Tag Archives: amplifier

Seeburg Jukebox SHFA1 amplifiers

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Location: Littleton, CO
Symptoms: Two amplifiers, both barely working.

Of all of the amplifiers I’ve rebuilt, I’ve never encountered an amplifier with so many bad capacitors as I have on these amplifiers.  Compared to Wurlitzer, Seeburg must have been trying to use the cheapest capacitors they could get their hands on.  I suppose a lot of it has to do with how many hours the jukebox was on and the temperature inside the amp chassis.

I replaced all of the electrolytic capacitors on both amplifiers.  As I pulled each one out, I was hard pressed to find one that was within 20% tolerance of its original value.  And many were not even close, such as a 50 uF reading 0.6 uF.

In addition to the electrolytic capacitors, many of the paper/foil capacitors have been bad as well. These capacitors play a variety of roles, mainly used to block high voltage and couple the audio from each vacuum tube stage to the next. Failures with these capacitors, usually high current leakage, will usually cause a tube to be biased incorrectly. This is especially important in the final power amplifier stage and in the “cathode follower” stage just prior to the volume control.

Also, with the SFHA1 in particular, the Automatic Volume Compensation (AVC) circuit will cease to work.  And when this happens, most people pull the 6BJ6 tubes to disable the AVC circuit, which explains why many amps of this vintage are missing these tubes.

One might be tempted to replace every capacitor, but given there are nearly 80 capacitors in this amp, I prefer to replace all of the electrolytics (21 capacitors), since they have a finite life, and then replace the others (which, in theory, should have a longer life) as needed.

Below I list some of the problem non-electrolytic capacitors. If you read on, you should have a schematic to refer to.

Preamplifier, prior to the volume control

On both of these amplifiers, C108 and C147 (0.22 uF, 400V) were leaky, causing attenuation of the audio signal, even when the AVC tubes (6BJ6) are removed.  This is usually the reason one channel might be louder than the other. These are very large and mounted to the rear side of the larger circuit boards near the sides of the amplifier chassis.

On both of these amplifiers, C121 and C160 (0.005 uF, 400V) were leaky, causing the voltage on V103, V109, pins 7 and 8 to be too low.  This causes clipping and thus distortion of the audio waveform.  The voltage on the schematic for pin 8 (cathode) is listed as 90V.  Before the bad capacitors were replaced, it was reading around 14V.  This circuit is the “cathode follower” I mentioned above.  These capacitors were among the first to be installed and are buried deep under everything else. These capacitors connect between Pin 7 of the tube and the wiper of the treble control.

One amplifier had some distortion in the waveform in one channel that was caused by C110, C150 (0.005 uF, 400V).

Automatic Volume Control

When there is no signal coming into the amplifier, the voltage across C115 (1 uF) should be zero or within a 1 or 2 volts of zero. If it isn’t, then C111, C148  (0.01uF 400V) should be replaced.  One channel on each amplifier had a leaky capacitor.

Likewise, when you have a 4 – 8 mV signal on the amplifier inputs, and pin 3 of muting socket is grounded,  the voltage across C115 should be 30 to 50 volts.  If it isn’t, and everything else is working, then C115 itself is the problem and should be replaced.  One amplifier had this problem and the voltage would never go above about 6 volts, in spite of being nearly 50 volts of signal at the cathode of the selenium rectifiers.

Final Power Amplifier Stage

The 6973 tubes are used in pairs for each channel. They operate in a push-pull configuration.  If the bias of the grid is not correct or if one tube is weaker than the other, the circuit doesn’t work very well.  It’s like a teeter-totter, where the people on it should be nearly the same weight to have a good balance.

On the 6973 tubes (V105, V106, V111, V112) you should see -33 to -35 volts on pins 3 or 6 of each tube (with no input signal on the amp).  A leaky capacitor will cause the voltage to be more positive. The capacitors associated with each tube in the order listed above are C137, C138, C175 and C176 (0.05 uF, 600V).  On one amplifier, all 4 capacitors were bad.  On the other amp, all were good.

Tubes

Usually I don’t see many bad tubes in jukebox amplifiers. Almost all problems are related to bad capacitors. But…

I had a bad 5U4 that was shorting and blowing the fuse.  I had one pair of 6973 that was bad on each amplifier (these should be replaced in pairs).  I had a bad 12AX7 on one amplifier.  And one amplifier had missing 6BJ6 tubes.  With the exception of the 6BJ6 tubes, there are companies still making vacuum these tubes.  Try finding a company still making vintage transistors.

I highly recommend Vacuum Tube Supplies in Denver for anything related to tubes and tube amplifiers.

Seeburg SHFA4 Jukebox Amplifier

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Location: Lakewood, CO
Symptom: Loud hum on speaker outputs, amplifier rebuild.

As usual with an amplifier of this vintage, all of the electrolytic capacitors were replaced. This fixed the problem with the loud hum.  Also one of the electrolytic capacitors was causing a bias problem with one of the preamp transistors, causing that channel to be weak.

After the capacitor replacement, I was testing the amp with the oscilloscope, there was still one channel substantially weaker than the other, and both channels had non-linearity distortion. I traced this to the AVC (Automatic Volume Control) circuit.  The AVC circuit is used to level the volume between different records and different sides.

The AVC circuit uses the resistance through diodes to achieve this. The resistance of the diodes changes with the amount of current flowing through it.  The louder the song, the more current flows through the diodes, the lower the resistance, which lowers the volume.

AVC block diagram (click for larger).

AVC block diagram (click for larger).

The problem with this vintage of amplifiers is they used selenium diodes.  These diodes seem to fail with age.  In this case, diodes in both CR103 and CR104 were bad.  One was nearly open, another nearly shorted.  The resistance across the other diodes was high.  My multimeter couldn’t properly read them, so I relied on just measuring voltages across them.

Small selenium diodes are no longer made.Silicon diodes can be used, however, you have to use more than two in series for each selenium diode you replace.  I started with using two 1N914 diodes for each diode in CR103 and CR104.  The resistance was too low and my signal going into V101 was too weak.  So I doubled them, using four 1N914 diodes for each diode in CR103 and CR104.  That was 16 diodes total.

Old selenium diode pairs.

Old selenium diode pairs.

Rework showing the strings of four 1N914 diodes in the AVC circuit. One string is exposed, the others are in the green shrink tubing.

Rework showing the strings of four 1N914 diodes in the AVC circuit. One string is exposed, the others are in the green shrink tubing.

I also replaced the CR102 diode pair.  For this, a single 1N914 can be used for each diode in CR102. After taking some measurements, I could have used three diodes for each, instead of four.  But with four, I get a little more input into the V101 tube (about 120mV) without causing clipping on the output.

The amp is now working great!

So if you’re rebuilding an amp with selenium diodes in the AVC circuit, replace them with 3 or 4 1N914 diodes in series for each diode in CR103 and CR104 and use single diodes for CR102.  It ends up being a lot of diodes, but they are small and cheap.

 

Seeburg Jukebox Amplifiers, MRA4 and SHFA1

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Location: Loveland, CO
Symptoms: SHFA1: one channel not working well; MRA4: generally not sounding good.

Both amplifiers were brought back to my office for bench testing and repair.  It’s really the only way to work on an amplifier.  A known signal, usually a sine wave is injecting into the input.  A dummy 8-16 ohm load is connected to the speaker outputs.  With the oscilloscope, I start at the speaker outputs and observe the signal.  If it looks distorted or weak, I work my way back to the inputs to find the fault.

The SHFA1 had one channel that wasn’t working well.  I found that the output of the first stage 12AX7 wasn’t outputting as well as the other channel at the same point.  The grid of the weak channel had a more positive bias on it of a couple of volts.  I traced it to a leaky 0.22uF capacitor.

Once that was repaired, now the weak channel was much stronger that the other.  I traced that to a bad 12AX7 just before the final output stage.

This amp had some previous work done on it, some capacitors had been replaced throughout, but interestingly, some of the most common ones that would normally be replaced hadn’t been touched, like most of the electrolytics.

The MRA4 hadn’t ever been service.  It still had the original paper and wax capacitors used prior to the 1960’s.

When rebuilding an amplifier, I usually replace every electrolytic capacitor.  If the amplifier is from 1960 or earlier, I usually replace every paper/wax coupling cap that has high voltage across it. I will usually leave tone control and other low signal voltage caps.

Prior to the cap swap, the MRA4 had a weaker output than I normally see.  I traced this to a leaking 0.05uF capacitor in the coupling circuit to the final 6L6 tube.  This caused the tube to be biased so that it wasn’t operating in a push-pull configuration.

This amp still had the original 6L6 tubes installed.  For fun, since I had some brand new 6L6 tubes, I installed those and they didn’t deliver the output that the original tubes did. I put the original tubes back in.  I rarely replace tubes unless there is a good reason to.  And this little experiment proves why.

Both amps are working great!

Wurlitzer 2204 Jukebox, 532 Amp Rebuild

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Location: Denver, Colorado.
Symptoms: Amp rebuild.

When I listened to the amp, I didn’t really hear anything in particular that was wrong with it.  But the owner was unhappy with it.  Someone had previously done some work on it, replacing a few capacitors, including the main filter capacitor on the output of the 5U4 rectifier tube.

I brought the amp back and bench tested it.  It failed.

Amplifier working normally at 50% volume.

Amplifier working normally at 50% volume.

 

Amplifier breaking down and oscillating at slightly higher volume.

Amplifier breaking down and oscillating at slightly higher volume.

 

Amplifier in bad shape at full volume.

Amplifier in bad shape at full volume.

The problem was primarily in the power supply as these waveforms were measured at the Aux Amp output, which is essentially the output of the preamp stage.  The final output stage was loading the power supply and breaking into oscillation.  Most often when an amplifier breaks into oscillation, there is a faulty capacitor someplace.

I decided to do my standard “re-cap” where I replace all of the electrolytic capacitors and some of the rolled capacitors of smaller values subjected to high voltages.  I tested some of the old capacitors once they were out of the circuit (except in rare cases, capacitors can’t be tested while they are in the circuit). There are two large can capacitors that contain four capacitors each.  All of the capacitors in one of the cans were dead (reading very low).  All of the capacitors in the other can were better, but all were out of tolerance. Those cans probably were the culprits of the oscillation.

Unmatched 6L6 tubes, not even from the same era. The one on the left is military surplus. The one on the right might be original.

Unmatched 6L6 tubes, not even from the same era. The one on the left is military surplus. The one on the right might be original.

Both Wurlitzer and Seeburg amplifiers of this era use 6L6 tubes in the final stage of the amplifier. From a theoretical point, the two tubes should be gain-matched because one tube drives the upper half of the waveform, the other tube drives the lower half of the waveform (a.k.a. push-pull amplification).  But these amplifiers seem to be pretty forgiving if the tubes are not matched. I once saw a Seeburg operating with one 6L6 tube missing.

The owner and I decided to go ahead and get new output tubes.  The nice thing about 6L6 tubes is that they are still being made today due to their popularity for use in guitar amps.

After all the work was finished on the amp, it tests perfectly at all volume levels.

 

 

Amplifiers, Amplifiers, Amplifiers!

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In an odd quirk of fate, I’ve had a rash of jukebox amplifiers to work on, as well as some Seeburg control centers.  The amplifiers are a Wurlitzer 532, Seeburg TSA1, Seeburg SHP3, and an amp from a Rockola 424.  The 532 is the oldest and the SHP3 is the newest.

Testing a Wurlitzer 532 amplifier.

Wurlitzer 532

At the customer’s house, the 532 was exhibiting all kinds of problems.  The sound was popping, cutting in and out, and most of the time was severely distorted.  One of the items that was causing all of the popping was a bad connection in the octal socket between the mechanism and the amplifier.  The power to the amp runs out, and back in, through the connector (except for the tube heaters which are on all of the time).  Also the amp has a rectifier on it which supplies power to the mechanism.

Each female pin on the octal socket is shaped like a “U”.  Over the years with the plug being unplugged and replugged, the top part of the “U” spreads apart and doesn’t make a good connection with pin. I took a sharp ice pick and poked it down into the edges of each hole and bent each arm of the “U” so they would be closer together.  That solved that problem.

However, there was still a lot of distortion.  I opted to bring the amp back for a bench test.  With a bench test, I can inject a known signal and trace it through the circuit with an oscilloscope to see where the signal breaks down. In this case, it was bad at the output of the first tube in the chain.  It wasn’t a bad tube, but a bad electrolytic capacitor connected to the cathode of the tube.  The amplifier will be getting re-capped (replacement of all electrolytic capacitors).  It’s overdue.

Seeburg TSA1

At the customer’s house the amp was distorted especially in the lower frequencies.  Changing the volume had no effect.  It seemed to be in both channels.  There wasn’t good stereo separation.

I brought the amp back for bench testing.  There is a coil in the amp that connects the left and right channels together.  I’ve seen this in other jukebox amps of the late 1960’s, and it doesn’t make sense.  After I disconnected it, I discovered the distortion was only in a single channel.  And it took me a while to discover it as I had to drive the input fairly hard for it to show up.  And it was more prevalent at bass frequencies.  It looked like someone was taking a bite out of the lower side of the sine wave (it wasn’t at the peak).  After checking the bias and the caps around the preamp stage where the problem was, I concluded it had to be the transistor.  I replaced it, and the problems cleared up.  I chalk that one up to weird transistor failure.

Rockola 424 amplifier

The service manual calls this a 40276A.  The amplifier itself has 40218 labeled on it.  Regardless, one channel was completely dead.  The mono switch didn’t have any effect, so the problem was in the output stage.

I brought it back for bench testing. I quickly discovered the 1200uF capacitor used in the feedback circuit had a broken lead.  This amp had been previously worked on by somebody else and they had replaced the capacitor without securing it well.  The vibration of moving the jukebox to Colorado from the east coast probably caused it to break. Since the lead was broken right at the capacitor, it needed to be replaced.

While checking out the amp, I noticed some electrolytic capacitors where leaking.

Electrolyte leaking from capacitors.

Those 100uF capacitors got replaced, as well as the main filter capacitor (2940uF was replaced with a 4700uF).  After cleaning the preamp wirewound pots (not easy or effective), the amp is working well.

The Seeburg SHP3 amplifier will be covered in a future post about a Frankenstein jukebox.

Wurlitzer 3500 “Zodiac” Jukebox

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Location: Longmont, CO

Symptoms:  Would not return after playing record.  Sound playing out of only one speaker (out of 4 speakers total).

As with other Wurlitzers of this era, the trip switch fails intermittently. I haven’t found a source for replacement switches, yet.  But with past experience, opening the switch and spraying contact cleaner inside seems to solve the problem.  The switch is glued together and to open it requires breaking it open.  This is done by inserting an X-acto knife under the edge of the top cover at the opposite end from where the trip wire attaches.  Usually about a half inch of the cover breaks away cleanly, allowing it to be re-glued or taped together again.  The contacts are directly underneath where the cover is removed.  I spray the contact cleaner on the contacts and cycle the switch dozens of times.

When playing records, I noticed the pair of top speakers (tweeters) were not working, and one woofer in the bottom was not working, leaving only one speaker that was working. No wonder it didn’t sound very good.  Wurlitzer, instead of referring to the right and left channels, refers to them as “A” and “B”. I swapped the speaker leads and determined that channel “A” of the amplifier wasn’t working. At the same time, when moving the “A” speakers to the “B” channel, the tweeter of channel “A” worked fine.  So there were two problems with the sound, channel “A” of the amp wasn’t working and the tweeter of channel “B” wasn’t working.

I decided to tackle the amp first.  I swapped the input cables from the tonearm to make sure the problem was not the cartridge or the tonearm wiring.  With my oscilloscope, I traced the signal from the input to the output of channel “A”.  The signal was fine until it got to Q6 and Q8.  Beyond that point, it was dead.  I checked the bias voltage between the base and emitter of each transistor. Q8 was 0.05 volts, which is way below the 0.6V needed.  Q6 was 1.2V, which was double the 0.6V that it should be.

At this point, since I knew I had a bad transistor and that it would have to cross-referenced and a replacement ordered, I decided to take the amp back to my shop for final repair.

Before I removed the amp from the jukebox, I wanted to find out what was wrong with the tweeter on channel “B”.  After checking continuity of the speaker connections with the ohmmeter, and checking the coil resistance of the speaker, the only thing left was the 8 uF capacitor in series with speaker connection.  I bypassed the capacitor with a jumper wire and it started working.  The capacitor is used to block the bass frequencies from coming out of the tweeter.  Somehow the capacitor had failed in an open condition. I added this to the list of parts to order.

With the amplifier at my home shop, I unsoldered both Q6 and Q8 from the printed circuit board to isolate them so I could test them individually.  I checked the base-emitter junctions with the multimeter in “diode” mode.  The junctions on silicon transistors should look like a diode, 0.6V one way, open circuit the other way.  In the case of Q6, it was open both ways.  Definitely bad.  Q8 checked OK and didn’t have any shorts between any pins.

As usual, with Wurlitzer, it is difficult to cross reference transistors because they used their own part numbers on them.  According to the service manual, Q6 is a 130537-5. Sometimes the first place I’ll go looking for a replacement semiconductor is NTE.  In this case, I typed in the part number 130537-5 and got NTE289A.  The basic specs seemed applicable to the circuit, so I was confident it was a good cross reference.  Another source I’ll use sometimes is this page that shows some of the cross references for Wurlitzer transistors.

As I was working on this amp, I noticed some leakage on two of the capacitors.

Capacitors (C19) on both channels are leaking electrolyte.  Click for larger.

A lot of people advocate replacing all of the electrolytic capacitors in a solid state amp of this vintage.  While electrolytic caps do have limited life, I prefer to wait until there are visible or audible symptoms.   The gray caps in the center of the above photo look burnt, but that is just residue from burning dust on some power resistors not shown in the photo.  Also, jukeboxes have spent most of their lives in establishments that had a lot of cigarette smoke, which leaves a residue.

The two leaky capacitors were replaced.  Since the new caps were much smaller with shorter leads, I soldered them directly to the back of the circuit board.  With the transistor replaced, the amp works fine.

 

Wurlitzer 3110, “Americana” Jukebox

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Location: Boulder, CO

Symptom: Sound in one channel stops working after 20 minutes of use.

I have worked on this jukebox in the past for other reasons; it gets a lot of daily use.  When I first arrived, I couldn’t find anything wrong. I could hear sound coming out of each speaker.  I checked all of the connections.

We were on the fourth or fifth record, and I was about ready to leave, when it stopped working. It started with static sounds, then after about a minute, the sound was totally gone.  After further checking of connections, I learned it was sensitive to vibration.  I could tap the final stage of the power amp with the handle of my screwdriver and the problem would change in severity, but wouldn’t begin working completely.  My initial thought was there was a bad connection in the socket for one of the power transistors.

I brought the amp back to my home shop and removed each power transistor and replaced the mica insulators and cleaned the pin contacts.  I also ordered and replaced the larger electrolytic capacitors in the power supply circuit and the final power amp circuit.

Electrolytic capacitors gradually lose their capacitance with age.  I assume this is because the electrolyte paste eventually dries out.  In some cases, especially in high voltage vacuum tube amps, the paste will leak out and the capacitor eventually shorts out.   This amp is over 50 years old, and although the capacitors seemed fine, it’s just a matter of time.

However, after rebuilding the amp, the original problem still existed.  I isolated the problem to Q15.  This transistor drives the transformer that phases the final output stage drivers.  As a double check, I swapped the channel “A” Q15 with the channel “B” Q15, and the problem moved to the other channel.  I don’t know why it’s sensitive to vibration, but there must be a problem with an internal connection inside the transistor case. In any case, it needed to be replaced.

One of the difficulties working on solid state Wurlitzer amplifiers is that Wurlitzer used their own part numbers for transistors, probably even specifying to the manufacturer to print their part number on the case.

Power amplifier stage for channel "B" showing transistors with Wurlitzer part numbers. Note the 6648 number, which is the date code: manufactured the 48 week of 1966.

It’s difficult to figure out what the transistor is and to find a suitable replacement.  Fortunately, with some help from Google and Bing, I found out this same transistor (Wurlitzer part number 125721) is also used in their electric organs.  Somebody figured out it matches an NTE121, Germanium PNP Transistor, Audio Frequency Power Amplifier.

I was surprised to find that somebody was still making germanium transistors.  (99.9% are made from silicon.)  Keep in mind that the forward bias voltage on a germanium transistor is 0.1V, compared to silicon forward bias of 0.6V.  This is something I had forgotten.

The replacement part was ordered and the amp is now working fabulously.