Tech Soundsystems Black Cat; transformer death x2 this could be terminal... unless

[3below]

7 minutes ago, warwickhunt said:

I'm only going to test the valves for basic operation to eliminate them + test the T250Ma fuse is OK (don't trust visual).  

 

As a side note - the tech who has done the work and fitted the new transformer(s), heat trip and fuse, would like the amp back (no further charges to me) as he won't be beaten on this and wants me to be fully satisfied.

That is the sort of tech you need, I know that feeling of 'not going to let this one beat me'.   I look forward to hearing what the cause of these failures finally resolves to.

[BigRedX]

I have a feeling that they are going to have to draw up the pre-amp circuit (that should be fairly easy since IIRC it's all point-to-point wiring), then work out exactly what should be going on at each point in the circuit and test until they find where it's doing something different which will probably be where the problem lies.

[3below]

30 minutes ago, BigRedX said:

I have a feeling that they are going to have to draw up the pre-amp circuit (that should be fairly easy since IIRC it's all point-to-point wiring), then work out exactly what should be going on at each point in the circuit and test until they find where it's doing something different which will probably be where the problem lies.

Without the schematic this seems the only way forward.

[warwickhunt]

I've trawled the net for the schematic and emailed Eich amps again (Thomas Eich founded Tech Soundsystems)... nothing from them after the first reply saying they no longer manufacture these amps and keep no spares or records.    

[LukeFRC]

1 hour ago, warwickhunt said:

I've trawled the net for the schematic and emailed Eich amps again (Thomas Eich founded Tech Soundsystems)... nothing from them after the first reply saying they no longer manufacture these amps and keep no spares or records.    

@disssa- is Tecamp in your schematic archive at all? 

[BassmanPaul]

As the transformer has a 12V winding you could find that the heaters are wired to pins 4 and 5, heaters in series for 12.6V, with nothing on pin 9.  that being the case the combined draw would be 450mA so the transformer current rating of 800mA should be fine.

Edited March 6, 2023 by BassmanPaul

[TimR]

9 hours ago, warwickhunt said:

@BassmanPaul I'm not sure if I need to whoop or groan!  

 

I whipped the valves (x3) out, as I knew they should be 12ax7 valves but I wanted to see if they were matching as the first one was blank and no brand.  As I took them out I gave them a good examination and lo/behold as I drew out #3 it was discoloured reddish brown!  I have no idea if this means it is dead but when my multimeter arrives, I'll run a test; Google seems to indicate pins 9 / 4&5 (correct me if wrong but I'll be testing all 3 the same way so should tell).

 

Did your amp tech not check these as a matter of course. That's probably the root of the failure. 

[agedhorse]

Did the tech check to be sure that the measured input current was less than the transformer's rated input current x0.65?  This would confirm that the load was safe for the transformer.

[warwickhunt]

3 hours ago, TimR said:

Did your amp tech not check these as a matter of course. That's probably the root of the failure. 

 

He assured me they were checked,that's not to say that all of these faults are based on one problem; I've been around long enough to not get tunnel vision.

[warwickhunt]

Got all excited as the fuse tested 'dead' with continuity meter and all of the valves tested OK across 9/4/5.  I found fuses this morning that I could run a test with (not exact match T500mA as opposed to T250mA but fine to test power up and output) but no joy... silence from the outs.  

 

I took out the test fuse and checked it was still OK (yes) and I plugged all of the leads into another amp and checked those to be sure it wasn't a different issue, no probs.  

 

The tech wants it back on his bench and no charge for labour but if a separate unrelated failure has occurred I've said I'm happy to pay for parts.  

 

I really do appreciate everyone's input and I'll report back when he's had a look at it. 

[agedhorse]

What is the primary current rating of the transformer? I'm about 99% sure it's well under 250mA full load at 230V, meaning that if there wasn't a problem with  the transformer itself before powering it up with the higher value fuse and there was an issue in the circuitry downstream, you now have another problem in addition to the original fault.

 

This is why DIY repairs, and repairs by techs who are under-qualified for the work they are attempting end up the way they do.

[BassmanPaul]

A thought that struck me was that if the heater winding fed into a rectifier/capacitor to run the heaters at DC Did the designer multiply the current draw by 1.4 to allow safe operation of that supply. That would equate to  630mA . That's much closer to the 800mA limit of the 12V winding.

Edited March 12, 2023 by BassmanPaul

[warwickhunt]

5 hours ago, BassmanPaul said:

A thought that struck me was that if the heater winding fed into a rectifier/capacitor to run the heaters a DC Did the designer multiply the current draw by 1.4 to allow safe operation of that supply. That would equate to  630mA . That's much closer to the 800mA limit of the 12V winding.

 

Cheers I'll relay to tech.

[warwickhunt]

Update... 

 

It seems to be operating absolutely great.  Fingers crossed for long term operation.    

 

[BassmanPaul]

I hope your troubles are over, if not keep us informed.

[warwickhunt]

22 minutes ago, BassmanPaul said:

I hope your troubles are over, if not keep us informed.

 

Gawd I hope so.  Thankfully the additional work hasn't cost me!  

[agedhorse]

With that wiring, I would be wary of intermittent short circuits that load down the secondary. 

[warwickhunt]

9 hours ago, agedhorse said:

With that wiring, I would be wary of intermittent short circuits that load down the secondary. 

 

Can you expand?  

 

[Chienmortbb]

12 hours ago, warwickhunt said:

 

Can you expand?  

 

I don't know what @agedhorsewould say, but from my point of view, the cables soldered to the transformer should have been threaded through the hole on the terminals and either wrapped around or at least bent back on itself. It would also be a good idea to put sleeving or heat-shrink over the terminals.

 

Those joints where the cables are just laid on the terminals are not ideal. They rely on solder, but solder does not make a good mechanical joint.

[warwickhunt]

3 minutes ago, Chienmortbb said:

I don't know what @agedhorsewould say, but from my point of view, the cables soldered to the transformer should have been threaded through the hole on the terminals and either wrapped around or at least bent back on itself. It would also be a good idea to put sleeving or heat-shrink over the terminals.

 

Those joints where the cables are just laid on the terminals are not ideal. They rely on solder, but solder does not make a good mechanical joint.

 

Ah OK I get where you are coming from.  I'm competent with a soldering iron, so if it is a case of making connections more secure I can do that!  I can cable tie, shrink wrap etc anything that looks like it may put strain on a joint.   

[agedhorse]

How are the secondaries attached to the bridge rectifier?

 

Theres a lot of sloppy wiring and terminations showing in the picture.

[warwickhunt]

5 minutes ago, agedhorse said:

How are the secondaries attached to the bridge rectifier?

 

Genuinely sorry but you lost me at 'How are the...'.    

 

If you can use the image I posted to highlight the areas I need to address I'll have a look.  

 

I'm not trying to be obtuse, I'm a bassist who's knowledge of soldering et al is making guitar/speaker leads and soldering the odd speaker in/out of a cab.

[atsampson]

It's a long shot, but I would be tempted to have a careful look at the electrolytic capacitors at the right-hand end of the preamp board. If the amp is 18 years old, then (a) they're most likely past their intended service lifetime and (b) they were originally fitted just after a time when some manufacturers were using a dodgy electrolyte formula that caused early failure. If, say, you've got a filter capacitor across the HT or LT supply that starts acting as a short circuit once it's warmed up a bit, that might explain the fault you're seeing...

[Chienmortbb]

10 hours ago, warwickhunt said:

 

Genuinely sorry but you lost me at 'How are the...'.    

 

If you can use the image I posted to highlight the areas I need to address I'll have a look.  

 

I'm not trying to be obtuse, I'm a bassist who's knowledge of soldering et al is making guitar/speaker leads and soldering the odd speaker in/out of a cab.

The black components inside the blue rings are the bridge rectifiers. A bridge rectifier is just four rectifier diodes arranged so that an AC voltage is rectified to a DC voltage. The electrolytic capacitors are the large yellow and black devices to the left of the bridge rectifiers.

 

 

 

 

[warwickhunt]

Bearing in mind the insightful comments from @Chienmortbb @atsampson @agedhorse @BassmanPaul (and others along the way) I might get the amp checked over while it has a fully functioning transformer.  

 

I suppose my dilemma is that the tech doing this work has charged me not a lot of money to do the original transformer replacement and then zero charge for the 2 subsequent replacements, as he felt obliged to warranty the work; now finding someone I can trust to take this to the logical next step of possibly looking at the capacitors etc is not going to be easy.