Grampian 636 reverb repair

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turntable

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May 3, 2012
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8
Hi guys

I have just been given a Grampian 636 reverb unit which is in some state of disrepair, so I have decided to try bring it back to life as my first repair project. The only problem is that resources are very low on the ground, in fact I have found nothing, so I need to approach this in a different way. Has any one got any advice or general things to try with this kind of unit. I am just getting a loud continuous noise when the device is on and off, and there is no indication whether it's actually powering up or not.

Any help would be fantastic.

Thanks

Lt
 
I always check the power supply before braking my head... It simple to check and repair and casues a lot of trouble in old devices...

JS
 
just be very careful when checking for power, use a good voltmeter and prove it works on a known power source/proving unit to satisfy yourself the meter is actually working.
 
Hi

Thanks for the replys.

The unit is run off 2 nine volt pp9 batteries which I have replaced the battery clips for. Other than that I am clueless as to what to test next or what voltage etc I should expect to read as I can't find a schematic.

Thanks again
 
You could track the signal inside, put some signal at the input and with a prove listen to the input circuit and go getting inside and/or insert some signal with a prove and listen to the output, maybe you could see where is the problem happening...

High res photos may help we to help you, to have an idea what you are seeing...

JS
 
grampian636-1.jpg

heres a drawing of my modified grampian 636. parts of it mite look a bit weird. without delving back into it (where i was for ages already) i can say that it works very sweet. theres several opotunities inside the circuit for gain changing. the mic stage gain control is very strange. i might be wrong about that part.goes well tho. when the gain is low (-) not much mic amp noise gets thru to the next stage.

the board is a messy one. badly planned out on variboard. my one anyway. it was a mission to draw it. all good tho.  is yours a printed board or variboard?


note the power supply end. 
im not absolutely sure ive drawn that right. i remember having issues when using a centre tapped supply.  i think it went ok when i had two seperate 9v supplies - stacked.  18 volts from ground to hot. and a clean 1/2 of that fed just for the tank driver.  the 2000 uf probably arent even needed if you using batteries? anyway. try some things out. have a think. youll work it out.

also note that this is a positive earth unit.
good luck.any discuss about the circuit is welcome. germanium sounds so good. its all kinds of noisey but so so juicey. wow.
hope this helps.
 
Good call Rob!

EKADEK, that schematic is a work of art! Beautifully drawn - love it!
:)
Stewart
 
ditto on liking the schematic :eek:

I thought it was a Picasso or MCEscher

That reverb can be a Great sounding unit, and well worth your efforts,  best wishes
 
I second what Rob said, replace all the electrolytic caps and check power rails. After all that is confirmed working then I would go through and check all the semiconductors.

 
Hi group, more on 636 repairs! Or..moron 636 repairs  ;D  I'm looking at my customers unit, I've fixed the power up and tightened mountings on transformer and input to the spring, which was completely adrift, put a new socket onto the balanced mic input. It's reverbing, but, and it's a big but, the overload light is permanently on & the two 1.8 ohm power resistors get hot & start to smoke. So..testing, thanks for the schematic, I have a couple of points I want to discuss. All the transistors test ok, no shorts. The two 2000uf caps test ok, no shorts. I've taken the transformer out of circuit and tested, it has one turning of 57 ohm on one side and one of 170+ ohm on the other. It looks like it should have two turnings either side, 3 legs, but I only get a reading from the middle to one side, not the other side, so it looks like a split bobbin with an open circuit on one turn either side! Labelled 'G13', could this be rewound? This doesn't quite match your schematic which shows one turn on the input side and two on the output..? So that's it really, non original power supply that I'm going to check out further..and I may have to re-work the spring driver cct. The DC power comes out to the power switch as one 22v rail, already rectified and regulated. Then it goes to the PCB at ground and +22, no middle rail. I'm thinking this should be AC switched, not DC, I can correct that easily. So, as you all talk about two 9v rails, has someone bodged up this power supply and made a mistake powering the board? Do you guys have three connections from power supply to PCB - 0v, 9v & 18v?? Thanks for any help peeps X
 
a pic of the dc to the board showing the missing '9v' connection
 

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and another showing a couple extra resistors that follow the large caps, starting wit hthe +ve and in parrallel with the leftmost and then connecting to the rightmost +ve following the wire link inbetween the caps on the component side. I think these are incorrect, possibly to provide a midpoint in the power rail..
 

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hi there. just noticed your posts ourmanflint. im a bit confused - so i'll look at my gramp and the circuit as soon as get a chance.
 
So, another Grampian 636 to fix up. This one has been taken apart and all the front panel and spring tank wiring is disconnected. Can someone post up a photo of the front panel wiring to help me put all this straight, especially around the reverberation pot and the switch and output jack??
 
Just picked up two of these .. . shall study up more before attempting to power up.
Seems like it should be a top characterful unit (if working!)
 
If anybody has a carcass/parts donor of one of these, I am seeking the cover for the overload indicator light. cheers.
 
just thort i'd update this thread with an original circuit for the 636.. yes . i got a couple of things wrong in my rear eng effort. no worries i hope.
 

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