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 Post subject: HELP! TMB problems
PostPosted: Wed Apr 10, 2019 10:32 pm 
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I'm 35 hours into my first amp build. I have had some hiccups, but have been going slowly, checked my work, done troubleshooting, found the problems, and recovered until now. Now I'm lost and need help. Here's the latest. I finished the build and went through first power up and voltage checks. Everything seemed OK, except voltages were quite low on all of the V3 pins (84 instead of 220, 19 instead of 52, 22 instead of 75, etc). I went through and triple checked my work, and found that I had mistakenly installed two 1M resistors where the two 100K plate resistors should have been (R16 and R17). (Rookie error reading stripes and making error reading decimal on multi-meter. THat also explains why I was short a 1M resistor on the cliff jack assembly.) I only had one 100K resistor left (which worried me), so I went to Fry's and bought a five-pack of 100K 1/2 W resistors. The problems started when I tried to remove the incorrect resistors from the turret board and install the correct ones. I struggled mightily, the work looks terrible, and worst of all, I burned one of the .01 uF coupling capacitors (C9 I think) with careless placement of the soldering iron. Hoping maybe its functionality somehow survived, I continued to power up the amp, and 30 seconds later realized I had forgotten to plug in the speaker. I shut it back down, plugged in the speaker, and turned it back on. I started checking voltages when I noticed the EL84 in V4 was glowing very brightly and way too hot. (It had not done that when I powered it up with the wrong resistors earlier). I quickly shut it down. I wasn't able to get voltage checks with tubes in, After discharging the capacitors, I did check ground continuity checks and, and checked resistance values, removed the tubes (except rectifier) and did another voltage check of B+ voltage. Voltage was good (over 400V), but it seems one of the new resistors (R17) is now infinite ohms. Also, the 470 ohm resistor (R18) seem to be dead now as well. I cannot get a reading across it at all. So, the problems I know about are: 1) C9 has a big burned cavity on the side and is likely toast, 2) R17 and R18 are suspect, 3) The EL84 in V4 now overheats when power is applied, and 4) I don't know how to de-solder and re-solder on a turret board. Please tell me me next move. I was loving this project until tonight. Thanks!!


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 Post subject: Re: HELP! TMB problems
PostPosted: Thu Apr 11, 2019 7:44 am 
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Holy Ghost
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Location: Brighton, Canada
Well, you should remove the suspect parts and test and get new parts.

To remove parts, Suggest that you get some sort of solder sucker. Basically a reverse syringe . Heat up the joint, suck as much solder away as you can and pull the part out with a pair of needle nose pliers.
Then go back to the startup procedure.
You can test the OT resistance, but you need to disconnect it from the impedance switch to do it.

++++++++++++++++++

If you suspect your OT for some reason, you can do a resistance check to get an idea of it's health.

Disconnect all the wires from the circuit, then use a digital ohm meter to measure the following resistances. Clip the probes to the leads, do not hold them with your fingers.

Primary
Brown - Red: 275 ohms
Brown/White - Red: 160 ohms
Blue - Red: 331 ohms
Blue/White - Red: 166 ohms
Brown - Blue: 606 ohms
Brown/White - Blue/White: 326 ohms

Secondary
Black - Orange (16): 1.0 ohms
Black - Green (8): 0.7 ohms
Black - Yellow (4): 0.6 ohms
Note, it is very hard to measure low resistance.

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 Post subject: Re: HELP! TMB problems
PostPosted: Tue Apr 16, 2019 10:17 am 
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Thanks so much! I ordered and installed newt parts and checked my work again. It's mostly good news now, but I still need a little help. The TMB channel is working! On the TMB channel, both inputs work, all knobs and controls work, and there's no hum, hiss, crackling, or popping. Even on a crummy 8-inch practice speaker at low apartment volumes it sounds great. However, I'm getting nothing from the normal channel. No 60 Hz hum when I touch tip of guitar cable, no sound from guitar. I have gone over the connections visually and can't find a fault. Are there some continuity or voltage checks I can do that would isolate where I may have a bad connection or a short? Thanks again for all the support.


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 Post subject: Re: HELP! TMB problems
PostPosted: Tue Apr 16, 2019 10:51 pm 
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Holy Ghost
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Should be easy. Trace continuity In steps from the tip of the cable to the input grid of the first tube. Make sure the signal is getting to the tube. Then confirm voltages are good and the signal path to the Phase inverter is good to.

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 Post subject: Re: HELP! TMB problems
PostPosted: Mon Apr 22, 2019 12:09 pm 
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Thought you might enjoy this photo, Stephen. Still working on the cabinet, but could not wait to hear this thing in action. I may still try some mods to try to squeeze out more gain. I needed some boost on the TMB channel to get that punchy Marshall tone I was looking for, but boy I found it! Using a Boss equalizer pedal to drive the front end, all may band mates agreed it sounded amazing!


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 Post subject: Re: HELP! TMB problems
PostPosted: Mon Apr 22, 2019 7:56 pm 
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Holy Ghost
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Location: Brighton, Canada
Nice! I guess the problems got sorted!

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