Melody HiFi problems

[pontiacpete]

This is a high end all tube Hi fi stereo amplifier, with 4 X el34s, 2 X 5u4s and 4 X 6SN7s. There are many rectifiers. There's a bridge rectifier connected to the 2 5u4s.
I have no schematic. If anyone has one please post it. I've tried contacting the company in Australia and the representative in Ca and no luck.
There are a couple of problems with it.
It came to me earlier this year and many tubes had gone bad and one el34 socket was burnt, which I replaced. I also found one shorted rectifier diode(1504?) in the bias circuit.
Anyway, I replaced the quad and the 2 5u4s and the diode and had it running .
Now a couple weeks ago the owner tells me the fuse blew as soon as he got it home months ago but didn't call me back until now.
This time around I found that the first main rectifier was measuring some strange resistances so I changed it out and the unit starts up. However after some time with the caps drained I tried switching it on without a limiter and the fuse blows. So I order up a KBU 1010 rectifier and it starts with the limiter but blows the fuse without it.
The mains fuse on the IEC connector says it is suppose to be an F3A but I only have T3A fuses and they are blowing.
I just tried a T4A fuse and it starts up without the limiter the last couple of times. I did this thinking the fuse is not rated high enough. I know should just calculate the fuse rating myself but I thought I'd would ask here first.
Any ideas on the fuse rating? Or for that matter what other issues could be causing the fuses to blow.

The other issue is with the volume pot on the bottom right of the picture. It got damaged in transport and needs replacing or repair. Any suggestions on what this might use. It is a stepped pot. I see several on ebay that are similar looking and are about 100K, is this about right for a replacement?
Thanks for reading and thanks any ideas in advance.

[Mick Bailey]

The volume control is a stepped rotary attenuator. The reason is to get a better approximation to an audio curve, by using individual resistors closer to the 'ideal' value than a pot may offer.

I've seen two types, one selects each individual resistor and the other type adds resistors in series.

In either case, there's no real benefit over an ordinary pot; you hear a volume level and the pot rotation may be a few degrees out compared to the more precise attenuator. Maybe there's less noise and a better matching between the channels but a decent quality audio pot can be substituted.

You may need to add the resistor values (or measure the good channel if there is one) to get the total resistance. Usually either 50k or 100k.

[pontiacpete]

Thanks for the info Mick. I should just put an ordinary pot in there, but I will have to check with the owner.
I assume it is a dual gang, right?

Any ideas about the fuse rating? I'm almost convince I should use the 4A fuse.

[pontiacpete]

I tried putting an ammeter in series with the mains on the back side of the IEC socket and found that at turn on it pulls a bit more than 2 amps but idles at about 1A.

[Jazz P Bass]

Seeing that these units go for what, $2000.00, I would attempt to contact the manufacturer as to a replacement attenuater.
Email : info@melodyamplifier.com
In the USA:
Melody HiFi, LLC
Torrence, CA
(310) 375-8938 (Mingus)
(901) 881-7411 (James)

The T4 fuse holding is strange one.
Why would the amp demand 3 amps at startup?

I would measure what the amp is pulling from the mains at idle.

[Mick Bailey]

It could be a lot more than 2 amps instantaneous current at startup - depends on whether your meter can display peak current that would be too fast for a display to react to. The tube rectifiers ought to give a soft-start but what the SS rectifiers are up to would need the wiring tracing through.

F3A seems a little bit 'quick' to me - the Mesa Tremoverb uses 2x5U4G and 4x6L6/EL34 and is fused at 4A SB for 120v operation. But if the amp is labelled up for F3A then presumably this was what the designer spec'd.

[pontiacpete]

Made a mistake it idles at 1.7A forgot about the relay that turns the amp on to operating mode.
Repeating with ammeter , I saw one time a surge of a bit over 4A. I'm surprised the 4A fuse survived.

I tried the manufacturer several times, no response. what's up with that?

[Jazz P Bass]

How about trying a different set of 5U4's.
NOS would be ideal.

I know the latest & greatest EH tubes sometimes go south.
Heater to plate.
You obviously do not have that condition but one of them may be trying to fail.

[pontiacpete]

Good point JP, the first visit I put in a couple american made 5u4s I had here and told him that they were used and didn't charge much. Now I have JJ and another without a label.
I'll order a couple NOS tubes this time and do this ammeter check, again

If I don't hear from the company I'll get a regular old dual gang 100K pot.
Unless I can fix the original. Had it apart yesterday, one of the 2 very small contacts(piece of thin 1/8" sheet metal bent into a 'V' shape) fell out of place. Don't know where it's suppose to go in the slotted wheel so I just took a guess and now there a large dead area of sound in the middle of the dial.

[52 Bill]

Do you remember what the original fuse value was when it was working before? Is the transformer set up for 120/240 line voltages. If this was made in the land down under could the fuse holder be marked for the 240 volt value?

[pontiacpete]

The first time it broke down the owner said he changed the fuse (with a T3A I recall was in when I got it) several times hoping the amp would work.
When I had it here I started it up with the limiter after doing the work. I definitely started it at least a couple of times with the 3A slo blo. This is what is confusing to me, I must have started it without the limiter, I always do. Anyway, he was anxious to get it back so I rushed the work a bit. And as the story goes he took it home and it blew the fuse.
The amp had worked for several years before it had the tube trouble but I have to ask, was that original fuse a 3A fast blo or something higher.

The amp has many secondary windings but there are only 2 wires coming out to the mains. There are no switches to change to 240/120.

With the amp pulling 1.7A , unless there is something still wrong, shouldn't the mains fuse be a 4 or 5A?

[TimmyP1955]

I believe that the reason for the resistor ladder instead of a pot is so that the two channels will behave identically - pots sometimes have pretty different wafers in the two sides.