Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: makjoh on March 26, 2021, 04:29:39 am
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I turned my little Laney TT20 on and heard it crackle and smoke a little....switching off and investigating inside the amp there was nothing obvious or burnt out. The internal fuse on the pcb tested fine and wasn't blown. The main fuse was obviously blown(T100 Ma) and replaced with a T100 Ma fuse as is required. I have power going to the amp, light showing , and current going to the pcb but no sound!?
Any assistance greatly appreciated. I'm wondering why this smoking and crackling occurred and why the fuse didn't protect the amp as designed? Although it blew and was the T100 Ma as required
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Fuse can't do what are not designed for. Main fuse protect your amp. Fuse blowing save you power transformer and more.
On tubes amp issue, check tubes first. Power tubes are first to suspect.
If you have to replace Power tube, buy the same brand, if you bought another brand, you must check and adjust tubes bias
There are 2 fuses inside the amp, not only one .
Owner manual; trouble shouting page 9
http://pdf.textfiles.com/manuals/STARINMANUALS/Hohner%20-%20Laney/TT20,%20TTH20.pdf
Service manual download here ; https://elektrotanya.com/laney_tt20_sm.pdf/download.html#dl
I do.
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More great help from you Latole :worthy1: I'l go through the information you sent and let you know. Thank you again
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Good luck .
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The main fuse was obviously blown(T100 Ma) and replaced with a T100 Ma fuse as is required.
The schematic owner's manual shows the main fuse should be T1AL or T500mAL, not T100mA.
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:w2:
Schematic ;
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Clearly there's a discrepancy between the schematic and the owners manual. I'd trust the numbers stamped on the actual chassis. Either way, the main fuse is not 100mA.
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Clearly there's a discrepancy between the schematic and the owners manual. I'd trust the numbers stamped on the actual chassis. Either way, the main fuse is not 100mA.
100% right.
What is write on the Makjoh amp chassis ?
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Sorry guys....my mistake...the fuse I thought was the mains is the HT fuse. All three fuses are correct in their ratings. However,...I have taken the chassis out to get some readings from the transformer. The readings were with the tubes out of the amp
1. The heater wires are reading 85 volts! (should be standard 6.3?)
2. The L.T. wires (Orange) are reading 22 volts (schematic says 20)
3 The Red H.T. wires read 260 volts each.
Even more alarming was the reading with the tubes in
1. The heater wires were showing 358 vdc!!?
I switched off and here I am again
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You may do some mistake with reading on heater?
With 385 volts, you blow tubes heater !
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On an unaltered amp, either an error in measurement, or the PT has been compromised. . .
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Heater voltages can't be different with tubes out ( 85 V) and with tubes in ( 385 V
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1. The heater wires are reading 85 volts! (should be standard 6.3?). Something wrong, must read 6.3 AC voltage
2. The L.T. wires (Orange) are reading 22 volts (schematic says 20) Readings is good
3 The Red H.T. wires read 260 volts each. Reading is good with tubes out
Try some more reading, transformer with all wires unplug from the circuit . Transformer only .
Report
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Ok.....Today I have disconnected the transformer wires from the pcb
Here are the readings after first checking the supply voltage of 240v on the meter
Red wire 1...I read 22 volts with just the + probe connected (?) when I ground the other probe it jumps to 76 volts!
Red wire 2...same readings as red wire 1
Orange wire is 10 volts (again just one multimeter probe) grounded with the other probe it shows 55v?
2nd orange wire shows 6 volts and 46 volts (As above)
Red HT wires show (1st) 51 volts and (2nd)150 volt when grounded.
I checked the schematic and noticed the black grounding wire.... I had been grounding to the chassis. So when I rechecked using the black wire as ground, all the voltages for each wire were correct.
I'm a little confused as I thought ground was common on everything?
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are you measuring VDC or VAC? and if you're measuring with the transformer disconnected from the rest of the circuit, you should be measuring VAC between red and red, etc
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....I'm a little confused as I thought ground was common on everything?
In the final amp, probably.
In the naked transformer, probably not.
As thetragichero says: measure end-to-end of each winding.
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are you measuring VDC or VAC? and if you're measuring with the transformer disconnected from the rest of the circuit, you should be measuring VAC between red and red, etc
Yes I had the meter set to VAC. The transformer is testing fine with all correct voltages. Time to reconnect and remeasure
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Everything reconnected, the tube sockets cleaned and the amp is working, BUT......massive amounts of hum coming from the speakers. I tried the chopstick test on the tubes. EVERYTHING is microphonic. Every little tap on the tubes, amp chassis, transformers, even the reverb send and return wires pick up the taps to be heard from the speakers. The tubes have tested fine in another amp. Any suggestions and advice greatly appreciated guys
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Bad ground contact or ground is missing somewhere.
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I just seem to locate any bad earth or missing earth?
When I switch the amp on there is a loud hum coming from the speaker which almost disappears when I turn up the volume to 4 and 5. It then increases with a different kind of hum from level 6 onwards. Even muted the hum is evident through the speaker. The bright button seems to increase the hum from level 4 onwards. The pentode mode button amplifies the hum even more.
Thanks guys
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In the schematic there is no ground reference for the heaters. Or do they share the centertap with the high voltage winding?
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In the schematic there is no ground reference for the heaters. Or do they share the centertap with the high voltage winding?
Basic wiring don't show everything, schematic show artificial ground with pot.
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I Just can't find the fault....damn.... :BangHead: