Source Manufacturing - C-Mark, Thomann and Audiohead Amps

[grahamprie]

Hi,

I have a faulty Audiohead AH-1000 amp which is the same internally as the C-Mark and Thomann units - which were made ( in China ) by Source Manufacturing and distributed in Europe by anyone who wanted to buy them. The name on the front panel was added by the distributors, but the top cover is embossed with the manufacturers name and a non-functional website address.

My amp has one dead channel which keeps frying the replacement transistors I fit despite having tested every component on the board seperately. ( yes, I really have taken them ALL out and checked them ) I think I've got a hairline crack in the board but I can't find it...

My next step will be to try the channel module on a smaller supply - preferably with some current limiting for safety - I can't afford many more devices.

Does any one have a schematic?

Source Manufacturing won't supply one, and so far I have had no luck with any of the companies in Europe who bought them.

The circuit is really simple - a dual op-amp on the input which floats with the signal and a pair of driver transistors followed by 4 pairs of power devices, but I can't seem to get a handle on the fault.

Any help gratefully appreciated.

Graham

[picturevalve]

I'm in the same boat, I have a ah1400 for repair with one channel down - all o/p devices shot. I emailed Thomann and got this reply :

Hello,
sorry, schematics are pateted,
We can help you if there are any problems via mail
mfg, best regards
Reinhold Meißner, Werkstattmeister
Musikhaus Thomann
Musikhaus Thomann
Inhaber Hans Thomann
Treppendorf 30, 96138 Burgebrach
HRA Bamberg Nr. 8988

Have you tried asking them directly about the particular problem ? [ Pretend your amp is a Thomann ]
I emailed
werkstatt@thomann.de.
Let me know if you get any insights, I've ordered spare devices from Profusion as they were cheaper than Farnell - as to whether they're any better...?
Wish I'd found the other thread on this topic before I ordered, though.

[picturevalve]

Hi Graham, have you looked at the schematic for the QSC RMX 1450 ? The output devices are the same and the 3 pots are bias and 2 x current limit. Worth a look, -from another thread on here about the behringer a500.

http://www.qscaudio.com/support/libr...ms/rmx1450.zip

[grahamprie]

Thanks for the links - I did eventually get a circuit for the Audiohed amp from a firm in the west midlands who wouldn't give me a name I could find in directory enquiries - it seems that they were service agents for these amps for a while, but don't do it any more... Their engineer who now only works one day a week had a copy of the generic diagram for the whole range, but said they never knew where they were with these amps because every batch was different...

By the time the diagram arrived, I had already traced out the entire circuit and devised a method of test using one pair of output devices and some large external resistors as current limiters.

The fault turned out to be an intermittant open-circuit on the bias adjuster pot, so of course, it measured OK with no current flowing...

The circuit designer should be shot for putting the pot in the wrong way, so that a failure blows up the output stage. I remember back in the '60's when vbe multipliers first appeared in amps, the advice was to put the adjuster in the 'fail safe' leg of the voltage divider - pehaps that never got translated into chinese.

The replacement power devices I used come from Semelab ( through Farnells ) and are relatively cheap. So far, I've had no more problems. I think the originals were genuine Toshiba parts, though I've heard that there are some cheap copies around...

If I can help anyone else, feel free to contact me through grahaprie@aol.com.

[picturevalve]

If you could let me have a copy of that generic diagram, I'd be very grateful - I've now got an AH2400 for repair as well.

[grahamprie]

Hi,

I'm afraid all I have is a very poor photocopy which probably won't scan, so I'll need your postal address to send it.

I've now looked at the QSC reference mentioned earlier, and the circuit is significantly different - for a start, there are a lot less devices in it, and the centre-tap of the power supply IS the output terminal - the common connection of the power devices is actually GROUND. i.e., the whole PSU swings with the signal! Naturally, each channel has its' own PSU...

Send me your contact details to the mail address I gave earlier, and I'll post you a copy of the diagram.

Graham.

[picturevalve]

Yes, I only posted that as you were having problems with the 3 pots - in another thread. Address emailed and thanks.

[picturevalve]

Hi Graham, no reply from you on that email address, is it still current?

[davemud]

Hi Graham

Just found this topic and have an AH2400 with intermittent channel problem can i get a copy off the circuit diagram have tired the manufacture rout to.

regards davemud

[AudioSystemsEng]

hmm - i didn't really want an england flag