Sherwood Car Amp

[Armadillo]

Just a thought - I pulled a couple of the power transistors a while back and checked them. When I put them back, I cleaned and reapplied thermal grease to both sides of the mica. The thermal paste I have looks like a silver metallic. Would this conduct from the collector to case to cause the voltage?

Aside from the mica, Screws needs plastic sleeve that you have to install. Check continuity between collector and the heatsink, if you are worried.

I would recommend as attached.

first, ensure that you have 10.5V at pin 12 of IC501.

then, use a digital logic probe, probe pin 8 and pin 11 for pulses. Any sign of life?

if yes, then you continue probe the Q510 and Q511 bases.... and continue .... until you find no pulses.

logic probe is not expensive, $10 I think.

Note: set logic probe to cmos, tie positive to Battery+ and negative to ground.

[jheatac]

Just a thought - I pulled a couple of the power transistors a while back and checked them. When I put them back, I cleaned and reapplied thermal grease to both sides of the mica. The thermal paste I have looks like a silver metallic. Would this conduct from the collector to case to cause the voltage?

Aside from the mica, Screws needs plastic sleeve that you have to install. Check continuity between collector and the heatsink, if you are worried.

I would recommend as attached.

first, ensure that you have 10.5V at pin 12 of IC501.

then, use a digital logic probe, probe pin 8 and pin 11 for pulses. Any sign of life?

if yes, then you continue probe the Q510 and Q511 bases.... and continue .... until you find no pulses.

logic probe is not expensive, $10 I think.

Note: set logic probe to cmos, tie positive to Battery+ and negative to ground.

I had a little higher than 10.5v. It was 10.65 or so.

I had a pulse from each. I had voltage out of the supply but not enough.

The wave, if I remember correctly, had a considerable flutter in the middle. It would cross 0v and rebound about 30% then go negative. Not that I know exactly what I'm supposed to see, but it didn't look right.

Sent from my SM-G920V using Tapatalk

[Armadillo]

I had a little higher than 10.5v. It was 10.65 or so.

I had a pulse from each. I had voltage out of the supply but not enough.

The wave, if I remember correctly, had a considerable flutter in the middle. It would cross 0v and rebound about 30% then go negative. Not that I know exactly what I'm supposed to see, but it didn't look right.

Sent from my SM-G920V using Tapatalk

What is the voltage at;

C518 =
Q511 Vbe =
Q510 Vbe =

Q512 Vbe =
Q513 Vbe =
Q514 Vbe =

Q515 Vbe =
Q516 Vbe =
Q517 Vbe =

[jheatac]

I will hopefully have parts to put this together tomorrow. Then I can get you some measurements.

Can someone explain to me how the to error amps in the 494 ICC work with a common output? I'm not very knowledgeable, but I can't seem to grasp what is supposed to happen or what to expect the output to be

Sent from my SM-G920V using Tapatalk

[floobydust]

TL494 has two error-amplifiers, wired together in a "OR" configuration.
"... With both outputs ORed together at the inverting input node of the PWM comparator, the [error] amplifier demanding the minimum pulse out dominates..." {datasheet}

So Error Amp 1 OR Error Amp 2 can lessen or stop PWM.

In the Sherwood, Error Amp 1 is used to regulate the DC-DC converter output (feedback to pin 1) compared to pin 2 as the setpoint voltage.

But pin 2 is used for another thing.
I think Q520/Q519 ON instantly shuts down PWM. So REMOTE ON turns them off to enable the TL494. Kinda confusing. As long as pin 2 can have its ~4.3V setpoint for Vout.

Error Amp 2 doesn't do much, pin 16 GND and it seems to limit max. duty cycle less than 50%.


You should be able to run the TL494 from REMOTE only (leave BATT unconnected), unless Q520 gets in the way.
It is one way to check signals with DC-DC driver/output transistors unpowered. The pre-driver stage Q508/Q509 should still be working.
The TL494 should just go full tilt, max. PWM trying to bring up the DC-DC output voltage.  R556/C521=33.3kHz I would expect.

[jheatac]

TL494 has two error-amplifiers, wired together in a "OR" configuration.
"... With both outputs ORed together at the inverting input node of the PWM comparator, the [error] amplifier demanding the minimum pulse out dominates..." {datasheet}

So Error Amp 1 OR Error Amp 2 can lessen or stop PWM.

In the Sherwood, Error Amp 1 is used to regulate the DC-DC converter output (feedback to pin 1) compared to pin 2 as the setpoint voltage.

But pin 2 is used for another thing.
I think Q520/Q519 ON instantly shuts down PWM. So REMOTE ON turns them off to enable the TL494. Kinda confusing. As long as pin 2 can have its ~4.3V setpoint for Vout.

Thank you for taking the time to explain that. If I read it a couple more times it may sink in.

Error Amp 2 doesn't do much, pin 16 GND and it seems to limit max. duty cycle less than 50%.


You should be able to run the TL494 from REMOTE only (leave BATT unconnected), unless Q520 gets in the way.
It is one way to check signals with DC-DC driver/output transistors unpowered. The pre-driver stage Q508/Q509 should still be working.
The TL494 should just go full tilt, max. PWM trying to bring up the DC-DC output voltage.  R556/C521=33.3kHz I would expect.

[jheatac]

I have another situation now.

I pulled the 6 power transistors out - Q512-Q517 - thinking they were all the same.

The parts list says they are 2SD1213-S (with the exception of 1 2SP1213-S, which I think was a misprint)

I goofed and ordered -R, based off of the one I had in hand when ordering. I realize now, that 3 were -R (Hfe 100-200) and 3 were -S (Hfe 140-280)

My problem now, is I don't know which came from where. I thought I compared one at each side before removal, but now I am questioning that.

Based off of the parts listing, I would think it was 2 of the S and 1 R on one side and 1 S and 2 R's on the other. Can I put all 6 as R's?

At this point I'll replace the Rs with new, and put the Ss back in, however by my testing the Ss were more out of spec.

[WaveyDipole]

I would have expected them to all be identical spec rather than a mixture of R and S types although this is not likely to be much of a problem in practice. I don't think putting in R types in all 6 positions would be a problem and having all of them the same spec would be preferable.

I had a pulse from each. I had voltage out of the supply but not enough.

The wave, if I remember correctly, had a considerable flutter in the middle. It would cross 0v and rebound about 30% then go negative. Not that I know exactly what I'm supposed to see, but it didn't look right.

Sent from my SM-G920V using Tapatalk

So you are observing this with an oscilloscope?
When measured with reference to V+ (pin12) the pulses at pin 8 and pin 11 as well as the base of Q508/Q509 should be identical but in opposite phase. When measured with reference to GND, the pulses at the base of Q510/Q511 should also be identical but in opposite phase.

I was going to ask at which point in the circuit did you observe the wave that had the flutter? Of course, once you replace the transistors if the situation changes then it may not matter. Lets see what happens when that is done first.

[jheatac]

I am getting -35v and +28v at the amplifier section now

I have replaced all 6 of the 2SD1213-R(or S) with 2SD1213R (Q512-Q517)
I have replaced both of the D600F with NTE373 (Q510 & Q511)

I have attached a shot of the scope on pins 8 & 11 of the 494C chip with reference to pin 12.

It looks like it is maxed out to me, but I am not knowledgeable.

The output wave heading to the rectifier looks like hell to me, but again, I am not knowledgeable.

[jheatac]

Here is the waveform at the input of the step-up transformer - one pole referencing the other pole

The other shot is the base of the power transistors referenced to ground.

[floobydust]

The waveforms look right to me for an 80's bipolar push-pull switcher with no snubber and Darlington output trannies.

TL494 pins 8, 11 go low to turn on the transistors so the scope trace shows they are off most of the time= low duty cycle, idling. It seems to be ~2usec PW but base-drive scope trace different suggests ~8usec on time so not sure why the difference. Point is the PSU is not stuck working really hard and things are symmetrical.

Are the speaker protection relays going click?

I'm concerned the rails are not so equal +28V vs -35V.
The +ve rail is usually a bit lower than the -ve rail because the prot. relays are powered off it.

The power amp sections draw current from +ve to -ve rails (no current flow to GND unless SPKR connected) so they should not be able to imbalance the rails.

I would look and see if the four power amp sections are balanced, having near zero volts on their outputs.
Just to see if anything is wonky and loading the rail down to +28V.  Or the rectifier diodes or filter caps might be off.
Make sure D521/D522 are OK and you get 220R from input DC GND to Audio GND. The amplifier side uses this as a reference point.

I think this is a good old school Class AB amplifier, I tried to buy something like it but everything nowadays is Class D with higher distortion. It's worth repairing. Must be pretty big, like Alpine 3544.

[jheatac]

Okay. So I made some progress. Some of it was solving my own stupidity, but still... progress.

I had the amplifier functional last night. Once I figured out the voice coil was blown in my test speaker and got a second speaker, it started working.

I was playing with it a little more and noticed I had about a 3v wave coming from the output with no input. I put a 5W, 4.7ohm resistor as a dummy load (with no input) and it did not change this.
It was identical on two channels and about half as much on the other two channels. (Scope pic attached)

But in the process of poking around something went wrong. I don't think I did anything, but I noticed that the amperage draw went from 0.69 to maxing out the 5amp bench supply)
The pulse from 8 & 11 have a large spike in the middle of the off cycle.

I have to go to work but I will work on it later this evening.