Saturday, February 11, 2017

Restoration work V-7 VTVM, HW-100 SSB XCVR


I tracked down the line cord defect to loose screws inside the wall plug - but the unit still isn't working. I don't even see filaments glowing in the two tubes. There is power coming from the power transformer secondary on the filament windings - but the two tubes are not glowing.

The tubes work fine on my tube checker, so the problem lies somewhere on the circuit board between the pilot light where I measured the voltage and the tubes themselves. Sigh. I had expected this to be a mostly working unit to restore, but the reality is different, as so often true with ebay purchases.

At this point, I will partially disassemble the meter to get access to the board upon which the tubes are mounted and do some continuity and solder joint quality testing. I quickly found the problem. The PCB had cracked, breaking three traces including the filament voltage to the two tubes. 

Board cracked, three traces lost continuity
I bridged the three cracks in the traces with short jumpers, but took the time to inspect and test the components on the board in case any of them warrant replacement. I also saw plenty of oxidation on the rotary switch contacts, which has to be cleaned to return this to useful condition. 

After bridging those traces, I discovered a secondary teeny crack that might be visible in the left side, about middle height in the picture. It interrupts the filament voltate to the tube socket that is off screen, as well as another trace. Will repair those and continue checking. 

Jumpers in place, now time to check out resistor and capacitor values on the board. Just in case, I will replace all six capacitors on the unit. The resistors all seem close enough to spec to be good. Firing up the unit, I found that tubes now lit, the meter did its swing up and down as the 12AU7 dual triode warmed up in the bridge circuit.

I tried the ohms with a dead short and used the VOM resistance measuring device to inject a DC voltage. Behavior seemed reasonable. I have two frozen potentiometers - the zero adjust and ohms adjust dials on the front. I can't get them to move even with shots of Deoxit inside and pliers on the shaft. I suspect these will need to be replaced as well.

I have ordered replacement pots, plus the capacitors, for delivery in a few days. Digikey again, when I can tolerate a few days shipping delay and don't seek near instant gratification.

Meanwhile I cleaned up the contacts on the rotary switches and got this into as good as shape as I can prior to receiving the replacement components. I will set it aside for now.


I received my Digikey shipment of the remaining parts necessary to apply all the recommended modifications to the transceiver. I can't do live testing until the power supply I just bought arrives, but I certainly can install the mods while I wait.

I had been shopping for the power supply itself - HP-23A/B - but realized that it also comes mounted inside the SB-600 station speaker, thus it was worth looking at those auctions. I realized that I could buy an SB-600 for about the same as the power supplies were closing at, but also get the speaker and enclosure. 

By early evening, I had made all the modifications to the audio board and was moving on to other boards. There are many wires hooked to the boards, making it quite difficult to work on them. They have to stay in place on the chassis, which leaves limited access for soldering irons and other tools. 

Each part has to be laboriously unsoldered and removed before a replacement can be put onto the board. In a few places, additional parts are added which at least means no desoldering first. For example, a .005 uf filter capacitor was added on a voltage rectifier tube to sop up high frequency noise. 


We have set a plan for the testing of the fixes I created - this coming Friday during our next Alto session. Hopefully this will get everything working well enough to begin archiving all the cartridges. 

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