M100C, arrived and already I'm in trouble!

Following on from my previous thread where I was looking for advice on purchasing a M100C, it has now arrived.

Plugged in, everything worked, but as per previous post, the volume was very low.

I saw that at some point it had had an external speaker (extra speaker wire was still connected - I removed this from the connectors on the amp). So needed to check the speaker switch was set to 16 watts as per manual. It was, although not easy to see, as the amp seems to be painted in a horrible silver paint which is very annoying. (And, yes, thanks to Ron I know the AVC tubes are missing - that's for another day)


Then, on advice of Rob, put meter on this green resistor - it read only 1257 ohms, but it that because I need to unsolder one end to get a true reading?
I need to unsolder it first?

Also, I noticed two wires in parallel off the main speaker going underneath the speaker, so took the speaker off and its already got a tweeter installed (with a capacitor) inside the main one,

So put the speaker back, played a song, and suddenly it was loud - I never really touched anything as far as I can see. Played the whole song with great sound. But on the next song, it started to crackle then go low volume, crackle, back up high. It played loud for a few seconds, then I was sure me turning the volume knob caused to crackle and go low, but then it went loud again and me fiddling with the volume made no difference. Same seemed to happen with the bass and treble - I was sure me turning them caused the crackle and change at one point, but then again it settled into loud and however much I twiddled them made no difference (weird!).

Then it played loud for a minute and I carefully prodded around power/speaker/etc wires, etc to see if something was loose causing this, but nothing affected it.

Then, volume when completely, and very loud hum. I then noticed the 5U4 tube glowing very red! So I switched it all off (there was a bit of a smell too). Switched it back on (for one second only) as loud hum started again, switched, off and scared to switch on again!

I am sorry this is so verbose and basic, but I wanted to include as much as I could as I have no idea what is relevant and what is a red herring.

Any thoughts before I call the local repair guy - he's good and fair, and no issue calling him, but thought it was worth posting here first so I can try and learn as I go.

Thanks in advance.

p.s. Apologies if this is all explained in the troubleshooting guide, but only just ordered it & manual and has not arrived yet!

The glowing 5U4 indicates excessive drain or short on the B+ line. Hopefully the small was just an overheated resistor.

The first order of business is to check the existing circuit against a schematic. Since there have been at least some modifications done to accommodate the non-field coil speaker, you need to get a view of what was done. I have seen some really stupid hacks to electronic equipment, sometimes by people who should have known better (fellow broadcast engineers and theater tech's). When people are operating outside their normal area of expertise, they make mistakes based what they think is supported by theory.

Above all else: Do not plug it in until you find the short.

Rob.

That resistor looks like an old 5k ohm. (green body) Resistors drift UP in value with age/use. Not down. Yes you will have to pull one end to check it. The rectifier will have a red glow. I don't know if what you are seeing is normal or not. Google dim bulb tester and make one. Sounds like you need to clean switches and pots.

did it for you:
http://www.antiqueradio.org/dimbulb.htm

BEFORE using a "dim bulb" check the amplifier ( and all other) fuses for proper type and size ! Do NOT operate any equipment with "over-sized" fuses !! Ron Rich

There are soem things here that need to be corrected:

-The resistor in question is a --wirewound-- type. they do not drift either up or down. They fail by opening.
Carbon composition resistors can go lower in value when use in pulsed circuits, especially when high voltages and low currents are present.

Rectifier tubes with have dim orange glow only in the heater or cathode -never on plates. "Showing color" indicates a gross overload.

Rob/NYC

Rob,
I agree--except for one minor detail--I have seen 5U4's "glow/flash", when there were no other problems in the circuit--the tube itself was "bad" (shorted ?) Ron Rich

Ron Rich wrote:Rob,
I agree--except for one minor detail--I have seen 5U4's "glow/flash", when there were no other problems in the circuit--the tube itself was "bad" (shorted ?) Ron Rich

5U4's, 5AS4's, along with other rectifier tubes, can also "flash" during warmup if someone has increased the 1st filter capacitor above the tube's ratings.
47uF 450V is the most I'd use as a 1st filter.

I agree with Matt's comments here. this is called "flashover" and it occurs when the inrush current of a capacitor present a near short to the tube as the cap 'forms". It is blue and within the plate structure.

What Max described is a typical of gross overconduction causing the plates to glow red. That is usually a short on the tube load.

Rob

Thanks everyone for your input, as some of you all have guessed already, I am no engineer! While I have taught myself to a decent level on EM pinballs, all this caps/filters/B+/tubes/etc is way, way more difficult - I have ordered manuals, reading everything I can on-line, but it would take me years (and probably a degree in elec/sound engineering) to get anywhere close to understand everything that gets said here. So...I really appreciate all your patience.

That said, I have called a tech to help out on this issue, but he's not coming for over a week, so I'd like to see what I can do, safely, for my own education. (He's the same guy that I called for my 1st EM, good guy, but since that visit, I have fixed EM's on my own - I know the learning curve will be longer and steeper on this).

So, one step at a time:

1. I can certainly unsolder one end of that resistor and check - but is that even worth it given the box sounded great, albeit only for a minute or so, and Rob said if these fail, they fail open which I assume (?) means it would not be causing the short and huge load to RU4 tube (it was glowing very red!)

2. I can build the dim-bulb tester. I assume any incandescent bulb from home depot is fine, and start with 40w, then 60w then 100?

3. Is this the first, easy, logical test: remove RU4, switch on – if 40w bulb glows bright, the RU4 is not the issue, if the 40w does not glow, try 60 then 100 and if still just a faint glow, then RU4 is probably the issue.

If the above is logical, I'll start with that, and report back?

Max, wait for the manuals and preferably your tech too (if he understands amps).

The problem you described can easily result in burned out transformer(s).

Be patient.

Rob.

Rob,

Spot on! As always, I was being far too impatient! I'll wait for the tech and report back.

(My wife said - "I told you that, how come you listen to Rob-NYC and not me" )

Hi Max;
I cut my teeth on building vacuum tube guitar amps which, I believe, gave me the confidence in restoring the amplifiers and receivers in jukes and jumping to this level from EM & SS pins.

I do think that having a qualified tech look at your MRA3, this time, is a good idea. There are dangerous voltages in these amps.

If you'd like to expand your knowledge of these vintage electronics, I'd recommend this site
http://www.ax84.com and a series of youTube videos by "Uncle Doug".

I'd start with this series of youTube videos by "Uncle Doug" that I found helpful in understanding this technology.

Here are his tutorials on amp power supply circuitry
https://www.youtube.com/watch?v=hJsPQdAhTkQ
https://www.youtube.com/watch?v=dl729dEJ3To
https://www.youtube.com/watch?v=x5SSKX74DKg
and
preamp and power amp circuitry
https://www.youtube.com/watch?v=901iaPVVzY0

With respect to a more in depth tutorial on vacuum tube amp design, check out this page
http://www.ax84.com/p1.html

Under the Current Materials section of the AX84. com site, you'll see the P1 Theory Document. This explains the design of a simple "single ended" guitar amp called the P1. The P1 is an amp similar to the early guitar amps of the '50s and thus representative of the technology of the day. Although the Seeburg MRA3-L6 has a "push pull" design to the power amp portion, the basics (vacuum tube theory, power supply theory, the role of capacitors and resistors etc.) are the same.

The 10.10.04 P1 document is the P1 schematic. You might want to reference this while going through the P1 Theory Document. You might note that there is no rectifier tube in the P1. This is replaced by solid state diodes.

I wouldn't worry about the math, graphs and tables. This explains why certain values of capacitors and resistors are used in the design of the P1. Seeburg engineers did this for us back in the 50's.

Why is it people will refer you to a manual or "take it to a "tech" until someone tries to help, then advise comes from everywhere?

eddie wrote:Why is it people will refer you to a manual or "take it to a "tech" until someone tries to help, then advise comes from everywhere?

I can only write for myself in answering this question.
My responses are based on how the question is asked., If the question tells me that someone is in unfamiliar territory -and- their question might imply a serious problem I do recommend getting written material so that we can be "on the same wavelength".

If the person seems experienced or the question has a simple solution, I'll go in-depth more quickly.

It is a kind of fine line between encouraging "tinkering" which can cause costly damage and defeating the purpose of this forum by just telling everyone to "go to a tech".

Rob/NYC

I agree. But I see it all the time, a new member will ask a simple question on an adjustment and the answer is "The manual should cover this" or "Read the manual". Many times followed by "Take it to a qualified tech".