Blackstar HT Soloist 60: almost no-sound in channel 2 and 60 Hz hum

[rickalcala]

Greetings dear fellows:


I'm Rick, from Peru. This is only a request for advice or guidance, because there is no Blackstar technical service in Peru: I have a HT soloist 60 combo, and I used it only a couple of times in 5 years. Suddenly, before my first disk recording, after messing around with the loop channel, the distorted channel (channel 2) lost all their volume, it's almost inaudible at high gain and volume, and a relatively loud hum noise appears: I think is a 60 Hz hum, that increase with the master volume. BUT, I can barely hear the guitar at high gain and volume, if I put my ear close to the speaker, sound like it has all its high gain...but with an extremely low volume!

The clean channel works perfectly and has a very good volume response, also the reverb works fine. I tested the power tubes using the effect loop and they work perfectly and clean and with no hum. The hum comes when I plug the guitar into the front jack. Then the logic told me to bought a new pair of 12AX7 pre-amp tubes, however the problem persisted after I change the tubes (almost inaudible channel 2 and 60Hz hum). I brought my amp to three (3) different technicians and I have spent on them $ 300 bucks, but they didn't solve the problem. One of them says that the problem is the chip that controls the footswitch (IC10 -PIC16F57 ) and he asked me to buy it....but the footswitch works flawlessly, and respond very well when I change the channels, no pops and everything fine and correct, and that gives me some serious doubts about his diagnostics.

Please, someone could give me a clue of what could be the faulty component and why? I cannot afford to send my amp to UK or USA, and I do not want to throw it away or selling it, because It is MY amp and I love it. And I do not want to test things with no clue due to the extremely dangerous potentials this amps have. I am an engineer with a lot of safety certificates, and I know that I do not want to play and messing around with a power connected tube amplificator. I only want to change the faulty components.
I will really appreciate some guidance, clue, advice about the possible problem: Mosfets? Caps? Resistors?

Best regards and thanks in advance. Please help.

Rick Alcala.


PD.: I'm sharing the HT Soloist 60 schematics.

[The Dude]

Do you have a scope? If so, insert a test tone and follow it through CH 2. See where it drops off. CH2 starts at R17. Are all of your filaments lit?

[g1]

Tracking down the faulty component will require testing in the live circuit. Are you familiar with the safety procedures required for testing high voltages, and comfortable with performing them? If not, there are too many components to just try and replace them without testing under working conditions.
A good place to start would be DC voltages at pins 1,3,6, and 8 of V2 tube.

[Enzo]

And it may not be a faulty component at all. A broken or cracked printed circuit copper trace could do this. Also could failed solder connections. AANy connectors in the signal path can do this. And cutout contacts on insert jacks or FX loop jacks.

[rickalcala]

The Dude: Sorry, I think I do not know what a scope is. I am an English learner. I have a multimeter. Every filaments are bright and red in the power and preamp section. What voltage I should expect in the R17 resistor? I am a geologist, I only know about rocks psychology, I know something of electronics because I had 3 courses in applied physics with lasers and oscilloscopes...

[rickalcala]

G1: Thanks a lot, I have to check out what voltages I should expect there....that the part that I am very afraid of....messing around with high voltages. Maybe pre-amp tubes use low voltages?

[rickalcala]

Enzo: thanks Enzo. I messed around with the loop section and they looks fine apparently. I know from YouTube that they could be a headache. But I will check for every soldered connection. Some place where I can start? Only channel 2 has a very very very low volume, but sounds crunchy.

[Enzo]

A scope is an oscilloscope.

https://en.wikipedia.org/wiki/Oscilloscope

Let us just say the oscilloscope is to electronics as a ground penetrating radar is to geology.

It gives us a visual indication of conditions.

[rickalcala]

Thanks a lot Enzo. I usually use more seismic generated signals but I understand how the GPR works and I love it. It is less destructive and do not use dynamite as 3D seismic does which I usually work with. There is a lot of signal processing in geology/geophysics too. Oscilloscopes here are very expensive. I am familiar with oscilloscopes but buying one to repair only one amp will create a lot of criticism from my wife. I do not believe that the problem is the IC that control the footswitch as the last technician here in Peru says...I do not want to buy another component from abroad (IC chip) until being clear what happens. I think I will start with the Enzo suggestion and measure the behavior after R17. Luckily the schematics tells the amount of voltages in the CH2 section. And sorry for the ignorance, but: Does the mosfets that usually burns out in the Blackstar amps belong to the Pre-Amp or the Power section? Or Am I crazy and I should return to try to understand rocks? Thanks again BTW.

[Enzo]

Don't think in terms of what happens "most often". We just track down what happened in THIS instance.

I am more of a geomorphology man myself, but I do enjoy... rock music.

[g1]

G1: Thanks a lot, I have to check out what voltages I should expect there....that the part that I am very afraid of....messing around with high voltages. Maybe pre-amp tubes use low voltages?

Tube V2 pins 1 and 6 would commonly be a couple hundred volts DC, pins 3 and 8 would be under 10V.

Does the mosfets that usually burns out in the Blackstar amps belong to the Pre-Amp or the Power section?

They are in the power amp section so they are not the problem as your clean channel is working properly.
But there are other Jfets doing switching functions that could be responsible.

Attached are the parts of the circuit that are only for the overdrive channel. Your problem must be in one of those areas.

[J M Fahey]

So I guess you are a Mining Engineer or a Geologist?
No absolute need to *buy* an expensive oscilloscope if you can borrow or get temporary access to one.

Universities always have some in the Electronics Labs, maybe you can ask for access by showing your Engineer credential and smiling a lot, I did that myself at Buenos Aires UBA back in the day.
Or at the company you work in/with.

Seismic analyzers have a built in screen, so not usable for you, but maybe they have an associated calibration/testing Lab.

Or, you buy from China a cheap basic scope module, very low specs, you only need to*see* an Audio waveform there are some very basic but usable,a bare board with a screen, some buttons and a connector for U$20 or so.

Search Alibaba for digital scopes.

Maybe you can buy one of those at Mercado Libre inside Peru.

As a side not,and not to rub salt on your wounds, but I often suggest Musicians outside US/UK/EU NOT to buy complex amps such as yours but very basic, classic and hard as nails models, such as old Marshalls (Plexi or JCM800), fender Black/silverface , up to,say, Peavey Classic , Fender Deluxe or Deville, nothing more complex, which are standard, well known and repairable any day of the week, and add any extra flavouring and tricks using pedals or pedalboards, which worst case can be discarded and replaced.