Tannoy Sytem 800 distorting but cleans up on power off

[petercornell]

Hi all

I have a Tannoy System 800 LH active monitor that sounds quite badly distorted and is unusable. It is about 12 years old and I intend to recap it anyway.

So it is really distorted and unusable, but when I turn it off, as the power drops away it cleans up for a second or two before fading to nothing.
Any idea what it could be? I am assuming it is the PSU caps and I am going to recap the PSU anyway, but could it be anything else?

I have not opened it up yet, just waiting for parts to come in. I recently serviced its stereo mate and I did not find any dry joints or bulging caps on that one, so I am ASSuming that these caps are OK as well, but I will change them anyway. There was no conductive glue on the PCB to cause any problems.

Thanks in advance
Peter

[Enzo]

It is biamped, meaning there are separate amplifiers for woofer and tweeter. Are both affected or only the woofer?

Due to your description, I suspect some point is DC offset. First check your power supplies. -VP and +VP are for the power amps. Are both present and roughly the same voltage? Then VSS and VDD are for the op amps, are both up to voltage and clean? I don't know what voltage they set, but 15v seems reasonable.

If supplies are OK, then look at your op amps. The two TL074s have the outputs on the corner pins. ANY of those sitting at DC? Pins 1 and 7 on IC1 are outputs, same question.

ANd I have no idea what plugs onto the 8-pin JP1, controls maybe?


And of course check for any DC offset going to the speakers themselves.

[petercornell]

Hi Enzo
Thx for the reply.
I am not sure if is affecting both the HF and LF channels as it is a dual concentric speaker and difficult to tell, even trying to listen to high and low frequencies.
As soon as the unit is open, I will look at the voltages.
All the best
Peter

[Enzo]

I would disconnect the woofer section and then only the tweeter would sound.

[petercornell]

So the speakers came in both with the same problem. The distortion was present all the time and not only on shut down.

There are a pair of 317L and 337L regulators for the opamps and they run really hot. The 10uf caps C8 and C10 are close to the regs and the solder has been melted and one leg of each cap was not connecting to the solder pad.
Replaced the cap (ESR was high) and will get some TO92 heatsinks for the regs on Monday.
Happened on 2 individual speakers, same age, from around 2014

[J M Fahey]

There are a pair of 317L and 337L regulators for the opamps and they run really hot.

WEIRD.
There are 10 op amps there in total, equivalent to 5 x TL072 which "eat" 3.5mA each, so total IC consumption shoould be around 5*3.5=17.5mA
Each regulator, starting with some 30/35V rails (please measure and post actual value) will dissipate around (35-15)V*17.5mA=350mW , well within ratings.
But you not only say they run Hot, I even see a browned PCB area around them ... plus the not small detail of caps:

The 10uf caps C8 and C10 are close to the regs and the solder has been melted and one leg of each cap was not connecting to the solder pad.
Replaced the cap (ESR was high)

STINKS of very lossy caps , maybe even installed backwards (are they original?) pulling gobs of current, maybe even pulling op amp rails way below expected 15V
I bet replacing them (triple check orientation) will solve the problem.

and will get some TO92 heatsinks for the regs on Monday.

Won´t hurt but hot regulators is the symptom, not the cause.

Happened on 2 individual speakers, same age, from around 2014

Were they bought new in 2014?

Just curious, what are the chipamps used?

[petercornell]

Sorry for missing your post JM. These speakers us TDA7294 power amp chips.
I also found out these units were actually bought in 2003....
Peter

[nevetslab]

The date code on that TL074CN Quad BiFet op amp is 9927.....1999, 27th week, so manufacture date of the speaker is probably within a year of that.