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Old 15th May 2008, 07:56 PM   #1
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Default H&K Tubeman Circuit Questions (added schematics!)

Hey guys!

I've been around this forum for a week or too now, having got interested in a diy - guitar amplifier project!

I have not much experience with electric circuit design, so I'd like to ask you seasoned pros for some guidance on the following problem:

I've acquired this '92 Hughes & Kettner Tubeman recently, an amazing recording tube preamp. Unfortunately I used a 19V AC power supply instead of the suggested 12V AC.... It worked for a few hours very decently, but now it's dead. I'm an idiot I know, but now I want to fix it very desperately.

So I got out my trusty old multimeter and checked some diodes and stuff, and I noticed that two diodes (D1 and D2) were not working properly anymore, they were conductant in both directions.

Now my questions are

a) What could these diodes, in connection with the rectifier B1 just below, be for? I can't make any sensse of this.

b) Could it be that using a 19V supply fried them (and/or any of the ICs? I hope not! :-( )

Thanks in advance!!

Attachment:
http://people.ee.ethz.ch/jrichner/hktubeman.pdf
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Old 15th May 2008, 09:02 PM   #2
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Default Re: H&K Tubeman Circuit Questions (added schematics!)

Quote:
Originally posted by El-Ahrairah


So I got out my trusty old multimeter and checked some diodes and stuff, and I noticed that two diodes (D1 and D2) were not working properly anymore, they were conductant in both directions.

Now my questions are

a) What could these diodes, in connection with the rectifier B1 just below, be for? I can't make any sensse of this.


Hi

Have you tried to measure D1 and D2 out of circuit?

If not, the conduction in the two directions is caused by conduction through the bridge B1.

This diodes and bridge B1 are for clipping distortion sound, or any other sonic effect ...
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Old 18th June 2008, 06:41 PM   #3
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Thanks! I have measured D1 and D2 in the circuit indeed, out of it they seem fine...

Do you think I, as a person with little knowledge about electronics, do actually have a chance to find the problem? It appears to me that the "input circuit" (rectifier, 12, 15, 300V) is O.K., and that all the outputs (mixer, guitar amp, power amp) deliver no sound makes me think the problem is somewhere in between these stages

How would a professional tackle such a problem? I've been trying to get in touch with real life pros but they're all way to busy to help me repair an old pedal, or even give hints on how to do such a thing!
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Old 2nd December 2008, 02:54 AM   #4
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Default Pretty Please...

I have been looking for this schematic for the longest time! Could you please PM the schematic because I tried the link and it is dead.

Thanks!

Tom
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Old 2nd December 2008, 03:10 AM   #5
llwhtt is offline llwhtt  United States
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The link is dead, please email the schematic to me and I'll be glad to help. Depending on the voltage ratings of the parts (capacitors, regulators, etc.) it could be very simple.

Craig
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Old 2nd December 2008, 04:33 AM   #6
Trout is offline Trout  United States
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I have that exact unit, those old ones were far better than the newer ones.

Have you tested the tube?

I could sure use that schematic also. I fear the day mine might get sick.

Please please?
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Old 2nd December 2008, 04:22 PM   #7
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I've done all sorts of troubleshooting:
replaced tubes
socketed IC and swapped them in and out of working circuits
traced the signal path with signal probe

but! no dice, the problem is that the cricuit is quite compact and hard to trace b/c of the daughter riser card. In short, no obvious problems power is where it is supposed to be and signal makes it through the jacks but no serious gain on the other end...

Totally scratching my head! This is a fantastic pedal and i've tried about twice a year to get HK to sell me a schematic but they always blow me off! I've been trying for about four years and nothing. Terrible customer service, IMO. At least they could say we no longer support legacy products. I've tried the german, english, french and american "service centers".

Anyway, please keep my in mind if you ever come a cross one!

Thanks in advance!

Tom
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Old 2nd December 2008, 05:21 PM   #8
Trout is offline Trout  United States
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I just tried the link above again and it is now working. It is indeed the early (Best Version)

If it fails to download PM me, I will email a copy of the download..
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Old 2nd December 2008, 10:09 PM   #9
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Guys,
Still working on this?
Putting 19V AC into the 12V AC socket would:

1) 12.6 V DC to tube heater would go to about 20V - check that the tube still "lights" and you haven't burned out the tube heater.

2) +12 and -12 V DC rails would have tried to go to about +20 and -20V respectively BUT should have been clamped by zener diodes D16 and D17. These zener diodes would also have acted to hold the +15 and -15V rails at about the right voltage. You would have been getting excess dissipation in those zeners and they may have overheated and eventually blown.

If they did blow the +12 and -12 would have risen to those +20 and -20 V levels.

The NE5532 (IC1) will cope with that (+/-22V max) BUT TL071 (IC2) won't (+/- 18V max) and is almost certainly "cactus".

3) +15 and -15 V DC rails would have gone to +25 and -25 V respectively. The TL084 (IC3) will not cope with that (+/- 18V max) and is almost certainly "cactus".

4) The initial high voltage supply at C44 would have gone way over the 450V DC rating of that cap and so it may be "blown up".

If it were mine I would replace D16 and D17 Zeners and check R59, R60, R61 and R62 for any evidence of heat stress. that should restore the +/- 12V and +/-15V supplies.

Then replace IC2 and IC3.

Then remove C44 and check carefully to see if it has blown out its "vent " hole, if so replace it too..

Then check the 300V DC rail, if it is not there replace C45 as well and then if still not there replace all of D10, D13, D14, D15. Note that if the high voltage supply is good it will go quite a bit over 300V until the tube warms up and then settle back to about 300V.

Hope this helps.

Cheers,
Ian
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Old 3rd December 2008, 11:55 AM   #10
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Great analysis gengertube! I will have to wait a few days until I get home and try out the knowledge. AND

I got the schematic! tried the link in the middle of the night and it worked! Thank you world of intredpid DIY'ers!
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