Peterson Model 420 Strobe Tuner advice on the Repair

[TD]

Hello again, to everyone on the EEvblog!

I would like to start out by making it clear I am not a professional in electronics repair. I do have a nice Amateur lab with brand name Osiloscopes, power supply's, bench meters 6.5 digits, Electronic load, cheap signal generators, and some other decent gear. However, I am just an Amateur who is enjoying the path of learning electronics and repair for my personal benefit and enjoyment.  I am still far down the Amateur ladder and have a lot to learn. especially when it comes to understanding schematics and following signal paths. I always feel ignorant when I take on a repair. but I think that is how most amateurs feel in any hobby or field they are still new too!

I have played guitar off and on for many years. I found a Peterson model 420 for sale and listed for a fair price on Nov 26th. Finding one for a fair price is rare, most people think they are made of gold or something.
The seller did not have the ability to test the unit. I thought there could be an issue, the photo's listed for the unit did show it powered up. However, I noticed the strobe disc appeared to be stationary. I was not sure if that was from the frame rate of the camera or if it was not spinning. I ask the seller he stated it seemed to be spinning and working fine.  I decided to purchase it only if I could get it at a price that would make it okay if the unit needed repair. I made an offer to the seller, The offer was excepted. I was now the proud new owner of a Peterson strobe tuner model 420.

To make a long story short. My wife works on the front lines of the Covid epidemic. It just so happened that on Nov 30th she was tested positive for Covid. A few days later I had a fever of 104.6 on Dec 3rd. I ended up in the hospital and diagnosed with pneumonia. I was very sick for a couple weeks. It was a while before I started to feel good enough to take a look at my new strobe tuner, the 17th was the first day I felt like taking a look at it.  Unfortunately, My first gut instincts proved to be correct yet again. The strobe disc,  i.e. Hurst synchronistic motor was not spinning. when I powered on the unit, the motor would jump moving about .25" then stop dead.

This is when I decided to reach out to a friend and get the schematics for the unit and then started to troubleshoot.
I did find one thread on the model 400 with a similar issue however it is not the same unit and did not help me with my issue.

Troubleshooting.

My first instinct was to go straight to the motor drive PCB. I probably should have started on the transformer rails but I did not. I pulled the motor drive PCB. Then I pulled all 4 transistors on the board the (Q1) MJE711 the (Q2) MJE721 the (Q3) 2N3414 and the (Q4) 2N3405  all 4 tested good. I tested the electrolytic Caps the 3uF & the  Dual 10uF Capacitor (good),  The dual Cap is one I have never seen before, it's an axial dual cap with one negative leg on the one side and two positive legs on the other. Testing in parallel as 10uF each leg and series as 5uf. I believe this is correct and means the cap is testing as (good). I don't see any sign of leaking or swelling of the caps.

Next, I disconnected the Hurst synchronous motor. the motor has two black leads and one red and one white lead.  I ohm tested the motor wires test (R) & (W) to (B) test at 32Ω, (R) & (W) to frame test OL or open circuit. I believe this means the motor is testing as (good) and is not shorted and in working condition. I am not familiar with these motors but I think the black wire is connected inside the motor. The motor does spin freely by hand and the bearing feels good.

Next, I went to the Mic Preamp PCB. on this board there are two 150nF 25V Illinois Axial caps one of the two caps had a lead broken off. it was detached from the body of the capacitor. I happen to have a few 150nf 25V TTA Illinois caps in my parts bin. I replaced the cap with the broken lead. However, everything seems to be good on this board other than the one cap I replaced.

Next, I pulled the power supply PCB with the goal of testing all the electrolytic capacitors. on this board, there were one more of the 150nF 25V Illinois caps. two 2000nF 50V electrolytic caps and one 20nF 350V cap. All capacitors on this PCB tested (good). There is also a transistor (Q1) I pulled and it tested (good) as well.  I do have an ESR tester, However, without any leaking swelling or signs that any caps are bad I did not see the need to test the ESR I would have to look up each cap datasheet and find the proper test frequency. At this point, they all appear to be good so I have not done an ESR test on the caps. 

All capacitors and transistors that were tested,  were desoldered, i.e. removed from PCB Tested then reinstalled back to PCB.

I was also thinking that this type of gear is not like amplifiers that are used hard cranked up and left on for thousands of hours during their life. The strobe tunes seem to have a much better life and only turned on when needed and even though they are old from the '70s and have external damage they have very low hours of use compared to other musical gear. This one seems to follow in the outlook as the internals look great and lightly used.

At this point, I am confused and lost I don't know were to go from here. I would have thought that I would have found a problem by now I would think one of the components I test should have tested as bad and started me down the path of the repair. 

Next, I am going to put all PCBs back and put the unit back together. I have installed a new cord with an earth wire attached to the chassis. I am going to check the transformer rails from the schematics there should be a +400v, +15v, +25v, -25v, -12v rails. This is what I will do first thing in the morning.

I could use any advice I can get. As I said I am only an Amature and it showed on this repair. I have uploaded the schematics for the model 420 with the wire sheet for the hurts motor. I hope this will give someone the info I am missing.
 I would like to fix this on my own without having to send it to Peterson for repair. That will bring up the total cost to more than I can afford. I have always wanted a good reliable tuner and this is a dream tuner for me! I hope I can get it going again,.

[bob91343]

Q1 and Q2 collectors should show a sine wave to drive the motor.  Once you get it together, maybe it will run.

[Renate]

I'm putting my money on C1 & C2 on the motor driver board.
They are using C1 as a non-polarized electrolytic (the center lead is disconnected).
I always say, show me a non-polarized cap and I'll show you a cap that needs replacing.
C2 is also high stress. It's part of a capacitive divider for the phase shift.

Q1 and Q2 collectors should show a sine wave to drive the motor.

Maybe a square wave?

Edit: If you can get a finger in there, give the disc a little spin (with it powered).
I once had a lawn mower like that.

[bob91343]

You once had a lawn mower like that?  Did you also once have fingers on that hand?

[Renate]

You once had a lawn mower like that?

Nah, I just gave it a quick and careful kick.
(Don't try this at home.)

[TD]

I did not get a chance to work on her this morning. However. I am jumping into the shower right now at 7pm then I am going to put her back together and see what happens. I will let you all know what happens either tonight or in the morning, depending on how long it takes me to put the tuner back together.
Thanks for all the replies and advice so far. It is appretiateded.

 

I'm putting my money on C1 & C2 on the motor driver board.
They are using C1 as a non-polarized electrolytic (the center lead is disconnected).
I always say, show me a non-polarized cap and I'll show you a cap that needs replacing.
C2 is also high stress. It's part of a capacitive divider for the phase shift.

On (C1)  I can't seem to find a Sprague Dual 10uF capacitor for a replacement.  Do you think I could use (2) 10uF ("non-polar") or even (2) (polarized) 10uF capacitors and just tie the negative leads together then put the positive lead as they were with the dual cap? Would that work just like a dual cap? As for the Negative lead of (C1) Dual cap. Renate you are correct about the negative side is Soldered to an open, not in circuit location on the PCB, it just seems to be soldered down to help hold the capacitor in place. So would it be okay for me to put two 10uF caps in its place with the negative leads tied together?

[Renate]

Well, I think the two possibilities is that there is no signal coming out of Q1 & Q2 or that the 2nd phase is not good.
If you can try starting it spinning by hand that would be good proof that the rest of the circuit is working.
If you don't have a scope, you could hook an earphone through a small cap (like 0.1 µF) to the Q1 & Q2 output and see if you get a pretty loud low frequency buzz.
(Don't put it in your ear!)

Yes, you can put two 10 µF back to back.
C2 is important also.
The phase on the second winding is not so exact, since the frequency goes over a 2:1 range anyway.

[TD]

Well, I think the two possibilities is that there is no signal coming out of Q1 & Q2 or that the 2nd phase is not good.
If you can try starting it spinning by hand that would be good proof that the rest of the circuit is working.
If you don't have a scope, you could hook an earphone through a small cap (like 0.1 µF) to the Q1 & Q2 output and see if you get a pretty loud low frequency buzz.
(Don't put it in your ear!)

Yes, you can put two 10 µF back to back.
C2 is important also.
The phase on the second winding is not so exact, since the frequency goes over a 2:1 range anyway.

I do have a scope & a true Isolation transformer. I put it back together after all the testing. I don't have the lights now. However, I believe that is just an old wire broke. No smoke. I did notice R1 3.3Ω 2W Sand block resistor was pretty dam warm I would say Hot. I did not put the Thermal imager on it but my finger said it was hot. lol. However,  I have some caps and transistors, ON order right now.  I order TWO 10uF 100v caps. ONE 3nf 100V cap. All the 150uf 25v caps. Transistor Q1 Q2 Q3 Q4.  Once they are here.
 The motor:
will free spin power off great.  when power on it will not free spin it feels like a magnetic lock if I were to describe it in lamen terms.  like two poles pulling on each other not letting the motor spin. If I spin by hand it will only move as long as my mechanical force is in play the sec my finger lifts off it stops with a wiggle like it's being pulled back into place. Like a magnet would feel.

[TD]

I will be sure to update one Capacitor and transistors are delivers. Once everything in my last post arrives I will update.

I want to say thanks everyone who has given me advice. It is appreciated. Thank You!

[Renate]

Oh, I missed the part about you having a 'scope.

I try to avoid pulling any component until I'm dead sure that it's a problem.
First, there's a problem of incremental damage to the PCB.
Then there is also the chance that I screw up and put something in backwards.

This is basically a simple circuit.
The transistor oscillator at the top runs at around 10 MHz. More like 2 MHz
That goes through a dozen probably 9 bit counters of various modulos for a frequency of 20-40 kHz. More like 4-8 kHz
That goes to IC1 on the motor driver board, which is presumably a 10 bit (or 3 decade?) counter for 20-40 Hz.
The rest is just like an audio amplifier (for a square wave).
The C1 & C2 is just to get it spinning.

Since it's cogging (magnetic pulling) we can see that the output is stuck in one polarity.
Is the input signal missing or is the amplifier blown?


If I owned this, I'd probably be a philistine and get rid of the whole divider scheme with its LC oscillator and "fudge" resistors.
I'd run a nice little AD9850 digital synthesizer on a 10 MHz TCXO (vs the normal 125 MHz).
That would give me a frequency resolution of 0.0023 Hz, close enough for jazz.
For extra credit, get the 10 MHz from a GPSDO!

[jdragoset]

Times Have Changed.
When I was in high school, the band had an antique Conn strobo-tuner that stopped working (changed some electrolytic cans) but was some piece of work.
The synchronous motor was run by a vacuum tube amplifier driven by a 60 Hz tuning fork oscillator.
The motor drove an oil filled gear box that had twelve shafts with a pitch wheel on each.
The strobe was from a rectangular neon tube driven by an audio amplifier and microphone.

[TD]

Oh, I missed the part about you having a 'scope.

I try to avoid pulling any component until I'm dead sure that it's a problem.
First, there's a problem of incremental damage to the PCB.
Then there is also the chance that I screw up and put something in backwards.

This is basically a simple circuit.
The transistor oscillator at the top runs at around 10 MHz.
That goes through a dozen probably 9 bit counters of various modulos for a frequency of 20-40 kHz.
That goes to IC1 on the motor driver board, which is presumably a 10 bit (or 3 decade?) counter for 20-40 Hz.
The rest is just like an audio amplifier (for a square wave).
The C1 & C2 is just to get it spinning.

Since it's cogging (magnetic pulling) we can see that the output is stuck in one polarity.
Is the input signal missing or is the amplifier blown?

Thank you, Renate!  You have the circuit laid out correctly. However, some IC's don't seem to be located on the PCB that the schematic shows them on.  or I am reading it wrong.  My main confusion in the original post was the fact none of the components I checked initially,  tested as bad. I went that route because I would have bet cash money on C1 or C2 or Q1 Q2 testing as shorted.  I agree with pulling parts and putting them back in wrong. I did take photos on part position before pulling. It is funny you mentioned it. I have made this mistake in the past. 
 I will follow your recommendations. However,  I am going to wait until all my parts get here. I went and purchase the Caps and transistors. even if I don't need them now. They could fail at any time and it will be nice to have them. 
 
As for IC1 on the motor drive schematic, and IC1's physical location.  That has tripped me up because IC1 is not actually on that motor drive PCB. as the schematic shows. I'm sure I am missing something and it is my bad.  I think IC1 for the motor drive is the IC on the PWB 300060 synthesizer board. it is the only 14-pin IC with a (test location) like its shows on the motor driver PCB. I'm not 100% on this but I will be able to put it on the scope and see what signal is coming from that test point. I will upload photos of all 4 main PCB's. Then a set of pics with just the two with the motor Driver PCB and the other PCB I think holds IC1 for the motor driver.  the other boards are physically connected to (Note selector switch) and the Contrast potentiometer.

   At the end of this repair, I will upload photos of all components needed in the repair. plus photos of the hard to find components with links on where to find them if possible. Just to help out the next person in need.   All the parts I orders should be here by DEC 30th - JAN 2nd. I will be sure to update the post then. Thanks again!

If I owned this, I'd probably be a philistine and get rid of the whole divider scheme with its LC oscillator and "fudge" resistors.
I'd run a nice little AD9850 digital synthesizer on a 10 MHz TCXO (vs the normal 125 MHz).
That would give me a frequency resolution of 0.0023 Hz, close enough for jazz.
For extra credit, get the 10 MHz from a GPSDO!

Nice! This is not a bad idea!
 LMAO! (Close Enough for Jazz!!)

[Renate]

Yeah, the schematics don't match the boards.
The boards are newer.

It uses an AMI S50241 top octave generator (the 16 pin DIP) instead of the two ICs on the schematic.
It's still using stone-age PMOS and uses a negative power supply (here the -12 V).
The data sheet says it typically uses a 2 MHz clock.
That puts its outputs in the range from 4-8 kHz

I can't read anything off the 14 pin, but that must be the output divider.

It's easy enough to check with a scope.
Just hit any (or all) the pins on the 12 pin connector.
They all should have 4-8 kHz or so on them.

Then the test point should have 20-40? Hz on it.

[TD]

The parts I ordered on running on with shipping all was expected for 28th delivers. However, it is looking like half will not be here until the 2nd. I will make sure to update the repair as it happens one parts get to me.

[TD]

Update. 
I got my components in. I have confirmed one thing the other day. The problem is not on the motor drive board. At least not on the board that holds the dual 10uF and the 3.3uf cap and the (2) oscillator transistors MJE721 + & MJE711 -, I have not changed any components on that board. I happen to have a friend that lives up north by our cabin, he has the same model 420.  We were driving up for a couple days & I knew he had one so I thur that PCB in my backpack. While I was up there. I swapped the PCB with his & his fired right up and worked fine with my motor drive board. I wish I would have thought to through my Fluke 287 in my pack. I could have taken some Hz measurements at the same time. but I'm a tart sometimes.

I am back home now.  Remember that AMI S50241 16 pin top octave generator is not on the same PCB as the schematic shows. Therefore. I am going to have to start troubleshooting the other circuit boards starting with the test point on the AMI S50241 top octave generator!

[Renate]

Well, it should be easy to figure it out.
You only have oscillator -> top octave -> divider

[TD]

Well, it should be easy to figure it out.
You only have oscillator -> top octave -> divider

10/4! I have time today, to work on it. Wondering which one it is, what components is the issue! just to make sure I could find a replacement if needed. I dug up a Top active IC replacement, did not purchase I only check If I could find a replacement.  I have never troubleshot one before. He'll, the only thing I know of the uses a top octave is a Synth. I am going to dive in today. Will give a heads up once I get a lock on the issue. I have not had any free time in a while, so I'm looking forward to diving back into this repair!