Help troubleshooting Elekit TU-8500 preamp

asdlk33

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Hi all, new to the forum, new to assembling electronics in general. I got an Elekit 8200 stereo tube amp kit and an 8500 preamp kit back in December which was my first project. I have an old-ish technics mk 3 turntable and a pair of JBL 4312 compact monitors hooked up to the amp. Everything sounded great and was working fine until last night. No sound! The main amplifier still works on its own.

After removing the cover from the preamp, I can see the heating elements in the tubes are lighting up, so I'm assuming they're fine. The fuse isn't blow, or doesn't appear to be anyway. Does anyone have any suggestions for how to go about troubleshooting the source of the problem? Like I said, I am pretty new to this and don't have much experience with electronics so I'm not even sure where to start.

Any help is greatly appreciated, Thanks in advance!
 
Hi,

1. It would be helpful if you could post a Circuit Diagram. Its not there ion the site at: https://www.tubedepot.com/products/elekit-tu-8500-stereo-tube-preamplifier-kit

2. If you have checked the connectors and interconnects and that the Power Amp is indeed working, its probably an electronics failure.

3. Since BOTH channels are not working, I would suspect the DC Power Supply ... Its the only section common to Both Channels. If you have a multimeter, please check the High Voltage DC after the High Voltage Rectifier.... & The Low Voltage DC.


There are 2 rectifiers just next to the Power transformer.... Check the DC output at both these.

The Tube filaments are probably driven directly by AC from the transformer, so they are glowing OK.
 

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IndianEars,

Thanks a lot for the reply, I just got really busy at work so haven't had time to try your suggestions yet. I will reply when I have.

Thanks again!
 
Hi, asdlk33 did you manage fixing it?

Unfortunately I have a similar problem with my Elekit TU-8500. All the line inputs are working fine, but I'm not getting any sound out of phono input from turntable.

I have checked all 3 voltage check points on the board and they show correct values. Please can anybody suggest where to continue or what parts I should measure?

I'm attaching the circuit schema and board.
Thanks
 

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schema.jpg
Check the Following:

1. Vcc ( +12 Volts) & Vee (-12 Volts). I have indicated these points in Green Rectangles. If not, revert, but IC-5 or IC 6 may need to be replaced.

2. Check the Voltages at the 4 points circled in red. Each of them should be close to 0 Volts... If not, that IC is probably to be replaced.
 
Thank you IndianEars for a quick response, I really appreciate it.

I measured again the VCC and it shows around -11.8V instead of +12V, VEE shows around -11.5. So this indicates that IC-5 or IC-6 need to be replaced?

For the 4 points that you highlighted above I measured on all of them around -10.5V. I haven't plugged anything into phono, and I used the ground next to VCC/VEE - is this correct?

Thanks,
Jiri
 
Your Vee is OK.

IC-1 pin3 which shows MINUS 11.5 Volts seems to have blown.

Please check voltage at Pin 1 of IC-1
and revert.
 
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On the PIN 1 IC-1 is -10.49V. Yesterday I ordered two new op amps. After I replace them I'll get back if it helped.
Thanks,
Jiri
There may not be ANY Problem with any Op Amp. Please DO NOT replace the opamps.

Your Problem seems to be the Power Supply, Vcc which should be 12 V and you are measuring MINUS 11.8 Volts.
Most likely the Vcc IC ie IC -5 (Voltage Regulator IC .... LM2940C) has failed and needs replacement.

Please Measure Voltage at Pin 1 of IC-5 (Voltage Regulator IC .... LM2940C) . It should be (very approx) 18 Volts DC.

Please Do NOT Change the Op amps in haste, unless the Power Supply is fixed. You could blow the new ICs.

Note: My Bad Typo.... In My previous post, I meant to type "Please check voltage at Pin 1 of IC-5 (Voltage regulator) and revert."
 
" I measured the Pin 1 of IC-5 and it shows 17.3V"
This means that your IC5 needs to be replaced. For good measure also replace ZD2 (BZX79C12 ) which is a 12 volt zener and C26 (220 mfd, 25 Volts).

Your Phono stage should start working after the replacements. :)

"Should I also measure the IC-6, "
Nope. Your Vee (Minus 12 Volts) is working Fine.
 
Hi, today I replaced all three components, but when I measured again the Pin 1 on IC-5 it shows 17.6V
On VCC I can still measure around -11V instead of 12V. Could you please advise if something else can be malfunctioning or what else I should measure?

Thanks,
Jiri
 

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