Yaqin MC-100B Help Needed!

[bobrown14]

Hi all,

I'm new here and looking for some help with a Yaqin MC100B Chinese tube amp.

History is:

Bought new - right channel dropped for no good reason.

Since it was new and all I decided to take it to a tech to have the issue resolved (specially after taking the bottom cover off and seeing the bad solder job - very shabby to say the least). Solder joints all looked like the temp on the solder iron wasn't hot enough or something ... holes and grey not shiny solder etc.

Ok on to my issue - had the bad solder joints red-done, found the bad one that dropped a channel and fixed that.

One more thing ... there was a "loose" wire up in the front near the input selector switch that the tech "re-attached". When I picked up the amp I didnt check to see if the "pre amp" portion was working. Of course when I got home and tried the 250mV side there was a LOUD hum with distortion that got louder with more volume.

My temp fix was to plug in a transformer box to one of the 250mV RCA jacks (preamp - not really any preamp in these but hey). The transformer cleaned up the circuit noise and hum to like new sound, but it's temporary. It verifies that the ground wire (possibly for the input selector switch) is not grounded properly) I want to find out where that "loose wire" is and where it it suposed to be plugged in.

Does anyone have any pics of this amp with the bottom off up near where the 600mV/250mV switch is and also pic of the back of the input selector switch or thereabouts? I have the amp back together for now but will be digging back in again this weekend and I want to find the ground that was put back wrong.

Help and comments much appriciated.

Cheers,

Bob

[planet10]

I know that there is a schema posted somewhere on the forum... if not i have it at home. I may have some inside pictures, don't know.

You likely want to get comfortable inside this thing... it is one of the Chinese products that can be considered a pre-built kit... you'll be itching to start stweaking it up

dave

[toufu]

Here is the schematics:

http://www.elektor.fr/StreamFile.lyn...Schematics.pdf

[bobrown14]

Originally Posted by planet10

I know that there is a schema posted somewhere on the forum... if not i have it at home. I may have some inside pictures, don't know.

You likely want to get comfortable inside this thing... it is one of the Chinese products that can be considered a pre-built kit... you'll be itching to start stweaking it up

dave

Hi Dave and all - thanks for the link to the schematic. I've been inside a few times and getting more comfortable. I haven't pulled the board out yet. I want to get it going properly first. If anyone has some pics of the inside up in the front near the 600mV/250mV toggle and/or the input selector I'd be VERY grateful.

Thanks again.

Cheers,
Bob

[planet10]

I checked my library of pics and have none of this Yaqin. Lots of the EL34

dave

[Globulator]

Originally Posted by bobrown14

when I got home and tried the 250mV side there was a LOUD hum with distortion that got louder with more volume

A strange thing I found with a Sheer Audio phono pre-amp was that the gold plated phono plugs were soldered directly to a PCB via big tabs - but one or two of the tabs were partially isolated from the pcb.

I.e. the phono grounds were soldered in, measured fine with a meter, but did not make proper contact. I had it working when I touched the phono with my finger, but not on it's own. The cure was to file down the ends of the tabs (to get to the base metal) and re-solder with a hot soldering iron and fresh solder, to join the newly filed metal to the PCB.

A very odd fault, difficult to find, but the symptom was the same - loads of hum!!