Michaelson Audio Odysseus Problem.
Hi this is my first post here. I'm hoping someone can help.
I recently got hold of a Michaelson Audio Odysseus valve amp. As far as I know this is a class A ultra linear amp. It has 8 EL34 power tubes, 2 ECC83 and 2 ECC85 pre-amp tubes. It outputs around 45 watts.
This is a photo of one:
The original owner had not experienced problems.
When I received the amp it worked and very nicely so I am determined to get it working again.
I have a little experience of fixing my Fender Twin guitar amp which I completely rebuilt (it was also an ultra linear - not so common for Fender Twins). However I have forgotten some know how since then and this Odysseus amp is obviously a little more complicated.
This is what went wrong and what I have done so far:
The right channel of the amp stopped working and fused. There are fuses connected to the screen grid circuits. I looked inside the amp and immediately saw some problems. The main problem being burning around the high watt wirewound resistors at the filter caps. Bear with me because I have forgotten some knowledge about what everything does. You will understand what resistors I am talking about here - whatever they are called. These resistors read 23.5 ohms on my meter (so bad a state that reading the colour code was impossible). They are not the same type as appear in the original Odysseus. They look too small and low wattage and were almost touching the PCB. The right channel had such a bad connection here, due to heat, it was loose. I sorted these by replacing with suitable value high wattage wirewounds (I actually stuck two 47 ohm in parallel here - which I presume should be OK) and I checked all connections, PCB tracks etc. No other obvious problems in this respect. I also noticed that the wirewounds on the power tube cathodes were in terrible physical state. These were 470 ohm wirewound. They were only marginally off but again not the same resistor types as was obviously on the original amp. Possibly too low wattage again. Originally, it had the standard oblong shaped ceramics here. I replaced these resistors. I also discovered a completely blown screen grid resistor. Once again these resistors don't look the same as the originals. Possibly too low wattage yet again but I have left them, only replacing the dodgy one for the time being.
As you will now deduce, work has been previously done to this amp. What I know of is that the Filter caps were replaced along with 100uf caps on each power tube cathode and two 220uf caps in the preamp stage. This is documented as being done two years ago. I have not yet tested all the caps because this amp is hell to work on. The replaced caps are all correct spec as far as I know. Some resistors were also replaced but not the ones I replaced. All resistors seem OK. I've tested them all.
Next stage was turning the amp on again. I make the assumption an EL34 valve had gone because it's screen grid resistor immediately went up in smoke and the amp fused again. I replaced this tube (I was provided with new spares of the same Harma valves when I bought the amp) and the amp then turned on OK except I get serious motorboating. I even returned the original resistors (the ones that were working but not entirely satisfactory as far as I can tell) to return the amp to how it was when it worked but to no avail.
My questions start with what causes motorboating and where should I go from here? Testing all the caps may seem like an obvious start but unfortunately that is going to be physically hell on this amp due to it's design. What caps, if any, might be causing my problem? It it the filter caps? Did the blown valve or loose connected resistor damage anything else? My knowledge is not that good but it's more a question of brushing up what I once knew so I can provide reasonably detailed info.
If anybody can help me get this amp working it wold be greatly appreciated. I can supply photos if that is of any use.
Thanks a lot.
Welcome to the forum - shame about the amplifier. I once had a Michaelson & Austin TVA10. If you so much as looked at it in a funny way (or removed its input or loudspeaker load) it would oscillate violently at RF and the output valve anodes would immediately glow red. I suspect you may have the same problem here. RF oscillation causes heavy power supply currents and that can often provoke motorboating. The motorboating may be because a different value was fitted when those HT capacitors were replaced. The amplifier clearly has a serious fault that has been there for some time and is going to need an oscilloscope and a fair degree of intelligence to fix.
I looked after a few of these amps in the early 90s when I was working for an MF dealership. Don't remember any instability problems with this particular design - we had one pretty much permanently powered up in one of the listening rooms which was used with a wide range of different equipment.
They all ran really hot though, so 15 years on I would be looking for dried up filter caps and probably more cooked resistors. Iffy soldered joints on the pcbs are likely at this age as well.
It would be worth going through the whole amp and replacing the components that are past their sell by date - caps, power resistors etc. Check the tube sockets for the EL34s and the soldered joints between these and the pcb below.
check resistors,electrolitics and joints :D
OK, thanks for the replies.
I tested as many of the caps as possible but my multimeter can't go high enough uf to test them all.
The latest is that when I run each channel and test two valves in it. The amp runs with no motorboating. If I hook up 4 valves in any channel then I'm back to motorboating. Maybe this can give a clue for you to help me track down what is wrong.
Another update here.
As I've said, when I run 4 valves I get the motorboating. There are two separate PCB boards with the valves attached and related circuitry such as cathode resistors and caps, grid resistors etc. These are connected to metal sections which also house the output transformers so when open these parts of the amp can be physically moved away from the main PCB. This movement cures the motorboating but with 4 valves in a channel I get a lower level hum (doesn't occur with 2). The amp has some kind of insulating spacers which are not there while I work and a lot of connections had shrink wrapped insulation which is obviously removed as well. It is possible some noise would be eliminated when rebuilding the amp but the motorboating does not disappear unless the valve PCBs with transformers are moved well away from the main PCB.
Apart from the electrolytics which I can not test with a simple uf on my multimeter, all the rest of the amps caps and resistors appear to be OK and I can not find any dodgy connections. A possible problem is burnt PCB around where the power resistors were but it is not serious. The PCBs seem a bit weak. some of the copper tracks around the power resistors were damaged and I had to bend the new resistor wires back across the PCB to make good contact but I can't see anything that now looks like a dodgy connection.
You mention about the HT caps. I have discovered that I think they were replaced with a different value. The only thing I have to go on is that I have an invoice for the previous repair. The invoice says that 6 470uf 220v caps were replaced but in the amp these caps are actually 560uf 250v. I know sometimes you have to go higher because of lack of availability of the correct value. I make the assumption that 470uf caps were what was originally in there but they used higher value even although the invoice says different. Could the problems have stemed from this? I also have no idea about the make of these caps. All the other Electrolytics are Jamicon. Should I simply upgrade all the electrolytics? What are good quality or at least worth the cost? The 6 big ones may well be poor quality and I don't think Jamicon is that good either from what I remember.
Also. Oscilloscopes. I have considered getting one of these before. Any ideas about what is a good oscilloscope to buy? Not too expensive but a good make. Something that I might find on ebay for example.
This is my first valve hifi amp and I've had a taste of what a great sound it is. I'm prepared to go to a fair degree of expense and time just to get the sound back and better if possible which I'm sure it is. This amp had a gloriously powerful and detailed sound.
You can try this number , not sure they still in the business or not ... they own the Michaelson & Austin .
Papworth Audio Technology,
32 Fendon Road, Cambridge, CB1 7RT
Telephone (01223) 416078
I telephoned Musical Fidelity who serviced the amp a couple of years ago. This is Michaelson's company. They wouldn't give me info. Basically they wanted me to send the amp for servicing. Judging by an invoice I have here from the service two years ago I will definitely not be sending it. Huge expense to replace a few caps, volume control and selector switch. I want to learn anyway.
One thing I can't understand at the moment is some resistors on the screen grids. They have colour code for 10 ohm yet all read around .9 ohm. I'm sure they are not the original resistors either but I have no schematic so I have no idea what was here originally. Two of these have gone. One was gone first time I looked in the amp (it had literally cracked apart). and another went up with the dodgy valve. They are tiny little wirewounds. At least that's what they look like to me judging by the one that has blown apart. I've not seen resistors like this before.
When I've said valve I mean tube of course. I presume non UK people know that we call them valves? It might sound confusing.
I've ordered Valve Amplifiers by Morgan Jones (I guess everyone will know we call them valves then). I need to learn more before I start doing anything else with the Odysseus. My guess at the moment is Filter Cap problems but I need to make sure all other components are in order.
I was able to completely rebuild my Fender Twin amp and do some repairs - change the balance control to bias etc, but I found that much easier to understand. Easier to see the wiring rather than looking at a circuit board and this Michaelson is a nightmare to work on unless you simply dismantle the whole thing and take it all out the chassis.
both side not working or just one side ? self bias or auto bias ? when u turn on the amp ,did the valve turn red ? or flashing ?
|All times are GMT. The time now is 07:02 AM.|
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio