Exposure 7 pre amp help

My newly acquired Exposure 7 pre amp just blew up after I connected it to a power supply

On each pre amp board, I can only see a burnt out resistor marked "R77". One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground. It seems to me that the power supply ground is "floated" at a certain voltage about the signal ground.



I would be very grateful if someone who has a Exposure 7 pre amp can tell me the value of the R77 resistor. If you have a circuit diagram of the pre amp, I would also like to have a copy. Many thanks.

Hon

 

Was it the correct psu for the pre ?

list the colour bands ...or go to the rs web site and use there resistor calculator.

Are the photo's pre or post failure ?

 

Unfortunately, I didn't test them before I paid for them:cry: because there is no power supply cable. The seller sold the pre/power amp to me in good faith that it would work even after over 10 years of storage.

Yes, it was the correct power supply for the pre amp. The separate power supply gives +/-24V using a 317 and a 337 regulators. The power supply also went wrong, I found that a decoupling tantalum cap for the LM317 was shorted to ground which also killed the 317 regulator for the +24V rail. The -24V regulator survived.

The pre amp shown in the photo has LM317 and LM337 for a second stage regulation. It looks like these regulators have survived.

I am not sure the cause of the damage: whether it is the shorted tantalum cap from the separate power supply or the pre amp itself.

The photo was taken a couple of days before I powered up the pre amp the first time. Even at 3Mbyte resolution, I cannot read the bands of the resistor. I removed it and measured 3.3K, but I don't believe the measurement because the resistor is burnt out. By tracing the circuit, the power supply ground is floated above the signal ground using this resistor. My gut feeling is that there may be more things wrong other than the resistor. The tracks on the pre amp PCB back along the LM317 +24V output and the LM337 -24V output look a bit brown, so there must have been large current through the pre amp regulation circuit.

I checked the few transistors ZTX214 connected to the LM317 and they seem to be alright.

Hon

 

can you pop up a pic of the psu and pre post problem.

 

There is no "pre" problem because the power supply and the pre amp were powered up by myself after 10 years old storage by the seller. I just got a cable to connect the power supply to the pre amp, powered it up, then both blew up. I did check that there were +/-24V from the power supply before I connected everything, so I assume the psu was working.

Here is a photo of the +/-24V power supply. It is quite simple so I managed to find the shorted tantalum cap next to the LM317.

 

what cable and connections are in use.....dins?

Check the cable for shorts using a continuity meter

The psu looks simple enough .....

 

Zanash

I have fixed the psu by replacing the shorted tantulum cap.

It is the pre amp which is a challenge. I just checked the LM317 on the pre amp, the output voltage should be +18V, but it is not giving the correct voltage. I'll buy a few LM317 on Monday from RS and see if that will fix the fault. However, I still need to know the value of R77.

Thanks anyway.

Hon

 

Studying the pic take a look at r28 on the left of the board, on the face of it its doing the same job ..... could be of equal value. the pics are too washed out to get a good colour match to read the bands.

 

Zanash

R28 is a 10K resistor which connects to the central pin of a 2 way rocker switch (for choosing MM or MC input). The other end of R28 goes to the signal ground.

I emailed the service department of Exposure to ask for a quotation for fixing the 7 pre amp. Tony Brady, the service engineer now lives in France and he will answer my query hopefully. He only travels to England twice a month to fix things for Exposure so I may take this route to get it repaired if the cost is not too much.

Tony won't be in UK until the 1st week of April, so I will give myself a few days to repair it.

Hon

 

As far as I can see ...and its a bit difficult ! its the same ground plane. If you have a meter you could check.... but 10k is normal sort of value to "float" the ground.

But sounds like you've got the main man on the case ....hope it gets sorted !

 

I temporarily (bravely) connected the repaired psu to the pre amp to check the voltage of the LM317 and LM337 on the pre amp boards. I am sure both regulators have gone faulty since the outputs are not +/-18V as calculated using the two resistors for defining the +/-ve voltage, being 260 and 3.5K ohm. The 337 was too warm to touch after a few seconds. I will get some new regulators on Monday and see how it goes.

Hon

 

Zanash

I got the bugger working at the end after working on it intermittenly for nearly 6 days! Tony Brady of Exposure replied to my query this morning about the resistor and the repair necessary to bring the 7 pre amp back to life. He is a great guy.

The R77 resistor should be 27 ohm which makes sense. Too high a value will create a large potential difference between the power supply and signal grounds. Now the resistor measures 0.68V which means that the preamp only draws about 25mA idle current.

Tony told me that the 10uF tantalum caps tend to short circuit which is exactly what I found, 5 out of 8 tantalum caps for the left and right boards were shorted. He suggested to replace them with electrolytic ones. I went to my local RS trade counter which only stocks Fairchild LM317T and LM337T regulators and cheap Panasonic 10uF 63V caps. I bought these cheap parts instead of waiting another day for some designer's caps like the Starget and the higher spec LM317AT.



The pre amp worked immediately after I powered it up. The regulators are a bit warm to touch without any heatsinking. The original ones were glued down to the pcb using Araldite which I avoid to use just in case the regulators blow up again.

I am waiting for the new parts to "burn in", then I will write a report about the sound of the Exposure 7 pre amp with the 6 power supply.

Regards
Hon

 

Thats great news.....you should like the stargets ...an honest cap with honest preformance! Looking at the pre from a modding perspective ....if you ever thought about a rewire you would have loads of scope as some of the signal wires are of lofi pretentions. But leave that for another day !