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Messages - batbob

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1
I measured the DC on the speaker out and it was under 10mV so I'm happy that it's all behaving nicely now.
It sounds awesome turned up loud again, instead of the humming and crackling I was getting.
Many many thanks for your help and support.
I've added some chip points for each of you that have helped as I appreciate your assistance a great deal.
Cheers, from a slightly deafened, Bob

2
Thanks Phil, that's an ace answer and easily understandable.

I'll check the DC on the output tonight when I get home.

If it is out, I'm guessing its the trimmer pot that would affect the Bias?

Thanks and I'll keep you informed as to whether it blows again!.
Cheers,
Bob

3
Hi all, got the two TIP's (142 and 147) all soldered in and working on the headphones like a good 'un.

Just as a final check to make sure I don't blow it again tomorrow after work when I thrash it a bit, is there anything about the connection to the Cab that could cause an issue?
I've measured resistance at the in and out of the cab, both 7.1 ohms, the Amp is an 4-8amp output so I don't see there being an issue.
Just wondered if there was anything I should look out for, someone mentioned DC on the output. I havent checked it yet but is there an acceptable level or should it be 0v dc?

Thanks for all of your guidance and support!
Bob

4
Parts ordered from CPC, now awaiting the TIP147,TIP142 and BD238.
I'll let you know how it goes.

I was looking at the TIP147 and 142 with the new package but thought that they'd be a similar problem as the TIP147T, now I've been through and checked the ratings I see why they're ok.

On Farnell the 147 had an hfe of 1000 and the 142 had an hfe of 500. The knacked transistor has a 500 so I bought two the same, I figured that it's not good to have two different gains through the circuit...

Cheers, Bob

5
Thanks Phil, you may be right.

What's everyones thoughts on replacing the TIP 147/142's with TIP 147T/142T's?
I know early on someone said I shouldn't do that, but looking into the spec sheets, they're the same, except for Package Type.
I ask this as I can't find any TIP147's and TIP142's that are the same size as these two in the amp without paying through the nose and waiting for too long..

Cheers,
Bob

6
Ok, I unsoldered the connections from the transformer and I'm getting the right voltages out, so it's not knacked.

I removed R47, R48 and R43 to try and narrow it down and then remembered that the +40 and -40 rails come off before that so then I started looking at what could short out in the poweramp stage again and tracked it down to transistor T6, a TIP 147 that I thought was buggered originally but seemed to be working ok once the IC had been changed.

I've checked it in circuit and saw a short between emitter and collector, thinking it might be something in the circuit other than T6 I unsoldered it to check it, it's a dead short on the desk so I'll need another TIP147 by the look of it.

I don't know if this is the last fault with it but I may try to find a decent TIP147 and bang it in. That will deffo be the last attempt. I'm reluctant to spend any more on it...
Anyone have any good contacts for TIP147's?

7
Thanks g1 and Phatt, We are on 230v, I didn't adjust bias trimmer.

I can't test anything really as the fuse went in the rear of the unit, 500mA. I changed it and then the 2 inside 2000mA went too. I've rigged up a current limiting lamp in the middle of the power lead and it's at it's brightest.

When I put an ordinary cable in, it blows the 500mA fuse in the rear. Even when the leads from the Transformer are disconnected from the board.

I think that the transformer is a dead 'un for some reason.

I've measured resistance across the primary at 22 ohms, as it's a 0.44A primary at 230v, I reckon it should be 522ohms.
The secondaries are 0.4 ohms and 0.5ohms (0.9 across both). Sounds like the transformer is now a deceased transformer.

I'll have to take a look around for a 230v 0.44 primary, 28-0-28 1.5A Secondaries transformer.

I'll let you know what happens from there. This might be the point I stop spending on it to be brutally honest.....
Thanks for your help too.
Regards,
Bob

8
It was on WARP when it went. It was working earlier when I had the volume a bit lower. Turned it up to halfway had a play on clean and then switched to warp then got about a minute into A muse song then it went off.

9
Bollox.
Just got it up to half vol for a bit of Muse Psycho and it stopped working, 500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob

10
Cool, thanks Phil. You're a gent. I'll bear that in mind in future with any other projects I take a look at.
Regards,
Bob

11
Well, it's a big thanks to you all, g1, Jason, Jazz,Phil and Tony for all of your help.
After the IC arrived today I soldered it back into the board and then tried it, and wouldn't you know it, all worked!
I banged in the T8 jfet too and it was all still working superbly. :dbtu:

It's all back together and plugged into the H&K 4x12 and sounds good, but I can't test it at the high end yet as the family were watching tv earlier and now I've just come back in from shooting its 11:30 at night!

Many many thanks again and I'll be hanging around on here from now on to see what other stuff people get up to, and look at trying some of those cool electronics/home brew pedals projects.

I've added a couple of chip points to you all as we've been going through this but it's a very little gesture compared to my gratitude.
May you all have a superb 2018 and I'll hopefully see you all on here again.


12
Thanks to you all from r your help so far. I’ll be receiving my new IC on Wednesday hopefully and I’ll let you know how I get on.
I’ll put the ic in and try it before I put TC8 back in the circuit too.
Thanks again

13
IC5 removed and pad voltages are all 0v apart from 4 (-15.8) and 8 (16.3v)
I guess that this means it was goosed and the weird voltages were a symptom of this.
To move forward in the circuit I went back to the drain of T8 pad, that was -15.6v before. It reads 0v now so I guess that the IC was sticking some strange voltages across T8, causing the hum too.

If it is the IC as we suspect, what explains the sudden gain achieved when the signals were large? Is there any logic in that effect happening like that? I suppose that it could be the inside of the op amp starting to function for some reason...


Thanks,
Bob

14
g1, regarding post #31, I've just been testing that and it appears that I had my drain and source mixed up. G=-6.9v, D=-15.6v and S=0v.
For some reason I'd watched a video on JFETS and they had the source as the top and drain as bottom on the diagram. I see where I went wrong now.. Sorry

Jazz and g1, 14v sounded wrong to me, I'll get unsoldering and let you know.

Thanks,
Bob

15
So, forgive my thinking on this, I might be 100% incorrect. I was worried about the fact that if I replace IC5 it might blow again.
I checked the voltages around IC5 inputs.
for earlier stage for Channel 1 treble/presence I found that the + input was .203v and the - input was .226v, output was .226v. All seems legit.

I checked the input voltages for the problematic op amp and the + input was -14.23v and the -input was -7.75v, output was -15.6v, looks dodgy to me, not sure why though.
I measured voltages back from the +input and it was -14.4 that side of C16 (a 220nf 63v cap), the other side where it went off to fx loop and R46 was 0v. I can't find why it would ground to 0v so now I suspect that C16 is knackered, when I capacitor test it with my DMM it shows as 230nf so it looks ok.
Just as a double check, I think that this is normal behaviour to block the DC but I wanted to check with you guys and girls that this looks normal.
If you think that the op amp IC replacement will work then I'm happy. Just worried about these weird voltages......


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