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We used to check source (output) impedance (mainly 50 Ohms) by applying a non-reactive variable load resistance until the measured open circuit voltage was exactly halved, the applied load would then be equivalent to the source impedance. So one must assume that a 50 Watt amplifier with say an 8...

Had similar with a pair of their so-called cryogenic double triode 12AX7's, one side of one went open circuit after 10 minutes then 5 minutes later the opposite side of the other one went open circuit. Watford Valves just said "Tough! It's the modern production problem", no money back. Just...

I recently tried to improve the poor performance of the Yaqin MS13 VU meter circuit. It is not powered except for the amplifier output to the speakers. I modified a circuit found on the internet that used diodes to try to emulate a Log Amplifier, it did improve the original circuit somewhat and...

Having replaced the bias parts the amp was put back on test but straight away something was not right, slight smell of over heating and those 5k1 resistors in the B+ line were the ones getting all hot and bothered. The voltage across them was far too high but as there did not appear to be any...

Hey Guys! I had an arcing sound with bright flashes in my own 100B but after checking capacitors and wiring, I put it all back together as I found nothing wrong, well the arcing had stopped. Then I had another 100B in for service with constant arcing which was down to the -70V bias wire breaking...

Some info here may help https://www.thetubestore.com/lib/thetubestore/schematics/HH%20Scott/HH-Scott-222-b-service-bulletin-Schematic.pdf

There sure is, the 6Z4's would sometimes spark over at switch-on. You may have noticed that it is a full wave bridge with one side using the ubiquitous 1N4007 and the 6Z4's on the other side. I reckon fitting a 1N4007 across each of the 6Z4's would make no difference to the sound but improve...

You can test as a 12AX7 but put 6.3V on pins 4 & 5, pin 9 is not a filament centre tap but a connection to an internal screen. With a Va of 250V, apply -2V to the grid and you should see a Ia of 1.2mA. Gm is expected to be around 1.6. I find in practice that they have a much lower gain...

I remember drawing that schematic, all done in MS Paint :rflmao: It's all in the pdf here:- http://www.g4cnh.com/public/A_LOOK_AT_THE_YAQIN_MC100B.pdf

Thanks for the info Mauro, I have put all the photos on the last pages and given the correct acknowledgement.

Resistor values changed, many thanks.

I added some notes on the schematic, thanks for the info as it may help others. http://www.g4cnh.com/public/A_Look_At_The_Xiang_Sheng_728A.pdf

Here is how I did it, see pages 14 to 17. http://www.g4cnh.com/public/A_LOOK_AT_THE_YAQIN_MC100B.pdf

100mA is normal I believe for single end class A and yes you identified the input leads. They go to their respective input tube grids via 1k grid stopper resistors R1 and R2. That sure looks in a hell of a state, I would be inclined to remove components, clean up the board and then re-build it...

The input from the volume control can be seen in the photo on page 4, where a single 2 core screened cable has White going to Lin and Red going to Rin and its screen to GND. The three coax lines from the rear connectors use the same cable type (Not keen on it myself as it must encourage...

I have added more photos of the component side. The schematic is correct and I added the changes I made to the iTube 182 I had in for repair. http://www.g4cnh.com/public/FatMan_iTube182.pdf I hope the mods allow the link, it is so I can continuously update as and when required. No financial...

I had a chance to reverse engineer one of these and put it onto the internet as I know circuits are hard to come by for this model. The tube types are shown on the schematic I drew. http://www.g4cnh.com/public/A_Look_At_The_Xiang_Sheng_728A.pdf

I realise this is an old thread but I recently had a FatMan iTube182 in for repair, it had a badly burnt printed circuit board. Full story here with a more accurate schematic. http://www.g4cnh.com/public/FatMan_iTube182.pdf I could only obtain on both channels around 7.5W per channel before...

Here is a box I made myself, not sure if I can load the pdf directly but perhaps the mods will allow a link to my web storage space where you can download. No commercial gain of any kind just a helping hand to anyone who was thinking the same. By the way, we have used the box for looking at the...

Yep - that was with a 1kHz square wave input. Of course other units may vary, this was a test on just one unit but others have confirmed similar results.

Just good engineering practice to put shields around the tubes on a phono amplifier as the gain is high. But some like to see the tubes lit up and find that there is no local interference that requires shields to be fitted. There should be no danger to the tubes as the spring tension is very...

The shields have a bayonet fitting. Push gently down and rotate counter clockwise then release. The shields should pop up and then can be removed. The tubes can be removed by gently pulling, helped a little by slight rocking North-South and East-West. For best results have the MS23 modified. Are...

Wish you could buy the old fashioned carbon pile regulator :D. Used them on 1943 WW2 Radar sets :rflmao:

Data here about the 728A I came across. Probably the only close as possible schematic on the Planet. http://www.g4cnh.com/public/A_Look_At_The_Xiang_Sheng_728A.pdf

Is this problem with a 3 tube unit? If so it could be the 12V regulator shutting down due to getting to hot. This happens sometimes due to excessive output from the transformer and the cure is to add a dropping resistor ahead of the regulator. You should be able to see this happening as a slight...