233D Transmitter Restoration Project
The 406B-1 wasnâ€™t as bad as expected; when I took off the lacing cord to trace all the leads and draw a new schematic, I discovered that one of the output chokes leads had been attached to the ground sound of the output capacitor. (Spade lug, so someone took it off and then put it back on the wrong terminal.)
No load outputs are now 312 and 306 VDC, which is what I expected.
Back in the rack, dialing the phone dial on the tone chassis did make some relays on the telco chassis click, but still nothing on the impulse relay to step the telco relay. I was going to hook up the scope to see how much output was going to the telco chassis, but I decided to screwdriver the â€śdialâ€ť pot on the tone control chassis and then the impulse relay starting working. The telco relay appears to step with the number of pulses from the phone dial, so it seems to be working. No response when I dial â€ś1â€ť but all the rest of the digits seem to work. The â€śhomingâ€ť function works occasionally when I change from â€ślocalâ€ť to â€śremoteâ€ť and but not consistently. I expected that every time a dialed a new channel, the telco relay would home, but that may not be the case. I think the documentation covers how all that works. Might be that we need the contact closure from the exciter to make everything work.
The power off function does not work when I dial â€śA0â€ť. The documentation says that both the dial on the transmitter and the remote dial must be dialed to shut down the power. That doesnâ€™t make much sense, but it probably isnâ€™t real important.
The Tone Control chassis stays powered up even when the filament contactor is off. The telco relay continues to respond to dial operation. But I canâ€™t get the filament power to come up with dial operation. I think when I select a channel with the phone dial, the transmitter should be turned on.
Noticed Iâ€™m getting about 100 ma deflection on the modulator grid current meter (I think). Also getting some deflection on the VU meter.
Mike bringing in 3 phase variac to bring up the plate transformer gradually. Guess that means Iâ€™d better start working on the HV bleeder. Iâ€™ve found some Ohmite 5K, 5% 10watt resistors that are reasonably priced. Iâ€™m planning to put 8 in series to replace the open 30K WWs currently on the board. The increase the bleeder resistance from 120K to 160K and should result in somewhat less power being dissipated but still should not impact regulation. The board will double in size, but shouldnâ€™t interfere with anything.
| April 29, 2014