Tag Archives: SHFA1

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 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!