Metropoulos Forum

Hi there, i just finished building a JTM45. The build went well. I'd been checking my work as i went and reviewing the previous day before moving on.

Yesterday i got my circuit board all wired up then my mains, gave everything another look and moved on to the burn-in check.

Step A: Fused installed (3A for 120v, 500mA on the internal HT.
Step B: Plugged in and reading 119VAC at the switch and neutral
Step C: Powered on (no tubes). 6.5VAC across the filaments of V1-5 and 5.6VAC on V6
Step D: Rectifier installed. 323VDC at the standby switch and -44VDC at V4,V5p5

All good so far

Step E: Preamp tubes installed (V1-3). I've got really low voltages on V1,V2 (almost nothing), V3 is reading about half of what it should

I doubled checked the circuit board, circuit board connections and filtering and have yet to find anything wrong. I also confirmed my under-board wiring with my multimeter

I went ahead and installed the power tubes to check them and everything on the power tubes reads in the ballpark of where it should be. I was able to set the bias. I get very little out of the speakers but if i probe around the circuit board touching the lead of a component will produce a very low volume radio station (i live near a few broadcast antennas and this has happened before.

I've been looking around online and trying to troubleshoot but i'm not sure what else to look for here.

Here are all my voltages and a couple of pictures. I'm happy to post more detailed pictures or check other things too.

Wall Outlet: 119VAC
HV DC @ Standby Switch: 325VDC

V1:
Filament(VAC): p[4,5]/p9 6.52VAC
V1,p1: -.4V
V1,p2: -.58
V1,p3: .03
V1,p6: -.46
V1,p7: -.58
V1,p8: .03

V2:
Filament(VAC): p[4,5]/p9 6.5VAC
V2,p1: -.4
V2,p2: -.8
V2,p3: .004
V2,p6: 0
V2,p7: -.4
V2,p8: .35

V3:
Filament(VAC): p[4,5]/p9 6.5VAC
V2,p1: 108
V2,p2: 8.5
V2,p3: 15.4
V2,p6: 74
V2,p7: 11.8
V2,p8: 15.4

V4
Filament(VAC): p2/p7 6.5VAC
V4,p1: 0
V4,p3: 383
V4,p4: 385
V4,p5: -44
V4,p6: 385
V4,p8: .03

V5
Filament(VAC): p2/p7 6.5VAC
V5,p1: .001
V5,p3: 384
V5,p4: 387
V5,p5: -44
V5,p6: 386
V5,p8: .04

V6 (Rectifier):
V6,p2/p8: 4.7
V6,p4/v6: 623

(Pictures to follow in the next post)

My post from another forum to close this out:

i think i've got it sorted.

an amazingly subtle problem i blame on murphy - and yet another that won't happen on my next build. learning these lessons is always good though.

I wasn't getting anywhere further with my troubleshooting but I wanted to get a visual on my underboard wiring even though it was all testing good. I remove the nuts holding the board onto the mounting screws and pulled the board up a bit so i could slide a small mirror underneath and take a look.

near one of the mounting posts the underboard wire had deflected a bit. i manually moved it some with my finger, then with the board up off the posts i fired it up and checked - i had b+ where i was supposed to, everything tested good, i plugged in and had that great fender (er marshall) clean.

so anyway, i did some wrangling and remove the piece of under board wire running from the junction of the 100k resistors to the tube socket side of the 10k 2w and replaced it with a fresh piece making sure to route it well away from the mounting posts. I reseated the board, resoldered the components i had to pull and i'm all fixed.

upon inspection i found that the insulation had been crimped between the board and the metal mounting post - this also explains why i was generating the heat i was at those resistors.

It's late on a sunday night now so i'm going to let it rest until tomorrow. I'll post updated voltages across the board tomorrow and hopefully get everything all wrapped up and mounted in the head cab.

I'll chalk this one up to lead dress and make a(nother) note that i don't like building blind just following the paint by numbers. fender-style layouts for me.

This is really weird now.

You refer to posting it on Sunday - these posts both appeared today (Thursday!) without pictures!

Were you newly registered waiting for 'clearance'?

Snagging an under board wire between board is quite common, (often choke/screens end of the board).

On the kits it is generally best to use bus wire and the thick black fibreglass reinforced insulation and keep the wires short and to the point. I also suggest pulling the wire up and hooking it over the turrets to stop it ever falling back if you have to resolder from above when tweaking/modding or replacing parts.

Hi Neil - you're right in how the post was made.

Initially i had pictures linked to it but it complained about the resolution being too high so i had to remove them to get the post accepted. then it had to be, "approved" before it actually appeared on the site which took a day or so.

by that time i'd solved the problem so i just posted what my solution had been so i didn't leave it dangling.

I always make my under board wire runs as short as possible too. In this case it was a bad thing as a direct line between the two points went right over (or very close to) the circuit board mounting hole which is what caused the problem.

For the replacement piece i did pull it up through the board. the next time I build - which should be coming up soon - i'm going to do that with all of the under board wire.

I did the same thing on my first build. You need to keep those wires away from the posts. I caught the orange bias resistor wire between the board and the post nut. That blew the fuse.

A similar thing happened to a friend with a 12xxx build. One of the wires to V1 was not soldered completely. The insulation had gotten between the wire and where he was soldering. Shows that the smallest little thing can cause the amp not to work at all. Glad you got it worked out.