Fender Champ 12 Repair

Fender Champ 12 Repair
Addition 6/6/13
There is a big effect difference between voltage levels that break through the skin conductivity – which happens to be in the 400 to 500V DC range of our amp circuits! BE CAREFUL! Drain the secondary capacitors when the circuit is off and always check the circuit with a multimeter on both AC and DC sides before touching components or working on it – with the mains UNPLUGGED. If the Mains is plugged in, the plug socket and switch contacts are LIVE still, with the switch in the OFF position.
If you are testing the circuit after a change, make sure all you test leads, probes and croc clips are well insulated from your fingers and chassis contact points (chassis edges can be sharp too). I always connect the croc clips and meter and double check. I then plug in the fused mains side. I use the mains toggle switch to power the secondary with the indicator light connected so I can SEE power also, as well as checking the on/off position of the switch BEFORE I plug in the mains. I switch OFF and unplugged the mains, checking the meter voltage or current level has dropped to 0V, (or use a drain lead on the capacitors ASWELL) before I move any croc clips to a new circuit point for another measurement.
For the sake of a few extra seconds, why risk safety by not switching off and draining the circuit?
A 240V AC shock will certainly help you not make that mistake again if you ever forget any of this – if you are lucky enough to not get away with it! My last one left my right arm aching for an hour!
A 500V DC shock may mean you don’t get to worry about any of this ever again…but now you know why.

This was a fiddly amp to work on, as it’s a bit of a hybrid of older point to point wiring for the valve holders, and a PCB for the channel volume, tone, distortion, presence and reverb sections, with a separate small mains PCB:

It is a bit of a cheap build with the cardboard reverb tank and untidy wiring layout, but then it has had a bit of repair work done to it, as the input jacks had been rewired to the PCB from prior breakage, and a techie had diagnosed the current fault of duff output transformer, according the EBay seller, which turned out to be correct.
It would have been a really straight forward repair – replace the OT transformer, but the previous tester/someone had wired the heater wires/heater secondary wires back to the mains PCB incorrectly, so I had no valve heaters lighting up on the first test after connecting a temporary Danbury OT, which meant me spending time getting familiar with the power supply section of the schematic for this amp:

First I removed the OT transformer next to the reverb to check the alleged failure, which was the OT primary being open circuit. I spent some time looking for replacement options on Ebay, which are only via companies like Mercury Magnetics in the US, and are expensive at £35 plus shipping.
As I had ordered a spare 15W OT to experiment with for the Guitar Amp Handbook kit build from www.ampmaker.com I checked with Barry that the specs for this would work with a Champ. It does, as most 6V6/6L6 output valves that use an inductive anode load have an output impedance of 2k-8k ohms, so there is all these options for different OT windings on the transformers Barry has built for him by Danbury Electronics in the UK:

You can get up to 6k ohm across all the OT secondaries so it works with the Champ anode.
The problem is this tranny is twice the size of the original Fender part, made by Shumacker (EIA606).
I researched this on the Web – Electronic Industry Association manufacturers codes – 606 is Shumacker.
I found a guy called willycom1 on Ebay, based in Indonesia who winds transformers to the same spec. for use in Champs and other amps, at a good price of £10 + £9 postage:
This is half the price of the US parts, so I bought a couple to try out, and he was helpful enough to question on the impedance. He can also wire mains transformers, but the weight makes shipping prohibitive.
This should be the right size for the chassis. In the mean time I thought I would use the Danbury, to get the Champ going. I had already checked all the valves for this amp in the Aston, when doing the Fallen Angel repair, so knew that part was fine.
I soldered it in place, but as stated above had no response, as there was no heater glow and no 6.3V AC across the valve pins. I was worried at this point that the mains tranny was duff also. That would have been a pricey repair for this Ebay offering!
I de-soldered the mains windings to check them properly with the meter, and got sensible values of 50 ohms for the primary, and 1.5 ohms for the heaters.
As I had come that far, I thought I would look at the underside of the control PCB, where I found the input, line out and headphones jacks had been fixed previously:

This also accounted for why the messy wiring, and why reverb tank, speaker and OT wires had been soldered to the back side of the PCB – maybe by the previous tester, to save time until a proper repair was booked.

I took the time to tighten the valve holder pins as usual, as the 6L6 was nearly falling out when upside down with no spring holder, and gave them a blast with the now invaluable Servisol switch cleaner. I checked continuity across all the point to point wiring also.
Once I had worked out which contacts on the Mains PCB to use for the mains heaters and indicator LED, by studying the schematic and using the resistance meter, I got the wires back to where I thought they should have been, attached the speaker and re-tested with all controls to minimum. The indicator lit this time, and I got a lovely blue glow from the 6L6.

I had removed the footswitch from this amp as the cable needs repairing, so this amp is stuck on the overdrive channel – an oddity I had not thought about before – but if you short out the connection on the back of the PCB (pliers etc.) where the pedal wires go, to get the clean channel – it does not latch, so as soon as you remove the short, it reverts to OD again. As there is no front panel switch option, this is why the foot pedal is hardwired on this amp, and is not an optional jack plug in, which I thought about modding for this amp by drilling and adding a jack to the rear, so I could use the footswitch on other amps like the Fallen Angel. Hmm, not now I guess, as it would affect the resale price of this amp if I kept the footpedal probably, and not worth explaining the handy mod. I thought about this as I don’t like hardwired mains cables either – I prefer a kettle lead to plug in, as it makes testing that bit easier, with less wires around or to desolder/move when removing PCBs or chassis.
Anyway, it was enough for a brief test to assume all else is ok – the OD volume, gain, bass, treble and reverb is working. Until the Indonesian OT arrives, I will leave it on the bench and give it a full review then.
A long time coming to get to this – but the “egg frying” noise with this amp was just a duff 12AX7 pre amp, solved with changing it for new one:

WiFi Hub noise eliminated with metal screens:

Only things left to fix for this amp now is the MP3 input, and the 1V line out sections…oh, and the reverb feedback issue…maybe in another year…?!