Tektronix 575 Mod 122C Curve Tracer Restoration Project – Part 1

A while back I picked up a Tektronix 575 Mod 122C transistor curve tracer from Craigslist and just started digging into it. With some encouragement from reader petepdx, I’ve started actually looking into fixing it up. (Click the photos for a much larger view!)

Inside, there’s some pitting and build-up, and some mildew visible around the CRT socket. There’s also a good bit of detritus floating around, from what looks like an insulator or spacer near the base of the CRT which disintegrated.

Nearly every tube was cracked in the socket. This must have taken a heck of a beating at some point. Dropped, maybe?

Using a shop-vac with a miniature brush, most of the dust and grime and bits of gasket cleaned right up. It’s actually not in as bad of a condition as it seems initially, at least cosmetically.

This doesn’t look great, though:

I do have a spare CRT for this unit, which seems to be in good shape. I haven’t pulled this CRT to see if it’s damaged.

There was a bit of dust, and white whispy build-up which could have been mildew on the paper insulation of most of the transformers.

Lots of surface corrosion in random places, too.

The inside of the Collector Sweep power supply doesn’t look too bad, though…but there’s that white discoloration on the main collector sweep power transformer still. Up top, not too bad. petepdx noted there should be a spool of silver solder inside – which there is!

It is desirable that only silver bearing solder be used on the ceramic terminals and for tinning the iron. Ordinary tin-lead solder may be used but repeated use will break the solder-to-ceramic bond. See your instruction manual.

Big power supplies on the bottom.

Lots of surface rust on one side of the main power transformer. On the panel-mount rectifier diodes, too.

Up top, there’s the horizontal and vertical drive circuits and the step generator circuits.

Finally, underneath is the CRT anode power supply, among other things. Some more white build-up here.

I also have a handful of the front panel switches, a full set of good pulled tubes, and a CRT from a 575 standard variant if I need them for spares. Overall, this is a bit of an oddball. There’s the mildew on the transformers, some mildew around the CRT, some evidence of water damage in the corners…the fuse holder is missing its cap, and was full of mud, and there’s some corrosion on the bottom deck neon bulbs. Otherwise, though, it’s actually surprisingly clean.

I’ll be going through it with a damp cloth shortly to remove even more grime and dust, and will post some photos of that step, then start looking into component replacements.

Possible plan for the resto:

  1. Thorough cleaning of grime and dust, oil fan bearings, clean/replace filter.
  2. Check and replace carbon comp resistors as needed.
  3. Replace tubular capacitors. Counted 18, may be some more hiding in there somewhere.
  4. Replace neon bulbs which appear in several circuit locations.
  5. Evaluate and replace as needed panel mount and ceramic terminal strip silicon diodes.
  6. Evaluate replacing HV power supply rectifier tubes with silicon diodes.
  7. Replace electrolytic capacitors. Quite a few of those.
  8. Visually inspect CRT.
  9. Test and replace tubes.
  10. Power it up with a Variac and hope nothing catches on fire.

It’ll be a big job to give it a full overhaul, hopefully nothing is too badly damaged which would make it impossible…

Philco 39-45 Retrofit to 40-201 Cabinet Vintage Radio Repair

Cross-posted from the Rain City Audio Repair Blog:

A local customer approached me with his family’s Philco 40-201 vintage radio to be restored back to working condition. It had sat for several decades, very well preserved, but ultimately needed some service after the years.

Very unfortunately, however, the original chassis had a bad case of the notorious Philco coil rot and while its audio section was fully intact and the power supply in good condition, the RF coils were rotted and couldn’t be repaired. Thus it was time to find another chassis to put into the opening.

The oscillator coil, along with most of the RF coils, were the defective components on this one. This was a particularly insidious failure: the coils tested okay with a multimeter, but failed to oscillate due to green coil rot inside the windings, shorting turns together and killing the resonance. It took quite a few hours of diagnostics to narrow this down, since it was so subtle.

There are several chassis that will fit the opening, but the one I happened to find was from a Philco 39-45. They’re identically shaped, have the same dial scale, but some circuit differences. While the 40-201 uses primarily loctal tubes with the exception of the rectifier and #42 power output tubes, the 39-45 uses a mixture of octal and pre-octal tubes. The 39-45 uses a three-gang tuner and a tuned RF amplifier stage, which actually makes it a bit better of a performer than the 40-201.

The replacement chassis came partially recapped and verified working; I replaced the remainder of the capacitors and did an alignment.

Finally, it was time for an alignment. It peaked up pretty close.

Finally, back in the case!

On the first trial run, it sounded fantastic – picking up AM stations across the dial, and very surprisingly about a half-dozen shortwave stations in Arabic, Spanish, English, and several other unidentifiable languages. It’s going to be a great performer – and a wonderful family heirloom – for a long time to come.

Rain City Audio Vintage Radio Repair

Harman/Kardon Control Amplifier A-401 Restoration

This little Harman/Kardon Control Amplifier, model A-401, just came through the shop for a full overhaul. It was in a pretty sorry state to begin with – one channel distorted, one barely gasping for air, and a world of scratchy controls in general need of attention. Despite the electrical shape, though, it was in great cosmetic condition and definitely worth a full rebuild.

Physically, it’s a very unassuming piece of gear with elegantly understated styling, the height of ’70s simplicity. Featuring up to 20W output per channel with low distortion, it’s a great small desktop amp or perfect for pairing with a vintage hi-fi system and will fit in with nearly any decor. No wonder the owner wanted it fixed up!

The inside was untouched, which is always nice to see. It’s a very open, accessible construction design. All of the PCBs could be accessed by removing the top and bottom covers without removing any boards from the chassis itself, and every component was easily accessible. After 40+ years, the original 2SC1030 transistors had tarnished pretty significantly but otherwise everything was in decent cosmetic shape.

For whatever reason, this amp had two tantalum electrolytic capacitors – the blue components on the right of the vertical board – but the rest were all standard aluminum electrolytics. I set out for the component replacement which was very straightforward. As expected, the caps were failing and had started to leave some residue on the boards.

All the electrolytic and tantalum capacitors in this amp were replaced with Nichicon Fine Gold audiophile grade electrolytic capacitors. These are much better components than anything that was available at the time and really help to bring out the best in these vintage amps.

Finding drop-in replacement capacitors is pretty tough, but I got lucky with these. Left, the original 6800 uF 35V capacitors, and right, a pin-compatible drop-in replacement of 6800 uF at 100V. Easy! Running massively de-rated like it is, and with the advantages of modern construction techniques, these main filters will almost certainly last decades. It’s always possible to make modifications to use whatever capacitors are available, and sometimes it’s unavoidable (10 mm-spacing screw terminal radial capacitors come to mind)  but it’s much easier, and cleaner, when there’s a drop-in replacement available. This amp could take up to about a 10000 uF capacitor in that position and still be within tolerances, so there’s some flexibility.

Even after all these repairs, it sounded fantastic – from one channel. The other channel was cutting out intermittently and had hum and distortion. While probing, the amp blew both its power supply fuses. It turns out that one of the mica insulators under the output transistors was damaged and wasn’t insulating very well; there was a conductive path to the heat sink. These transistors were pretty worn, anyway, so the owner approved a minor upgrade to a modern, new manufacture TO-3 output transistor with much improved ratings – the MJ15003.

During the troubleshooting process, I did identify the defective 2SC945 transistor in that channel’s driver circuit which was causing some distortion. It was a part of a push-pull pair, but only delivered about 1/10th the gain as its compliment on the other side, thus causing the distortion. Replacing it cleaned up the worst of it, but the channel was still popping and snapping a bit for several minutes after powering on. A quick check of the driver transistors in that channel found a few that were weak, intermittently conducting, and poorly matched – so they got replaced, too.

That took care of the problem!

After replacing the parts, everything checked out! It was time to adjust the bias to ensure it’d be a safe operation, then on to performance characterization.

I was very impressed with the results of the tests. This amplifier was flat +/- 0.3 dB  through my distortion analyzer’s signal generator range, 10 Hz – 20 kHz. The MJ15003 is also much more capable than this amplifier is asking of it so there’s quite a lot of headroom, and the more efficient signal path due to the up-rated components plus more powerful output transistors resulted in 50% more measured power output at very low distortion.

At 1 kHz, THD was below 0.1% through most of its original power rating of 20%, beating its specification by 67%. This one, however, was able to deliver 30W with a 150 mV input at maximum volume, still with a THD below 0.5%. That’s audible to many ears but still listenable for the few times you might really need the extra power.

This was a fun project, and it’s going to be a great little amp for a desk or an apartment for a long time.

EICO Stereophonic Integrated Amplifier HF-81 Repair

From the late ’50s, I had this nice example of an EICO HF-81 amplifier come through my shop. These are nice little stereo amplifiers with a built-in phono pre-amp and RIAA equalization and feature about 14W of power output per channel from a pair of push-pull EL84s each at about 1% distortion.

EICO was a popular brand of electronics kit maker back in the day. They made a lot of hobbyist or entry level shop test equipment (oscilloscopes, generators and testers) but had a handful of hi-fi stereo kits as hi-fi listening grew more and more popular. The fact they were assembled as kits makes them pretty straightforward to service, too – the layout is clean and there’s a lot of room on the chassis. It had been worked on a few times in the past – several coupling capacitors were replaced with early film caps in what looks to be the late ’60s/early ’70s. Just recently, the owner reported it was giving noise from one channel and took it to another local shop where the technician replaced a 50 uF capacitor (orange, bottom left) with a 100 uF cap that looks to me like it was probably an old-stock capacitor (blue, bottom left.) They’d clipped the old cap out of circuit but left its shell intact; not exactly what I’d call a best practice. More problems started cropping up shortly thereafter as the rest of the original capacitors degraded, so it came to me for an overhaul. After replacing the parts, there was still noise,  distortion, and lower volume on one channel seen on the oscilloscope screen in yellow. Tracing through the circuit, it turns out the CRL couplets in the tone network were failing.

I fabricated new PCBs with the same values to replace the damaged parts and mounted them to the shield.

That fixed the problem right up! After some final tweaks and a burn-in test, she’s ready to go home. Lots of parts out of this one:

It’s going to sound great for years to come!

Rain City Audio Tube Audio Repair

Esteemed Bose® Model 901 Active Equalizer – Serial #230 Repair

Cross-posted from Rain City Audio:

I had the distinct pleasure of working on one of the earliest  Bose® Model 901 Active Equalizers from Series I’s first production run. Serial #230, in fact! It’s made entirely by hand and shows traces of hand re-work as the design was being tweaked – really a piece of history!

This particular equalizer was a gift from Dr. Amir Bose himself to its current and only owner as an undergrad on a factory tour in the early ’70s, and after a lifetime of enjoyment it was time for a rebuild.

Incredibly, this one still has the “Acoustic Suspension Loudspeaker” decal in the center of the treble contour knob.

The owner reported it was having several issues, and opening it up one problem was obvious: the bass contour inductors had broken free of their mounts and were rattling around inside.

It was fairly similar to the later Series 1 Early boards I’d serviced, but had different style inductors and a few extra jumper wires.

It did have discrete Zener diodes installed instead of the reverse-connected transistors. And as for the transistors – the early production models used the 2N3393, and on this one they were so worn out they were only delivering about 20% of their normal gain tested out of circuit. That’s definitely no good!

It cleaned up very nicely, though:

This equalizer is restored back to full performance for its owner to enjoy for another few decades. With all new Nichicon Fine Gold electrolytic capacitors, 1% resistors, new miniature wirewound inductors, rectifier diodes, transistors, Zeners, and a thorough switch cleaning this one is good as new and will sound great for a long time to come.

Rain City Audio Bose® Active Equalizer Repair

1934 RCA Victor 119 Antique Radio Repair

I just finished a new project over at Rain City Audio – 1934 RCA Victor 119 Antique Radio Repair

An example of the 1939 RCA Model 119 radio made its way through my shop recently. I like RCA’s engineering – they were pioneers of radio and even their entry level models like this set had great performance. The styling is classic mid ’30s, too – great in almost any home….

Check it out at http://www.raincityaudio.us/blog/1934-rca-victor-119-antique-radio-repair!

Oregon Electronic Model A3 Variable Voltage Regulated Power Supply

Here’s another project I just finished at Rain City Audio:

This vacuum tube power supply from an engineer’s test bench can replace a radio or test circuit’s power supply to help with troubleshooting or experiments, with up to 400V for B+ and 6.3V 5A for the heaters. It needed some new capacitors, then fired right up and works perfectly!

Oregon Electronic Model A3 Variable Voltage Regulated Power Supply