Beogram 6500 RIAA Module damage

[alf]

Hi All,

i was hoping some of you can help:

while upgrading the RIAA module I accidentally overlooked the mains plug was still plugged in .

in the the process the module solder-side touched the chassis the the fuse blew straight away – a short happened.

I can still get all the voltages on the module but no audio signal, only a continuing pulse-like blop blop….the moment the mute switch opens.

the caps and transistors seem ok – could I have blown up one of the ICs on the module ??

thank you so much

ALF

can some of you provide me with some pointers ?

[Mark-sf]

There are 2 voltages going to the board – 22v and 12v. First, I would unplug the board and check that the power supply is outputting the correct voltages. Then after removing the power cord, I would reinsert the RIAA card and check the voltages on it per the service manual. Make sure you check for 12v on each IC. Since each IC is amplifying both channels, a bad one can impact both. If you have an oscilloscope, you can check the nature of the noise and the point in the curcuit that it is originating from.

[alf]

Thanks for your reply Mark,

yes, all 3 ICs are getting the 12V and I have the 22V supply.

I checked the in and outputs on IC2 and I C3 and noticed that periodic pulse I hear on the

inputs 2 and 6 at IC3 when mute is off.

Both inputs are also connected to ground via C4 & C12.

these findings are obtained with the PU arm lowered, meaning mute disengaged.

as the is no signal coming from a record I should not see any audible pulse or am I wrong here ?

ALF

 

[alf]

One thing I should mention – after the “accident” I by-passed the RIAA board to check whether

the problem could be at the muting board?

I had a perfect audio signal going through an external phono amp.

ALf

[Mark-sf]

Actually, pins 2 and 6 are the feedback loop inputs and not the signal inputs. Pins 3 and 5 are the inputs and 1 and 7 the outputs. I don’t know what you mean by “checking the inputs” because putting a probe on an input of a high-gain stage will produce an audible side effect. Given how inexpensive these opamps are, unless you find a burned part, it is easier simply to replace them as long as all the voltages check out.

[alf]

Oh, I see those inverted inputs are also called feedback loop.

Well, I hear you and know those op amps are not that expensive which isn’t the problem but as they are smd versions the soldering is the far bigger challenge here.

what I really meant was tracing the unmuted signal coming from the PU arm and it gets lost ??

that is a mystery to me  because in the RIAA by-pass scenario I get a proper signal so it has to get lost so

somewhere on that RIAA board, meaning just replacing the op amp may not solve the issue. I get back

after some further investigation with the help of the oszi.

ALF

 

 

 

[Mark-sf]

It’s not really a mystery as a short will not take out your cartridge coils since they are not conducting current. If you are reading 6V DC on each of the + and – inputs then the passive components are likely fine, and your issue is the ICs. You should use hot air and flux to replace the ICs. There are plenty of YT videos showing how.

[alf]

Hi Mark,

just a quick and interesting update:

Assuming the op amps had been damaged I replaced both with LT1364 typ op-amps and was fortunate

to manage the smd soldering.

but here’s the interesting bit – on board 11 which holds the mute switch the were two wire jumpers

at the bottom & one at the top on the board – sadly that periodic audible puls was still present without any audio from the PU arm.

Revisiting the SM had me scratching the head when looking at board 11 with RIAA fitted boards and a “CUT”

indicator…….well, I removed those two jumpers and audio signal coming from the PU arm was restored.

I seriously wonder if that was causing the initial issue all along and both op amps were indeed still ok ?!?

anyhow, I am not re-installing them.

whether both LT1364 were the best choice I would not be sure but at least I can list3n again to a record.

thank you for all your advice

ALF

[Mark-sf]

ALF, those jumpers most definitely need to be removed as they are shorting the inputs to the outputs which will cause oscillation. Glad to see you got it working, Enjoy!

[Author]

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