Pilot light on - everything else dead. Blackstar HT-60

[Steve Blackdog]

I have been doing some testing and found d36 (IN4007) appears to be dead. I will replace and see what difference it makes. R217 next to it appeared dead, until I plugged the Jack in which showed r217 to be ok.

Blackstar have kindly sent me the test spec, which may also assist me.

I am going to put the project to one side for a few days - I am getting a bit of schematic blindness, if there is such a thing. A fresh mind will hopefully help!

cheers

steve

update. Just snipped a leg of d36 - out of circuit all is fine.

[g1]

What do you mean when you say 'appears dead'? R217 is the cathode resistor for the power tubes. It should read 1 ohm. D36 protects R217 if a tube shorts. So D36 should also read 1 ohm because they are in parallel. Plugging into a jack should not make any difference to those resistance readings. Which jack?

[Delta362]

Steve, what do you mean by - D36 out of circuit, all is fine? Is the amp's bias circuit giving correct steady voltages now? Make sure you have one installed as g1 explained its purpose.

[eschertron]

Steve, what do you mean by - D36 out of circuit, all is fine? Is the amp's bias circuit giving correct steady voltages now? Make sure you have one installed as g1 explained its purpose.

I'm guessing D36 failed short? could that explain the symptoms?

[Delta362]

With D36 replaced, is the amp fully working as it should?

[Helmholtz]

Actually neither an open nor a shorted D36 would noticeably change actual operating conditions.
But if the diode has gone low resistance, this could cause a lowered and fluctuating test point voltage.

[eschertron]

Actually neither an open nor a shorted D36 would noticeably change actual operating conditions.
But if the diode has gone low resistance, this could cause a lowered and fluctuating test point voltage.

I haven't looked at the schematic. But based on your comment, the components at the cathode are only to provide a test point, not current regulation?

[g1]

I haven't looked at the schematic. But based on your comment, the components at the cathode are only to provide a test point, not current regulation?

That's right, it's a 1ohm cathode resistor for measuring idle current. And a protection diode across it. Hot-Rod Deville has the same setup.

[Steve Blackdog]

Hi folks

here my short update: d:36 was a red herring. I removed it and tested it out of the circuit and it was reading 600. Back refitted it read 1 ohm, matching the resistor next to it. So I have eliminated that as a cause of anything.

I tested pin 5 on the el34s. Both showed a constantly changing measurement, between 20 and 120. How should I be testing this? Valves out or in? Pin 5 versus ground?

the most likely culprits are things I have done, so let me recap.

I got the main pcb as new old stock. It had been damaged in that some of the pcb mounted pots had been snapped off.

I built my own aluminium chassis. I removed the remaining pcb mount pots, switches, leds and sockets. I attach a couple of pictures. I have rewired the back boards since these photos so they are a bit neater.

I reused the pots, wiring them to the pcb, rather than direct mounting.
I used McMurdo type valve sockets, wired to the pcb.
I built the back boards, namely fx loop and speaker connections etc as per the schematic.

so I am guessing I might have made a mistake.

any thoughts greatly appreciated.



steve

[Helmholtz]

here my short update: d:36 was a red herring. I removed it and tested it out of the circuit and it was reading 600. Back refitted it read 1 ohm, matching the resistor next to it. So I have eliminated that as a cause of anything.

600 what? Ohm or mV? And what about the other direction/polarity? Reading the diode in circuit makes no sense.

I tested pin 5 on the el34s. Both showed a constantly changing measurement, between 20 and 120. How should I be testing this? Valves out or in? Pin 5 versus ground?

I guess you mean between 20VDC and 120VDC at the grids?
Yes, grid voltage needs to be measured wrt ground. Valves in or out shouldn't make a difference.

Check the bias circuit as well as C122/123 (for polarity and leakage).

[Steve Blackdog]

600 what? Ohm or mV? And what about the other direction/polarity? Reading the diode in circuit makes no sense.


I guess you mean between 20VDC and 120VDC at the grids?
Yes, grid voltage needs to be measured wrt ground. Valves in or out shouldn't make a difference.

Check the bias circuit as well as C122/123 (for polarity and leakage).

thanks. 600 Ohms - as per the diode setting on my digital multimeter. Not tried other polarity - assumed it would not read at all?

why does it make no sense to read the diode in the circuit?

yes I mean 20 - 120 vdc. delta suggested that the voltage should be static rather than wavering.

I will check out c122/123.

Thank you!

[Helmholtz]

600 Ohms - as per the diode setting on my digital multimeter. Not tried other polarity - assumed it would not read at all?

why does it make no sense to read the diode in the circuit?

In diode setting, DMMs typically measure forward voltage. Forward resistance is very variable and depends on measuring current/meter. Reverse resistance should be very high for a good diode.

When measuring the diode in circuit, the meter will only "see" the parallel resistor as the diode's resistance is much higher in both directions. Only a completely shorted diode would show.

[Steve Blackdog]

Danke!

how do I test whether the 450v caps c122/123 are leaking without a capacitor tester? Do I have to take that out of the circuit as well?

[Helmholtz]

Danke!

how do I test whether the 450v caps c122/123 are leaking without a capacitor tester?

Good question.
But there is a simple test you can do: Just lift one end of each and monitor grid voltages for changes/fluctuations.

Yes, caps are tested out of circuit, lifting one end is sufficient The DCR should be infinite (DMM reads O.L.). Often a badly leaky cap shows some high resistance in the MOhm range. But even a leakage resistance of 20M can cause problems and leakage current may increase at higher voltage. Observe correct polarity when measuring ecaps.

BTW, power tube grid voltage needs to be negative wrt ground. You didn't specify the sign.

What is the voltage at TP13? Is it stable?

[Steve Blackdog]

What is the voltage at TP13? Is it stable?

no it’s not stable. I will confirm the voltage when I get back home later. If I recall it was around 10mV. Should it be stable?

thanks for your patience