SOLVED EQ Section Troubleshooting

rectifier

New member
So I got myself a pedal platform amp, or at least something that I think will work nice with pedals, a Blackstar HT-5TH. Got it cheap off ebay, it arrived and I found out that the EQ on the overdrive channel is not working.

Now the funny thing is:

Bass Pot - Not working
Middle Pot - Not working
Treble Pot - Acts as a 2nd volume control o_O
ISF Tone Pot - Works but also in a weird way, turning it down decreases the overall volume

I have been studying the schematics and the OD channel is not that complicated, It's not even tube based. I measured all the components in the EQ section and didn't find anything wrong so far. But maybe someone knows more than I do.

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Any ideas what components could be responsible that the treble acts like a Volume? For me it seems that the signal may go straight through the pot
 
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Mcknib

Well-known member
The ISF control shifts the entire eq section turned down Its more US style turned up British amp style, kind of interactive with the eq I think



Blackstar themselves have pretty good customer service I've fixed a few of their HT pedals but parts were hard to find I contacted them to see where I could purchase pots and they sent me some free

of the 3 pedals I've messed with 2 were faulty pots the other a shorting power fet I had to change 3 pots and a mode selector which fell apart, the shaft came out completely! albeit they look good quality they just appear to fail after a few years

You can't really accurately check components in circuit, having said that you can still check pot resistance across 1 and 3 and between 1 and 2 and 2 and 3 to make sure it changes ie low resistance between 1 and 2, high between 2 and 3 CCW and visa versa CW that will indicate it functions, it looks to me like the 4n7 would be the treble cap but I'd check all the pots in the network first

I'd advise emailing them they may even say what they suspect to be faulty as I say they were really good with me
 
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rectifier

New member
Measured the pots:

Bass 1 + 3 tuned down: 1.2 Ohm, turned up 95.7k
Mid 1 + 3 turned down: 1.2 Ohm, turned up 2.37k
Treble 1 + 3 turned down: 41.9k, turned up 42k

Bass: 1 + 2 turned down 0 Ohm, Turned up 95,9k
2 + 3 turned down 0.1 Ohm,´Turned up 0.0 Ohm

Middle: 1 + 2 turned down: 1.2 Ohm, turned up: 2.37k
2 + 3 turned down: 0.0 Ohm, turned up 0,0 Ohm

Treble: 1 + 2 turned down: 1.6 Ohm, turned up: 42.0k
2 + 3 turned down: 41.9k, turned up: 1.7 Ohm
 

Mcknib

Well-known member
Those pots are behaving how you would expect with Bass and Mid wired as variable resistors and the Treble as a voltage divider

I'd do the same check on the ISF pot where one half, the half attached to R80 is a variable resistor and the other half a voltage divider

I should have looked properly earlier to see how they were wired, with the variable resistor half you only need to check resistance changes between 1 and 3 as you rotate the shaft with the voltage divider half do all the checks

I'd also check visually components common to Bass and Mid C80 etc if they're both not working it's likely to be something they're both connected to I'm assuming they're SMD components?
 

rectifier

New member
I called an amp technicial over here in Berlin today, he kinda didn't want me to help, I probably got more help here and on other DIY forums. So since I didn't find anything wrong I decided to reflow the solder joints of the 3 EQ posts - Guess what the EQ is working again :ROFLMAO:
 

rectifier

New member
Same problem again, after amp heated up, contacted Blackstar for a new treble pot, lets see if they can deliver
The is something fishy going on, I suppose the pot is cracked
 
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