Marshall 2204 CSA HT Fuse

[bobloblaws]

Hey guys, I'm changing filter caps in a circa 1985 JCM 800 (2204). It is the Canadian import version, sometimes referred to as a "CSA" model. One of the differences is that it has a 500mA fuse on both sides of the PT secondary as opposed to just one after the rectifier. To my surprise when I opened it up I found that one of these fuses was blown, given that the amp seemed to be working normally. In this state I measured the B+ at about 440V. I replaced the burnt out fuse and the B+ increased marginally to about 450V. So I'm curious how it was still able to operate with that leg of the PT secondary disconnected. I'm guessing it has to do with the fact that the secondary has a grounded center tap but I was hoping someone with more electrical knowledge could enlighten me. I'm also curious if there was potential for damage to the PT or elsewhere in the amp if it was operated like that at high levels and/or for long periods of time. Thanks!

[bobloblaws]

Anybody?

[Helmholtz]

I think it's like this:

The open fuse left the HT power supply with half-wave rectification (only one active half of HT winding). At low load (idling) the voltage will be almost as high as with two-phase rectification (actually DC voltages should be identical at no load) but should sag more with increased output power. Other consequences are higher ripple (60Hz !) and increased PT heating.

Two-phase rectification using both halves of the HT winding doesn't double DCV compared to half-wave rectification but rather "fills" the gaps between the 60Hz pulses.
OTOH, full-wave bridge rectification (doesn't require a CT) would double DCV.

[Jazz P Bass]

The nearest schematic that I can find that has (2) 500ma secondary fuses is the 2205.
2205 amp.pdf
If this is the same setup as yours, then, with 1 fuse open, you were running on 1/2 of the transformer.

[g1]

It would be the same scenario if 1/2 of the standby switch failed in the schematic Jazz posted above.
Agree with what Helmholtz said, it should be more noticeable at high power under load (where it would be saggy, lower power, and hum at 60hz rather than 120hz should be noticeable).

[bobloblaws]

Thanks for the replies! That is helpful. Owner texted me yesterday and said he had noticed increased hum recently, so that lines up with the blown fuse. I know that he had it in to a tech previously for a blown internal fuse, but I don't know if it was one of the same ones or one of the heater fuses or the additional mains fuse that the CSA amps have. Assuming that it is periodically blowing the HT fuses would it be feasible to bump them up to 750 mA slow blow or 1A fast blow instead of 500mA slow blow? Obviously the ideal thing would be to address the root cause, but if it is only happening once in a blue moon that could be a tall order I would imagine.

[Helmholtz]

I don't recommend to increase fuse ratings as they are meant/chosen to protect the PT. Check rectifier diodes, filter caps (leakage!) and power tube dissipation and use quality brand slow blow fuses - and make sure that a matching speaker load is used.

[bobloblaws]

I don't recommend to increase fuse ratings as they are meant/chosen to protect the PT. Check rectifier diodes, filter caps (leakage!) and power tube dissipation and use quality brand slow blow fuses - and make sure that a matching speaker load is used.

Ok. What is an effective test to check the rectifier diodes?

[Helmholtz]

Ok. What is an effective test to check the rectifier diodes?

https://www.wikihow.com/Test-a-Silic...h-a-Multimeter

[bobloblaws]

https://www.wikihow.com/Test-a-Silic...h-a-Multimeter

OK, so nothing out of the ordinary then. According to that tutorial I can reliably check in circuit, which I wasn't sure about. I thought you might be suggesting something different than a simple check for a shorted or open diode since the scenario in question was one where the fuse was generally stable but popped on occasion.

[g1]

Most common cause of HT fuses blowing is power tubes. You said you are already going to replace the filter caps, so if the diodes check ok, power tubes could still be suspect. And faulty power tubes can be very intermittent, so be sure to at least do a tapping test on them under heavy load.

[bobloblaws]

Most common cause of HT fuses blowing is power tubes. You said you are already going to replace the filter caps, so if the diodes check ok, power tubes could still be suspect. And faulty power tubes can be very intermittent, so be sure to at least do a tapping test on them under heavy load.

Thanks, good to know. As it happens new tubes are going in along with the new filter caps.

[Helmholtz]

If everything suggested above checks ok and the amp works as it should, the currents through the fuses will never even get close to 500mA.

[bobloblaws]

If everything suggested above checks ok and the amp works as it should, the currents through the fuses will never even get close to 500mA.

Thanks. It turns out the owner had it in to yet another tech on a different occasion for the same issue (hum + blown fuse). The fuse that I found blown appears to be a fast blow while it's brother appears to be a slow blow (looks like a coiled up spring inside), so I'm speculating that the only real issue is that buddy put the wrong type fuse in it last time.

[The Dude]

I'm not sure there are enough fuses in that thing. Three HT fuses is simply not enough. I'd add a couple more just to protect the fuses that are already there.