Revox B77

A Revox B77 MKI managed to find its way into my collection of audio toys.

From the Revox Classics page

Included in the trade...

The QC checklist - 1982.

The Specifications p33

Specifications p34

This machine was originally the Low Speed version, with 3¾ - 7½ IPS Record/Playback speeds, later converted to a High Speed, 7½ - 15 IPS. This modification required a high speed capstan motor, in addition to various other electronic modifications.

Before powering it up for the first time - it was in storage for a couple of decades - I removed it from the case to look for any signs of burnt, bulged, discolored, or missing parts. I was curious about user experiences with this machine - e.g. common failures, tape and maintenance recommendations, etc. A fantastic place for this is Tapeheads.net.

Tape! Not something that you can find at your local audio store... so I ordered a reel of ATR Master Tape. Another source is Full Compass.

Once I got the tape, I hooked the machine into a Gaines audio converter, to feed the Crown D45 amp, driving a pair of B&W DM602 S3 speakers.

D45

All is good - for awhile...

Electronic components age - just like anything else. One of the most common issues is the failure of electrolytic capacitors - as they age, they open, short, or turn into some resistive value between zero and the infinite.  This can cause any number of problems - anything from not passing audio, to flames jumping from the power supply! The other issue is the tantalum capacitors. Tantalums like to explode - that's another story...

I wanted to make a recording. The machine had been working perfectly - yet this time, the audio was extremely low in level. Often, reseating cards and various connectors, exercising switches, can solve the problem - as contacts oxidize and signals get messed up. Not this time!

The next step was locating a service manual. First, I checked the main power supply voltages - all were within specification. Then, I pulled out each circuit card to look for visible damage.

Well that was easy to spot...

Burnt resistor! Now, why did it burn...

R6 is the burnt resistor, upper right...

The underlined component, a 47Ω resistor, was toast. The circled component, C6, is (well, was...) a 220uf 25v electrolytic capacitor - that due to age, turned into a 20Ω resistor - causing excessive current to flow from the +21v rail to ground - and taking out the resistor.

At this point, I made made the decision to simply replace all of the electrolytics - so I made an inventory - comparing the actual circuit cards to the service manual, and ordered the parts.

Once I get the parts and get everything replaced, I'll post an update.

Wobbulator

An unusual piece of test equipment found its way into my collection of worn, forlorn anachronous  electronic items.

The Triumph Manufacturing Company of Chicago made this wonderful Model 830 Oscillograph Wobbulator.

An excellent description of this item can be found here... http://richardsears.wordpress.com/2011/09/08/triumph-830-wobbulator-oscillograph/

When I first started to examine it for possible restoration, it was clear that it would require more hacking than I'd hoped, some key parts have been removed. I will simply clean it up and use it as an unusual bookend.

The most beautiful thing I found inside is the CRT - I may display it in a little case. It is a beautiful example of early CRT technology.

Dumont 2501A3 CRT

This is a Dumont 2501A3 CRT, a 3" medium-persistance green fluorescent tube, using electrostatic focus. Inside the glass tube near the base, some hand-painted numbers suggest a manufacture date of January 1941, coinciding with the earliest appearance of data that I have found for this tube.

Decisions to make...

I was unable to find low voltage multi-section capacitors to replace C39 and C105 (dual section 1500uf@16 volts). This forces a restoration decision - gut the old cases and hide the new capacitors inside the can, or leave the new caps exposed.

These two multi-section capacitors have a cardboard shell, and I did not want to attempt gutting the shell, as it seemed much less sturdy than the typical metal cannister. Thus, I decided to abandon a strict cosmetic restoration. For my Atwater Kent and Crosley battery radios, I did take the time to hide new components inside old cans, or discretely underneath the chassis. For this project, I am more interested in function than museum display.

It isn't pretty. It just works. Purists will cringe.

New low voltage filter caps - nowhere to hide...

I was able to find new high-voltage multi-section electrolytics.

New multi-section 20uf@475v, next to the old.

C30 - initial solder removal.

I was able to avoid removing the originally connected components for the most part. This required careful desoldering to preserve the length of the original component lead, saving time in the long-run. The components were tested, and were within expected tolerances. Any component can pass simple tests and fail under power. I suspect that these will be fine.

I did remove two of the original resistors for C1, simplifying installation of the new one.

Original C1

New caps

Original input section paper caps...

New caps.

Chester remains unimpressed

I am waiting for a few more parts.

C39...

I chose the easiest part to remove to get started... C39. First, I documented the original component.

Easiest part to remove...

I also made a little drawing.

Is it art?

There are several ways to do this. Since the connecting wires are of sufficient length, I simply cut them close to the terminals of the capacitor. I did use soldering equipment on one wire. This gave me a sense of the proper temperature setting for those cases where I do not want to clip a lead. The solder used in this beast flows very nicely. Another important consideration is whether the device uses silver solder - which has a higher melting point. You do not want to use lead-based solder in that case. Some of the old Tektronix scopes used a silver formulation, and even included a small length of the stuff mounted inside the chassis.

Choose the proper tip

Since the tabs of the capacitors are flat metal, a chisel-tip was selected - the third one from the left. This will transfer the heat in an efficient manner, promoting complete flow of molten metal in the desired area.

Until the restoration is complete, I mark the component and save it. Sometimes replacements have different dimensional characteristics, so it is handy to have an old one if you don't yet have the replacement. There is one more cap like this one - two sections of 1500uf @ 15 volts. I haven't purchased these yet, so off to a supplier website...

New Project - HP 400L

An HP 400L  VTVM found its way into my collection of amusing, anachronous test equipment.

Click for larger view

It came with the operating and service manual, which was printed in 1963.

This VTVM measures AC voltages in twelve ranges - .001 VAC to 300 VAC, Decibel range -72 to +52, with a frequency range of 10 cps to 4 mc.

It weighs about 18 pounds, or in terms of our cat, 1 Chester Unit.

1 Chester Unit

Unlike a lot of old stuff, this came with the original box. It had been sitting on a shelf, unused, for decades. It is exceptionally clean, so I don't have to deal with rust, dust, or other annoyances.

What's Inside?

The following two images show the jumble of parts.

Left interior jumble...

Tubes!

First up...

Never plug-in and turn on old junk like this. Yikes. Since I dislike refinishing metal and all the cleaning, this is a nice project to work on. To make it operational, I will replace the electrolytic capacitors, a few Bumblebees (another common old capacitor type) and other random components as I check out the circuitry. Unlike the Hallicrafters restoration (see the previous project), I will use modern multi-section electrolytic capacitors.

The Suspects...

Bottom of multisection capacitor C1, with associated resistors

I will take several pictures of each component prior to removal. Each part directly connected to the capacitor will be checked out-of-circuit. There are no tell-tale bulges or evidence of heat stress in this area.

Selenium Rectifier

I will research the need to replace this selenium rectifier - I vaguely recall some post in another restoration forum about these things.

Getting the Stuff Together

Which meter to use...

 I was too lazy to look for the test leads for the tube-based Simpson 303. Is it a shock (so to speak) to imagine I have two of those? Not pictured - the Weston 931...

Solder, desoldering stuff...

The Powerstat - to gently ramp up the voltage for testing - in a few weeks!

Next: Deciding where to start.

New Photography Blog

In addition to restoration of old radios, test equipment, and other electronics, I like to play with film and paper based imagery - cameras that are old, unusual, home-made, or modified in some way will be explored. If you have an interest in film, check out:

http://anachronousphotography.blogspot.com/

 

Self Portrait, San Francisco October 2011

Hasselblad 501CM, Kodak Ektar Film

 

 

 

 

 



Hallicrafters SX-96 Mark 1 Manual

Here's the manual!

There was one discrepancy that I noticed when comparing the parts list with the schematic. For any restoration, it is worth verifying part values and circuitry - this may save time. It is frustrating when you discover that you missed one or two components, and have to make additional orders. There will be surprises along the way; however, I like to minimize the interruptions and take the time to get a good idea of parts needed. This also helps locating parts.

Modifications pose additional challenges. Manufactures sometimes substitute parts - similar components may look different due to manufacturing changes, or substitution of similar parts from another source. Values may differ slightly - note whether the part is within the stated tolerance requirements of the specific circuit location. Manufactures also make changes after the schematics are printed - there may be additions or deletions to enhance performance, or reduce part count to reduce manufacturing costs.

The other changes are the result of someone trying to fix something. Some well-intentioned modifications may be baffling and sometimes dangerous. I've encountered mods that don't do anything at all!

The Tiny Mystery

When comparing the parts list to the schematic, I couldn't locate C-86 (.01 mfd 20% 600 V molded paper - parts list page 20A) on the schematic. From the schematic, I noticed that C-85 ( .01 mfd, in the AC line primary after S3B, schematic page 19B) was not in the parts list. The values matched, the component type matched, and there were no other lost capacitors, so the mystery was easily solved. Trust me - it isn't always so simple!

Click on each image for a larger view.

That's all!

Next - look for the HP 400L Vacuum Tube Voltmeter restoration.