in both channels. Then I measured the output voltage of the drivers in the two channels. Without load, both channels were almost the same. But with a load of 4.7 ohms ......on the pnp driver of the defective channel, the negative voltage increased. And this It is the same reason for the loss of pnp power transistor. Of course, one of the parallel pnp power transistors always burns and sometimes it also destroys the npn power transistor.
The resistors were tested, but I did not replace them, but I tested and replaced the rest
Without the power transistors, the loop is broken and only an integrator remains through C408. Impredictable results may occur.
I do not recommend such an approach.
Replace the power transistors with small signal (TO92) transistors with a 1kΩ resistor soldered to their emitters. The feedback loop is restored and if they burn it's not a big loss. A 'serious' load for these ss-bjt's would be some 100-470Ω to mimic the LS.
I do not recommend such an approach.
Replace the power transistors with small signal (TO92) transistors with a 1kΩ resistor soldered to their emitters. The feedback loop is restored and if they burn it's not a big loss. A 'serious' load for these ss-bjt's would be some 100-470Ω to mimic the LS.
This indicates another origin of the problems: VT406. If it leaks from E to C, VT408 and VT410 starts conducting lots of current. The output voltage will drop and the feedback loop tries to lift it again with VT403 to VT405 and VT407//VT409. It would fail to manage this and still resulting in a voltage drop at the bias circuit and the burning of the output NPN's too. Check thouroughly.
Friends.......voltage test....replacing semiconductors....capacitors....bias potentiometer....testing the correctness of the copper circuit... test osiliation ......no results . by connecting the speaker With an ohm less than 8, the bias changes and the pnp power transistor... is destroyed... should I replace the resistors at this time? The ohm test is correct... but maybe they should be changed after 40 years. Does anyone have experience?
Hi hooman,
Though I doubt any of us have direct experience with this Sony TA1150 Sony model, many of us here have years (or even decades) of experience. Each of us probably has different ways of approaching this sort of problem and this explains why you are receiving varying recommendations.
You need to recognize that a 4 ohm load is a very heavy load and likely to further damage an amp that isn't already working properly. A short is even more difficult--- something has to fail, and it won't be the short. This amp does not feature any short-circuit protection other than fuses. With luck, it will be a fuse, but from my college days I remember a corollary to Murphy's Law: "A transistor, protected by a fast-acting fuse, will protect the fuse by blowing first."
The configuration you show in post 14 is convenient, as I believe you've removed the 4 power transistors. Install a 51k resistor between the emitter of VT406 and the base of VT402. Do not load with a 4 ohm load! The added resistor subs for the R420//R421 feedback and leaves the output node and the difficult-to-drive Zobel network dangling without harm. Apply power and the emitter of VT406 should bias to about 0V. This is a variation on Citizen's suggestion in post 15. Assuming bias behaves as hoped, you should be able to vary VR401 and observe operation of the bias spreader. We'll do some additional tests before restoring the outputs.
Would you describe any test equipment you might have? Thanks.
Though I doubt any of us have direct experience with this Sony TA1150 Sony model, many of us here have years (or even decades) of experience. Each of us probably has different ways of approaching this sort of problem and this explains why you are receiving varying recommendations.
You need to recognize that a 4 ohm load is a very heavy load and likely to further damage an amp that isn't already working properly. A short is even more difficult--- something has to fail, and it won't be the short. This amp does not feature any short-circuit protection other than fuses. With luck, it will be a fuse, but from my college days I remember a corollary to Murphy's Law: "A transistor, protected by a fast-acting fuse, will protect the fuse by blowing first."
The configuration you show in post 14 is convenient, as I believe you've removed the 4 power transistors. Install a 51k resistor between the emitter of VT406 and the base of VT402. Do not load with a 4 ohm load! The added resistor subs for the R420//R421 feedback and leaves the output node and the difficult-to-drive Zobel network dangling without harm. Apply power and the emitter of VT406 should bias to about 0V. This is a variation on Citizen's suggestion in post 15. Assuming bias behaves as hoped, you should be able to vary VR401 and observe operation of the bias spreader. We'll do some additional tests before restoring the outputs.
Would you describe any test equipment you might have? Thanks.