Lowrey (Rodgers) C-32 restoration

[Scott Dennison]

Greetings all. I'm just beginning the process of restoring a Lowrey C-32. For those that aren't familiar with this model, it was built for Lowrey (in a Lowrey cabinet), by Rodgers back in the mid-1960s, with all Rodgers internals. It has 2 sets of individual transistor oscillators and a full 32-note pedal board, so it should make a wonderful practice organ if I can get it going again.

The bad news: it's DOA. Thanks to fellow Forum-ite "td", though, I've got a copy of the power supply schematic, which I'll insert here.



But I think I'm making progress with it. What's happening is as follows: powers up but no audio. All supply voltages are good except -12VDC for keying and preamps - 0V there. (Obviously that would explain "no audio.") Immediate cause for no -12V supply is that it's blowing fuse F3 (1.6 amp slow blow). From there, I think I've traced the problem to zener diode D1: it's reading open both directions, out of circuit. Fortunately I've found a modern equivalent, NTE137a for about 40 cents apiece.

My question though: what are the odds that the bad diode might have taken down Q1 with it? According to the service manual, the function of the diode is to provide the reference voltage for Q1, which in turn regulates the -12V supply. But I have to admit I'm at the limit of my electronics knowledge at this point.

So, should I start looking for an equivalent to Q1 (2N1544) while I'm ordering parts?

[jbird604]

It might be wise to have that transistor at hand because it's hard to say just what might have been damaged by the shorting of the zener, if that is indeed what triggered the whole event. Be sure the 2000 mfd caps are not shorted or open as well. It wouldn't hurt a bit to go ahead and replace them while you're in there. New ones will be quite cheap, and don't have to have the same form factor as the originals. Just clip out the old ones and solder in new ones, mounting them in any convenient way. You can easily check the two resistors in the circuit with a meter, and look at them to make sure they aren't burned.

My next fear would be that even after I got the -12 up and running it would blow the fuse again when connected to the organ circuitry, because you can never tell just what made the fuse blow in the first place. Could be a short circuit somewhere up the line. You might test the various devices and circuits that use the -12 with a simple VOM reading to make sure you don't have a dead short out there somewhere before you go to all the trouble of fixing the regulator.

Put the red lead of your meter on the -12 output lugs of the power supply, and the black lead on the system ground. You should get a reading of perhaps 100 ohms or more, depending on just what the -12 was driving. If you get a near-zero reading, you have a problem to trace down.

[Scott Dennison]

Thanks very much for the guidance, jbird. I did the test you recommended and the results were encouraging: just shy of 90 ohms from the -12V supply lugs to ground. So I'm hoping this means the problem was localized to the PS itself, and specifically to the area already under discussion (D1, Q1, and the two 2000 uF electrolytics.) Also, the resistors in question all tested within reason - all a little high, but nothing out of the ordinary for 50-year old resistors. And no visible signs of overheating, so the fuse clearly did its job well.

Unfortunately the zener diodes are on back-order, and the modern equivalent for transistor 2N1544 (=NTE121) goes for about $30 apiece! So I plan on putting in the new diode and seeing if the fuse blows again, before ordering the transistor. (I'm also picking up a new meter with a transistor test function to see if it can tell me anything about the old transistor; my present meter only has continuity testing, and I wasn't 100% sure how to interpret the readings I was getting.)

In the meantime, do you have any tips on checking those two caps? A simple continuity test shows they're not shorted directly to ground, anyway.

Thanks again for the help.

[toodles]

2N1544: http://store.americanmicrosemiconductor.com/2n1544.html for $14

https://www.tedss.com/2028001966 for $4.00

Larger value caps of some age tend to do better than smaller ones (like used in keying or sustain circuits), and back then these filter caps tended to be of good quality. If they aren't shorted, I suggest not replacing and see if they perform.

[jbird604]

Without an actual capacitance checker, and without removing them from the circuit, it's hard to test the caps. If you put a probe on each end while in the ohms mode with your meter, and you get a low ohms reading at first, then then reading goes up and up and up, the longer you keep the probes in place, that is a good indication that the capacitor is charging up, which is what it is supposed to do. If you don't see this, try reversing the probes, as there could be a diode draining the capacitor in one polarity or the other.

As toodles says, you can leave them in there and try it out. If the caps are weak, you will probably notice hum in the system. The sure-fire way to test them with the power supply up and running is to put a probe on each terminal of the cap with your meter in the AC voltage mode. You shouldn't read very much AC voltage across any filter capacitor in a power supply. Readings vary, depending on a lot of factors, but I normally find only a few hundredths of a volt max of AC across a capacitor. When a capacitor fails by opening up, the AC voltage across it will be quite large, several volts.

[indianajo]

I say, there is way too much energy put in to exact matching the transistor and zener.
Zener, the new one should be same voltage rating and same wattage or higher.
Transistor, you initially test them with the base out of circuit with the diode tester function of a DVM. Or the ohmsx2000 scale. From base to emitter of silicon npn transistor, should be .6 v (600 ohm) forwards, infinity backwards. from base to collecor of npn transistor readings should be to same. Transistors tend to fail shorted (blown junction) or open (melted die to pin wire).
When replacing transistors, I try to match or exceed the specs. NPN for NPN, PNP for PNP, Fet for Fet etc. In junction transistors New Vce>=old one, Ic.= old one, Pd >= the old one. In power circuits the SOA is important, but organs tend not to stress that design parameter too hard. Gain can be important in some old amp design, but you as a repairman/individual can't purchase transistors in specific gain classes. A factory could have. Power dissipation should be the same, if the new package doesn't match the old heat sink, buy a new one. Use thermal grease on heat sinks, which is poisonous, don't get in eyes or mucous membranes. I tend to pick the actual transistors I buy by specifying package, then the minimum Ic, Vce, I will accept, then push the select button and see what is cheap today.
For more detail on transistors & diodes, see Thomas Floyd, Electronic devices the electron flow version. I have Edition 2 I bought for $2 at Goodwill. Meter use is covered in there, also.

[Scott Dennison]

Many thanks for the suggestions, guys - especially the sources for 2N1544s. Unfortunately those haven't come in yet, but I did get the NTE137a diodes. (I-J, your point is well taken about not needing precise component matching, but orders had already been placed.) Anyway, I replaced the bad diode and... the PS still blew the fuse. Rats. Oddly though, it didn't zap the new diode (?)

So now I don't quite know what to think. Summary below:

-Initial problem: no keying supply (-12V) due to blowing fuse F3 (1.6 amp slow blow).
-Found diode D1 open; replaced - but still blows fuse. New diode OK so far though.
-3 ohm, 5 ohm, 270 ohm resistors show no signs of overheating; all test within reasonable range on a meter (in circuit).
-Transistor Q1 tests ok on a meter/tester, out of circuit.
-2000 uF caps don't seem to be shorted to ground, but exact condition uncertain as of yet.

So, what would you guys do next? Go ahead and replace the transistor? Pull the caps for testing? Or just replace them regardless? Since it's blowing the fuse in just a couple seconds I can't do any meaningful voltage tests in that area. Also: I had assumed that the one 4000 uF cap (right after the rectifier) was probably fine, since I am getting the -25V supply. But should I be looking at that (or anywhere else for that matter)?

I'm resisting just doing "shotgun" replacements since I won't learn anything that way. So, I will really appreciate any further guidance you can offer on the "right" way to proceed with diagnosing this. Thanks guys.

[indianajo]

you need to put the lightbulb limiter in the power line to allow you to debug. Or replace everything around there (cheaper).
A light bulb limiter is a grounded metal box with a circuit breaker, with a 3 prong Ac cord. The The safety ground of the AC cord goes to the box. The neutral (white) goes through to an AC receptacal cord. The Hot (black) of the AC plug goes to the circuit breaker. The load side of the circuit breaker goes to the center of a medium edison light bulb socket. The outside of the lightbulb socket goes to the hot of the AC receptacle cord.
You plug the amp (organ) into the receptacle and plug the AC plug into the wall or a breaker strip.
usually a 60 watt bulb works for most amps, although especially high power ones use 100 W.
Then if the light bulb lights and goes out, usually the fuse did not blow. If the light bulb stays on, the light bulb is protecting the fuse and something is sucking a lot of current.
After fixing a pair of 197? Allen 301 power amp with no sound (hot, produces 2 W sound out cold), with four $7 rail caps and two $.10 power up reset caps, I laugh at all the people desperate to save their Rodgers and Allen electrolytic capacitors from the seventies. PLPLPLPLPLPLTTTTH!!
Of course, the transistor could be okay at 2 v test voltage and not at 30v rail voltage. I've had a few of those, too. I Used to bounce weak transistor steel cases off the ceiling until I built a light bulb box.
PS I buy my used circuit breakers for mounting in project boxes, from apexelectronic.com of LA, the surplus house. Or Electronicsurplus.com of Buffalo.

[Scott Dennison]

you need to put the lightbulb limiter in the power line to allow you to debug. Or replace everything around there (cheaper).
A light bulb limiter is a grounded metal box with a circuit breaker, with a 3 prong Ac cord. The The safety ground of the AC cord goes to the box. The neutral (white) goes through to an AC receptacal cord. The Hot (black) of the AC plug goes to the circuit breaker. The load side of the circuit breaker goes to the center of a medium edison light bulb socket. The outside of the lightbulb socket goes to the hot of the AC receptacle cord.
You plug the amp (organ) into the receptacle and plug the AC plug into the wall or a breaker strip.

This is interesting... I had never heard of such a device, and it took me a minute to get my head around it, but I think I get it now. What you're describing is basically a current limiter, no? If I have this right, a fuse is essentially a current-limiting safety device for the amp/organ; then in turn this lightbulb box is a current-limiting device for the fuse. So it sort of "absorbs" the excess current by giving it a load (the lightbulb filament), which then brings the current down to a point where it won't damage the amp, so the fuse doesn't blow. That way, you can probe the amp "live" for troubleshooting, but without risk of further damage. Very interesting, thanks for the suggestion.

[tucsondave]

A variation of that is to use a #211 automotive lamp in place of fuse F3 using a couple of clip leads.

td

[Scott Dennison]

Hmm, I think I'm moving in the right direction. Now I've got it to where the fuse isn't blowing any more, but I'm only getting -6V at the -12V keying supply. Should I suspect Q1 at this point? I'm especially wondering if it's drawing too much current because the 270 ohm, 2W resistor feeding its base is getting hot. (Able to detect this now that the fuse is holding.) It's not charring or anything - just getting hot to the touch, and fairly quickly at that. Not sure if that's to be expected or not.

A variation of that is to use a #211 automotive lamp in place of fuse F3 using a couple of clip leads.

Thanks for this tip - can a #211 stand in for other fuse values, say from 1A to 5A?

[indianajo]

Thanks for this tip - can a #211 stand in for other fuse values, say from 1A to 5A?

Having mains voltage and current flying around on the end of a clip lead is not part of my maintenance plan. It would be less dangerous after the transformer where current would be limited by that device. The grounded metal box with circuit breaker described previously provides some control of dangerous mains current if a wire comes out from under a screw, for example.
I test transistors with the double diode test listed previously. Transistors that previously exploded without the current limiting box I found to have C-B junction drop of less than .46 volts.
However, excess current can also be caused by a shorted component downstream from the regulator. Pulling boards while measuring current may highlight which board is using too much current.
At this age, I usually replace every electrolytic capacitor in a device containing the consumer grade of these- ie all consumer audio electronics. You can change them one at time as they fail, but this leaves your device broken most of the time or at least frequently. The replacements, I buy ones with a service life >3000 hours, and 10000 hours when I can get it. I've had to change the electrolytic caps for low voltage four times in an amp I bought in 1970. I used to buy capacitors in the open stock in a local electronics store frequented by TV repairmen. I wasn't high volume enough to deserve counter service in those shops.

[tucsondave]

Hmm, I think I'm moving in the right direction. Now I've got it to where the fuse isn't blowing any more, but I'm only getting -6V at the -12V keying supply. Should I suspect Q1 at this point? I'm especially wondering if it's drawing too much current because the 270 ohm, 2W resistor feeding its base is getting hot. (Able to detect this now that the fuse is holding.) It's not charring or anything - just getting hot to the touch, and fairly quickly at that. Not sure if that's to be expected or not.


Thanks for this tip - can a #211 stand in for other fuse values, say from 1A to 5A?

What is the voltage at the base of Q1? Q1 can't draw current since it's a series voltage regulator. If the base-emitter was shorted or the collector-emitter was open then current would flow through the 270 ohm resistor instead of the collector-emitter of Q1.
Is Q1 in a socket? If so remove it and check the voltage across D1.

The 211 lamp is only for 12 volt supplies around 1-2 amps.

The rule of thumb for a series light bulb in the AC line is use a bulb rated at half the wattage of the unit under test. So if the organ draws 200 watts use a 100 watt incandescent lamp.

td

[Scott Dennison]

Aha, thanks for this, td - your comments helped me to understand the circuit a little better.

The voltage at the base of Q1 is -6 VDC, same as at the keying supply terminal (which is at the emitter.) But I'm getting about -28V at the collector - wouldn't that mean the C-E junction isn't conducting? Don't know if this is meaningful, but I also measured for DC current across the 3 ohm 12W resistor and didn't get anything.

Unfortunately they didn't use a socket here, but I can pull the transistor if necessary. Thanks again for your help.

[tucsondave]

I just noticed in a previous post that you used an NTE 137A zener. That is 6.2 volts.
You want a 12 volt zener. Or if you have two of the 137's put them in series.

The base should be -12, or the zener diode (D1) voltage rating and slightly higher than the emitter voltage.

If you measured no voltage drop across the 3 ohm then there's no current flowing so your probably correct that the C-E junction is open.
You do have the load connected don't you (keying and pre-amps)?

td