Ten Tec Delta Recap: Part 1

Figure 1: With capacitors in hand, recapping begins.

The recapping of the Delta 580 is a project that I’ll document, even though I’m not sure others will find the information interesting or useful. As time goes on, there are fewer and fewer of us old analog radio aficionados. Let’s face it, old analog radios had an Achilles heel. Unlike digital radios, in analog radios the analog signal path is subject to signal purity degradation all along its path. Digital radios mostly have that problem only at the front and back ends. So, with an analog radio comes a bunch of signal conditioning devices in the form of electrolytic capacitors. The Delta has thirty nine of them.

Figure 2: Board 1: 4 caps, 3 bad. one good.

As I mentioned in the introduction of the 580 blog post, the old Delta was sounding pretty raspy. I took a look at the signal path, and decided to recap the TX path first. I think most would agree that is the most important signal purity issue. So, glancing at the block diagram, I decided to pull the SSB gen board first (shown in Fig 2). I hope it is not a harbinger for the rest of the boards, because of the four electrolytics on it: one was thoroughly trashed, two had objectionably high ESR, and the last capacitor tested good. I’d kind of like to see the ratio working the other way around (hihi).

Figure 3: TX carrier signal much improved after replacement of bad cap on board 1.

The biggest capacitor on the SSB gen board (a 33 uF job) had a 50% change in capacitance value, a double ESR, and a little leakage. Perhaps that is why the signal was 100% improved on the scope shot taken after the board was reinstalled back into the Delta 580. I can’t stop now though LOL. I already bought the caps, and I’m in the mood to do it. I have to say that Ten Tec’s modular board arrangement makes this easier than it’d be on a monolithic board rig. I could be done now, and I’ve not have taken the entire circuit out of the case. It takes a couple minutes to pull a board in the TT rigs, and a extra minute to put it back in (“in” is harder than out because you have to keep the spacers from falling off hihi). I probably have ten minutes in the SSB board’s recap desoldering and resoldering procedure.

So, the straight scope display does not show any appreciable hum modulation. But, the scope input taking the AM detector output from my test jig does indicate a touch remains. So, we sill continue plunking away …

Figure 3: The IF Amp board is the next board on the signal path

So, I moved on to the IF Amp board. It’s the next board on the path that contains electrolytics. If one looks at the picture in figure 3, one gets a bit of the impression that says he’s looking at a DIY board. I kind’a like that about Ten Tec’s earlier stuff. It’s about what I might do if I were doing an HB job on the board. Definitely not mass produced. I have a vision in my head of the little ole lady stuffing the board at a table in Sevierville, TN in 1981.

Will give the results of board #2 on the next post. 73s. – Ron / WB8LZR


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