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Old 6th Jan 2020, 10:59 pm   #1
IsquaredR
Tetrode
 
Join Date: Apr 2017
Location: Minocqua, Wisconsin, USA.
Posts: 86
Default Hammarlund HQ-180A I.F. Help Sought

I've been restoring a Hammarlund HQ-180A and run into a snag. After replacing the resonance capacitors inside of IF transformers T3, T4 and T5 the receiver is dead. I've double-checked the wiring to each of the inner IF can coils and they are fine however I've discovered an oddity with the B+ voltage being fed to the 455 kc Gate (V17 6BA6) and the 1st Converter (V3 6BE6). While the plate voltage on these two tubes is supposed to be 250 volts DC, it is only 140 volts DC. The voltage coming from the OA2 is good (around 270 volts DC) and the plate voltages on all the other tubes are correct. My problem seems to be in the circuit vicinity of I.F. transformer T3.

There are two red wires going to T3's center pin (Pin "B" on my photo) that measure 270 volts DC; ostensibly this is the B+ input to the transformer. BUT yet the voltage being fed from the T3 (Pin "D" in my photo) to both plates of the 6BA6 and 6BE6 is only 140 volts DC.

Here's another anomaly of transformer T3: Pin "B" has NOTHING connected to it inside the can! Obviously either something is missing or it is simply an unused pin and Hammarlund used it as a tie point. Using the pin as a tie point makes no sense because a straight piece of wire would have been more logical. If there was any tap or coil inside the can connected to the pin with the red wires I would assume that the coil had opened, shorted or developed a high resistance internally. But there is nothing connected to that pin inside the metal can. Perhaps there might have been a short jumper that is not present on my T3 that connects the tie-point pin to Pin "D" (on my photo) which is then fed to the plates of both V17 and V3. But I have no such jumper.

By the way there also appears to be a schematic error with the tap for the B+ voltage for tubes V17 and V3 coming off of a grid bus instead of the B+ bus which feeds the other tubes.

I would be grateful if anyone has their HQ-180A on their workbench and would kindly compare my diagram of the underside of T3 with theirs. There may have been something omitted on my T3 that would account for the plate voltage drop from 250 VDC to 140 VDC (such as a jumper wire between two of the pins on T3. My diagram shows all that is found on T3.

Any help appreciated.
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