Laney tfx 200 drive channel silent; unusual supply voltages

[Carl45Adams]

Hello, I am working on a laney tfx 200 tube fusion amp that belongs to my cousin. I have never heard it in its working state.

The primary problem is that when engaging the drive channels there is no sound at all. The clean channel gives some sound but in my opinion it is very quiet for an amp of this wattage. The tone controls also seem to do very little. The effects do seem to work a little bit but I notice the chorus seems to give little periodic blips.

I have checked the fuses and I have checked/swapped the tube/valve. It works as expected in another amplifier. Voltage seems good at the tube.

Headphones and line-out are consistent with results when using standard 4-Ohm speaker.

I have used contact cleaner on the pots.

Okay: here is where I am confused and also where the schematic seems to lack a bit of information. Running with 120 V household AC. Checking the voltage from the transformer on the power amp board P1 (orange) and P2 (white) are both 25 V rms relative to P3 (black and yellow) as it says on the transformer (BTL Industries 6131-2, can't find any info, different that the one suggested on schematic). Now: how to get the +15V and -15V supply? P1 and P2 are connected a bridge rectifier, P3 is connected to ground. But when I measure the supply voltage from the transistors 58 and 59 and from the connectors between the boards I get +34 and -34 V. Now that does agree with sqrt(2) x 25. Is it supposed to be the Zener diodes (D16 and D17) in the little insert circuit to the right hand side of the page that keeps the voltage at +/- 15V? Could they be (part) of the problem? (wonder how many op amps are fried!)

Any advice would be appreciated since I am not really familiar with some of the design aspects.

I don't have a scope here at my house but I can easily get one. I can probably also lay hands on some DC power supplies and transformers to reroute on the board.

Thanks,

Carl

[Enzo]

Not sure of your description, but your 25vAC coming into the bridge rectifier will rectify and filter to make about 35vDC, which you seem to have. That is power for the power amp stages. Those 35vDC supplies not only feed the power amp, but they also pass through pins 5 and 6 of the 6-pin connector to the preamp. From ther they flow through teh 330 ohm 5 watt resistors to zener diodes D16, D17. You apparently know all this.

So what about all that is not right? Do you have or not have about 15v across each of those zeners? Why do we think op amps are blown out? If a zener opens, then your 35v could make it to an IC, but they generally fail shorted rather than open. In any case, if you have the two 15v supplies, then the power supply is working.

The tube has its own power. First, do the heaters light up? Next do you have high voltage on the tube?

[Carl45Adams]

Thanks for your response.

Yes, I forgot about the 330's. Checking at the zener diodes everything is +/- 15 as it should be.

Tube lights up. For stage 1, I have 158 V at the plate (anode), 0 on the grid, 1.14 at the cathode. With a 1 k cathode resistor drawing a current of 1.14 mA and 100 k plate resistor the HV supply is 272 V. Those numbers seem to compare to the ec883 data sheet.

Stage 2 is a bit different. Plate is 130 V, grid 0, cathode 0.85 V. With a 0.82 k cathode resistor drawing a current of 1.04 mA. However this seems to imply the HV supply is only 234 V with a 100 k plate resistor. I haven't been able to get at the HV supply lead yet to check it.

[Carl45Adams]

And whoops, had the wrong schematic! The tf 200 and tfx 200 are close but differ in that there is no analog reverb tank and setting (and other things as well I am sure.). I am going to try and trace the input signal with a scope. https://elektrotanya.com/laney_tfx20...wnload.html#dl

[Carl45Adams]

Just as a follow up on this. Using the oscilloscope, I tracked down the issue to the potentiometers. I suppose I could have checked those first but they were a bit difficult to access. The 100 K ones that control the overdrive circuit were open circuit on the centre pin so no signal was ever reaching the valve section of the pre-amp. The 100 K for the clean volume and the bass controls for the clean and distorted channels were also bad (but in a different way, centre pin had a higher resistance than 100 K). I did these checks with the pots still in the board so the actual values may be off. I was also suspicious of the other pots for mid and treble so I just replaced all of them with units with logarithmic type taper. I bought them from Mouser electronics, manufactured by Bourns, PDB181 series. I am at least getting signal through all the channels and have tone control.

My next plan is to test and possibly replace the pots for the effects loops. I bought linear ones hoping that is right choice. Need to put things back together as well.