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schematics question (with picture attachment)

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fairchild

New Member
I'm trying to fix my roland space echo. The reverb stopped working. I'm trying to poke around with my multimeter set on continuity, but i've just recently started trying to learn electronics and i'm limited with reading schematics.

In the space echo schematics that i've attached, i first can't figure out which end of the reverb unit (outlined in RED) is IN and which is OUT. I've connected one of the multimeter probes to the IN (or OUT... again, not sure which is which) RCA cable of the reverb. Using the schematics i can see there is a connection at point #33, so i'm following the path and i'm not getting continuity after the component that i outlined in blue. Not sure if there should even be continuity.

Can someone point me in the right direction?

i color coded the connection points on the reverb. Hopefully someone can tell me which is which. hopefully i've supplied enough info.

Thanks!

**broken link removed**
 

Snowgoose

New Member
Firstly, the TA7200P is an audio amplifier IC so its output, 7v p-p is connected to pin 32. This output signal passes through an electromagnetic coil (green mark) which inturn vibrates magnets attached to a spring. The reverb bit. The other end of the spring also has a magnet and coil (yellow mark) which produces the audio signal for the single stage transistor amplifier Q10. The "component in blue" as you call it is a DC blocking input capacitor for the input of the transistor amplifier. The final reverberated output sound appears on pin 34.

As Mr goodwin points out above, a multimeter isn't much help, suggest you disconnect the reverb unit and connect a speaker directly between pins 34 and E. If you get a sound you know the IC amp works then connect pin 34 to pin 33 but control the gain.

To test if the reverb unit is the problem you could connect the output of the reverb unit to the input of another amplifier and feed it with a seperate audio signal. If all else fails check power supply connections, reverb connections and that the spring/magnets/coils move as they should, there not stuck or for loose components etc.
 

fairchild

New Member
I actually do have a really old oscilloscope that was my grandfathers. Just turned it on for the first time in at least 20 years. It's a Tektronix Type 453. I'm guessing it would need to be calibrated.

The reverb unit definitely works.

Can I alligator clip 34 and E to the tip and sleeve of an instrument cable then the cable to a guitar amp? And couldn't i just do the same from 33 and E? Is this how i could diagnose the point where my signal stops? This is kinda what a scope does, gives a visual of the signal, right?

Thanks for the help. I'm slowly learning.
 

audioguru

Well-Known Member
Most Helpful Member
The transistor amplifies the signal from the reverb a lot. Maybe your scope won't show the tiny signal at the input of the transistor. Try it and see.
 

fairchild

New Member
Audioguru, that's a good point. I haven't tried anything with a scope yet. I'm just using my multimeter to see if there is continuity between points. Are you saying that the signal might be too low before it hits Q10?

I'm thinking that TA-7200 or Q10 might have gone bad. Are these types of components prone to failure?
 

audioguru

Well-Known Member
Most Helpful Member
I think this circuit is so old that an electrolytic capacitor has gone bad.
Spring reverbs were used in the 60's, about 44 years ago.
 

fairchild

New Member
Everything else in the circuit works fine except the reverb. The echo sounds great. Any tips on diagnosing the problem capacitor?

Spring reverbs are still in use today. Probably more so than the 60's, i would imagine.
 

#1supertech

New Member
re: your inop reverb there

Hello,

Quick input here - as I was just browsing the site, and I came across your reverb problem / circuit.

First of all - what kind of Multi-meter are you using? What make and model? The Tektronix 453 scope you have there - depending on which model you have - might not even have any CRT vacuum tubes or "solid state Triodes" in it, which means it's a better upgraded model with FET's in place of the older CRT vac tubes! I grew up on the older TEK models when I worked for Emerson UPS Co. They were real workhorses for sure! They were great scopes in their hay days - and though they are now very old and outdated - production on them stopped in ~ 1982 or there abouts - they still are great scopes if they work right. If yours still works OK it might just need a bit of tweaking on the ASTIG - the GAIN CONTROLS, and the VERTICAL STEP ATTENUATORS is all to just get by for your use there. I wouldn't pay to have it professionally calibrated, as for what you are using it for it doesn't need it - you are just looking for AF signals is all.

PCB wise -

Use good safety practices when testing the PCB circuitry while having AC power applied. Same goes for making in circuit measurements that need the AC power removed!!

Having said that -

It would appear that GREEN is your OUTPUT - as coming out of IC-1 at pin 2 (AF OUTPUT), with the VCC coming in at pin 3, and PCB point 38 being approx 12.5 volts DC and Grd being at pin 1. I would also check your AF INPUT signal at pin 10 of IC-1 as well. All related DC voltages - as noted regarding IC-1 have to be there otherwise you need to T/S IC-1 first, and R&R any external components that are found bad. (ie: C-41/43)

Again - the AF signals need to be checked with an O-scope, as a DVM is useless when looking for AC type AF signals.

CAP's can be checked separately for proper value using a good CAP meter & resistors can be checked using a DVM, unless that is both are standard features on your better DVM there.

Your DVM - if you are using a good quality one - should give you good Continuity between points 32 to Grd, and also 33 to Grd as well. No matter what the coil "Impedance" windings are they MUST give you good resistive Continuity between both those 2 measured points otherwise you either indeed have bad coils, or you have bad connection points!!

It would appear that by what you said regarding point 33 and C48 - that C48 looks to be OK unless it is totally OPEN or bad? It still may have to be R&R'd from the PCB and checked with a CAP meter. Same goes for C47 & C49 & C50 as well, as all are Electrolytics, and could be bad just from age! I would also check R59 if it is a Power resistor type. Check its tolerance % rating, and if it's out of spec by any degree change it out with the exact value and wattage rating! You can always go higher on the wattage rating just as long as the resistor leads fits through the existing PCB mounting holes.

If you somehow misconnected (switched) your INPUT and OUTPUT it's possible that you may have blown out IC-1, but again - a good O-scope check will rule that part out right away!

YELLOW being the base INPUT to Q10 I would check point 34 for proper collector AC signal OUTPUT O-scope wise, as well as any improper DC voltage level there using a DVM.

With NO power applied you can try to measure Q10 in circuit, but chances are it may have to be R&R'd to do so - if the diode junction test proves to be inaccurate.

IC-1 as noted on the schematic is a 10 pin SIP package AF PO Amp rated at 4.2 watts, and the old ECG crossover p/n for it is a 1153 just for reference should it actually be bad.

I would look it over first to see if it has any signs of overheating such as by a cracked case, etc. Also check the underside of the PCB to make sure you have no micro-arching - as from a bad solder connection standpoint. Any heat driven component such as these can cause that after that many years of use and age!

Signs of severe browning (burning on the PCB itself) under where a power device is located (ie: high wattage resistor - Zener diodes - Power Amps & Transistors, etc) are a sign that that/those devices are being driven way too hard, or else they may be going bad as well.

I would not connect the output on pin 34 to the input on pin 33 as someone had suggested, as that makes no sense if Q10 is working OK!

Everything else being said it doesn't look to be too complicated of a repair!

Let me know what you find?

Best regards,

Frank
 

fairchild

New Member
Thanks for the reply #1supertech! glad you found my post.

I'm using a BK Precision 2407A:

Model 2407A - Mini-Pro, Autoranging - B&K Precision

I don't know much about my 453. I opened it up... i'm not seeing any tubes.

I don't really know how to use it. I don't even have probes for it! Got any recommendations? Maybe some advice on using it?

My multimeter doesn't test capacitors unfortunately. I think i would just re-cap anyway.

I think with what you've explained (once i figure out how to use the scope) i should be able to diagnose the problem and fix it. I might send you a PM from time to time if you don't mind :)

Thanks for your help!
 

audioguru

Well-Known Member
Most Helpful Member
My multimeter does not test capacitors. It just measures their value.
 

ke5frf

New Member
You can use an ohmmeter in the megaohm range to check the capacitor's ability to charge, outside of the circuit. Observe polarity of the test leads with electrolytics and make sure it is discharged before testing.

As the cap charges from the meter's battery voltage, it will eventually approach infinity on the scale as the charge builds up to (near) full capacity. The higher the capacitance value, the longer it will take to build the charge. A shorted capacitor will just read dead short and never approach infinity and an open capacitor will never register anything but infinity if it is appropriately discharged.
 
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#1supertech

New Member
Fairchild,

You're welcome on the post reply btw.

Thanks for your reply as well.

Couple of things here first -

As for your Tek 453 scope there - seeings it has no VAC tubes inside - that looks like to be a good thing, as it now looks like yours may well have been a later model towards the end run of those models.

You can contact Tektronix CS directly, and see if they have a downloadable Users Manual, and/or a Main Service/Calibration Manual for it. That or else go online and see what you can find there.

Sorry to hear that it had no probes with it, but I'm sure you can find those online say at Ebay for instance - fairly cheap I would think. You'll definitely need at least (1) X1 probe to start out with, so you can at least do a simple probe CAL prior to using the scope. You'll see a PROBE ADJUST test point right next to (or very close by) where you connect the probe's BNC connector. I'd just start out with a basic X1 probe to start with, unless you can find a good X10 probe for cheap.

The standard twist-lock BNC probe connectors should all be the same as my older X1 probes use the same BNC connectors as do the newer X10 probes so I think you are safe there as for that part of it.

If someone out there has changed over that part of the probe connection scheme then I'd stay far away from those, so make sure you know in advance what you are looking for first of all. If it's a Tektronix brand probe you should be good to go. Any other brand name then I can't swear by it. Buyer beware I always say!

I'd just make sure that the probe(s) you buy used don't have cables that are KINKED in any way or form, or frayed as from abuse! AND make damn sure that the center pin on the BNC connector isn't broken off or bent!! You may even want to do a continuity test on the cables to make sure the shield and center wire "point to point" are good and making a solid contact with NO BREAKS!!

My Tek 2215 scope - that I bought in the early 80's - came complete with 2 of everything - fortunately for me - (except for the service manuals that is) - as I bought it brand new when I worked in-house at a hospital as a BMET II repair Tech. Fortunately for me at that time the service manuals that came with it were very well written and included everything! Schematics - Calibration procedures - great T/S section right down to the very component level! Can't beat that part !

I even managed to keep a few extra older style Tek scope probes (and misc probe accessories) from a couple past industrial type Tech jobs, but they are only a X1 and not a X10. I currently use the 2 original Tek X10 probes myself, as that way I can look at very small signals without any loading effects to the circuit under test.

When I worked at that hospital Bio-Med Tech job I also made up various scope test leads for less critical aps where loading is no biggie, and where I'm just looking for raw AC/DC signals and HI/LO logic voltage levels.

Sorry I can't give you any more scope training help, or advice other than what I already have prior, or any help on using yours there, but I'm sure if you check out your public library where you live they are sure to have a book or two on how to use one. It's somewhat of an art after you learn the nuts and bolts basics. Kinda like that with using a DVM properly for the 1st time.

Speaking of which -

That's a better then average BK 2407A DVM you have there btw! Not bad looking either from looking at the link you included and checking out the Specs on it. Not bad at all for the ~ $58 price tag I saw online. Not sure what you paid for it ?, but sometimes far less I'm sure if someone online or locally had a great deal going on it at the time!

Accuracy could be a lot better I would think, but I guess you don't get much for $60 these days, even though BK has some pretty good test equipment out there. I don't care for the autoranging feature myself, and neither of my 2 DVM's (Wavetek 85XT & Meterman/Beckman 320B) have that feature - as I like to switch functions and ranges manually myself.

I noticed that your meter there has a CONTINUITY CHECK
Buzzer Threshold of < 320 Ω which seems a bit high for my purposes - as compared to my 2 meters, which are at < 75 & 80 Ω respectively using a lower range base overall. Of course your meter has higher overall range base using 320 instead of 200 like both my DVM's. Yours is more of an industrial type, as some even use 400 as a base. Of course when you can see the LCD display while doing an Ω check who cares - right! Sometimes my longer meter leads are stretched out doing Continuity checks and hearing that buzzer quicker is a nice feature. Oh well - to each their own.

My CAP tester is an older LED display BK model btw, and I got it free when someone was trashing out some storage lockers. Had the original box and all paperwork too! I added (4) 7.2 volt rechargeable Ni-MH batts that I had lying around, as rechargeables are all that I ever use in all my batt driven devices around the house. I also added a perfectly matched spare Casio brand 6-volt @ 200ma AC/DC wall adapter as for the external DC power source to recharge the new batt pack that I added, as no doubt BK would want an arm and a leg for their version I'm sure. Just had to add a matching mini-jack plug to that Casio wall adapter and it was good to go - worked perfectly!

Sorry I can't help you out further on the scope part.

It looks like you have a game plan figured out in so far as for the CAPS R&R part. Seeings that there are only 4-5 CAPS in ? there, and that they are fairly cheap to replace - I guess that is your next step. If that doesn't produce a fix then I guess I'll hear a OH **** next….hahaha

Next step would probably be to get that scope fired up after taking a crash course in running one up and using it.

Update me on your findings and progress even though I can't see what you are working on there. Some digital PIX would be nice, but I'll leave that up to you.

Stay in touch and best regards,

Frank
 

#1supertech

New Member
To audioguru & ke5frf,

As to both your replies you are both on the same page as far as for just testing the CAPS value with a DVM that has that feature built in - in so far as displaying the CAP's value that is. That will at least tell you if the CAP under test is within it proper capacitance rating. If the reading is grossly out of spec then pitch it and replace it with a new one of proper circuit value and voltage rating.

As with testing the CAPS on a DVM doing a simple charge test - well - not as accurate, as a LEAKY CAP will always give you some varying readings more or less, and still won't tell you what the CAP's actual capacitance value is, or how far it's drifted out of spec.

Best to invest in a CAP meter if you are doing a lot of CAP change outs!

I've seen some fairly inexpensive DVM's in the $30 range that have a CAP measuring feature built in, and if they are of good quality then that's not a bad price to pay.

Just a thought…..

Frank
 

fairchild

New Member
Frank,

I think once i get some probes and experiment with what i know so far, i should be good to go with the scope.

I convinced my boss to buy a DMM for work and that's how i got that BK meter. It was hard finding something inexpensive with all the features i wanted. I think this meter will be handy for now, though i think having that CAP tester is essential for my new audio repair hobby.

Here's a photo of the circuit i'm trying to fix(best i can do with my cell phone camera):

(The reverb section is on the top right)

**broken link removed**

Here is the reverb section from the diagram in the manual:

**broken link removed**

Thanks for all your help and input!
 

#1supertech

New Member
fairchild,

I got your reply notice email just now and thought I'd answer you back before it gets too late.

The first of 2 lower resolution cell-phone pix you posted didn't do me any good, as I can't really see anything close up in that actual PCB pix. The 2nd pix only shows the part's mobo reference #, which again does me no good. Some Cell-phones aren't too bad for taking general pix, but when it comes to close up's they aren't worth a plumb nickel!

What you really need to do is use a good digital camera and take a couple mobo pix of the area in ? and post those so my sharp eyes can possibly see if there is a bad cap or possibly a bad resistor, etc. Sometimes a good quality pix is worth its weight in gold as it were!

If you have a good quality digital camera take and make the posted Jpegs around 250kb in size and no bigger!! That equates to a camera setting of (SQ1 at the 1280 x 960 NORMAL) mode setting.

Hopefully you can do this as it would help.

Regarding your DVM there - good move on your part twisting your boss's arm! Got a great deal there!

Hope you find some cheap but yet good scope probes and work out the/any bugs with the scope. Make sure you get the ground wires with the probes (sometimes they get lost) elsewise you will have to buy separate ones, or else make some up, or else just use 1 common ground from the scope's common grd point to the PCB ground under test. The latter is what I made up using an extra long Black test lead wire with a Black male Banana plug on the scope end, and an added Black Mini Test Clip at the test point end.

Keep us updated as to your progress there!

Best regards,

Frank
 
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