26-07-2020, 07:43 AM
I forget why I bought this a couple of years ago. It was missing the strap and knobs. Then shortly later I saw another with the mains adaptor on the back. It proved to be a wreck internally; the ferrite rod aerial was broken in two places and the wavechange switch was broken. Perhaps it had no valves? I can't remember. Anyway I got out the chassis and contemplated the switch needed to test 1R5/DK91, DK92, 1L6 or DK96. I think all the RF/IF amps can work in the same socket except the DC90: 1T4/DF91, 1U4/DF904, DF906, DF92, DF96 or DF97. I have all of those except the DF906. Maybe a DC90 can be tested as a poor sort of IF amp? I do know a DF97 and DC90 can both work in the same socket as a VHF Mixer/Osc without realignment as the DF97 is triodised in that application.
There are two variations of connection for the common detector / preamp: 1S5/DAF91, 1U5/DAF92 and DAF96. The 1U5/DAF92 swaps diode and anode, which is a layout matching anode on the other types. The DAF961/1Б1П is in the 1.2V family and is a 3rd layout. I'll ignore it.
Then there is the output type; there are about 10 different ones. Some differ only in filament. The 1.2V types are different. Likely the 1S4/DL91, 3S4/DL92, DL95/3Q4 and DL96/3C4 are the most common.
Anyway, I hadn't restored the complete one other than putting the knobs and PSU on it because access to replace caps looked hard. It turns out that there is really only one paper cap actually really under the chassis. One of the three chassis nuts is under the 16uF 150V HT cap. It can be pushed aside if the tag is unscrewed. It's best to unscrew the speaker and the back plate for the PSU so everything can be connected on the bench.
No IF. Replacing the 1F1 Mazda with a DF96 fixed that. Now no RF. It's a shiny Bentley DK96 with the mysterious white stuff inside at the base. I've seen that before. Getter on top black and shiny. Still nothing with a known good DK96 (ha!). Eventually I decide to check the LO coils. Two coils on a 6 pin former.
One tag isn't connected. The coils can be deduced from the circuit. So the unconnected pin is the common of the primaries. either side is HT feed and 0V. Both LW and MW secondaries are open circuit!
I examine the coil in the other chassis which I'd removed the tuner from. It too had open circuit secondaries! Examination revealed that two wires had snapped at the tags. Not enough slack. One is the common between the MW & LW coils. So I remove the coil set in the model under restore and see it's the same! It's nearly 1 am. Time for bed. Either I'll add a solid pair of wires for lower down tags, or add something fine and flexible.
There are two variations of connection for the common detector / preamp: 1S5/DAF91, 1U5/DAF92 and DAF96. The 1U5/DAF92 swaps diode and anode, which is a layout matching anode on the other types. The DAF961/1Б1П is in the 1.2V family and is a 3rd layout. I'll ignore it.
Then there is the output type; there are about 10 different ones. Some differ only in filament. The 1.2V types are different. Likely the 1S4/DL91, 3S4/DL92, DL95/3Q4 and DL96/3C4 are the most common.
Anyway, I hadn't restored the complete one other than putting the knobs and PSU on it because access to replace caps looked hard. It turns out that there is really only one paper cap actually really under the chassis. One of the three chassis nuts is under the 16uF 150V HT cap. It can be pushed aside if the tag is unscrewed. It's best to unscrew the speaker and the back plate for the PSU so everything can be connected on the bench.
No IF. Replacing the 1F1 Mazda with a DF96 fixed that. Now no RF. It's a shiny Bentley DK96 with the mysterious white stuff inside at the base. I've seen that before. Getter on top black and shiny. Still nothing with a known good DK96 (ha!). Eventually I decide to check the LO coils. Two coils on a 6 pin former.
One tag isn't connected. The coils can be deduced from the circuit. So the unconnected pin is the common of the primaries. either side is HT feed and 0V. Both LW and MW secondaries are open circuit!
I examine the coil in the other chassis which I'd removed the tuner from. It too had open circuit secondaries! Examination revealed that two wires had snapped at the tags. Not enough slack. One is the common between the MW & LW coils. So I remove the coil set in the model under restore and see it's the same! It's nearly 1 am. Time for bed. Either I'll add a solid pair of wires for lower down tags, or add something fine and flexible.