Hartke 3500 Bad Tube Preamp Output

[x-pro]

What does the minus lead of the voltmeter connect to when measuring? Is there a direct connection between the "Common" on the board and the "Common" power supply?
It looks like you soldered 071 instead of 072 . Which has power on pin 7. So the chip is not receiving it, because it is supplying +18V to the unused pin 8. Pin 4, on the other hand, is fine.

[misterc57]

My ground probe is connected to the body of the compressor switch which is at ground (I have confirmed continuity to ground). The preamp board is grounded to the cabinet chassis via a wire (see pic).

I have measured 1M to ground from pin 3, ground probe on the compressor switch. Confirmed by a second reading with my ground probe on the amp board ground.

The chip I installed says TL072 (see pic).

Learning a lot on this one. Thank you

[x-pro]

The minus 16V voltage on IC101 pins can be due to 2 reasons:
1. malfunction/breakdown of the output (pin 1 ) of the operational amplifier to the power bus.
2. when there is no plus supply voltage at pin 8

[misterc57]

The minus 16V voltage on IC101 pins can be due to 2 reasons:
1. malfunction/breakdown of the output (pin 1 ) of the operational amplifier to the power bus.
2. when there is no plus supply voltage at pin 8

Will check V at pin 8 again. If there I will swap IC101.

Looking at my supply of TL072 this particular one installed is not the same brand as the batch I have. So I am not sure where it came from.

Thank you

[misterc57]

Thank you all !!!

Good news. Installed a new TL072 and all is working, original issue resolved. New chip has low mV readings at pins 1,2,3.

In the end; new C101, C104, IC101

The only odd thing is that it seems to take a while for the tube side to work (long warm-up). I have not timed it but it seems longer than it should be. Like 2+ minutes. I actually thought it was not working at first but when I go back to the tube side it is working fine.

Mark

[g1]

Bottom of the power supply area of schematic shows heater DC supply. Either the voltage shown or the pins it goes to seem to be in error, but you may have leaky caps at C185, C187 which could cause slow warm up.

[misterc57]

Bottom of the power supply area of schematic shows heater DC supply. Either the voltage shown or the pins it goes to seem to be in error, but you may have leaky caps at C185, C187 which could cause slow warm up.

Slow warmup issue seems to have gone away. I am seeing about 6 VDC on top of C186 (0.1).

[g1]

The 6VDC there makes sense. The 10V they show on the schematic did not make sense.
It's possible that those caps have now re-formed if this amp had been sitting around unused for a long time. I guess a dirty tube socket, or bad solder connections at the socket could also be responsible. If you still have access it would be good to clean and re-solder the socket.

[Helmholtz]

The 6VDC there makes sense. The 10V they show on the schematic did not make sense.

I think the 10V in the schematic refer to the line above, which must carry a higher voltage.
While the 6V after the dot means the heater voltage.

[g1]

I think the 10V in the schematic refer to the line above, which must carry a higher voltage.
While the 6V after the dot means the heater voltage.

My thought was that the 10.6VDC was meant for series wiring of heaters, and maybe some changes were made and they forgot to correct the voltage listed.

[misterc57]

I had reflowed the tube socket a while back. What is the purpose of those heater caps C185 and C187? Here is a screen shot from the "other" 3500 schematic (in this case C408 and C409)....

Thank you

[x-pro]

Try to answer this question yourself.
What voltage will be after the diode bridge without these capacitors?
It's not about the numerical value.

[Helmholtz]

What is the purpose of those heater caps C185 and C187?

C187, R230 and R187 provide smoothing and filtering of the DC heater supply (unfiltered DC for the heater would be just as "bad" as AC).
R230 is part of the filter arrangement but also drops around 3V and slows down heat-up.
The voltage before R230 should be 9V to 10V. It's used for LED supply.

[misterc57]

I suspected filtering but thought that filtering caps had to be connected to ground. I guess at some point that other side is at ground.

[Helmholtz]

I suspected filtering but thought that filtering caps had to be connected to ground.

Not necessarily.
Filter caps need to be connected across the voltage to be filtered. That voltage could be floating.
But I'm seeing a ground symbol at the negative side in the schematic.